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October 16

Trackside thing

What is this?

Over the past few years, thousands of these things have appeared alongside UK railways. What are they? They are about five feet tall and in groups of maybe 30-60 spaced about 20 feet apart. This one was quickly snapped on my phone near Milton Keynes.--Shantavira|^{feed me} 08:02, 16 October 2013 (UTC)

They are trackside lights for night inspections. SourceMike (talk) 13:53, 16 October 2013 (UTC)

Ah, that's interesting. Thanks.--Shantavira|^{feed me} 15:53, 16 October 2013 (UTC)

No problem! I found it just as interesting! Mike (talk) 15:54, 16 October 2013 (UTC)

Children crying because they can't do something?

Is it common for children to cry because they find something difficult? I recall crying about age 10 because I found it too hard to join up my writing (my cursive is now pretty neat IMO) which seems a strange thing to get upset about. --129.215.47.59 (talk) 10:43, 16 October 2013 (UTC)

The Wikipedia article titled Crying states "Crying is believed to be an outlet or a result of a burst of intense emotional sensations". The inability to complete a task can bring on intense stress, which would be an "intense emotional sensation". --Jayron32 10:48, 16 October 2013 (UTC)

If people around you are doing something effortlessly, and being praised for it, but you can't do it yourself, that's pretty frustrating. Happens to adults too. I'm in my sixties and still can't do joined up writing, not legibly anyway. Fortunately one develops a sense of perspective with time.--Shantavira|^{feed me} 11:06, 16 October 2013 (UTC)

A child might well cry because he does not understand some homework requirement such as finding the common denominator in order to add fractions, or writing a book report. Crying can also be a learned response, or a form of manipulation, to get a parent to do the homework for him, or to just avoid doing it at all. I did not see in our article about crying mention of crying as a learned or intentional tactic, or a form of manipulation,or a for of emotional blackmail to get something or to avoid something, although books say it sometimes is. Edison (talk) 21:48, 16 October 2013 (UTC)

I suggest that you pretend to cry, record yourself, and play it back to see how well you did. My guess is that you, like almost everybody else, would be laughably bad at it. --Bowlhover (talk) 05:10, 17 October 2013 (UTC)

I can cry on command, it's very lifelike and I doubt anyone who didn't know me extremely well would be aware it was fake. A lot of my ex-girlfriends, and a few ex-boyfriends (women seem better at this), could do the same with varying degrees of realism- but all on the believeable end of the spectrum. Then again, I know a lot of actors, and while not all of them were, this may bias things a bit. At any rate, it isn't that difficult to do consciously, and if it is a learned response, it might not even be being done consciously- in other words, it's neither implausible nor infeasible.Phoenixia1177 (talk) 06:47, 17 October 2013 (UTC)

When I was four the ice cream truck came around, and when I asked if I could have some I was told to get the money off the counter, which I then handed to my dad, since I had never bought it before myself. He told me to go to the truck and buy it. I said I thought he was going to help me, and burst out crying. It was more traumatic than having stuck my fingers in an electric socket and than having sat on a fire ant nest, neither of which seemed like a betrayal. I don't believe I ever cried like that again, although a few years later I did throw up when we had a substitute teacher who was administering a spelling test without giving us enough time to write down the answers. μηδείς (talk) 02:59, 18 October 2013 (UTC)

Power used by mobile phone and radio towers

When a load is connected to the secondary of a transformer, the power drawn by the primary from the source increases because of the magnetic coupling. Will something similiar happen when a mobile phone is switched on? Suppose a thousand phones get switched on (and hence get 'connected' to the tower), will the tower use more power? The same question applies to radio transmission towers. Does a radio tower consume a more power when radios get tuned to it's frequency (and hence gets 'coupled')? - WikiCheng | Talk 10:53, 16 October 2013 (UTC)

From first principles of physics, we know that a transmitting antenna's effective impedance does change due to the presence of a receiving antenna, even if the receiver is many miles away. But that effect is tiny - you can do the math to verify. In the case of ordinary telecommunications, radio antennas operate in the far field (as opposed to near field). Definitionally, this means that the effects of the receiving antenna are too far away to matter.

A much more prominent effect is that modern digital telephones use a bidirectional protocol. Telephone transmitters are not broadcast towers: they are nodes in a many-node, asymmetric full-duplex communication. The transmitter has more work to do when multiple devices are attached. Perhaps the easiest protocol to intuitively understand is time-division multiplexing; adding more telephones would require a higher duty cycle; in other words, the transmitter is on for a longer part of each time interval, and therefore uses a higher average power.

Depending on where you are, and which company runs your mobile telephones, time-division multiplexing might be supplanted by more advanced digital communication protocols; but in principle, whichever scheme they choose will have the same general relationship between number of users and total transmitter power usage. (Thanks to the rule-of-thumb about circuit design, the gain-bandwidth product, we can relate the engineering tradeoffs between time- and frequency-multiplexing of the transmitter design back to first principles of physics, and the conservation of energy, and so on). Whether you spend the power over a wider frequency-spectrum during a short interval (typically, using complex digital codings); or if you use a narrow spectrum for a longer interval (using time-division scheduling); the same power-bandwidth product gives the same signal-to-noise ratio. In actual designs, practical details may shift the optimal choice in one direction or the other. So, this gives the engineers who design radio protocols a little room for flexibility, and lets them pick the best-available scheme that is implementable in today's electronics technology.

In closing, I should mention that the concepts of base load and variable load also apply to transmitters; it is plausible that for a large transmitter, the base load is so close to the variable-load that the transmitter's power supply cannot reasonably switch modes, or otherwise deliver a variable quantity of power. Such large power-supplies are difficult to design efficiently, and this is an active area for new engineering research and development. Now that software can switch transmitters on and off as fast as, say, once per millisecond (!), power supplies need to be designed that can toggle between peak and idle at rates very close to those software latencies. This seems trivial to the engineers with backgrounds in software and digital systems, but as the power supply designers need to build capacitors and inductors and so forth, they are constrained by device size and switching time. So, while controlling a digital signal at two gigahertz is very easy using today's computers, swinging a couple hundred kilowatts on and off at even one kilohertz is quite difficult. Compound this difficulty by the fact that your cellular tower is sometimes in a remote area that might not connect to a utility electric grid: it might have its own diesel engine or gas turbine... Nimur (talk) 13:37, 16 October 2013 (UTC)

Thank you - WikiCheng | Talk 06:03, 21 October 2013 (UTC)

A geometrical analyze of camera capture blur?

If one takes a picture with a camera like the Canon EOS 5D Mark III which has a image sensor with a size of 36 x 24 mm and 5760 x 3840 pixels. That captures an object that moves 0.178 meter sideways during the exposure time at a distance of 180 meters from the photographer. How many mm or pixels will the light from the object traverse during the image exposure? I suspect the distance from the middle of the lens to the sensor plays role here but don't find any data to calculate with. Electron9 (talk) 12:15, 16 October 2013 (UTC)

You have not supplied one vital piece of information. You need the focal length of the lens in use. The pixels traversed will obviously be greater if the lense is set for a higher zoom-in. 120.145.145.144 (talk) 12:43, 16 October 2013 (UTC)

I updated the distance to 180 m. And found this EXIF info which I hope completes the input data set:

Parameter	Value
ApertureValue	7,00 EV (f/11,3)
FocalLength	120,0 mm
FocalPlaneResolutionUnit	Inch
FocalPlaneXResolution	3942,5051
FocalPlaneYResolution	3950,617
XResolution	300/1
YResolution	300/1

Perhaps the EXIF "FocalLength" is another type of focal length ? Electron9 (talk) 14:51, 16 October 2013 (UTC)

Assuming that focal length is correct, the object's image would have moved 120(.178/180)=.1187 mm during the exposure time, which at 160 pixels/mm amounts to 19 pixels. The 36 x 24 mm sensor size is the same as the standard 35mm image size, so at least there's no distinction here between the real focal length and the 35 mm equivalent focal length. Red Act (talk) 17:16, 16 October 2013 (UTC)

It's occurred to me that your phrase "another type of focal length" might possibly be due to your finding the comma in "120,0" to be confusing, because it looks like two numbers separated by a comma, or a number in the thousands with an inadequate number of zeroes after the comma. It actually just means 120.0 ; a comma is used instead of a decimal point in many parts of the world, including most of Europe and South America. See Decimal mark. Red Act (talk) 21:01, 16 October 2013 (UTC)

Considering the camera dimensions are 152 x 116 x 76 mm, I find the 120 mm focal length rather large. Electron9 (talk) 06:10, 17 October 2013 (UTC)

In the picture in the link you gave, it looks like the camera's largest dimension is along the optical axis. And the dimensions don't appear to be for just the body of the camera without the lens, because 76mm would be an extraordinarily thick camera body. So the 120mm focal length seems quite plausible to me, because it's 32mm less than the 152mm total size of the camera along the optical axis. Red Act (talk) 11:57, 17 October 2013 (UTC)

Can natural remedies also be synthetic?

Among the natural occurring substances sold as pharmaceutical drugs (take for example, melatonin or 5htp), could it be that you find several synthesize substances? That would be funny, since some people take them because they want to avoid artificial substances. OsmanRF34 (talk) 15:34, 16 October 2013 (UTC)

Your terminology is wrong -- in the US those things are sold as dietary supplements, not as pharmaceutical drugs. My understanding is that in the US, if a chemical is synthesized, it is treated as a pharmaceutical drug, and the manufacturer has to provide proof of safety before it can be sold. If it is derived from a plant or animal, it is treated as a dietary supplement, and the burden of proof is in the other direction. Looie496 (talk) 15:50, 16 October 2013 (UTC)

I doubt that's correct Looie. There are synthesized "artificial" products that are not drugs, such as many "artificial flavors." There are specific definitions of "drug" and so I can't speak to them all, but at least some of those do not turn on whether a product is synthesized. I would need to see a good cite before I believed Looie's comment. Shadowjams (talk) 23:07, 16 October 2013 (UTC)

See the section about the DSHEA in [1], it discusses the shift in burden of proof to the FDA- the initial segment discusses that drugs must undergo stringent testing. Here's the actual text of the act (DSHEA), [2], see section 4 about the burden of proof issues; this, also from the FDA, [3] mentions that the FDA is required to take action after it is on the market. This from the FDA, [4], states that supplements do not require approval. For an FDA def. of a drug, see section 321(g)(1) of [5]- in that section, it mentions a distinction between drug and supplement relating to section 343(r), which can be found in this document, [6]Phoenixia1177 (talk) 09:08, 17 October 2013 (UTC)

By the way, this is in relation to the burden of proof claim- natural -vs- synthetic does not appear to enter into the issue, it appears to be related to what is being claimed. I haven't done the research on that specific aspect, but nothing I've read seems to draw a distinction, it is not directly in the definitions- indeed, the linked ones would appear to refute the claim, unless they were amended somewhere.Phoenixia1177 (talk) 10:09, 17 October 2013 (UTC)

But the lines gets blurry when the same chemical can either be produced by nature or in a lab. StuRat (talk) 19:57, 16 October 2013 (UTC)

This makes me think of 1080 poison, it is used to eradicate possums, which an introduced pest in New Zealand. If I'm not mistaken, being sodium fluoroacetate, it is a salt of a "natural substance", isolable from certain plants. So 'natural/organic/non-synthetic' are utterly useless terms when determining the toxicity of a substance. Plasmic Physics (talk) 00:29, 17 October 2013 (UTC)

If the absolute structure of a synthetic substance is exactly the same as that of the naturaly derived substance then it is considered bioidentical and as long as it is very very pure then it is as "good" as and as "healthy" as the purified "natural" one. This is essentially an economic desision, is it cheaper to extract from the natural source e.g. morphine or synthesize e.g. ephedrine and pseudoephedrine. Die Antwoorde (talk) 08:07, 17 October 2013 (UTC)

<rant>I find the "natural = good for you, artificial = bad for you" philosophy one of the strangest perversions of logic of modern times. Firstly an artificial substance that is chemically identical to a natural one is indistinguishable from it - it doesn't remember how it is made, it is, literally, the same thing. Secondly the statement is patently, demonstrably ridiculous - arsenic, mercury, uranium, snake venoms, hemlock, amatoxin, and all infectious diseases are entirely "natural", and not one of them is good for you. Paracetemol, aspirin and hundreds of other drugs are entirely synthesised and, if taken appropriately, cause little harm compared to the amount of good they do. Thirdly it is not even applied consistently - I have often seen vaccines disparaged because they are "artificial" when they are in fact one of the most natural therapies I can think of - priming the immune system to respond to previously encountered antigen is entirely in harmony with how the immune system naturally works. It is certainly more natural than many alternative therapies, take accupuncture - in what situation in nature are needles inserted in extremely specific locations on the body? This is not even getting into the strangeness of the philosophical position that regards humans and what they make and do as "not part of nature".</rant> Equisetum (talk | contributions) 11:24, 17 October 2013 (UTC)

For natural vs nature identical)The difference resides in the impurities present!

For compouds that are "human designed"?)It is mainly a hstory of safe use thing. And it is not like we even know all of whats naturally in everything or if cretain "human designed" desigen compounds naturally occur in something! Die Antwoorde (talk) 12:55, 17 October 2013 (UTC)

Fair enough - if the impurities make a difference then they make a difference, but then the issue is the impurities, not the natural vs. artificial origin (if you could replicate the impurities as well the artificial stuff would be just like the natural). Likewise with the history of safe use - the issue is the history of safe use, not the origins, a newly discovered natural substance is more risky than an artificial compound with a long history of safe use. I accept that natural vs artificial can often broadly correlate with both these things, but I find the masking of the real issues behind the facade of natural vs artificial to be unhelpful at best and profoundly damaging to people's ability to make rational decisions at worst. Note that I'm not accusing anyone in this thread of this at all - in my opinion it's predominantly the mainstream media and the food and supplement industries that are responsible (along with the generally abysmal standard of school education in critical thinking and the assessment of evidence). Equisetum (talk | contributions) 14:40, 17 October 2013 (UTC)

What about enviromental etc differences between extraction of natural vs synthetic not just the bottom line $$. 122.111.240.138 (talk) 15:37, 17 October 2013 (UTC)

Yes, agreed - where it is environmentally better to naturally extract a compound that should be taken into account (and often isn't - I didn't claim that I'm in favour of pharma and agritech's approaches to the issue either, quite the opposite!), however where it is environmentally better to artificially synthesise a compound (such as where extraction by e.g. distillation uses a lot of energy, or where the substance originates from an endangered species), that should also be taken into account. Again, all I am saying is that natural vs artificial is a poor proxy for the real issues such as this. Equisetum (talk | contributions) 16:12, 17 October 2013 (UTC)

But, wouldn't the probabilities of impurities be lower in the case of artificial compounds? It seems intuitively easier to synthesize a pure compound than extract a pure one from some organic matter. OsmanRF34 (talk) 15:03, 17 October 2013 (UTC)

Yer but the impurities will likely be not natural stuff with no history of safe use.

A fair point - now we are getting to real issues - I am indeed willing to accept that in the specific case of a substance which has long been safely used in a natural preparation, the risk of using a new artificial preparation is possibly elevated due to the new impurities (assuming that the preparation can't be demonstrated e.g. by mass spectrometry to be effectively impurity free). It may also be less effective due to a missing beneficial impurity. However, this is equally true of a new artificial synthesis method which replaces an older one - it's about the change of synthesis method, not natural vs artificial, the trouble is, lots of thinking on this is clouded by the fact that most older, more well established preparations happen to be natural due to the relative youth of the field of synthetic chemistry. Equisetum (talk | contributions) 16:01, 17 October 2013 (UTC)

Yes, this is what I mean about general, broad correlates. It's important to bear in mind though that a) artificially produced substances will have impurities at some level as well, such as reaction intermediates and degradation products, they will just be different ones to the natural products (and they can be in any amount - it entirely depends on the degree of purification, it may be easier to get a pure compound artificially, but the purity you end up with still depends on how much effort you put in to purification) and b) impurities, whether natural or artificial are probably about as likely to be bad for you as good for you, and many are going to be completely neutral (I don't have any source for this, it's just naive logic - I would be interested to hear if anyone has done studies on this point). Equisetum (talk | contributions) 16:01, 17 October 2013 (UTC)

There is also the possibility of introducing new impurities (even into the natural products} through the purification processes. How pure does the synthetic caffine that goes into coke have to be I wonder U.S.P ?. — Preceding unsigned comment added by 122.111.240.138 (talk) 16:28, 17 October 2013 (UTC)

Caffeine has to be at least 98.5% pure to meet the USP standard apparently [7], so still some room for impurities there, but not that much. The added impurities thing was what happened to Dasani, a UK brand of bottled "purified" water produced by Coca Cola - they took municipal tap water and ran it through a purification process which turned out to oxidise the bromide in the water to potentially harmful levels of bromate. That was a fairly depressing episode - they took a perfectly good product (tap water) and turned it into an unnecessary, environmentally damaging, harmfully contaminated one before selling it at a profit. Equisetum (talk | contributions) 16:51, 17 October 2013 (UTC)

• Looie and Phoenixia gave the correct answers. The "distinction" here is an arbitrary one dreamt up by legislators and lobbyists. There is no such thing as a "natural" chemical which can't be synthesized--cost is the only real problem (An exception was Cannabinol, a complex molecule which it was yet unknown how to synthesize when I was an undergraduate.) The notion peddled to the FDA in the nineties was that if a substance could be concentrated from a living organism and wasn't marketed as a drug it should be legal to sell without a prescription. This is a legal distinction, not a scientific one. μηδείς (talk) 20:10, 17 October 2013 (UTC)

Antibiotics in routine lab work

I was wondering how/why certain antibiotics are selected for use in lab work. Why penicillin and streptomycin and ampicillin and kanomycin? Is their use or disuse in medicine a consideration? --129.215.47.59 (talk) 16:17, 16 October 2013 (UTC)

I'm not sure what you mean by "lab work", but generally speaking the antibiotic effects of these drugs are a consequence of their chemical properties, which can make them useful in other contexts. For example, penicillin breaks down a component of the bacterial cell wall -- it also causes epileptic activity when applied to brain tissue in high concentrations. Looie496 (talk) 16:27, 16 October 2013 (UTC)

"Lab work" meant such work in a life science research facility, forensics lab or a myriad of other establishments employing such techniques as bacterial transformation and/or culture for production of plasmid DNA, among other things. --2.97.26.56 (talk) 21:39, 16 October 2013 (UTC)

Almost certainly price and availability factor in greatly, but there is also a lot of mindless rote tradition in biology. If someone does a demonstration once and it works, the next will tend to do the same thing, and the next, and the next... Wnt (talk) 04:11, 17 October 2013 (UTC)

More mindless than in chemistry and physics? 129.215.47.59 (talk) 10:35, 17 October 2013 (UTC)

I'd agree there is a lot of rote tradition in biology - but it isn't exactly mindless most of the time - if something works well you stick with it unless you have a good reason not to. This is for at least two very good reasons 1) It lets you more easily compare your experiments with others both in the same lab and outside it 2) you don't generally have to spend nearly so much time optimising protocols if they are already well used and characterised. I quite deliberately try to use "standard" techniques when I can because of this. It does often become mindless though when something doesn't work well for a particular system and people stick with it anyway because it is "what everyone does". With the antibiotics specifically, they are often used as a selection agent when transforming bacteria etc. (i.e. you include a antibiotic resistance gene in your construct so that you can select for those bacteria that have taken it up by plating them on antibiotic media). For this you need to use one which has a cloned, characterised and readily available resistance gene. In practice, since people don't tend to design their own vectors if they can use one "off the shelf" the choice is usually made for you when you choose the vector. I have never come across medical use as a specific contraindication for using an antibiotic (I presume you are thinking that you don't want to go around playing with resistance genes for medically essential antibiotics in case of accidental release). If you are using proper biosafety procedures it should not be to much of an issue in any case. I'm still not sure that I would be comfortable using a "last ditch" antibiotic such as vancomycin in the lab, although I have come across one protocol in the literature which used it (as a "quick and dirty" way to eliminate the gut microbiome of a mouse when you don't have access to germ free and gnotobiotic mice). Equisetum (talk | contributions) 10:58, 17 October 2013 (UTC)

I used to have a construct that used chloramphenicol as a control. It was the horrible cut-and-paste vector that had been spliced together from a number of different commercial plasmids, and passed down from lab to lab. No one even had a map of the whole vector, and it had duplicate restriction sites in completely illogical places, including several in the middle of the antibiotic resistance gene! Completely mind boggling that anyone used it. Part of why I'm glad I don't do much of any molecular biology these days.(+)H₃N-Protein\Chemist-CO₂(-) 11:15, 17 October 2013 (UTC)

Yes - that'll be the mindless part alright (not on your part - you recognised it was bad) - jesus, I can imagine using that, grudgingly, if I absolutely had to, but an incompletely mapped vector is entirely too much flying blind for my taste. Another thing I particularly "like" about molecular biology is the fact that no-one at all follows the protocols in the standard reference book (Molecular Cloning, A laboratory manual) because a PhD would take half your life if you did, but there is no standard reference for the quicker protocols and shortcuts that everyone actually uses! Equisetum (talk | contributions) 12:10, 17 October 2013 (UTC)

(edit conflict)What Equisetum said, more or less. - It's because you're using commercial vectors, so you pretty much have to use whatever antibiotic resistance is already built into the plasmid. Cloning out and replacing the antibiotic resistance gene in a bacterial expression vector is not particularly convenient, since the more useful restriction sites (places that restriction enzymes can cut the DNA) are near the multiple cloning site (where you put the gene), making it harder to actually excise the antibiotic resistance gene without accidentally ruining the construct. As a result, most people just stick with commercial pET vectors for expressing stuff in E.Coli, which means sticking with the more common antibiotics. If you accidentally damage the part of the plasmid with antibiotic resistance, then even the successfully transformed bacteria will die when you try to grow them on antibiotics. The whole reason for using these genes is that it forces your bugs (bacteria) to keep the plasmid you've given them or die, that way as long as you keep them on media with the appropriate antibiotic they have to express your protein of interest. (+)H₃N-Protein\Chemist-CO₂(-) 11:09, 17 October 2013 (UTC)

It's kind of funny how it works. There's been no shortage of people making up their own vectors for this and that, even publishing them here and there. The choke point is that people see an article about a clever new technique, and then they think, yeah, but is it REALLY going to work? There's a crisis of "peer review" currently ongoing, where conniving private companies have three people glance over a paper and claim they should be rewarded by controlling the results of publicly funded research forever. We need a mechanism whereby people do a real 'peer review' where they actually try out the technique and candidly describe their trials and travails getting it going. Wnt (talk) 19:43, 21 October 2013 (UTC)

Bupropion: Elontril and Wellbutrin

Why has Glaxosmithkline two names for the same drug? I understand that Fluoxetine can be called Prozac or by other names, but these are from different companies. — Preceding unsigned comment added by 80.58.250.84 (talk) 17:24, 16 October 2013 (UTC)

It may be region specific. While GSK "owns" the drug it gets marketed by different partners which may offer it under different brand names. Mike (talk) 17:50, 16 October 2013 (UTC)

SourceMike (talk) 17:50, 16 October 2013 (UTC)

Right, it's region-specific. The name Wellbutrin is used in the US; Elontril is used in Europe. It might be worth noting though that Bupropion is actually sold by GSK under two different names even in the US alone: Wellbutrin and Zyban. The main difference is that the Wellbutrin formulation is intended as an antidepressant, whereas the Zyban formulation is intended to treat nicotine cravings. Looie496 (talk) 18:19, 16 October 2013 (UTC)

Lucid dreams

I'm quite interested in these sorts of things. Are there any particularly notable studies and/or papers I could read on lucid dreaming? Thanks! --.Yellow1996.^(ЬMИED¡) 18:21, 16 October 2013 (UTC)

Not exactly a reliable source however does have some good information: [8]Mike (talk) 18:48, 16 October 2013 (UTC)

Looks great (and anyone else is welcome to add what they find); thanks! :) --.Yellow1996.^(ЬMИED¡) 19:02, 16 October 2013 (UTC)

Omni Magazine published a Survey of lucid dreaming in April 1987 (Archived issue). This link summarises the results: [9]. --Auric talk 19:17, 16 October 2013 (UTC)

Cool - that one is really interesting. I'll read it in it's entirety soon. --.Yellow1996.^(ЬMИED¡) 19:28, 16 October 2013 (UTC)

While not about lucid dreaming this is a good article about sleep. Bus stop (talk) 20:21, 20 October 2013 (UTC)

is it true you can peer into the distant past?

is it true you can peer into the distant past by looking up at the stars? what does that mean? how far in the past? 212.96.61.236 (talk) 21:42, 16 October 2013 (UTC)

I don't even know how to answer this...no sorry! Mike (talk) 21:48, 16 October 2013 (UTC)

The speed of light is finite so when you look at the nearest stars you see them as they were about four years ago, and distant galaxies can be seen as they were billions of years ago.

What would be much more interesting is if they had huge mirrors and you could see what happened on earth eight or more years ago. They don't so we can't but hopefully aliens have recorded the Jack Benny Show or I Love Lucy or so we can enjoy them again. Even now aliens are recording and treasuring the The Rush Limbaugh Show or studying it in their equivalent media studies at university ;-) Dmcq (talk) 22:07, 16 October 2013 (UTC)

We recorded those shows ourselves, you know. StuRat (talk) 14:36, 17 October 2013 (UTC)

I hope they liked Doctor Who, there are still a few 'missing episodes'. 220 of ^Borg 04:33, 19 October 2013 (UTC)

• We know the distance to various stars and galaxies in light years. If Sirius is 12 light-years away it takes its light twelve years to get here. So when you look at it you are seeing it as it would have appeared 12 years ago to somebody in the same solar system as it. This applies to the Alpha Centauri system, which is just over 4 light years away, to the Andromeda Galaxy, which is 2.5 million light years away. Betelgeuse, which is in the process of dying, is 642 light years away. It may actually already have gone nova, but we just haven't seen it yet. μηδείς (talk) 22:14, 16 October 2013 (UTC)

You can see into the very recent past by looking at your hand...the light that reaches your eyes is a couple of nanoseconds old - so you're seeing your hand as it was a teeny-tiny fraction of a second ago - not as it is "now". By extension, everything we see is somewhat delayed due to the time it takes the light from that object to reach us. Our other senses are delayed by even more than that. Sound waves travel at around 700 miles per hour - so if you can hear something happening a mile away, you're hearing it from about 5 seconds into the past. When the island of Krakatoa exploded, it was heard 3,000 miles away - and those people heard an event that had already happened four hours in their past! So yes, when you look out at the sky, you can see into the past. To pick a concrete example - on a dark night and with the naked eye, you can just about see the Crab supernova - and what you see is what was happening there 6,500 years ago when the pyramids were being build in Egypt. Nobody knows for sure what it looks like right now, and we won't know that for another 6,500 years. SteveBaker (talk) 22:43, 16 October 2013 (UTC)

More prosaically, every time lightning flashes, you see it nearly instantaneously (some very small fraction of a second), while you hear the thunder a few seconds later - a handy gauge for estimating how far away the lightning is. And if you're some distance from a ball game, it's almost unnerving to see the batter hit the ball soundlessly, and then hear the crack of the bat as he's running toward first base. Seeing into the past, hearing from the past. ←Baseball Bugs ^{What's up, Doc?} carrots→ 03:02, 17 October 2013 (UTC)

You may want to try to spot M81 with the naked eye see here for directions. You will then look 11.8 million years back in time with the naked eye. Count Iblis (talk) 23:36, 16 October 2013 (UTC)

Although you can't see it with the naked eye, the cosmic microwave background radiation was emitted around 380,000 years after the Big Bang. We study it to find out what was happening in the universe 13.8 billion years ago, before any stars or galaxies had formed. It's not possible to directly look back any farther than that, because the universe was not transparent at earlier times. --Amble (talk) 00:13, 17 October 2013 (UTC)

Can things interact faster than the speed of light? If not, does "now" have any real meaning for distant objects?  Card Zero  (talk) 02:43, 17 October 2013 (UTC)

That's kind of the "God viewpoint", i.e. somehow being in more than one place at once. But speaking in mortal terms, if light took 4 years to get from Alpha Centauri to us, then we're seeing it as it was 4 years ago... and conversely, if some cognizant being is there and can see our sun, they're seeing it as it was 4 years ago. Barring some catastrophe in the interim, their "now" should be just as meaningful to them as our "now" is to us. Would they be exactly the same "now"? If you could have magically plunked down a pair of clocks, set to the same time, in both places 4 years ago, and then magically retrieved them from both places now, would they still be in sync? Maybe, maybe not. But they should be close enough for government work. ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:55, 17 October 2013 (UTC)

Andromeda paradox. Count Iblis (talk) 03:14, 17 October 2013 (UTC)

At least under the special theory of relativity, the concept of now (ie, events happening at the same instant of time) makes sense in a fixed inertial frame of reference. However observers in different frames of reference need not agree on whether two spatially separated events occur simultaneously or not; so the concept of now is not absolute. See relativity of simultaneity for further details. Abecedare (talk) 03:16, 17 October 2013 (UTC)

That's an oversimplification though. If one has the relative accelerations one can choose a frame of reference and calculate a now relative to it. Of course you won't be similtaneously aware of things outside you light cone. But existence and awareness are two different things. μηδείς (talk) 03:48, 17 October 2013 (UTC)

Yes, the universe is expanding. The rate of expansion and the speed of light is a Doppler phenomenon and is partly explained by the Hubble flow of the universe. Three dimensions is hard to visualize so a simpler model is to use a 2D model of the surface of a perfectly spherical balloon. If you image the light path between objects to great circles on the balloon, you will see that every point on the surface moves away from every other point as it inflates. The further the object, the faster it moves. As light has a constant velocity, this movement is reflected in a Doppler shift or commonly called the red shift. This is the way we measure distance to galaxies and stars as the frequency emmision of elements with zero relative velocity is known. The more the galaxy shifts to to longer wavelengths indicates that it is far away and moving away faster than nearer objects. The farther away it is, the older the system we see and therefore the farther back in time. It's fallacy, though to say we are "looking back in time" because there is no universal frame of reference (which point on the surface of the perfectly spherical balloon is the center?) We will never look back in time to our own sun, rather it is a measure of distance. It's 8 light-minutes away. For far away galaxies, it's more like saying the time difference and the spatial difference do not vary by the same amount through theories of relativity. They are moving away in time from the big bang just as we are but the speed and distance makes it look like they are closer to the big bang than we are., --DHeyward (talk) 09:55, 17 October 2013 (UTC)

If something happens on the sun, we see it 8 minutes later. Logic says we're looking back in time, i.e. we're seeing something that happened 8 minutes ago. ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:14, 17 October 2013 (UTC)

Ah, that's presuming you can be on the sun and the earth and observe both emission and observation and measure it. You cannot. Really there is only your "now" and everything else is a distance to you. Now matter how far you look, you cannot see yourself yesterday. I understand we separate time and space as a perception but for the most part, when we look at far away galaxies, we are looking at the newest parts of the universe as it expands. Our frame of reference is that we are always the oldest frame in the universe. It's like asking whether the earth is rotating around the sun or moving in a straight line in a curved space. We appear to pass the same point every 365 days but is that true? --DHeyward (talk) 16:21, 17 October 2013 (UTC)

I can't see myself yesterday, but someone else could, if they are one light-day away and have a sufficiently good telescope. I cannot literally be in both places at once, but I can imagine it, and that's sufficient. I can't literally be at my next-door neighbor's house and my house at the same time, either - yet there's no issue about their "now" vs. my "now". ←Baseball Bugs ^{What's up, Doc?} carrots→ 16:47, 17 October 2013 (UTC)

If you are one light-day away, they are seeing their own "now" and you as being one light-day in distance. If you use time as the reference, both observers looking at each other would say they are seeing the other one day in the past. How can both observers claim to be one day ahead of each other? These space separations of observers makes past/present meaningless terms. For far away objects where we only observe photons, it is light-like interval with the light cone propagating from the emission, then it's partly correct as distance and time are the same and we exist in the causal light cone that can be traced pastward to the orgin. --DHeyward (talk) 20:16, 17 October 2013 (UTC)

Who says both observers claim to be a day ahead of each other? No, they're in sync, and presumably astute enough to realize that, just as we are. It's effectively no different from seeing a tape-delayed ball game broadcast, 24 hours after the actual game was played. ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:24, 17 October 2013 (UTC)

Not quite. Space-time interval#Space-like interval --DHeyward (talk) 00:38, 18 October 2013 (UTC)

You lost me at the bakery. ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:55, 18 October 2013 (UTC)

Yes, it seems semantical, but it's really not. I understand the thought experiment that both are simultaneous and we vary in time through distance. But at a fundamental level it's simply not the case. There is really only one of an infinite number of inertial frames of reference that an event occurs at the same time. Whence time isn't the metric used, distance is. A simpler example is here: Relativity of simultaneity. So as a thought experiment, a supernova that is 4 light years away, the distance is accurate but no inertial frame of reference will agree at the time of the event. The problem is accepting that there is no absolute frame of reference. No one can tell you if that supernova happened before, after or during an event on Earth with any certainty. They are not causally dependant on each other. It's as perplexing as understanding whether the Heisenberg uncertainty principle is a description of the nature of the universe or the nature of measuring the universe. It's not very satisfying to not know what you are measuring. Even Einstein understood the dilemna and couldn't reconcile the distinction that measurement and reality where hopelessly intertwined and it doesn't matter whether the measurement affects the result or whether the result is randomly determined. God does not play dice with the universe but the reality is we cannot distinguish the difference. It's wholly unsatisfying as an intellectual exercise. But your bakery reference is an easier point to make: How far across the universe would I have to look to see you in the bakery yesterday? I can't. Our frames of reference are too close. Possible a supermassive black hole could bend your light cone so it appears I am seeing you in the past. Really though, I am only measuring a distance, not time.--DHeyward (talk) 05:04, 18 October 2013 (UTC)

DHeyward, I think you are mistakenly interpreting "time-intervals are not absolute" to something akin to "there are no such thing as time intervals". Under the special theory of relativity, at least, as long as one uses a fixed inertial frame of reference both time intervals and distances are well-defined and (in principle) measurable quantities. For example, I don't know what your statement, "a supernova that is 4 light years away, the distance is accurate but no inertial frame of reference will agree at the time of the event." even means, since (1) unless you have already (implicitly or explicitly) fixed an inertial frame, saying that the supernova is "4 light years away" is meaningless, and (2) once one has fixed the frame of reference, one can certainly (again, in principle) uniquely determine when, and how far, the event occurred in that inertial frame's timeline. Abecedare (talk) 05:26, 18 October 2013 (UTC)

Again, supposing some guy (I'll call him Dr. Ort) happens to be somewhere about 1 light day's distance away from earth, and has an incredibly good telescope that can see minute details from many billions of miles away. Today (in my time frame) I paint a sign on my bakery roof that says, "Greetings, Star Gazer!" About 24 hours later, Dr. Ort sees my message. Ergo, he's seeing me yesterday (in my time frame), i.e. as I was yesterday. Hard telling where I'll be 24 hours after painting the sign when Dr. Ort sees it, but that has no bearing on it. At any given point in time, Dr. Ort's "now" is effectively the same as my "now", he just can't see it until 24 hours later (and vice versa). ←Baseball Bugs ^{What's up, Doc?} carrots→ 05:55, 18 October 2013 (UTC)

Right now, Hubble's Ultra Deep Field IR can see as far as 480 million years after the Big Bang. source Ssscienccce (talk) 12:51, 18 October 2013 (UTC)

There is no such thing as "now". It is a concept. But it has limited applicability. Strictly speaking it has no applicability, because as SteveBaker points out above "you're seeing your hand as it was a teeny-tiny fraction of a second ago - not as it is 'now'." Bus stop (talk) 20:42, 20 October 2013 (UTC)

Quantum Relative Entropy

Is the quantum relative entropy between two pure states always either zero or infinity? — Preceding unsigned comment added by 81.155.161.54 (talk) 21:58, 16 October 2013 (UTC)

Yes, if I'm understanding Quantum relative entropy#Non-finite relative entropy correctly. Red Act (talk) 00:00, 17 October 2013 (UTC)

October 17

Second opinion

Light can break Newton's third law – by cheating

Can I justify believing the veracity of the claims made by this experiment.? Plasmic Physics (talk) 00:49, 17 October 2013 (UTC)

You can read the claims made by Ulf himself in his paper. It's rather beyond me, honestly, but he makes no claims about violating the laws of physics. And I wouldn't trust anything you read on New Scientist anyway. Someguy1221 (talk) 01:02, 17 October 2013 (UTC)

I mean, how falsifiable are his claims, and do they stand up to scientific scrutiny? Plasmic Physics (talk) 01:10, 17 October 2013 (UTC)

So far as I can tell, his claims are eminently falsifiable. Unlike earlier theoretical works regarding diametric drives, Ulf's does not require any exotic materials to function (it also doesn't necessarily do anything useful). Anyone with the right expertise and resources should be able to build it, though since I do not have the expertise I have no idea how difficult that would prove. It's worth noting that this was published in Physical Review Letters, which is considered one of the most prestigious journals devoted to physics. So it is a given that this work was reviewed by several independent experts prior to publication. Someguy1221 (talk) 01:24, 17 October 2013 (UTC)

Yes, it's falsifiable, and I see no reason to doubt that the experimental realization was as reported. Note that even in the science-fiction scenario of a negative-mass diametric space drive, you're still not really violating Newton's third law. Instead, it's the consequence of applying Newton's laws to something with negative mass. The idea of breaking Newton's third law seems to have been introduced in the New Scientist report rather than the article itself. --Amble (talk) 05:42, 17 October 2013 (UTC)

I don't see how it happens. The objective appears to want to create a mass difference between two objects. But conservation of energy still applies. There is no such thing as a "rest mass of light." It's not zero. If there is a frequency, it has energy and mass. If it doesn't, it's not light. I can almost grasp a situation where a mass imbalance occurs between the front and rear of a spacecraft but not as a free energy + mass on one side and - mass on the other based on an interference pattern. Maybe a massive amount of light forced into a material withe significant dielectric differences fore and aft but that would just recenter the center of mass and it would return when the beam was stopped. --DHeyward (talk) 10:28, 17 October 2013 (UTC)

The overall conclusion seems to be "something that behaves mathematicaly similar to mass, but allows some probably impossible things (negative mass), behaves as we would expect that impossible thing to behave". As far as I can tell, the "drive" effect is only on the propagation speed of the light pulses, with no effect on the material itself. MChesterMC (talk) 12:33, 17 October 2013 (UTC)

The paper is not talking about photons per se, but about some kind of quasiparticle that corresponds to the light pulses. That is, he's not talking about photons with negative mass, but a phenomenon that behaves like a particle with negative mass. Someguy1221 (talk) 05:38, 18 October 2013 (UTC)

We know the speed of light depends on the dielectric it passes through. Will changing the dielectric in different sections affect mass balance? I would tend to think equivalent mass, wavelength and energy would always balance. Even if a negative mass were created, it would balance. I've never been able to conceive of any type of propulsion that didn't require a loss of mass (i.e. energy). to move an object through a distance. That seems fundamental to me. Perhaps there is a way to make the distances shorter but it's hard to imagine. --DHeyward (talk) 05:27, 18 October 2013 (UTC)

Why do people tie off their arms when shooting up?

Thanks. μηδείς (talk) 01:03, 17 October 2013 (UTC)

The most obvious reason to me, would be to find a good vein. Plasmic Physics (talk) 01:08, 17 October 2013 (UTC)

Yes, it makes the veins pop up a bit. That's standard procedure at my clinic when they need to draw a blood sample. ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:18, 17 October 2013 (UTC)

That makes sense. I had thought maybe it had to do with preventing the drug from entering the bloodstream while they were busy injecting it. μηδείς (talk) 02:22, 17 October 2013 (UTC)

They used to tell us in school that the addict would let some blood come into the syringe chamber to dilute the heroin slightly, before injecting the whole mess into the arm. Thankfully, I have no first hand (or arm) knowledge of that process. But it fits with tying off the arm to make the veins pop up and to initially draw blood before injecting it back into the vein. ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:37, 17 October 2013 (UTC)

Intravenous therapy#Hypodermic needle mentions the practice of pulling up a bit of blood as a way to verify that the needle is actually in a vein. DMacks (talk) 06:10, 17 October 2013 (UTC)

Yer but just remember to release the pressure before you actually inject all you junky scumbags! :-) Die Antwoorde (talk) 07:35, 17 October 2013 (UTC)

"Only dopes use dope." ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:11, 17 October 2013 (UTC)

Apparently, [they don't always do it.] OsmanRF34 (talk) 11:16, 17 October 2013 (UTC)

This came up because of the ltest episode of Homeland, and wasn't meant as a venue for bashing people. μηδείς (talk) 16:25, 17 October 2013 (UTC)

I'm quoting Cheech and Chong, FYI. ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:21, 17 October 2013 (UTC)

If you donate blood (in the UK at least), they use a Sphygmomanometer to achieve the same effect. At a blood donor session a few years ago, I was asked if I wouldn't mind being used for training of a new staff member. When I agreed, the Cockney nurse called across the crowded room "Oi! Come and have a go at this one, he's got veins like drainpipes!" Alansplodge (talk) 09:44, 18 October 2013 (UTC)

I used to inject animals with drugs on a regular basis. When you're giving an intravenous injection you need to raise the vein to ensure that it's easy to find and you can get into it without the needle going through the other side. You can do this by applying heat to the skin, applying an irritant to the skin (not considered ethical these days), or applying pressure to the vein to temporarily stop the flow and raise the vein by back pressure. As said above, you push the needle into the vein and then pull the syringe plunger back to suck up a little blood to ensure you are in a vein, then release the pressure and inject. I have seen nurses use a disposable rubber glove to tie off the arm for this purpose. Of course if you miss the vein and inject anyway you can cause tissue damage at the injection site and the drug will not be metabolised at the correct rate. If you regularly inject into the same veins they eventually collapse and it gets harder to find them. That's why junkies are always searching for veins they haven't used before and end up injecting in all sorts of strange places. Richerman (talk) 12:00, 18 October 2013 (UTC)

Why do the veins collapse? Do they come back? Isn't that bad for circulation? And what if they're just drawing some (non-large) amount of blood, as nothing noxious or osmotically active is injected into the vein, wouldn't the vein stay healthy? Sagittarian Milky Way (talk) 22:56, 18 October 2013 (UTC)

To an extent, the blood system is like the road system, blood can find alternate return routes. (They can take vessels from your leg to reuse in your heart, for example.) But obviously shutting down a vein in your elbow is not good, and damage builds up. See krokodil and do a NSFW google image search for the fun you can have with impure drugs!) Even if the injections are harmless there is still mechanical damage from the needle. Someone else will have to comment on self-repair, but my suspicion is that there is a point of no return of scarring and so forth. I spent about six months with an IV which had to be moved at least twice a day, but after ten years I am unaware of any permanent damage. μηδείς (talk) 00:48, 19 October 2013 (UTC)

I don't understand -- what does a Russian satirical magazine have to do with IV injections? 24.23.196.85 (talk) 02:27, 19 October 2013 (UTC)

Hehe. I am sure there's a disambig link at the top of that page. μηδείς (talk) 02:37, 19 October 2013 (UTC)

Fish tank

Hi! I’m back again one more time… last days I pass by this little country hotel that have an awesome aquarium in the lobby and watching the fishes made me remember when I was a kid and have lot of fishes in tiny fishbowls… so one thing came after another and somehow I finished buying a couple of glasses and wow… it seems that I’m going to make a fish tank for my living room… of course it wouldn’t be as large as the one in the hotel, but I don’t know if, at my intended dimensions, the vertical water column pressure effect could be an important issue to consider.

my question is: how can I calculate the pressure that the glass and the glue/sealant will have to hold?

I know how to get the average pressure of the whole tank, but my principal concern is in the stress concentration at lower part of it

I’m thinking in something like 145cm width by 60 cm height by 50 cm depth more or less

thanks!!

201.220.215.14 (talk) 05:07, 17 October 2013 (UTC)

Go to your local pet supply shop and buy one that's already properly constructed. ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:09, 17 October 2013 (UTC)

Seconding Bugs. Apart from any questions of sufficient mechanical strength, how do you know what glues and sealants, which will be in contact with the water, will or will not have a poisonous effect on the plants, fish etc in the tank? {The poster formerly known as 87.81.230.195} 212.95.237.92 (talk) 13:50, 17 October 2013 (UTC)

Haha… no no I already bought the glasses, and besides let’s assume that here there isn’t anything like a “pet supply shop” plus building up the thing is part of the fun…

isn’t any physic or math formula that allow you to obtain the pressure in the edges of the tank?

the seller told me that the glass will hold up for that dimensions, my major concern is for the glue

the glue will be something silicon based

Iskánder Vigoa Pérez (talk) 13:59, 17 October 2013 (UTC)

P = ρgh. Where rho is defined as the density of water at a desired temperature, g is gravitational acceleration (choose standard or local), and h is the depth of water measured from the surface. This will yield the pressure as a function of depth. Plasmic Physics (talk) 14:09, 17 October 2013 (UTC)

While I agree that it seems odd to try to build an aquarium yourself, I will try to answer as best I can. Some thoughts:

1) You said you only bought 2 sheets of glass ? The usual aquarium has glass on 5 of the 6 sides. Do you intend to put something other than glass on the bottom and 2 of the sides ? Or do you intend to try to cut the glass yourself ? And do you have a lid ?

2) The pressure will be solely based on the depth of the water. However, you also need to consider that the glass will tend to bow out more, the greater the area over which the pressure is applied.

3) I suggest you add a physical support at the bottom, like a wooden frame to hold it all together. Then the load on the adhesive will be far less. Here's a top view of what I have in mind:

  +---+-----------------------+---+
  |   |         WOOD          |   |
  |   +-----------------------+   |
  | W |                       | W |
  | O |       AQUARIUM        | O |
  | O |                       | O |
  | D |                       | D |
  |   +-----------------------+   |
  |   |         WOOD          |   |
  +---+-----------------------+---+

You might also extend the wooden frame to have vertical posts along each of the 4 edges, and a repeat of the bottom wooden form at the top. Wood is far easier to work with than glass, and doesn't shatter if you mess up. StuRat (talk) 14:18, 17 October 2013 (UTC)

Not that it's any of my business, as it's your project - but isn't the kind of question an engineer would ask before buying the glass? ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:18, 17 October 2013 (UTC)

the man that sold me the glass told me that it would support the pressure and give me total warranty, the glass is almost 8mm, now I’ll buy the glue, so I’m asking

Iskánder Vigoa Pérez (talk) 15:58, 17 October 2013 (UTC)

thanks… that’s exactly the formula I was looking for

stu, I have the total glass area and know a man that will cut into the five rectangles that I need

thanks for the answers

Iskánder Vigoa Pérez (talk) 15:58, 17 October 2013 (UTC)

A silicone glue is normally used to hold and seal the panes together. You want to be careful to get 100% silicone - a lot of the caulks and glues sold will have a small amount of fungicides or other chemicals in them, because they're normally used in bathrooms or kitchens and it is nice not to worry about mildew growth. In an aquarium, the fungicide can dissolve into the water, potentially causing health problems for your fish. Do you have a solution for filtration and aeration? Katie R (talk) 16:54, 17 October 2013 (UTC)

You might want to try browsing through this magazine (or even take out a subscription) for informed advice in this area. {The poster formerly known as 87.81.230.195} 212.95.237.92 (talk) 18:00, 17 October 2013 (UTC)

Well… the height is 0.6 and the density will be in-between 995 and 997 so the glue in the bottom will have to hold max 5.98 kP… that’s 0.87 psi I’m right?

I’m getting something wrong here? I’m approximating everything toward my security and yet it seems so little…

katie as far as I know for the filtration all I need to do is buy a filter and for the aeration plants and a bubble device; someone told me the same thing about the glue…

Thanks for your time, and for the link to the magazine, it’s very interesting… — Preceding unsigned comment added by Iskander HFC (talk • contribs) 22:49, 17 October 2013 (UTC)

That is approximately correct. Plasmic Physics (talk) 23:12, 17 October 2013 (UTC)

And that's not as little as you think. Using dimensions for the long sides of 145 cm width by 60 cm height, or 57 inches by 24 inches, I get an area of 1368 square inches. Multiply that by an average pressure of 0.87/2 psi, and I get a force of 595 pounds. That's a lot of force to expect glue alone to hold (not completely impossible, though). Thus my suggestions for a wooden support frame. StuRat (talk) 03:10, 18 October 2013 (UTC)

You should also remember that the most important dimension that restricts the amount of fish you can keep in the tank is the surface area of the water. This is because the gas exchange nearly all takes place at the surface, so you can't keep as many fish in a tall thin tank as you can in a low wide one that holds the same volume of water - as it doesn't absorb as much oxygen see:[10]. Aerating the water with a pump increases the surface area by agitating the water but that is, of course, only effective as long as the pump keeps working see:[11]. Richerman (talk) 12:32, 18 October 2013 (UTC)

Note that for example this glue manufacturer does not recommend it being used for more than 30 gallons (114 l) and/or water height more than 18 inch or 46 cm. Ssscienccce (talk) 12:46, 18 October 2013 (UTC)

thanks for all the links… very interesting stuff… I’ll try don’t place to many fishes in there, btw the aquariums in the photos here in Wikipedia and at practicalfishkeeping are way better than the one I saw in the hotel, and [Aquascaping] has some beautiful photos too… I hope mine resemble some of those

Iskánder Vigoa Pérez (talk) 17:28, 19 October 2013 (UTC)

Good scholarly sources

Are Razib Khan and Dienekes Pontikos blog good sources when it comes to human race classification? — Preceding unsigned comment added by 70.31.154.71 (talk) 08:57, 17 October 2013 (UTC)

Blogs are not considered as reliable sources on Wikipedia unless written by people who are acknowledged experts in the field or are under editorial control. See WP:RS. Dmcq (talk) 09:17, 17 October 2013 (UTC)

Dienekes and Razib Khan are both experts — Preceding unsigned comment added by 70.31.154.71 (talk) 09:29, 17 October 2013 (UTC)

You seem to have decided already that they are "experts". This is highly debatable - see also this. The idea that "human race classification" is a matter for a Science reference desk - when so much is based on social and cultural factors - is itself somewhat dubious. Ghmyrtle (talk) 10:04, 17 October 2013 (UTC)

Razib Khan seems to get a little bit respectability since his blog is hosted by Discovery Channel. He also has been cited thoroughly through out Wikipedia.OsmanRF34 (talk) 11:27, 17 October 2013 (UTC)

Thanks for help, turns out neither are actually experts, and no one even knows Dienekes identity. Razib Khan does have some academic background but his science seems to be criticized a lot — Preceding unsigned comment added by 70.31.154.71 (talk) 15:43, 17 October 2013 (UTC)

Generally for a good scholarly source I would tend to set the bar a bit higher than wikipedia's definition of good sources (which is fine - since Wikipedia obviously needs to deal with plenty of non-scholarly sources as well). I would like to see something that has either been peer-reviewed or draws on peer-reviewed research in the making of all of its claims (such as a review article or monograph ). The University of Illinois generally agrees. Now that isn't to say that other good sources don't exist, it's the "scholarly" bit that is lacking in other sources, rather than necessarily the "good". I personally don't know of any blogs which undergo peer review, and even if they refer exclusively to peer-reviewed research, I don't think I could get away with citing a blog in an academic paper (unless the paper was on the phenomenon of blogging!), so I would say that currently no blogs are good scholarly sources. Equisetum (talk | contributions) 14:40, 18 October 2013 (UTC)

Drinking coke

As I understand correctly, 100 years ago, the favorite route of administration of cocaine was drinking it. Today it seems to be snorting it. Why the shift? OsmanRF34 (talk) 11:25, 17 October 2013 (UTC)

See nasal administration. It's faster, bypasses irrelevant organs that would try to digest the drug, and can also bypass the blood-brain barrier because the nose has a specially close connection to the brain. (This doesn't really answer why they didn't try snorting it in the first place, as was already done with snuff. Lack of imagination?)  Card Zero  (talk) 12:01, 17 October 2013 (UTC)

One important factor is that it was originally used as a medicine, not a recreational drug. (Coca-Cola was originally a weak concentration of cocaine mixed with cola, sold in syrup form, as a medication.) In that case, you want the slow release you get from the digestion process, not the quick release from snorting it. Indeed, if people didn't figure out that they could get high from refining it further and snorting it, cocaine might still be used as a med today. Also note that heroine and several other illegal recreational drugs also were used as medications originally. StuRat (talk) 14:25, 17 October 2013 (UTC)

What do they put it the fancy energy drinks these days??122.111.240.138 (talk) 15:55, 17 October 2013 (UTC)

Read the Wikipedia article titled Energy drink. --Jayron32 17:03, 17 October 2013 (UTC)

Yer but I dont mean the "generaly included" additives" I'm talking about the colonels secret recipe. And now to ease the side effects we can buy a relaxation drink and human designed additives may be added also maybe. 122.111.240.138 (talk) 18:10, 17 October 2013 (UTC)

Heroin, usually referred to in medical contexts as diamorphine, is actually still used extensively in the UK for the management of both acute and chronic severe pain. Cocaine is still used in several countries as a local anaesthetic in certain specialised situations. Equisetum (talk | contributions) 14:16, 18 October 2013 (UTC)

The crackpot doctor Wilhelm Fliess, a contemporary of Freud, pioneered the application of cocaine to nasal tissues.--Digrpat (talk) 20:41, 17 October 2013 (UTC)

Coca tea is still being sold. Count Iblis (talk) 23:58, 17 October 2013 (UTC)

Certain people seam too like applying the free base to their oral mucosa! 122.111.240.138 (talk) 03:53, 18 October 2013 (UTC)

Thousands of years ago, people chewed coca leaves. Later, Incas chewed leaves when making long treks at high altitudes. I think that Incas already had problems with people getting addicted to coca leaves. Then Spanish conquerors imported them to Europe. And then, centuries later, an American decided to make a medicinal drink with them. He used a cola drink as base, so he called it Coca-Cola.

Coca-Cola, get it? A cola drink with those coca laves that people had been using for centuries and millenia. Now in a more convenient format, faster to take, doesn't leave spots around your mouth, doesn't force you to chew for minutes or hours, lasts longer in storage, etc.

Moreover, cola has caffeine from cola nuts. Coca leaves have undesired side effects, like apathy. You can't work properly under their effects. Caffeine compensates those undesired side effects and makes you more active and awake.

These are the sort of things that made Coca-cola popular. In historical matters, context is very important. --Enric Naval (talk) 17:36, 18 October 2013 (UTC)

Megawats, generating capacity and consumption

A small West African country is said to have 90-100 MW installed generating capacity. There is a proposed bio-fuels project which will grow suger cane, produce ethanol, and use the ethanol to produce 30 MW of electricity, offering 15 MW for sale back to the national grid, and using the other 15 MW for the ethanol plant and related local installations. If I understand, the entire country now uses less than 100 MW. One project can increase production by 30%, but requires 15% of what the entire national grid produces and consumes, just to run the one project (of 12,000 hectares) producing 85,000 cubic metres of ethanol for export. Does that make sense? Are these numbers credible? Thanks if you can make this more understandable. — Preceding unsigned comment added by 193.173.50.222 (talk) 12:09, 17 October 2013 (UTC)

One thing to keep in mind is that it always takes more energy to produce a fuel than you get from it. However, in this case, one of the biggest energy inputs is the sunlight used to grow the sugar cane, so it might be reasonable to expect that the rest of the refining process would take half of the energy produced. I am skeptical, though, that this process is the optimal use of the land and sunlight. Selling the sugar cane instead, or some other crop, might very well make more financial sense. You could also place solar panels there instead, to create electricity directly. However, solar panels are a rather low efficiency way to make electricity, too. StuRat (talk) 14:33, 17 October 2013 (UTC)

Thanks.I'm sure I wasn't very clear when I put the question. I'm trying to understand if it is credible that one biofuel factory could be using 15 MW for its activities, while the entire country (Sierra Leone) presently makes due with less than 100 MW. I would think, even in a country with very low generating capacity, that one factory could only use 1%, or a fraction of a percent of all the electricity being used in the entire country. But I don't know much about electricity, (or ethanol production) ... so I was looking for some insight on that. I'm skeptical of the claims, numbers, publicly stated plans of the biofuel investors. Thanks for anyone who has additional information. — Preceding unsigned comment added by 193.173.50.222 (talk) 16:06, 17 October 2013 (UTC)

It looks about right to me, I'm sorry to say. This confirms an installed electricity capacity for Sierra Leone of 100 megawatt in 2010 (hover over the little graph). This confirms the figure for 2013 and says they want to increase production tenfold by 2017. This describes an Ethanol plant in Peru (also sugar cane) generating 37 MW (and potentially exporting 17 MW) to produce 35,000,000 US gallons (130,000 m³) per year. These figures are somewhat larger than the plant you describe but roughly in proportion. Thincat (talk) 20:36, 17 October 2013 (UTC)

How much time should I studying for the astronomy GCSE?

Hf245 (talk) 17:41, 17 October 2013 (UTC)This applies to the UK. I am studying GCSE astronomy as an extra qualification. However, I have only 1 year to complete the 2 year course. I have one lesson for about 1 hour a week. I need to learn the rest at home but I don't know how often I should study and for how long.-

After reviewing the official documentation from the UK's Ofqual website, specifically comparing the recommendations from the five exam administration corporations listed in the GCSE Guide, your first step should be to classify yourself as a "Higher," "Middle," or "Lower" performer; and then cover the syllabus outlined in the order presented. For example, one test administrator, EdExcel, offers the current Astronomy course specification in "linear syllabus" format. That specification is 60 pages long, nearly as lengthy as a high-school-level textbook on astronomy in the United States!

Standardized test assessment, (and standardized learning, for that matter), are complex topics. What do you expect to achieve by learning this material? If you hope only to pass a test, there are excellent study-techniques that will statistically improve your test performance. But if you hope to learn this material to build a baseline for future study and work in scientific fields; or if you enjoy the topic and want to enlighten yourself, your strategy should be entirely different. If you want to enlighten yourself and score at a particular level on the tests, there's probably a middle-ground. Nimur (talk) 17:59, 17 October 2013 (UTC)

A GCSE course is normally taught for between 2.0 and 2.5 hours per week (plus homework assignments), so I suggest that you should spend at least five hours per week to cover the course in one year. If you learn quickly, then you might need less time than this, but you should obtain past papers covering as many years as possible and test your learning on these. A keen interest in the subject often reduces the learning time. Dbfirs 18:22, 17 October 2013 (UTC)

You should not study like that. What you should to is start to study, make sure you can solve the difficult problems and when you are starting to make progress, you can have some idea about how much time it will take to completely master the subject. Studying by the clock is an extremely bad way of studying; the whole point of year long courses is that you can afford to spend a lot of time time on difficult problems that can be done in, say, 5 minutes when you've mastered the subject. Count Iblis (talk) 18:32, 17 October 2013 (UTC)

Yes, I agree (except you should start with the easy problems and go on to the difficult ones later). I assumed that the OP was wanting to budget his time in advance. A solid five hours would not be appropriate for most people. Dbfirs 20:34, 17 October 2013 (UTC)

I don't think we can advise this because it depends on the ability and enthusiasm of the student. Personal experience - I studied, took and passed Music O level in just 8 weeks. I also supported (much later on) a student doing Astronomy GCSE in a similar circumstance to yourself who was already an astronomy geek and we covered the syllabus in a college term (10 weeks). So my recommendation is to study the subject at your own pace. Do what you find interesting. --TammyMoet (talk) 20:40, 17 October 2013 (UTC)

Thanks a lot for your help, I've split the topic up into smaller topics and I do one of them around every other day! :)

Binaural beats

Is there any link between binaural beats, iDosing, and this track? — Preceding unsigned comment added by 184.99.143.31 (talk) 18:25, 17 October 2013 (UTC)

October 18

DNA variability within a single organism

How much variability is there in the DNA sequence from cell to cell within an individual organism (say a human)? There is some variation due to copying errors when new cells are built, right? I imagine the error is very small as a percentage of the whole sequence. Can this error be measured? Sorry if this info is easy to find on WP- I don't know much at all about this. I found Human genetic variation but it doesn't seem to be what I want. Staecker (talk) 01:46, 18 October 2013 (UTC)

You might find these related topics of interest: mosaicism and chimerism. StuRat (talk) 02:58, 18 October 2013 (UTC)

somatic mutation, gene amplification; see also V(D)J recombination for one regulated instance I know of (which is extremely important to any adaptive immunity). I have a nagging suspicion that there are a lot more regulated instances of DNA alteration in somatic cells we haven't discovered, but of course, zero proof for that suspicion. Wnt (talk) 03:53, 18 October 2013 (UTC)

What about between organisms? I read a while ago that humans and chimps were 98% matching in DNA. Our nearest bioloigical species. A little bit farther down it said we share more than 70% with fruit flies. Kinda puts it all in perspective. (andecdotal BTW as I can no longer find the source).

The common estimate is that the error rate in DNA replication for animals is about 1 error per billion base pairs. Since we have 3 billion base pairs, you expect each cell division to produce three errors. Unfortunately, measuring this is difficult. You cannot, for instance, take two random cells from your body, sequence their DNA, and state with confidence the genetic differences between them. DNA sequencing technology typically pools the DNA from many cells to produce an aggregate signal that drowns mutations which are rare within the sample. Technology to sequence the DNA of a single cell exists, but such things introduce more detection errors than DNA replication produces mutations, so most of the differences you find between two samples would be artifacts. Someguy1221 (talk) 06:22, 18 October 2013 (UTC)

Exactly what I wanted to know- thanks! Staecker (talk) 13:39, 18 October 2013 (UTC)

I think that number is way too low. The data here have been evolving over time, but my understanding is that the most recent value for the mutation rate during human cell replication is on the order of 1 per 50 million base pairs, yielding on the order of 50 errors per replication event. See for example http://www.nature.com/scitable/topicpage/dna-replication-and-causes-of-mutation-409. Looie496 (talk) 17:21, 18 October 2013 (UTC)

Hot food kills good gut bacteria?

Does hot (Spicy) food like Thai Tom yum soups, or Tunisian Harisa, or Capsaicinoids and similar substances damage good gut bacteria \ gut flora? Thanks (I also wonder if there exists any typical foods who can..). many many thanks kind helpers ! Ben-Natan (talk) 02:24, 18 October 2013 (UTC)

Well, so the short answer is, I don't know. Our capsaicin article does not mention whether the effects on bacteria have been studied.

But if you're thinking that, simply because these substances provoke a strong response in us, that they must be somehow generally corrosive or hostile to other organisms, no, that doesn't follow at all. Capsaicin appears to be a defense against mammals specifically, and activates a particular kind of response in mammalian nerves to produce sensations of heat and pain. Bacteria are not mammals and do not have nerves, and I see no reason to think capsaicin should have any particular effect on them at all — which is not to say it doesn't, just that there's no obvious reason to think it does. --Trovatore (talk) 02:38, 18 October 2013 (UTC)

Not just mammals, see the 2nd paragraph here: Pain in invertebrates. StuRat (talk) 02:54, 18 October 2013 (UTC)

This http://npic.orst.edu/factsheets/Capsaicintech.pdf says it is toxic to some bacteria. That is as specific as saying it is toxic to some eukaryotes. (Paradoxically it is believed to help prevent ulcers, since it causes the stomach to react to protect itself.) μηδείς (talk) 02:50, 18 October 2013 (UTC)

Think of alcohol. Not a lot of stuff grows in it. Pretty good disinfectant. But the body metabolizes faster than it can really affect gut flora. Digestion is a complex process. I personally am not willing to test whether what goes in one end is the same as what comes out the other even if "fiery fiesta" describes both . Suffice to say it wasn't transformed it into ice cream on the way through. Stupid body. --DHeyward (talk) 05:36, 18 October 2013 (UTC)

Same question was asked here;the answer quotes a study: "no inhibition effect is found when the concentration of capsaicin is less than 0.0125mg/ml". Not sure how much 1 cubic cm of capsaicin would weigh, let's assume 3 gram. Tabasco is minimum 0.2% capsaicin, based on this, so 6 mg/ml; one drop about 1/30 ml; if you add 10 drops of tabasco to 200 ml of soup, the content would be 2mg capsaicin, or 0.01 mg per ml soup, close to the 0.0125mg/ml, so I assume at least some spicy eaters exceed that concentration regularly... Ssscienccce (talk) 12:04, 18 October 2013 (UTC)

These people would have gone over that limit. Count Iblis (talk) 19:11, 18 October 2013 (UTC)

Hydrocarbon smell

Do completely pure straight-chain hydrocarbons have any smell? I thought I remembered reading once that they do not, and that the smell is always due to traces of impurities, which are extremely hard to totally eliminate, but now I can't seem to find this confirmed anywhere. The Wikipedia articles for hexane through decane at least say "gasoline-like" smell, which of course would be expected, but for that feeling that I read somewhere it was actually a misconception. Maybe I'm misremembering it? 86.128.1.149 (talk) 02:45, 18 October 2013 (UTC)

Well, natural gas is mostly methane and is mostly odorless, because of this an odorant is added to help detect leaks. I can tell you from experience that butane does have a smell when it comes out of a lighter, but how "pure" it is, i don't know. Vespine (talk) 02:51, 18 October 2013 (UTC)

I would imagine that normal industrially produced samples of all such chemicals would be some way off 100% chemically pure. 86.128.1.149 (talk) 02:58, 18 October 2013 (UTC)

Smell is a chemical perception. It all depends on the individual but there is a "normal" normal olfaction. Odorants are usually added not because the chemicals are "odorlss" but because you can breath lethal quantities before your brain detects it. Odorants are added to natural gas because it reaches an explosive density before your body registeres enough of a smell. By contrast, the "rotten egg" odor is detected long before it's lethal. I believe most aromatic compounds have a dectable smell at some level (even if it's past the lethal leval). By contrast, nitrogen doesn't appear to have any receptors and is inert. Why people chose cyanide over nitrogen as a method aof execution is a mystery to me. Body no likee cyanide. body will breath nitrogen at normal heart and respiration rates until brain death. --DHeyward (talk) 05:58, 18 October 2013 (UTC)

It appears that H₂S gas is lethal at lower levels than HCN gas (but this seams to be the subject of some dispute) but I think you can smell H₂S at much lower levels. 122.111.240.138 (talk) 06:38, 18 October 2013 (UTC)

What does that have to do with my question? 86.160.217.227 (talk) 11:00, 18 October 2013 (UTC)

• Ok. I guess probably not! Although I know that all the AR grade hexane I have ever smelt smells a bitDie Antwoorde (talk) 12:13, 18 October 2013 (UTC)

Google search for "smell" and n-hexane, n-heptane, seems to return results that mainly mention "gasoline-like", so... Propane and n-butane also have a smell, but the odor treshold is quite high, about 1500 ppm. For longer chains, the smell is stronger, n-pentane treshold is 1.4, n-hexane 1.5. (source for the odor treshold, see google for the smell) Ssscienccce (talk) 12:23, 18 October 2013 (UTC)

I wonder about the purity of the n-alkanes used in this investigation was and what were the impurities. There are some possible impurities with quite low odor threshold values.122.111.240.138 (talk) 16:54, 18 October 2013 (UTC)

I would assume (hope) that laboratories determining the odor threshold of these substances would not neglect something as obvious as the possibility of impurities present in the samples. In other words, if they say the odor threshold of n-hexane is 1.4 ppm, that really means n-hexane and not some other compound mixed with it. The concentration of the compound was determined by gas chromatography, only in case of standard gases (like sulphur dioxide) was the concentration displayed on the bomb used. Ssscienccce (talk) 13:54, 19 October 2013 (UTC)

Confirmed the levels of the compound under investigation yes but it says here that all alkanes, all alkenes except ethylene and all alkynes except acetylene are odourless [[12]] and [[13]].Die Antwoorde (talk) 14:03, 19 October 2013 (UTC)

From my recollection parrafin oil etc does not really smell much. Die Antwoorde (talk) 04:53, 19 October 2013 (UTC)

But look at what it says about the composition of AR grade "hexane" at sigma

grade ACS reagent, description Mixture of hexane isomers plus methylcyclopentane, assay ≥98.5%, impurities Thiophene, passes test

Yes and I remember it usually shows quite a few peaks in a gc trace!Die Antwoorde (talk) 12:39, 18 October 2013 (UTC)

• Anyone know what this [[14]] smells like? Die Antwoorde (talk) 12:45, 18 October 2013 (UTC)

I work with it regularly -- it smells like gasoline, but not as strong. 24.23.196.85 (talk) 02:19, 19 October 2013 (UTC)

Ta but there is still about 3% of other stuff which could contribute. Die Antwoorde (talk) 04:43, 19 October 2013 (UTC)

• Well it says this - "Principal intermolecular interactions between alkane molecules are London dispersion interactions. Due to weak forces, the C1 to C4 alkanes are gases, C5 to C17 are liquids, those with more than 18 carbon atoms are solids at 298K. All are colourless and odourless." here [[15]]. Die Antwoorde (talk) 12:56, 19 October 2013 (UTC)

Nazi gas chambers

I've read just the other day that in gas chambers where carbon monoxide was the asphyxiant (such as the ones at Treblinka and Sobibor), quite a number of victims survived the gassing and had to be shot or burned alive; in the article about the Treblinka camp, it was claimed that pregnant women made up a disproportionally large percentage of these "survivors" (I put "survivors" in quotes because they were killed anyway, just not by gassing). My question is, why? What is it about pregnancy that helps one survive carbon monoxide poisoning (assuming, of course, that the article is correct)? 24.23.196.85 (talk) 04:02, 18 October 2013 (UTC)

Biochemical life giving majic maybe stem cells. --122.111.240.138 (talk) 04:12, 18 October 2013 (UTC)

Fetal hemoglobin (HbF) has a higher affinity for carbon monoxide than does the normal adult hemoglobin (HbA) that is circulating in the mother. So the fetus takes up carbon monoxide faster than the mother, and releases it more slowly...so it's sort of a case of the fetus taking a hit for the team. see Oxygen–haemoglobin_dissociation_curve#Fetal_hemoglobin - Nunh-huh 04:23, 18 October 2013 (UTC)

Pregnancy can cure disease. 122.111.240.138 (talk) 05:02, 18 October 2013 (UTC)

On the other hand, people with anemia are more susceptible to CO effects, and pregnant women have lower haemoglobin levels. It might just be a myth or observer bias? Ssscienccce (talk) 12:39, 18 October 2013 (UTC)

Thanks! I'm inclined to think the fetal hemoglobin explanation might account for this. 24.23.196.85 (talk) 02:40, 19 October 2013 (UTC)

How to make a real 3D movie?

https://www.youtube.com/watch?v=dVHP7Nhsn4E

Let's imagine a high-resolution LED cube has already been invented. It's at least good enough to display NTSC quality images or videos.

I think this cube can easily display 3D computer graphic images. It is also good for display of X-ray computed tomography.

It is a shame that the cube is transparent. You can see an object's inside.

How to make a 3D camera for the cube display?

I think its camera shall be a 3D scanner.

Ordinary 3D cameras are certainly not for this kind of display. -- Toytoy (talk) 11:03, 18 October 2013 (UTC)

See Light-field camera, although I picture the ideal display seeming more like a window that you can look through, and moving around lets you actually peek behind things, and your eyes would be able to focus on things in the display the same way as in real life. Katie R (talk) 13:05, 18 October 2013 (UTC)

I don't think a light-field camera would help much here. It does give you some depth information but that's about it. You'd be better off photographing the scene with ordinary cameras from a bunch of different angles. There has been a lot of research on automatically converting such photos into 3D scenes, but I don't know much about it. -- BenRG (talk) 22:18, 18 October 2013 (UTC)

And actually for your specific display example (I can't watch the video, but your description makes sense), you're right, you would want a 3D scanner. I don't know if there are real-time color systems that are capable of producing the data for a movie of the object being scanned. Katie R (talk) 14:11, 18 October 2013 (UTC)

How long from TIA to CVA (stroke)

Once I've heard a paramedic who said that every person who has TIA (It's a little stroke that goes away in some hours usually) he must get CVA (stroke) in three month from this day, if he doesn't treat of his little first stroke. So, I would like to cheek it out, Is it right setting \ assertion? 95.35.242.34 (talk) 12:44, 18 October 2013 (UTC)

See Transient ischemic attack#Prognosis. Rojomoke (talk) 12:49, 18 October 2013 (UTC)

now it's clear. only ten percent (or according other cite one third) of TIA will develop stroke (maybe I didn't remember well what he said). Thank you. 95.35.242.34 (talk) 12:57, 18 October 2013 (UTC)

How rare is it to have no viewable eclipse for 5 2/3 yrs?

Where a random point in the sky has an unvarying ~35% chance of being blocked by cloud (and remember, it only has to be peek out for seconds of an eclipse hours long to count). But not counting half the penumbral eclipses due to undetectability or ones that must be seen between say 1 and 5:30 in the morning because people that have jobs can't really do that. I don't know if latitude makes much difference (it's very important for total solar rarity though), but 40N. Sagittarian Milky Way (talk) 22:22, 18 October 2013 (UTC)

October 19

CFL fixture problem

I have CFL bulbs in fixtures on either side of the porch. The fixtures each have a light sensor, motion sensor, and a timer, and are set to light the bulbs for a few minutes, if motion is detected, when it's dark out. Each works independently. One works properly. The other works when I first put a new bulb in, but, in short order, the bulb fails to light. Instead I get a faint red glow down by the base, and nothing more. The bulb does seem to be bad, though, as it doesn't work in another fixture, either. So, it looks like something is different about that fixture, causing it to destroy CFLs.

I could just put an incandescent bulb in there and not worry about it, but I'm curious about what could be causing the problem. The only thing obviously different between the fixture which works with CFLs and the one that doesn't is that the one which fries CFLs is closer to the indoor switch which provides power to both. Could there be some type of power surge coming from the switch which levels off over the longer distance ? StuRat (talk) 22:57, 18 October 2013 (UTC)

Some types of motion sensing fixtures have a dim down mode, where the light is dimmed to half brighness for a second or so before it is turned fully off. Such a feature will ruin CFL's. I have a fixture like this - it has a tiny switch on the base to enable or disable the dim mode. As all these motion sensing fixtures rely on a device called a triac to switch power to the globe, there is a possible failure mode where full voltage is never applied to the globe - again that will lead to failure of the globe. Triacs are designed to vary the power to an electrical load by delaying the start of each voltage cycle. They are so cheap they are used for on/off control as well. 58.167.234.97 (talk) 02:11, 19 October 2013 (UTC)

Thanks. So I had a bad triac in that fixture, and a fully functional one in the other ? StuRat (talk) 02:37, 19 October 2013 (UTC)

Not actually the triac itself, but the circuitry controlling the triac. If the triac itself fails, the globe will be permanently on or permanently off. 58.167.234.97 (talk) —Preceding undated comment added 04:56, 19 October 2013 (UTC)

Incandescent bulbs in general don't lose much life by being turned on and off a lot, but fluorescents only have so many starts before they fail. For good service life in a motion detector light, along with efficiency, a halogen might be a good choice. LEDs would also withstand frequent on and off cycles, but are still relatively expensive. They are a good choice for a bulb which is on a large percent of the day. Edison (talk) 03:43, 19 October 2013 (UTC)

True, but Stu said it fails "in short order". That implies the globe fails after at most a few dozen times maybe. Whiel CFL's are not desin=gned for short "on" times, it shouldn't fail that quick, so teh fault lies in the fixture circuitry. Or, he needs to turn the dim mode off, if fitted - he won't have noticed the CFL globe dimming, it it most likely will just go out. 58.167.234.97 (talk) 04:59, 19 October 2013 (UTC)

It has two 3-position switches, one of which controls the sensitivity to light, and the other of which controls the time it remains on after motion is detected. There's no switch to control whether it dims. It could just always dim, and not have a switch, but then the question comes up as to why the seemingly identical model on the other side of the porch doesn't seem to burn out CFLs. StuRat (talk) 02:54, 20 October 2013 (UTC)

Global warming and agriculture

According to the article on climate change and agriculture, plant yields are expected to decrease as temperatures rise because each plant has an optimal reproductive and growth temperature which if exceeded leads to reduced growth. Why can't we genetically modify crops, whether through genetic engineering or more conventional methods, to have a higher optimal temperature? 74.15.138.165 (talk) 01:44, 19 October 2013 (UTC)

We don't even have to do that, just grow the proper crops for the new climate. For example, places which are no longer subject to winter frost become available for lots of new crops, like oranges. Also, in other cases, an additional harvest may be possible in a year for a given crop. Global climate change is not universally bad, it just requires that we adapt to the new environment, such as moving inland to deal with rising sea levels. StuRat (talk) 01:48, 19 October 2013 (UTC)

Wikipedia is not a source. The Middle Age and Roman warm periods were great for crops in Europe. Temperatures reached a maximum after the last ice age around 2000-3000BC. We are now in the cooling trend of a new ice age, called the neoglaciation. Recent trends have been local blips on that curve. See Canadian ecologist E. C. Pielou's, After the Ice Age, which is searchable at Amazon and well worth reading. μηδείς (talk) 01:59, 19 October 2013 (UTC)

Just to clarify: no credible scientist believes the world is actually cooling, just that it would be cooling if not for CO2 emissions. I assume that's what Medeis meant, although the natural Milankovitch cycles have been completely drowned out by anthropogenic warming in the past century. --Bowlhover (talk) 04:09, 19 October 2013 (UTC)

One century is 0.1% of the cycle of the ice ages, so how is that relevant? (Alternatively, how is the cycle of the ice ages relevant? At any rate, these two things have little bearing on each other.)  Card Zero  (talk) 18:43, 21 October 2013 (UTC)

The problem with climate change is not only the higher average temperatures but also that the changes in the local climate will happen fast and the local climate may well become inherently more variable. Higher evaporation rates will lead to more droughts, while the average amount of rainfall will also increase. This means that you'll have longer and more severe periods of droughts, interrupted by huge and sometimes catastrophic amounts of rainfall. Agriculture under such conditions will become more difficult, this may well lead to the collapse of our civilization. Count Iblis (talk) 02:12, 19 October 2013 (UTC)

You have moved the bar from global warming to climate change. Neat huh? μηδείς (talk) 02:35, 19 October 2013 (UTC)

No moving involved, only clarification. Global warming is the primary and most easily measurable effect, but no serious scientist ever thought that a general warming would have no or insignificant secondary effects on climate. --Stephan Schulz (talk) 18:51, 21 October 2013 (UTC)

This discussion has been closed. Please do not modify it.
The following discussion has been closed. Please do not modify it.
Why are you still allowed to post here? 68.0.144.214 (talk) 03:33, 19 October 2013 (UTC)

It depends on the crop. Field corn, for example, loves heat. But it also needs water at the right times, and a minimal growing season. The length of the growing season varies from north to south in the US. Northern varieties are bred to mature more quickly. If the North's summer becomes a lot longer (which does not really appear to be happening anytime soon), the corn with longer time to maturity could be grown farther north, and total yields would increase - provided the water and nutrients are also there. ←Baseball Bugs ^{What's up, Doc?} carrots→ 04:04, 19 October 2013 (UTC)

Icebreaker

Three related questions: (1) Is it possible to make a hole in pack ice by firing a torpedo at it, as described in Ice Station Zebra? (2) Is it safely possible to use this method to free a ship trapped in ice? (3) Is it safely possible to free a ship trapped in pack ice by firing a (non-nuclear) cruise missile at the ice? 24.23.196.85 (talk) 02:35, 19 October 2013 (UTC)

The book synopsis for Under Ice: Waldo Lyon and the Development of the Arctic Submarine mentions "the Skate’s torpedo firing into ice", presumably USS Skate (SSN-578). As for (2) and (3), they sound rather dangerous. WAG: You'd probably have to fire very, very close to the ship to have any appreciable effect. Clarityfiend (talk) 04:19, 19 October 2013 (UTC)

As for using a cruise missile, an unguided torpedo is ever so many times cheaper than a Tomahawk missile at USD 1.5 million each. [16] It would be the world's most expensive hole in the ice. Also you'd have to get the thing to explode under the ice, which might not be as easy as it sounds - I'm not sure about that because they don't put the instruction manual on the internet. Alansplodge (talk) 08:35, 19 October 2013 (UTC)

Sometimes they do: [17] :-)Phoenixia1177 (talk) 08:45, 19 October 2013 (UTC)

Gosh! Alansplodge (talk) 19:14, 19 October 2013 (UTC)

(un-indent) Right, a torpedo is MUCH cheaper than a cruise missile -- but suppose there's a ship trapped in pack ice and being slowly crushed by the pressure, no icebreaker can get to it in time, and there's no submarine in the vicinity to fire a torpedo -- only a B-52 armed with cruise missiles (with conventional, non-nuclear warheads, of course). Is the trapped ship done for, in this scenario, or might it be freed without being sunk by the explosion in the process? 24.23.196.85 (talk) 19:32, 19 October 2013 (UTC)

With enough missiles you could free any ship I guess, not sure if one will do much good. A ship won't be crushed if it's trapped in a small isolated ice sheet, it's the pressure from the wind and sea currents on large ice sheets that can crush a ship, the same way it creates pressure ridges. An explosion on the ice will break up the ice in the vicinity and inertia will prevent the force being transmitted through the ice over long distances, but how many cruise missiles it would take and what the optimal distance (or optimal heigh of detonation) would be is anybody's guess...

Explosives were used in an attempt to free the Discovery, see Discovery Expedition: Second relief expedition. Ssscienccce (talk) 12:37, 21 October 2013 (UTC)

In other words, this method is doable, but inefficient and potentially dangerous. Thanks for the info! 24.23.196.85 (talk) 23:58, 21 October 2013 (UTC)

Largest and most used Gastroenterology textbook?

What is the largest and most used (by universities) Gastroenterology textbook for students? Thanks. 79.182.146.170 (talk) 02:59, 19 October 2013 (UTC)

Asymmetric reciprocal effects

What are some of the most famous examples of asymmetric reciprocal effects?

What I mean is, you have two parameters, say volume and pressure, or capacitance and voltage -- I chose these randomly, these may not be good choices -- and when you increase one of the two, the other will respond and change, too, by a factor that relates to the increase in the first quantity. But when you reciprocate, i.e., increase the second quantity of the pair instead of the first, the first quantity will not change by the same factor but by much less or much greater a factor.

Apologies for my clumsy (and probably incorrect) use of technical terminology. — Preceding unsigned comment added by 89.204.130.121 (talk) 05:39, 19 October 2013 (UTC)

Anything with a non-linear relationship in it somewhere will be non-reciprocal, as will anything that is what is in electronics termed an active device. For example, all electrical networks consisting of nothing but passive components (ie resistors, capacitors, or inductors) will be reciprocal. A field effect transistor is an example of an active device, it can be set up so that a certain change in gate (input) voltage will produce a large change in drain (output) voltage, but any change in drain voltage forced by external circuitry will not change the input voltage. In radio, probably the most famous non-reciprocal (and active) device is the coherer, used in the early days to detect radio signals and convert them to audible clicks. Coherers compised a quantity of loose metal filings in an insulating (eg glass) tube. The incidence of radio wave energy caused the filings to clump together and drop in resistance, allwing current to flow in headphones. An electromagnet was typically empoyed to bang the side of it and loosen the filling again, ready for the next radio wave pulse. Obviously, no amount of banging or tapping on a coherer will produce radio frequency energy, so the device is clearly non-reciprocal.

Active devices, and non-reciprocal devices, can be made with magnetic materials, as the response of magnetic materials is inherently non-linear. Before the invention and refinement of transistors, magnetic amplifiers where widely used wherever it was desired to control a large current with a small input current - the regulation of factory processes, control systems for aircraft and missiles (use of magnetic amplifiers in missiles for flight path controls began with the very first - the German V2), control of elevators in multi-story buildings, and many more.

A carbon microphone, used in telephones for about 80+ years, is an example of a non-reciprocal active device. Sound pressure in the air causes change in electrical resistance and thus electric signal current. But no amount of electrical signal will produce a sound pressure. Carbon microphones are also an amplifier. Because the energy for the output current comes from a battery, the electrical power output was greater than the sound power input. That was important, as it overcame signal attenuation in the lines, and losses in the receiver, and allowed the deployment of telephone company networks before radio tubes and transistors were invented.

58.167.234.97 (talk) 06:03, 19 October 2013 (UTC)

That is very useful information to have, thank you very much. But unless I misunderstand you, the examples you give are of non-reciprocal relations only, whereas I am looking for reciprocal but not symmetric relations. (I increase parameter A from 30 to 40 and parameter B consequently increases from 100 to 101. But when I increase parameter B from 101 to 102, parameter A increases only from 40 to 41.) --89.204.130.121 (talk) 06:22, 19 October 2013 (UTC)

I gave you exactly what you asked for. My examples were:-

• Coherer: parameter A is field strength of radio frequency; paramter B is DC current. A affects B but B has no effect on A.
• Field effect transistor: Parameter A is gate voltage; parameter B is drain voltage. Again A affects B but B has no effect on A.
• Carbon microphone: parameter A is sound power; parameter B is electrical signal power.
• Another example is magnetostriction: In certain materials, a magnetic field causes the material to change in length and so apply pressure to something. But applying a pressure change does not produce a magnetic field.

Your use of the term "symmetrical" is what engineers term "reciprocal". "Reciprocal" to a mathematian means dividing into one, but to an engineer it means the translation of energy goes both ways between port A and port B. If changing A from 30 to 40 causes B to go from 100 to 101, AND if changing B form 100 to 101 (or changing it from 101 to 102) causes A to change from 30 to 40, then engineers would call it a reciprocal device. They would call it symmetrical if ALSO changing B from 100 to 99 would cause A to change from 30 to 20. That is, the change (on either side) had the same effect in both polarities. Almost all active devices are non-symmetric to some degree, as they are to a certain extent non-linear. (non linearity is of course the source of harmomic distortion in amplifiers) Electronic tubes follow a 3/2 power law. FETs are square law at low currents.

Perhaps you could give a set of numbers showing what you mean by reciprocal" AND "non-symmetric", as all my examples illustrate an, albiet more at a greater extreme, principle embodied in the numbers you used.

58.167.234.97 (talk) 06:51, 19 October 2013 (UTC)

Perhaps I should say "mutually influencing" instead of reciprocal? Other than that, I can't think of anything else that I could say to make my question clearer. (And I've given a set of numbers exemplifying what I mean in my previous post.) --89.204.130.121 (talk) 07:19, 19 October 2013 (UTC)

(I see that you've added some text to your latest reply in between my reading it and my posting of a response. Thanks but my question still remains. And in case the answer is, No that is impossible, nothing in nature works that way: Is that so not only for two mutually influencing parameters directly coupled to each other but also if they are just two mutually influencing parameters in a complex environment with many intervening links in one or more (non-linear?) cause-and-effect chain(s)? --89.204.130.121 (talk) 07:27, 19 October 2013 (UTC)

I'll have another go at making this clear for you. Here are the basic principles:-

• I'll use the term network. A network can be a network of electronic parts, mechnical parts, or whatever. In many fields, the term is system.
• Parts can be passive or active.
• Passive means that there is a one-to-one correspondence between a quantity and another quantity. A pipe is a passive part - the quantity of water leaving equals the quantity entering. Change one and teh other must change equallly - in sign and in magnitude. An electronic resistor is a passive part. The current in it is directly proportional to the voltage across it. Change either, and the other must also change in proportion, in sign, and in ratio.
• An active device is a device where one quantity controls another quantity. One example is a water tap - the position of the tap handle controls the water flow. Another example is a catalyst - the concentration of a catalyst (you can call this your "a") control the rate of reaction (your "B") of two other chemicals. Another example is a filed effect transistor - the gate voltage controls the flow of current in the drain.
• Practical parts can be almost ideal passives (eg electronic resistor), almost ideal actives (a transistor), or a mix of the two (eg a thermistor).
• A part can be linear, or non-linear.
• A linear part means that a change of x% in one quantity always occurs with the same x% change in another quantity - regardless of polarity, magnitude and offset from zero. An example is a resistor - a change in x volts will always cause the same y amps in current, regardless of whether it is a change from a to a plus x, or b to b plus x.
• A non-linear part means that the change in one quantity varies in magnitude, even when the change in the other quantity is always the same, when the offset from zero is changed. An example is a diode. As voltage is increased, the current increases ever more dramatically.
• A non-linear device is always non-symmetrical - that is a negative change has a different magnitude effect than an equal positive change. — Preceding unsigned comment added by 58.167.234.97 (talk) 11:07, 19 October 2013 (UTC)
• A network or system can comprise any number of linear, non-linear, passive, and active devices.
• A reciprocal network or system is where two quantities (your "A' and "B") each have an equal effect on each other.
• A network or system comprising only of pure linear passive parts is always reciprocal.
• A network or system containing at least one non-linear part or at least one active part will have a degree of non-reciprocity.

If this does not satisfy you, I suggest you give a set of numbers that show EACH term that you used. That is, give a set of numbers that show what you mean by non-recipocity, and another set of numbers that show what you mean by non-symmetricality. Then I can give examples to suit each. 58.167.234.97 (talk) 11:02, 19 October 2013 (UTC)

(outdent) You're starting to discuss a great deal of terminology now, which may be a good idea (to make sure we're all on the same page) but maybe not absolutely necessary. Here is your set of numbers:

Reciprocal:

Change in Quantity of A	Resultant Change in Quantity of B
30 -> 40	100 -> 101

Change in Quantity of B	Resultant Change in Quantity of A
100 -> 101	30 -> 40

Symmetrical:

Change in Quantity of A	Resultant Change in Quantity of B
30 -> 40	100 -> 101
40 -> 30	101 -> 100
Change in Quantity of B	Resultant Change in Quantity of A
100 -> 101	30 -> 40
101 -> 100	40 -> 30

But in what I am asking for, numbers might look like this:

Increase in one Quantity	Resultant Increase in other Quantity
QA 30 -> 40	QB 100 -> 101
QB 100 -> 101	QA 30 -> 31
Decrease in one Quantity	Resultant Decrease in other Quantity
QA 40 -> 30	QB 101 -> 100
QB 101 -> 100	QA 40 -> 39

I've called this "asymmetrical", for want of a better word. (Again, A and B are measurable physical parameters, such as volume, pressure, voltage, magnetic field strength, temperature, quantity of an element or compound, acidity, salinity, or whatever else one can imagine.)

Now if you tell me, that's preposterous, nature cannot work that way (or: simple logic tells us this is impossible), then o.k., my follow-up question is this:

Can there be a sufficiently complex system, in which A and B are but two of many distinct but mutually influencing factors, in which the interactions work in such a way that ultimately, changing A impacts B differently than changing B impacts A, in a manner as shown in my table above?

And if my follow-up question is nonsensical because there is something wrong with its internal logic or understanding of nature, please point to the error in my thinking. 89.204.130.121 (talk) 15:11, 19 October 2013 (UTC)

Ok, that makes it clear what you are asking. And I should have picked up what you were getting at earlier. Your third case is what an electrical engineer would call a two-port device with dissimilar forward and reverse gain. "Gain" is the ratio of an output quantity to an input quantity. Devices with dissimilar forward and reverse gain are an important class of non-reciprocal devices. So important that most engineers would just say its non-reciprocal. Many devices and systems in electrical/electronics, mechanics, chemistry, etc have this property. The word assymmetrical is not a bad word for it, but in electrical/electronics, assymetry means something else as previously described. Such devices are not at all remarkable. The examples I gave earlier, carbon microphones, transistors, radio tubes/valves, magnetostrictive devices, chemical catalysts, worm-gear mechanical drives with regard to torque, etc etc, all have dissimilar forward and reverse gain. In carbon microphones, for example, the forward/reverse gain ratio is infinite. In worm gear drives, by altering the gear pitch, you can have pretty much any forward/reverse torque ratio you like. In transistors, the ratio is very high (>1000:1 in low power audio transistors) but you can increase or decrease it by adding circuitry around the transistor. In fact, for any device with any arbitary dissimilar forward/reverse gain ratio, you can in theory add feedback to get any desired ratio. For example, if you feed some of the output back to the input at the right level and in opposite sense/polarity, you can completely cancel out the reverse gain, producing an infinite gain ratio. This is called neutralisation. 58.167.234.97 (talk) 16:00, 19 October 2013 (UTC)

Thank you, this is helpful! 89.204.138.111 (talk) 20:06, 19 October 2013 (UTC)

But just for curiosity's sake: The examples you've given -- water tap, transistor, microphone, worm gear -- are all machines. Can you also name a couple examples of natural systems meeting this definition? 89.204.137.240 (talk) 01:03, 20 October 2013 (UTC)

Don't forget catalysts, which I mentioned. Catalysts can be natural as well as man-made. Biological systems use complex catalyst systems to extract energy from food and use it. There's probably a vast multitude of biological systems that display dissimilar forward/reverse gain. DNA: In the simplest cases, the quantity of an enzymes etc is proportional to the number of genes for that enzyme (other factors also affect the rate of enzyme production), however the quantity of any enzyme does not affect the rate of DNA production. Drugs: Narcotics work because they are closely similar to molecules naturally made in the brain. If you take a narcotic, the brain over time automatically cuts down its own production - that is one of the mechanisms of addiction, because if you stop, you're in trouble until the brain ramps up production again, driving you to take more of the drug. So there narcotic you take can be your Quantity A, the brain's natural production can be regarded as your quantity B. The gain or coupling in each direction is obviously not the same. Various climatic parameters are coupled in more or less complex ways with dissimilar gains. For example an increase in temperature increases average wind strength, and an increase in wind strength affects temperature, but depending on distance from coasts etc, the temperature may increase or decrease. 60.230.213.251 (talk) 01:26, 20 October 2013 (UTC)

That's very interesting. Speaking of climatic parameters, we know from paleoclimatic records that in Earth history, an increase in temperatures generally was followed by an increase in carbon dioxide, but not the other way around or not as often. Could the temp v. CO2 relationship be an example of dissimilar forward/reverse gain? You mentioned feedbacks. How well are all the intervening feedbacks between CO2 change and temp change that may damp so-called climate sensitivity understood? Can the principle of dissimilar forward/reverse gain help us better understand the lull in global warming since 1998? 89.204.137.240 (talk) 02:04, 20 October 2013 (UTC)

I don't want to take this any further as I am not a climate scientist. I am an electrical engineer. While engineers have a lot of good mathermatical tools that helps their understanding of non-electrical subjects, there is a danger in straying from one's qualified field. However, my impression is that climate science is not well understood, as the complexity of the calculations demand a super-computer. And even then, too many simplifications have to be made in order to make the problem manageable. I suggest you ask your question on climate modelling again, without all the preample that is in this question. Then any climate scientists in Wikiland may answer you. When questions and their answers get too long, it tends to inhibit others from reading it all. 60.230.213.251 (talk) 06:28, 20 October 2013 (UTC)

i ask you again:is there any picture or graphic of the mixed flow compressor from the Pratt and Whitney pw610f?

is there any picture of this compressorß cause i surfed the internet and found the patent but there is no picture about this compressor and google gives e nothing aswell and surfing the internet in general helped nothing. so i give you the question again is there any picture of this compressor?Saludacymbals (talk) 15:02, 19 October 2013 (UTC)

You can get patents from various online sources. The patent should have a diagram, but on some online sources you only get teh overview text. So, if you tell us the patent number, the patent titel and assingnee, and which site you got that from, we'll have a chance to find another online patent source that has the diagram(s). 58.167.234.97 (talk) 15:40, 19 October 2013 (UTC)

Assingee is PRATT & WHITNEY CANADA CORP. the patent number is EP 1322865 B1 the title is Mixed flow and centrifugal compressor for gas turbine engine but there is only the cuttet diagram and no picture of the compressor section and google and internet dont shows more Saludacymbals (talk) 20:57, 19 October 2013 (UTC)

Haven't found one... Ssscienccce (talk) 15:05, 21 October 2013 (UTC)

Special dietary treatments for Autism

Is there published research on the effectiveness of special diets, such as the Specific Carbohydrate Diet, in controlling autism/ADHD?

This is a field of medicine where research is scant. Many parents treat or control their child's autism with special diets they believe to work, or drugs like ritalin the consequences of which haven't even been confirmed - for example whether it changes the patient's blood pressure. Authoritative publications like Proceedings of the Royal Society B, The Lancet and so on simply haven't covered in in the same way as, say, lung cancer or Alzheimer's Disease. I'm looking for sources like professional journals and books. Thanks 92.28.225.238 (talk) 15:47, 19 October 2013 (UTC)

There is quite a bit of published research -- not all of it high-quality though. See https://www.aacp.com/pdf%2F2104%2F2104ACP_Review2.pdf for a systematic review that discusses diet along with other treatments. Looie496 (talk) 16:05, 19 October 2013 (UTC)

Note that, although both autism and ADHD can lead to social interaction problems, the two conditions are quite different. Also with high functioning autism, it can be strongly argued that it is wrong to try to correct it. A degree of autism allows high achievement in subjects requiring technical detail and is thus of benefit to society. Many autistics like themselves just as they are, thank you very much. Whether or not ADHD is a real disease is controversial. The incidence of ADHD diagnosis is high in cultures where parents and teachers are taught/counselled/compelled not to use corporal punishment, whereas in previous generations the offender would have got a good smack or the cane, and that would quickly end the problematic behaviour. This is not to suggest that their cannot be a disease or gene expression problem leading to ADHD, but it is very likley that many diagnosed with ADHD do not have anything wrong with them other than just needing discipline. 58.167.234.97 (talk) 16:15, 19 October 2013 (UTC)

Autism and ADHD are comorbid conditions - meaning that if you have one of them, the odds of you also having the other is significantly higher than for the general public.

There are ways to alleviate the symptoms of ADHD (eg with drugs like ritalin) but there is nothing that helps with Autism - other than teaching sufferers social skills and such - which helps them to live with the symptoms without in any way diminishing them.

We don't entirely understand why Autism happens - and it's really only a description of a collection of symptoms - not all of which apply to all sufferers, so there could be many underlying conditions that produce the symptom group that we call "Autism". Worse still, it's a spectrum condition - varying from people who are almost normal to people who are profoundly cut off from the rest of the world.

I have Aspergers' Syndrome - which is now 'officially' called "High functioning Autism". There are some common symptoms that I don't have (eg extreme sensitivity to certain sounds and smells) - but I do have the social interaction issues and the beneficial symptoms such as improved mental focus and the inability to empathically understand how other people are feeling. For all of that, I'm one of the lucky ones who doesn't want to be cured.

Having met people with many more of the symptoms - it's really hard to believe that there is just one underlying condition - which means that it's highly unlikely that there can be just one cure. For that reason, I am super-suspicious of "cures" that are just anecdotally supplied.

I think that many parents of the less severely afflicted kids are being quite cruel to their children because they don't understand what's really going on. I have seen parents who tried to "cure" the social interaction problems by creating a meet-up group where all of their children could meet up and play together. For someone with Asperger's, it's hard to imagine anything more horrible than doing this!

It's also worth mentioning that the idea that innoculations are the cause of Autism is busted, busted, busted. For chrissakes don't avoid innoculations for your kids in order to "prevent" them getting Autism...it won't work and it's a dangerous thing to do.

SteveBaker (talk) 20:36, 19 October 2013 (UTC)

Autisma is a spectrum disorder. No two autistic persons are alike in their symptoms. For some children, parentally or teacher driven social interaction may be cruel or it may not be. A couple of things:-

• Generally, high functioning autistics (Aspergers) gain good, even very good, social interaction later in life. That's because they have enough built-in social function to learn from their experience. The best time to learn is when you are young. So an autistic child may be uncomfortable in social groups, but the experience will give him a better adult life. Sometimes you have to be cruel to be kind. Like birds, who when hatched don't know how to fly and must learn from watching parents, all people have to learn how to do social interaction. It's just that for normal people, learning it is easy, but for autistics it is hard. High functioning autistics learn social skills and can end up as good at as anybody - it just takes them longer to do it. The earlier they start the better, just as playing piano is difficult - but start at age 5 (ie as soon as the arms and hands are big enough), practice every day and you can eventually be very good.
• Like other brain aberations leading to poor social skills, many autistics may have poor social skills and lack the ability to bond with friends. But that doesn't mean they don't crave friendship and human interaction. Some couldn't care less, but some do care. Often, childhood social interaction doesn't result in the autistic child gaining lots of friends, but the frienship bonds they do form can after a while be very strong and last a lifetime.

60.230.213.251 (talk) 01:42, 20 October 2013 (UTC)

Thank you. 92.28.225.238 (talk) 08:54, 20 October 2013 (UTC)

There are a handful of reports about "purine autism" (autism with hyperuricosuria), a pretty small subset of people who have specific issues with purine metabolism, and even a claim that the drug allopurinol can address it [18]. I think potentially a number of traditional Chinese gout remedies that work by a similar principle of inhibiting xanthine oxidase, such as cassia/cinnamon, would have a related beneficial effect, though not as potent, but I don't think any real parent would want to fool around with an experiment on top of an experiment where their kids are concerned, so I don't think there's any data on that! But though there's a lot of research about metabolic variations in autism, I don't feel like much of it has carried through, and of course simply diagnosing and subcategorizing the condition into several subtypes is a step that, though needed, isn't even being done yet on a regular basis. Wnt (talk) 16:05, 20 October 2013 (UTC)

To follow up, it looks like there is some Chinese traditional medicine basis for dealing with autism, though it is hard to tell the difference between truly traditional treatment for the "Syndrome of five delays" and modern ideas to repurpose existing medicines for other conditions [19]. Still, various sources of unknown veracity such as [20] lead me to think that there is an acknowledged relationship between autism and "kidney qi" and "liver qi" which seem otherwise involved in metabolic disturbances such as gout, and herbs recommended for it do indeed include cinnamon, among many many others. As hard as it is to pick through information like this, I suspect that any nutritional help for autism is more likely to come from a careful, skeptical, systematic examination of the existing body of semi-empirical observations than from some sudden inspiration based on the genetic or neurological data. Wnt (talk) 16:39, 20 October 2013 (UTC)

There are indication that vitamin D is involved in autism. The evidence is not conclusive and sometimes contradictory, but this may be because wat is considered to be a normal vitamin D level may still be way too low. If the hypothesis that normal calcidiol levels should be around 120 nmol/l or higher is correct, then the autism epidemic in the West may well be the making of the medical community itself. The medical community had issued RDAs for vitamin D that were too low by today's standards, and today's standards are not compatible with what is observed in indigenous people in Africa. Particularly during pregnacy there is a big problem.

Until very recently the official guidelines were that expecting mothers should not use any vitamin D (due to the now debunked theory that this could cause damage to the unborn child), so their calcidiol levels would be very low. But measurements of the calcidiol levels of pregnant Maasai and Hadzabe women that were performed last year show that they have very high calcidiol levels, some had levels higher than 250 nmol/l (until recently it was thought that 250 nmol/l is the highest level that is obtainable by natural means alone). Also, huge spikes in the calcitriol hormone levels were observed during pregnacy, extremely high levels that according to common wisdom should be dangerously high levels, are apparantly part of the normal pregnancy processes. Count Iblis (talk) 20:02, 20 October 2013 (UTC)

I'm underwhelmed by the study that article is based on, because it shows that people with autism have lower levels of vitamin D -- but that could be because those people simply spend more time indoors, for example. Wnt (talk) 01:26, 21 October 2013 (UTC)

Here are a few Cochrane Reviews on the subject:

• Gluten and casein-free diets for autism spectrum disorder. It concluded, "Research has shown of high rates of use of complementary and alternative therapies (CAM) for children with autism including gluten and/or casein exclusion diets. Current evidence for efficacy of these diets is poor. Large scale, good quality randomised controlled trials are needed."
• Vitamin B6 and magnesium in combination for children with autism spectrum disorder. It concluded: "Due to the small number of studies, the methodological quality of studies, and small sample sizes, no recommendation can be advanced regarding the use of B6-Mg as a treatment for autism."
• Omega-3 fatty acids for autism spectrum disorders (ASD). It concluded: To date there is no high quality evidence that omega-3 fatty acids supplementation is effective for improving core and associated symptoms of ASD. Given the paucity of rigorous studies in this area, there is a need for large well-conducted randomised controlled trials that examine both high and low functioning individuals with ASD, and that have longer follow-up periods.

You can find other autism related related Cochrane reviews here (not all about special diets/supplements). And I want to re-emphasize the message hinted by User:Looie496's above: unless you (ie the OP) are medically trained, or have justifiable confidence in your ability to properly survey and interpret the results, you should really be looking at only high quality reviews and ignoring individual medical reports on the subject. Abecedare (talk) 02:01, 21 October 2013 (UTC)

Quite right. But, again, correlation does not mean cause. It may be that kids with a degree of autism do better because their parents take an interest, and try lots of things - including forcing contact with other kids. And the kids will be aware of their parents' interest, and believe they can interact socially - the placebo effect. Kids whose parents take the easy road and give the child what he wants, i.e., stay in his room and persue his hobby interests, are never going to learn social skills. Heck, even completely normal children will not learn social skills if they don't have contact with other kids. 60.230.213.251 (talk) 05:31, 21 October 2013 (UTC)

Potential Energy and Kinetic Energy

Assuming two massive bodies collided with each other due to gravitational potential energy. In such case, it is expected that collision will result in heat loss. How do we (approximately) calculate this increase in temperature at the surfaces of collision? Can we say that: Heat rise = Kinetic energy difference = potential energy difference? --Almuhammedi (talk) 16:04, 19 October 2013 (UTC)

It's virtually impossible to calculate the temperature right at the surface of collision, because so many factors affect it. In fact it really isn't even meaningful, since the collision surface is obliterated at the instant of collision. Looie496 (talk) 16:08, 19 October 2013 (UTC)

Indeed. In the simple case of two uniform homogenous bodies where nothing gets torn apart and there is no elasticity, the deformation of the surface must take a finite time, and that tiem can be estimated. In that time some of the heat will flow into the bodies within and away from the deformed surface, so there will be a temperature profile. Following the collision, heat will continue to flow within the bodies until the temperature is uniform thoughout. You can easily calculate the final uniform temperature, by relating the kinetic energy to the specific heat. This is a not uncommon question in asked of students in physics courses. By knowing the thermal conductivity of the bodies, you can with a lot more math, calculate the peak surface temperature rise. In reality though, your two massive bodies probably won't be uniform homogenous, will exhibit some elasticity, the elasticity will be non-linear (ie not obey Hook's Law) and some of the material will be sheared/split/broken off. Then a proper cacluation is near impossible. 58.167.234.97 (talk) 16:37, 19 October 2013 (UTC)

You can calculate the total thermal energy added to the colliding bodies - that's easy. But figuring out how much of the heat energy ended up in each body...and precisely where within each body is virtually impossible. It depends on the shapes, the initial temperatures, the material it's made of, the ambient air temperature...a huge number of unknowns.

What you can say, easily and immediately is that the total heat energy created is equal to the sum of the kinetic energies of the colliding bodies - which (if this is a collision caused entirely by one object falling under gravity onto the other) is equal to the initial gravitational energy. SteveBaker (talk) 20:18, 19 October 2013 (UTC)

No, not correct. In purely elastic deformation, all of the kinetic energy is returned to the two bodies. Two bodies colliding dead on will rebound, heading apart again, without any conversion to heat. If two elastic bodies collide but with a glancing blow and not dead on, part of the knietic energy will end up as the bodies spinning while they fly apart again. For conversion to heat, plastic deformation is required, as Ssscience pointed out below. This generally means that calculating the amount of heat is not trivial, and temperature rise most decidely non-trivial, because plastic deformation, specific heat, and thermal conductivity are all non-linear in real bodies. Not at all easy. 60.230.213.251 (talk) 02:19, 20 October 2013 (UTC)

Without plastic deformation, the decrease in kinetic+potential energy will equal the increase in "heat energy". When plastic deformation has occured, part of the energy may have gone into breaking of inter- or intramolecular bonds, changes in surface energy, stored as internal stress... Ssscienccce (talk) 00:30, 20 October 2013 (UTC)

What you can do here is compute the relative speed just before impact. If this is much more than the speed of sound of the materials the bodies are made of, then you can use the same methods that are used to compute the effect of impacts in the Solar System. A lot is known about this from theory, simulations and experiments, there are ready to use formulas that are derived from these results using scaling relations. Count Iblis (talk) 14:53, 20 October 2013 (UTC)

Recrystalization vs. acid-base extraction

In an experiment in which benzoic acid is purified using two methods, recrystalization (heating and cooling) and acid-base extraction (which involved mixing benzoic acid with ethy acetate and changing its pH in a separatory funnel), which should one should result in a higher percent mass recovery (i.e. recovered mass similar to initial mass)? Also, I have no idea how, but my recovered mass from the extraction method was twice the initial mass. This doesn't seem right. --FutureTrillionaire (talk) 16:18, 19 October 2013 (UTC)

Extraction should give a MUCH higher percent recovery -- recrystallization relies on the decrease in solubility with temperature, so a good portion of your product will go into the waste with the solvent (been there, done that!) As for why your recovered mass is twice the initial mass -- the most likely explanation is that your extracted product still has A LOT of solvent in it (or maybe it somehow reacted with your solvent because of a pH excursion, though I think this is unlikely). 24.23.196.85 (talk) 19:25, 19 October 2013 (UTC)

Cool. Thanks man!--FutureTrillionaire (talk) 19:38, 19 October 2013 (UTC)

With a solubility of 68.0 g/l at 95°C and 2.1 g/l at 10°C (1.7 at 0°; 2.9 at 20°), recrystalisation could be quite efficient, depending on the amount of water used... Ssscienccce (talk) 21:10, 19 October 2013 (UTC)

Health effects of low exposure to pesticides

Is there any evidence of harmful health effects from consuming fruits and vegetables containing a small amount of residual pesticides? The article on health effects of pesticides wasn't clear about this. 74.15.138.165 (talk) 17:20, 19 October 2013 (UTC)

Short answer: yes, see paraquat for example. Nuanced answer: one reason the article is not clear is that the answer is not entirely clear -- among other factors it depends on what pesticide is involved and what amount. There is some reasonably solid evidence that exposure to certain pesticides can increase the likelihood of Parkinson's disease, for example (see our article). Looie496 (talk) 17:40, 19 October 2013 (UTC)

Well, that depends on how small is your small amount -- if you're talking about 1 part per trillion, then no pesticide can possibly have an effect at such a low concentration, whereas if you're talking about, say, 100 parts per million, then long-term harmful effects are very much possible, depending on the pesticide in question. 24.23.196.85 (talk) 19:51, 19 October 2013 (UTC)

What do you mean by exposure? Does that mean exposure at the typical concentrations that you would find on a vegetable in a supermarket? 74.15.138.165 (talk) 20:12, 19 October 2013 (UTC)

Birth defect rates were higher (in U.S., 1996 to 2002) when conceived in the period april to july, same period in which concentration of pesticides in surface water (and possibly tap water) is highest. Increase (in birth defects) was 3%. Doesn't mean causation of course... Ssscienccce (talk) 22:48, 19 October 2013 (UTC)

• There is sometimes a paradoxical measurable benefit from a small exposure to certain toxins. I am annoyed I cannot think of the term for this. Perhaps it will occur to someone else. μηδείς (talk) 00:44, 20 October 2013 (UTC)

Hormesis 89.204.137.240 (talk) 02:52, 20 October 2013 (UTC)

Or Desensitization (medicine), immunization and even homeopathy, 175.38.168.70 (talk) 03:02, 20 October 2013 (UTC)

October 20

Measuring tension in a belt

I have a long, thin drive belt running between two pulleys that are 1.2 m apart. I'm trying to estimate the tension in the belt - but it's hard to do directly. What I did was to buy a machine that fishermen use to weigh their catches - a little gizmo with a hook on one end and a digital display on the other. If I hook the machine over the belt midway between the two pulleys and pull on it until the belt deflects by 5cm - it reads 2170 grams.

Is this enough information to calculate the approximate tension in the belt? What would be the equation?

SteveBaker (talk) 04:38, 20 October 2013 (UTC)

I don't think you can calculate the tension in the belt when you aren't pulling it sideways - depending on the elasticity, it could in theory be completely slack. AndyTheGrump (talk) 04:49, 20 October 2013 (UTC)

Oh - yeah, I appreciate that. What's the tension when it's deflected? In this case, I doubt the tension is much different because drive belts are fairly inelastic and the deflection I'm applying is tiny compared to the overall belt length of around 2.4 meters. SteveBaker (talk) 04:59, 20 October 2013 (UTC)

(Hmmm - if it helps, I can measure the force to deflect the belt by two or more different amounts.) SteveBaker (talk) 05:00, 20 October 2013 (UTC)

(ec) You can approximate the tension by assuming the belt is a string (i.e., assuming its tensile forces are symmetric and isotropic). Then convert the measurement from your spring scale - you want to convert from grams (as indicated) to force (in newtons, or pounds) by multiplying mass times g (9.81 m/s). (Actually, the spring-scale measures weight: force on the spring, not mass, so it ought not display in grams!)

Then, you can draw out the geometry of your deflection: a force of (2170 · 9.81 N) gives a 0.05m deflection, let's say at a perfect right-angle to the belt, which is 1.2 m long. Draw the triangles, and solve for the tension along the hypotenuse - just straightforward trigonometry! The forces on an ideal string, in x- and y-, are exactly proportional to the deflections in x- and y- so you can use "similar triangles", or compute the tangent, or use whatever other favorite trigonometry formula you like.

The catch is, belts are not perfectly isotropic and symmetric; it is not "string-like; tension out of the plane is not perfectly additive with tension in the plane of the material. Even worse, your measurement is highly sensitive to error because of the tiny deflection you can realistically create. A tiny measurement error will be multiplied by an immense scale factor. (If you look online for belt tension gauges, as used in automobile maintenance, those devices work around this problem by controlling the geometry - they hold two points of the belt stationary while a second armature performs the deflection. No matter how long the belt is, the measured vertical deflection is always about the same size as the measured horizontal span, which keeps your calculations in a less pathological part of the tangent function; or put more simply, you want to avoid measuring a deflection using a long and skinny triangle!) My number-crunching, using Steve's data, estimates about 100 pounds tension in the belt: not impossible, but higher than I'd expect.

Among our many contributors, one of us has an ASME handbook; we can probably dig out an appropriate "engineering fudge factor" to adjust the simple equations, if your belt is made of a standard material, size, and shape... Nimur (talk) 05:01, 20 October 2013 (UTC)

Ah - so I understand the sensitivity issue. What I need to do to get more precision then is to perhaps add two fixed pulleys - maybe 10 cm apart and pull on the belt midway between the two to get the 5cm deflection. The problem with that is that the belt would need a lot of force to deflect it by so much over such a short distance. I'll play around with it...maybe I can figure out something better.

Many thanks! Having more eyes on the problem always helps. SteveBaker (talk) 16:59, 20 October 2013 (UTC)

I think you should lay out which alterations are possible and which ones aren't - also whether you need to know the tension moment to moment or if you are just measuring it once to position the pulleys. For example, if you have the option to anchor one of the pulleys by a material with Hooke's Law elasticity such as a spring, you can calibrate how far the pulley moves with varying forces you apply yourself, and then measure the position of the pulley moment to moment to know the tension. If you have the option to modify the belt itself, you could have a region of the belt with a known elasticity, with a small insert that is inelastic anchored to one end of the region, and have it report how far the other end is displaced. Wnt (talk) 15:56, 20 October 2013 (UTC)

I can't change the geometry or material of the belt - both pulleys are fixed (and moving them would alter the tension - which kinda defeats the object of measuring it). SteveBaker (talk) 16:59, 20 October 2013 (UTC)

Can you do the following?

• make two marks on the belt and measure precisely the distance between them
• take the belt off the pulleys and put clamps on the belt at the two marks
• apply measured tension to the clamps to stretch the part of the belt between the clamps, to the extent that the two marks are the same distance apart as when the belt was on the pulleys
• use the applied tension as an estimate of the tension of the belt when it's on the pulleys

--173.49.18.190 (talk) 08:34, 21 October 2013 (UTC)

Could you remove the belt, get another 2 pulleys, fix one, attach a newtonmeter to the other, and move it to the correct position (ensuring it is held by only the newtonmeter and the belt? The distances won't be 100% accurate, but it's probably closer than you'll get by the other techniques. If you set up some kind of fixed sand 120cm+x (where x is the length of the newtonmeter, plus a fudge to account for the extension while in use) from the fixed pulley, then you don't even need to hold it. MChesterMC (talk) 08:46, 21 October 2013 (UTC)

If the belt is sagging measurably by it's own weight, and you know the weight per unit of length of the belt, you could calculate the tension without applying springs or other devices by using the formulas for a catenary. But that's unlikely, drive belts usually have high tension compared to weight... Measuring the same way you did for a few other deflections (2, 3 and 4 cm, best not larger, don't want to stretch it) and comparing it could give you some idea. You have to measure the deflection in respect to a straight line between the pulleys, not from the belts "resting position" (because of sagging). To give approximate value, use 120*M/D with M= measured grams and D is deflection in cm. Not completely exact, but for small deflections it doesn't matter in comparison with the measuring accuracy. Ssscienccce (talk) 13:36, 21 October 2013 (UTC)

Is reading in your DNA?

Is the skill or tendency to be good/easily learn to read in human's DNA? Throwaway62 (talk) 04:47, 20 October 2013 (UTC)

The capacity for language is practically universal in humans. It's the exceptions and the struggles which get the attention, such as dyslexia. That suggests that language capability is hard-wired, as sure as the ability to walk upright. ←Baseball Bugs ^{What's up, Doc?} carrots→ 06:17, 20 October 2013 (UTC)

It's certainly both, as being exposed to reading early on also affects one's ability and willingness to read. StuRat (talk) 13:16, 20 October 2013 (UTC)

You can say it in a sense, because of course there are genetic learning disabilities that can prevent people from learning the skill. However, despite some interesting phenomena such as dyslexia which involve a more visual-level confusion that people can circumvent with training, I'm not aware of any genetic syndrome that specifically prevents reading while allowing other activities to work normally. This should not be surprising since reading was developed too recently for evolution to affect specifically to any great degree. Wnt (talk) 15:46, 20 October 2013 (UTC)

That last sentence doesn't make much sense to me. Reading is based on symbol recognition, and that ability goes way back in human evolution. In fact, it may even predate humans, as other animals seem able to recognize some symbols, too. So, there should be genes for that, which will affect the ability to read. StuRat (talk) 02:17, 21 October 2013 (UTC)

Well, if it comes to recognizing whether a symbol is a "b" or a "d", dyslexia affects that, and that may be very vaguely comparable to an animal trying to decide if an insect is an edible or inedible species. But recognizing that three letters together in a certain sequence, without spaces, but with spaces around them, have a particular meaning... it's hard to picture an equivalent from other animals. (There's a whole article on reading (process) which I should refer to rather than philosophize about from scratch, I suppose) Wnt (talk) 05:14, 21 October 2013 (UTC)

Though on consideration, it's possible I'm pursuing a No true Scotsman fallacy here... Wnt (talk) 17:12, 21 October 2013 (UTC)

Yes, I think very similar logic is used to determine that "that object is a chair" as is used to determine "that object is the word 'chair' ". It's all just various forms of pattern recognition. Heck, in some languages the word chair might actually be a picture of a chair. StuRat (talk) 17:23, 21 October 2013 (UTC)

But then there's grammar, and also the tendency of, say, Egyptian hieroglyphs to pull away from meaning exactly the thing represented in the picture (how exact that can ever be anyway is a side-track) into the domain of metaphor and sounds. You can say that symbols for sounds are combined into words and the words recognised as patterns representing things, but some of those things are very context-dependent abstract concepts or helper words, some of the words may be new but still understood, and it's all cognitive in fancy ways that amount to something more than pattern recognition ... arguably.  Card Zero  (talk) 18:59, 21 October 2013 (UTC)

This 2009 paper on the genetics of reading disability by by Tracey Petryshen and David Pauls says: "although there may be some disagreement regarding the genetic contribution to specific components of [reading disability], all studies demonstrate that [reading disability] is influenced by genetic factors. However, the specific underlying genetic mechanisms are not known. " Gandalf61 (talk) 09:21, 21 October 2013 (UTC)

Just a note that while the ability to read may be coded in our DNA, the skill is not. Skill in reading falls squarely on the nurture side of nature versus nurture and is not heritable. q.v. Epigenetics.--Auric talk 22:58, 21 October 2013 (UTC)

Dropping a Slinky

I was watching an episode of QI yesterday and on it Stephen Fry claimed that we don't know why the bottom of a dangling Slinky remains stationary until the top compresses down on to it at which point the whole thing falls to the ground. You can see what I'm referring to here. The man in that video though seems to suggest that we do know why this happens. Granted, it's simplified for laymen. Oversimplified you might say. I'm curious though and want a bit more explanation. I'm not looking for math formulas and things though because that would be over my head. But could someone explain it a bit more in depth? I have an idea in my head but I don't really know how to best express it. Something about the tension in the up vector, mid-drop, being greater than the force of gravity.

Note: This is not a request for medical or legal advice. Please do not hat this question. Thanks, Dismas|^(talk) 05:50, 20 October 2013 (UTC)

Isn't this the same as Steve's question above, about belt tension? Or is that just the delusion of generality perpetuated by the addled brain of a physicist...? Yes, tension in the real world is more complicated than the idealizations we teach in elementary statics!

The answer is quite obvious, and the video you linked above explains it very well. The tension at the bottom of the spring exactly counters the force gravity. At time t=0, when the top of the spring is released, the tension at the bottom has not changed. So, there is no net force on that part of the spring material; and it remains at rest.

Why hasn't the tension changed yet? Information about the dropping of the top of the spring should travel fast - at the speed of sound. In spring steel, that's something crazy fast, like eight kilometers per second! Even at 300 frames per second, that would take less than a frame of video to reach the bottom of the spring.

But slinkies are crazy contraptions! Tension between the coils is a totally separate entity from tension along the coils, and if we wanted to, we could describe the slinky as a "meta-material" with multiple speeds of sound! When energy propagates as a compression wave among the coils, (rather than as a compression wave along the interior of the metal material), the speed of propagation is quite slow.

And finally, since we have to satisfy action and reaction, consider the top of the slinky. At time t=0, the exact same rules apply. The tension has not changed and is still pulling that part of the spring downward; this is added to the force of gravity; and the contact force where the man was holding the spring has suddenly vanished. There is a net force: gravity plus downward tension. The top of the spring falls downward, faster than a free-falling object.

If you had enough time - say, you dropped the slinky off a tall building - that compression wave might reach the bottom of the slinky a long time before the slinky hit the ground, and the bottom would move. The wave might reflect, and the slinky would expand and contract as it fell; and then you'd get a wobbling slinky. Chances are pretty good that the entire slinky would start tumbling, unless you were absolutely perfect in the way you dropped it, and air flow was perfect, and so on.

If you study advanced (i.e., continuum) mechanics, your teachers will force you to solve you will have the opportunity to solve the equations of motion as a continuum - rather than treating the spring as a point particle, like you would in engineering statics (or "high school physics"). You will have to express the tension, and in fact the mass, as a function of spatial coordinate, and then construct an equation of motion that satisfies all the constraints; forces must be resolved, and you may also require that the spring remains intact (as a single, solid but perhaps non-rigidly connected object). Usually, this requires learning how to live and work in Lagrangian coordinates. Once you learn that technique, most of the rest of physics - even the weird stuff like relativistic motion and atomic physics - all seems pretty mundane! Nimur (talk) 06:09, 20 October 2013 (UTC)

Even though the speed of the actual movement of the object might be slow wouldn't tension in a substance like that cause effects that may propagate at about the speed of light.175.38.168.70 (talk) 08:41, 20 October 2013 (UTC)

(edit conflict) Further to Nimur's comments above, a simple way of looking at the situation is that the centre of gravity of the slinky falls as expected, but at the same time the slinky contracts because it no longer has a stretching force. Thus the top falls at twice the "expected" speed (because it has both gravity and internal tension acting on it) and the bottom doesn't fall at all until the contraction ceases. Dbfirs 06:19, 20 October 2013 (UTC)

Thank you both. I think I understand better now. And I don't read the Science Desk on a regular basis, so I wasn't aware of Steve's question above. Dismas|^(talk) 06:59, 20 October 2013 (UTC)

Going back to Nimur's answer above, in the HD video here at about one minute in, the slinky is dropped from a tall building and eventually - at about 1.30 - the top of the coil appears to overtake the base, which is still stationary. Richerman (talk) 10:02, 20 October 2013 (UTC)

The mathematical explanation can be found here . Ssscienccce (talk) 13:53, 21 October 2013 (UTC)

It really isn't counterintuitive. With a slinky you can see the upward force, i.e. the tension that is implied by the spring being stretched out. Since the area right above the bottom has the stretched shape, you know full well the bottom can't drop because it's held up by that. What's harder to accept viscerally is that the center of gravity really has to follow Newtonian physics. I mean, why does the very top of the slinky have to start moving at the exact rate needed so that the mass it has will move at the speed it does so as to make up for all the mass in the center and the bottom? I suppose I see that its speed will be faster from the same force the smaller its mass, and the force will be greater the larger the mass it supports, so it has to work out, unless it's hindered by air resistance or the speed of light or something in which case I suppose the Newtonian model doesn't apply any more, but it's still an unexpected place to see math poking its nose into. :) Wnt (talk) 18:49, 21 October 2013 (UTC)

Power use in simple remote control

Help! Recently my girlfriend went to a battery shop and got them to change the battery in her remote garage door opener. The man who did it told her that the battery would last much longer if she only used the remote when she was really close to the door, rather than out on the street. It sounded like the usual rubbish "technicians" come up with to me, but then a recent question on the Ref. desk about radio communication got me thinking that maybe I was too quick to judge! I assumed that simple remotes like this just broadcast a coded signal at a certain strength. What is the real answer? 122.108.189.192 (talk) 07:19, 20 October 2013 (UTC)

I think your first instinct is right, and that's BS. One exception is if she starts trying to use it when out of range and ends up pressing the button more than once. StuRat (talk) 13:14, 20 October 2013 (UTC)

StuRat is probably correct, but there are hundreds of types, makes, and models of door openers; the electronics inside them are usually subcontracted recursively to various small electronics companies; so it's nearly impossible for us - or the retail technician, for that matter - to answer with great certainty. Even if we had a lot of time, money, and skilled scientists and engineers, reverse-engineering garage door opener radios is not as easy as it sounds!

Most garage door remotes are simple unidirectional broadcast transmitters, so they expend the same transmitter power whether the signal works or does not work. However, there are some garage door openers with more advanced radios - probably not as sophisticated as the mobile telephone radio protocols we discussed last week. But it's plausible that your remote might "retry" broadcasting the command until it receives an acknowledge response from the base station. That would mean that it would waste a lot of power retrying (until it succeeds, or times out), if the reception is poor or the distance to the base station is too far.

Why does this matter? Because power savings of a few microwatts might extend the life of the battery by several years!

And, there are other reasons to care about garage door openers. Some years ago, I learned a lot more than I ever expected to know about garage door radio protocols, because (as you can read in this 2008 JIEDDO summary report), door opener remotes were a very popular and very sinister IED trigger in Iraq and Afghanistan. They're cheap, they're easy to buy, and the radios inside them are all totally different. Retailed door opener products just get slapped together by contract manufacturing designers with any radio circuit that happens to be available that week! That made detecting and jamming the bombs almost impossible for American electronics warfare soldiers. "JIEDDO has spent more than $2.3 billion to develop, procure, field, and sustain electronic jamming technology and techniques to thwart simple, cheap triggering devices such as two-way radios or garage door openers." As it turns out, when the lives are on the line, it is actually very hard to know everything there is to know about a garage door opener radio protocol. Nimur (talk) 05:48, 21 October 2013 (UTC)

There is nothing in the JIEDDO report that indicates some remote garage door systems send back a confirmation signal from the receiver. Doing so would double the cost and increase the current drawn form the remote battery, even if the first ping worked. And it is totally unnecessary: If your garage door doesn't open, simply hit the button again, a bit closer. As in recent years all such devices operate in the high UHF band, the range is consistent from one day to the next. The owner will soon learn how close he/she needs to be. 60.230.213.251 (talk) 06:45, 21 October 2013 (UTC)

You're right; the report I linked only establishes that a multitude of garage door radio exist; it does not describe them in detail; but that was an overview report written for politicians. Technical details would bore the politicians, so those sorts of boring details are kept in confidential reports that aren't available on the internet at large. Nimur (talk) 15:51, 21 October 2013 (UTC)

I have a Chamberlain garage door opener which uses rolling codes for security (so somebody can't just scan the code you use then come back later and replay it to open your garage door). Logically, they must send a confirmation signal, or how else would you keep the rolling codes in synch between the sending and receiving units ? Of course, if the confirmation code isn't received, then sending the code again and again automatically from the sending unit would be unwise, in case the button is pressed when you are nowhere near the garage. StuRat (talk) 16:26, 21 October 2013 (UTC)

Assuming you have a KeeLoq encoder, see Rolling code and this datasheet linked from it, particularly pages 16 and 17. The transmitter just increments its code by one every time you press the button - the receiver keeps track of the last code sent successfully. If the received code is within 16 of the last code, it's accepted. If it's more than 16 but less than 32k away from the last code, you need to resynch the transmitter and receiver by pressing the button twice, so that the receiver gets two sequential codes. If it's more than 32k away, the transmitter is locked out and you'll have to reset it. But no information is transmitted from the receiver to the transmitter. Tevildo (talk) 19:21, 21 October 2013 (UTC)

That's too bad. That method means it will accept something like 32 codes, making it much easier to hack, than if it worked the way I hoped. StuRat (talk) 20:59, 21 October 2013 (UTC)

Adaptive signal strength is useful in cel phones because of the length of the transmission, but implementing it in a car door opener seems unlikely, the two-way transmission would use additional power for the required receiver, besides there are legal limits on the signal strength of these devices so they're limited to short distances anyway. Ssscienccce (talk) 14:12, 21 October 2013 (UTC)

Can airplane runways be sloped?

I think you can save energy and time by landing while going up the slope, and take off while going down the slope. What is the problem with this?--朝鲜的轮子 (talk) 07:40, 20 October 2013 (UTC)

A Google search on "sloping runway" gives lots of hits, and some dramatic images. HiLo48 (talk) 08:09, 20 October 2013 (UTC)

Yes. Here are some tips on taking off and landing at sloped fields. Dismas|^(talk) 08:14, 20 October 2013 (UTC)

While it's possible, it's avoided where possible, and when not possible may result in quite some limitations. Civilian air traffic is highly safety-conscious (which I as a semi-frequent traveller very much endorse ;-). Part of that safety is achieved by regulation, training, and routine - to the degree that not only all air traffic controllers and pilots speak English, but that they are even expected to use a particular set of words and phrases defined by the ICAO. Having a non-standard runway layout decreases the value of that training and routine, and hence increases the risk of operating such an airport. Similarly, you have to define specific procedures for just that runway, and you would have to do a careful safety analysis that cannot draw on past experience from the large number of normal airports. Finally, most runways can be operated in two directions. Runway 21 is not the 21st runway of an airport, it is its single runway when operating in the direction that is 210 degrees off north. The very same strip of concrete (or asphalt, or dirt ;-) is known as runway 153 when when it's operated in the other direction. When there is significant wind, runways are always operated in a way that aircraft take off and land into the wind. Strong crosswind is a hazard that can even lead to an airport being closed - this is why many airports have runways in two different directions. --Stephan Schulz (talk) 08:35, 20 October 2013 (UTC)

Correction -- the other end of Runway 21 is Runway 3, not Runway 15. 24.23.196.85 (talk) 01:48, 21 October 2013 (UTC)

Right! Thanks for the correction! --Stephan Schulz (talk) 11:57, 21 October 2013 (UTC)

See also Courchevel Airport, one of the most dangerous airports in the world. Rather you than me.--Shantavira|^{feed me} 08:37, 20 October 2013 (UTC)

Thanks for the explanation on runway numbering. I've wondered about that when I've flown, but never enough to look it up once I got back home. The configuration of Chicago Midway International Airport presumably takes advantage of different wind directions, and obviously they can't use both parts of the "cross" at the same time - and it's clear that a slope would be of no use at all. In fact, I would think airplanes are designed with the assumption that they're taking off from a flat surface. I've seen smaller airports where some taxiing runways have a slight slope due to the natural topography, but that's not a problem. As regards Courcheval, I infer from the article that it's not too bad IF you're flying a small plane and IF it's a clear day. And it's not especially comforting that it's only the seventh most dangerous airport in the world. ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:59, 20 October 2013 (UTC)

You should have seen the sloping (and short, and rough, and high-elevation) runway at Matekane, Lesotho! 24.23.196.85 (talk) 01:48, 21 October 2013 (UTC)

Also note that over the length of a runway, building up even a modest slope would require moving massive amounts of Earth, especially if you don't want the ground to drop away right at the edges of the runway. So, the expense is yet another reason not to do this, in the case where the ground is naturally flat. StuRat (talk) 13:12, 20 October 2013 (UTC)

However, in a VERY few cases, a runway may be deliberately built on a slope, in order to allow heavily overloaded aircraft to take off downhill -- an example would be the airstrip at Scholkovo (east of Moscow, near the town of Zhukovsky) from which Chkalov took off on his flight across the Arctic Ocean. 24.23.196.85 (talk) 01:48, 21 October 2013 (UTC)

It would also mean that almost every single airport would need to double the number of runways if they were sloped. As an example if the airport has a 13/31 runway and the top of the slope is at the 13 end, then another parallel runway with the top of the slope at 31 is also going to be required or aircraft may be landing downhill and departing uphill. CambridgeBayWeather (talk) 06:51, 21 October 2013 (UTC)

The NAZI buzz bombs used sort of a sloped "runway". Since they didn't need to land, that eliminated the doubling of runways issue. I believe other unmanned aircraft use the same trick. Some are caught in nets, while others land on level ground. StuRat (talk) 16:51, 21 October 2013 (UTC)

A more recent example and also rather specialized is the Flight deck#Ski-jump ramp. Alansplodge (talk) 17:24, 21 October 2013 (UTC)

It's not obvious to me that you would save anything overall by taking off downhill anyway. You might reach take-off speed sooner, but at take-off you would be pointing in a less advantageous direction in terms of climbing to altitude, and it might take just as much fuel to correct that as was saved on the runway. 86.176.215.115 (talk) 19:49, 21 October 2013 (UTC)

Galvanised nails as plant water sensors

I'm a serial plant killer. I would like to make an arduino-based project to protect my herbs from drying out. How long would galvanised nails function as water sensors? Would they need to be recalibrated as they age? I have six pots, all on the same window sill. — Preceding unsigned comment added by 2.97.26.56 (talk) 13:03, 20 October 2013 (UTC)

I assume the problem is you forget to water them ? If so, you might want to get some of those inverted bulbs which slowly deliver water. I haven't used them myself, though, as I don't have that issue. Also, are you using any type of plant food/fertilizer ? StuRat (talk) 13:07, 20 October 2013 (UTC)

I've not used any fertiliser but these herbs didn't really last long enough to use it. I'm not sure how well those globes will fit but I'm going to buy some and find out. Thanks for the suggestion! I may still decide to make an electronic system, just for fun, but it can now go on the backburner along with everything else! --2.97.26.56 (talk) 13:23, 20 October 2013 (UTC)

You're welcome. Also, you might want to use some simple reminder method. For example, if you use a paper calendar, you could write a W in the corner of each day, then cross it out after you water the plants that day. Or, if you make coffee each morning, maybe you could put a "water plants" note inside the coffee can. StuRat (talk) 13:33, 20 October 2013 (UTC)

Responding, we should note that arduino is a sort of open source single board microprocessor; at least the original premise is presumably to measure how dry the plants are rather than to assume it. However, there may be a problem there, if I'm correct in assuming the galvanized nails would be intended as electrodes, because pure water is not a very good conductor, but ions make it conductive. So fertilizer, accumulated salinity, etc. would throw off the measurement. You'd need some sort of actual hygrometer. It seems like it could be as simple as a human hair ...... but I suddenly realize that I have absolutely no conception of what the "relative humidity of dirt" is, how to measure or even define it, even though it's something that we take as a matter of common sense every day! Wnt (talk) 15:39, 20 October 2013 (UTC)

• Our relevant articles are available water capacity, Hydrology#Soil_moisture, Soil#Soil_water, and Permanent_wilting_point. We could probably use a whole article on soil water content, if anyone is interested in putting a stub together out of the links above :) SemanticMantis (talk) 17:54, 20 October 2013 (UTC)

One could also use some sort of scales for this: you add water, system notes the maximum weight and plays the fog horn once the plant+pot is 50 grams (or whatever) lighter. 88.148.249.186 (talk) 17:27, 20 October 2013 (UTC)

And then you go to make a big meal and cut a lot of herbs, only to get a fog horn sounding right next to you. :-) Katie R (talk) 19:02, 21 October 2013 (UTC)

Err. Let us brake things down to their components parts. (1) Galvanised nails: Are these truly galvanised nails or nails that have been quickly and cheaply zinc plated and sold as being ' galvanised'? Because the thickness of zinc will affect their working life. (2) Although zinc is important for plant growth... too much is toxic. One does not leave this type of hydrometer in a pot all day and every day – unless one is a serial plant killer.--Aspro (talk) 22:15, 20 October 2013 (UTC)

There are very cheap and very simple Arduino soil moisture sensors available on Amazon - they're basically nothing but traces on the PCB. I suspect that over time they will corrode and lose effectiveness, much like a simple nail-based solution would. Katie R (talk) 14:57, 21 October 2013 (UTC)

Googling arduino soil moisture gives many discussions and several types of hardware. Seems like a common "almost anything above entry-level" model is to alter the metal to avoid corrosion. DMacks (talk) 15:12, 21 October 2013 (UTC)

Effects of high speeds in outer space

Since there are no g-force and air in the outer space, what are possible effects of, say, putting an arm (of course, in a space suite and in a isolated compartment) out of a window of a spacecraft travelling at cosmic speeds (say, 500,000 kmh and over)? Or just traveling externally fastened to a spacecraft, going with those speeds (excluding possible injuries from meteors)? --93.174.25.12 (talk) 20:12, 20 October 2013 (UTC)

Space is a near vacuum, not a true vacuum. So, you would be hit by some objects. Individual atoms or molecules presumably won't each put a hole in the suit, but will wear away at it, like dripping water wears a hole in a bathtub. I'm not sure how long this would take, though. Also, this would cause heating of the suit. And there are also micrometeoroids that would put a hole in the suit. StuRat (talk) 20:19, 20 October 2013 (UTC)

(ec)There are g-forces in outer space, of course. But apart from that: If you are in a spacecraft in free fall, there is no problem with sticking your arm out, or flying alongside, unless you come to really high speeds (when even the very tenuous gas and dust in interstellar space will start to become significant). --Stephan Schulz (talk) 20:24, 20 October 2013 (UTC)

At constant speed in an absolute vacuum and all other things being equal, none I can think of. This is interestingly related (Space travel using constant acceleration) not sure about the case here. Ok I suppose going out side the craft tethered would be like absailing at whatever g the ship is pulling. — Preceding unsigned comment added by 122.111.243.217 (talk) 04:32, 21 October 2013 (UTC)

@StuRat, "like dripping water wears a hole in a bathtub", what? where? when? Never in my life ... Caesar's Daddy (talk) 06:58, 21 October 2013 (UTC)

It would take a lot of drips, and might be more likely to happen to an old-fashioned ceramic or iron tub than something made of plastic as they are now. ←Baseball Bugs ^{What's up, Doc?} carrots→ 08:13, 21 October 2013 (UTC)

Not sure if plastic tubs are any better at resisting this. It might just be that they aren't old enough yet to show this effect. It takes decades. StuRat (talk) 16:18, 21 October 2013 (UTC)

Good point. Also, plastic doesn't rust. But a steady drip-drip-drip over many years ought to have some effect eventually. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:04, 21 October 2013 (UTC)

Yep, I'm old, and old-fashioned. I've seen that on old bathtubs. HiLo48 (talk) 09:39, 21 October 2013 (UTC)

Yes, so have I, though it's often combined with corrosion because of the constant damp. For pure erosion, the effect is more clearly seen in old stone sinks. For a more dramatic effect in nature, see Panhole (called swirlholes in the UK). Dbfirs 10:01, 21 October 2013 (UTC)

At that sort of speed, the atoms and ions will be like cosmic ray radiation and penetrate deep into that arm, causing radiation damage. Graeme Bartlett (talk) 10:49, 21 October 2013 (UTC)

500,000 km/h isn't that fast. Solar wind peaks at close to 3,000,000 km/h - prolonged exposure to peak solar wind outside the protection of Van Allen radiation belt will increase a person's lifetime cancer risk. And our solar system rotates around the galactic center at 700,000 km/h, while our galaxy is on a 500,000 km/h head on collision course with the Andromeda galaxy. Measured against the cosmic microwave background our galaxy zips along at 2,000,000 km/h[21]. It's all relative. 88.112.41.6 (talk) 15:17, 21 October 2013 (UTC)

Did the Nazis performed unethical psychological experiments too?

Think Monster Study, Stanford Prison Experiment without cutting it short, Learned Helplessness with humans, and so on. OsmanRF34 (talk) 21:01, 20 October 2013 (UTC)

I don't see anything listed in the Nazi human experimentation article. If anyone finds reliable info, please add it (with cites, obviously). DMacks (talk) 21:57, 20 October 2013 (UTC)

I don't believe so. Would they believe such psychology applied to them so what was the point for the war effort? They were mostly interested in how to treat various sort of trauma, transplants and diseases and viewed the prisoners as lab rats without even today's concerns. Dmcq (talk) 09:12, 21 October 2013 (UTC)

October 21

Metric

The Fisher information metric is understood to describe infinitesimal changes in the Kullback-Leibler divergence. However its quantum analog, the Bures Metric, is understood to describe infinitesimal changes in the Bures distance, which is defined with reference to the Fidelity, and not the Quantum Relative Entropy (the quantum analog of the KL divergence). What is the reason for this breakdown in analogy?

Has a metric describing infinitesimal changes in the QRE been studied? Does its behaviour differ from the Bures Metric? — Preceding unsigned comment added by 144.82.191.250 (talk) 10:51, 21 October 2013 (UTC)

Just linking some of the terms above for reference: Fisher information metric, Kullback-Leibler divergence, Bures metric, Bures distance, Quantum relative entropy. -- The Anome (talk) 11:21, 21 October 2013 (UTC)

Also Fidelity of quantum states. Red Act (talk) 17:18, 21 October 2013 (UTC)

Maybe the Mathematics reference desk is a better place for this ... Ssscienccce (talk) 14:27, 21 October 2013 (UTC)

This is unarguably within the remit of physics. — Preceding unsigned comment added by 86.134.240.24 (talk) 19:45, 21 October 2013 (UTC)

Regarding the absence of a very well known info on SEXUAL DESIRE on wikipedia

This question is related to sexual desire of males. In the article SEXUAL DESIRE i could not find a very well known fact. It is generally seen that in sexually active active males, the time gap between the ejaculations affects their sexual desire. For example, if a man masturbates or ejaculates regularly(say once a day or once every two days or twice a day) and suddenly stops it, his sexual desire(or what we call libido) increases..It is also observed that the orgasm after a period of abstinence are also intense and getting and erection in a short term abstinence period is quicker..Why does this happen..? And why does wikipedia not mention i under the factors affecting sexual desire ..? I know that males are made to release their sperms once in a while. But why could i not find a reason or mention of it on wikipedia. .. Please give a scientifically proven reason for your answer....many thanks — Preceding unsigned comment added by Ed beerman (talk • contribs) 11:46, 21 October 2013 (UTC)

If it's that well-known, you should have no trouble finding valid sources on it, and could improve the article yourself. ←Baseball Bugs ^{What's up, Doc?} carrots→ 12:42, 21 October 2013 (UTC)

You are generalizing with no links to any studies. Your statements amount to speculation and can be discarded.217.158.236.14 (talk) 13:24, 21 October 2013 (UTC)

It is noted (with cite) in Libido#Physical_factors. Libido is one of the concepts linked and discussed in Sexual desire, but I don't understand the distinction between those two concepts...seems like there is a lot of overlap between their articles. DMacks (talk) 15:07, 21 October 2013 (UTC)

• The most relevant article is probably refractory period (sex). Looie496 (talk) 16:05, 21 October 2013 (UTC)

yes,i have read about Refractory period(sexual)..But this article only deala with the time period after ejaculaion during which it is impossible for a person to get an erection..i am talking about a different thing(although we can relate it to refractory period)...i am asking about the reason that greater the gap between two ejaculations(at least in short run) , higher is the desire to ejaculate ...why is sit so..? — Preceding unsigned comment added by 117.207.94.35 (talk) 17:27, 21 October 2013 (UTC)

At a guess, I'd imagine that it has at least something to do with the changes in pressure in the ejaculatory system caused by the build-up and release of fluid, but that would be pure unfounded speculation on my part; there are so many mechanisms that could be related to it -- hormonal, nervous (voluntary and involuntary), muscular, vascular, psychological, learned behaviours, etc. etc. that it wouldn't be suprising if it was still an open question. Have you tried to take a look at the medical literature on the topic, e.g. via Medline? -- The Anome (talk) 00:42, 22 October 2013 (UTC)

Opportunities for Exploring Comets and Minor Planets

• List of missions to minor planets
• List of missions to comets

Where can I get information about proposed projects and/or theoretical opportunities of unmanned space explorations?

Is there a list of future "windows" for the exploration of solar system objects?

Launch windows are determined partly by the available propulsion power and the location of your launch facility. If you have an ion rocket and a very light-weight spacecraft and your rocket base is on the equator, you probably have larger windows for visiting every solar system body. Otherwise, you may have smaller or no windows at all.

I just want to see a list of the "ON SALE" (least expensive) days for visiting comets and minor planets. -- Toytoy (talk) 15:14, 21 October 2013 (UTC)

I think you will need to narrow you criteria; there are over 1 million asteroids and comets known. Rmhermen (talk) 15:43, 21 October 2013 (UTC)

Soda pop can exploding when frozen

Why do soda pop cans explode when put in the freezer too long? I've put 20 fl oz plastic bottles of cola in the freezer, but they don't explode. --209.203.125.162 (talk) 17:51, 21 October 2013 (UTC)

Aren't cans usually full to the top, with no room for the ice to expand, while bottles usually have air in the neck? μηδείς (talk) 17:56, 21 October 2013 (UTC)

Plastic bottles seem like they can stretch a bit from internal pressure, whereas metal can certainly deform but not as readily to increase volume? DMacks (talk) 19:00, 21 October 2013 (UTC)

Yes that's it. Soft plastics can undergo a great deal of plastic deformation (no surprise there), while metal isn't as flexible. StuRat (talk) 20:55, 21 October 2013 (UTC)

However, as I understand, the recent incidents of "dry ice bombs" in LAX were plastic bottles stuffed with dry ice and then capped tightly. ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:59, 21 October 2013 (UTC)

That's because it's easy to put dry ice in a plastic bottle and seal it. If you fill it completely with dry ice, then it will have to pop. For it not to pop would require it to be able to contain a gas at a pressure of about 1000 bar. Count Iblis (talk) 00:26, 22 October 2013 (UTC)

So if the cola inside is frozen, why does it explode all over the freezer? Why doesn't the can just burst open, leaving a frozen cola cylinder ? --209.203.125.162 (talk) 01:17, 22 October 2013 (UTC)

Not all of the cola is frozen. Enough freezes for it to break the seal, through which the remaining liquid escapes. 81.147.166.89 (talk) 01:31, 22 October 2013 (UTC)

The freezing process involves the unmixing of the water and the cola ingredients, you get pure ice from which the rest of the cola is expelled. The freezing temperature depends on the concentration of the ingredients, so, when a little ice forms the cola ingredients have been expelled from the ice, the concentration will have increases in the remainder. So the remainder will only freeze at lower temperatures. This means that at some given temperature, the freezing process will stop before everything is frozen. But of course, if the can cannot hold a large pressure it will break open. Most of the contents will be a liquid which then explodes out of the can as it was held under pressure. Count Iblis (talk) 01:35, 22 October 2013 (UTC)

Meat from unstressed animals

The claim that meat from unstressed animals tastes better is regularly made, but is there any scientific evidence that it is actually true? All the sources that I can find merely assert it without presenting any evidence, or use words such as "people say that...". 86.176.215.115 (talk) 19:42, 21 October 2013 (UTC)

Science has certainly backed this up. Lack of references may come from generations of our forefathers experience that take this as a given – so our forefathers didn’t require any authority to tell them what they already knew - so that's the origin of "people say that...".

• This 'patent' give some of the science bit as background (with refs) to explain the patent:[22]
• This link explains why it is important in to the meat industry:[23]

--Aspro (talk) 21:13, 21 October 2013 (UTC)

The type of food an animal eats supposedly can make a significant difference in taste (I've been told that deer that feed on corn taste a lot better than deer that have to resort to leaves and twigs), so it's not unreasonable to expect that the animal's general well-being could also affect the taste. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:01, 21 October 2013 (UTC)

Cline variation

What is meant when it is said that human variation is clinal not racial? — Preceding unsigned comment added by 70.49.46.102 (talk) 19:54, 21 October 2013 (UTC)

It basically means that the prevalence of specific human genes and phenotypes generally varies gradually instead of suddenly across geographic regions, making it difficult and arbitrary to try to meaningfully divide humans into discrete races biologically, if you consider humanity as a whole. See Cline (biology). Red Act (talk) 20:10, 21 October 2013 (UTC)

So essentially it is hard to draw the line where for example black race or white race or asian race starts? But it is easy to do that, we all know what an Asian, or African or white person looks like? — Preceding unsigned comment added by 70.49.46.102 (talk) 20:15, 21 October 2013 (UTC)

No we don't. There is so much regional variation those terms are almost meaningless sometimes. That of course doesn't mean there aren't' genetic differences between different populations, but they don't necessarily map to the traditional 'human races'. Fgf10 (talk) 20:49, 21 October 2013 (UTC)

A couple points:

1) It isn't always easy, with people of mixed ancestry.

2) The genes that describe things we think of as race (skin color, hair color and texture, etc.) are only a small number of the total genes. So, you could have Asian genes that control your appearance, and yet have non-Asian genes as well, which don't contribute to your appearance. StuRat (talk) 20:52, 21 October 2013 (UTC)

It can sometimes work reasonably well as a model to divide people into races, if you only consider the people who live in one geographic area, due to historical immigration patterns from separated locations which have obscured the clinal nature of the genetic and phenotypic variation among humans as a whole. For example, a lot of people arrived in North America from Europe, mainly Northern Europe and Western Europe, during the European colonization of the Americas, and a lot of people arrived in North America from Sub-Saharan Africa during the Atlantic slave trade. People in northwestern Europe and people in Sub-Saharan Africa tend to look rather different, so if you're only considering people who now live in North America, it works moderately well to categorize at least some of the population in North America into "white" vs. "black" races. However, if you take into consideration everybody in the world as a whole, what people typically look like varies gradually if you consider a sequence of populations extending from Sub-Saharan Africa up through northwestern Europe. For example, people from Egypt (in Northern Africa) don't typically look like they came from England or France, but they don't look like they came from Senegal, either; instead, they look very roughly somewhere in between (although the variation is more than one-dimensional). But if you're considering only people who are currently in North America, you aren't taking into account too many people with largely Egyptian ancestry, because there wasn't a large wave of immigration to North America from Egypt in North America's history. Similarly, the concept of "Asian" as a race also works somewhat in North America, because historically a lot of people arrived in North America from East Asia and Southeast Asia, but not as many people arrived in North America from Central Asia or Eastern Europe, so the clinal nature of genetic and phenotypic variations along the populations in between East/Southeast Asia and northwestern Europe is obscured. Red Act (talk) 22:18, 21 October 2013 (UTC)

This is even further obscured by our limited understanding of history. If a large wave of immigration to North America from South America (or vice versa) occurred before recorded history, we would have no knowledge of the event. --Auric talk 22:38, 21 October 2013 (UTC)

Thank you very much, that clears the confusion for me! 70.49.46.102 (talk) 01:11, 22 October 2013 (UTC)

Why are screw propeler more efficient than paddle wheels?

Couldn't well-designed paddle wheels (not just a flat board) be equally efficient? OsmanRF34 (talk) 22:00, 21 October 2013 (UTC)

Intuitively paddle-wheels seem less efficient because they waste energy lifting water and submerging air. Also, only a portion of the water they do move goes in the proper direction. StuRat (talk) 22:28, 21 October 2013 (UTC)

Is a screw propeller actually more efficient? They are superior for many purposes because paddle wheels don't work well if the water is not flat, but I would like to see a reference before accepting that there is an advantage in efficiency. Looie496 (talk) 00:09, 22 October 2013 (UTC)

The advantage of screw propellors is that the direction of flow imparted to the water is parallel to the screw rotation axis. That allows propellors to be made with blades of shallow pitch at the tips, gradually increasing to a more agressive pitch as you move toward the centre. In this way the velocity imparted to the water is the same throughout most of the blade length, even though the tangental velocity of the blade is a lot less near the centre than it is at the tips. Also, the blade tips can be rounded, so that in the last 5 to 10% or so of the blade, the water velocity is gradually brought down, avoiding a large shear, which would waste energy in turbulence.

Paddle wheels have the direction of water flow at right angles to the direction of rotation. That means neither variable pitch nor blade tip tapering can be used. As StuRat said, paddle wheels move some water in the wrong direction, they also move some of it at the wrong velocity, and the large shear at the bottom of the paddles causes much useless turbulence.

The problems of paddle wheels could be overcome to a limitted extent by using co-axial nested paddlewheels of huge diameter, with the inner wheels turning faster, and the very outside wheel turning very slowly. Or by using some sort of linkage to keep the paddle blades vertical. But its never going to be as good as a propellor, with anything like a practical robust assembly. Why would you bother, when a simple propellor does the job. 60.230.213.251 (talk) 00:43, 22 October 2013 (UTC)

There are various tracked amphibious vehicles which propel themselves using their tracks ("some sort of linkage to keep the paddle blades vertical"). The DARPA Captive Air Amphibious Transporter looks fun.  Card Zero  (talk) 01:04, 22 October 2013 (UTC)

besides every movement in a propel contribute to the ship displacement, in the wheel you have to waste energy raising up the paddles over the water surface, plus the wheel weight more and require more space, etc.

Planes have to be very very efficient, so they has “propel like” devices (helices turbofans etc)

Iskánder Vigoa Pérez (talk) 02:13, 22 October 2013 (UTC)