Wikipedia:Reference desk/Archives/Science/2013 April 26

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April 26

Experimental results following theory rather than vice versa

I don't have much for you to go on, but I'm hoping my description of this will jar someone's memory.

I read a chapter in a book that was written in English at least a decade ago by a theoretical physicist, if I remember correctly Japanese. It was a cautionary tale about how our expectations can color our science. A parameter in particle physics (I forget which) had a possible value from 0 to 1. At first theorists expected zero, and experimentalists got results compatible with that. However, theoretical expectations of the value gradually increased until they reached one, and the experimental results were always compatible with theory. Each result agreed within the margin of error with that of the previous team of researchers and with theoretical expectations, but the results at the end were incompatible with where they started. This pathological relationship was the opposite of what one would hope for in science, and the chapter was an instructive illustration that the hard sciences are still human and not necessarily objective accounts of the world.

Does anyone recognize where this came from? I only ever had a copy of the one chapter. — kwami (talk) 01:32, 26 April 2013 (UTC)

This sounds very similar to Richard Feynman's account of Millikan's experiment as an example of psychological effects in scientific methodology. (However, not Japanese, not a decade ago, and not between 0 and 1 :) ) --Dr Dima (talk) 01:58, 26 April 2013 (UTC)

Yes, that's another good example. In this case I think there was a graph plotting both theoretical expectations and experimental results over time; you could see that the error bars always overlapped with each other and with expectations. I think that would make it somewhat more accessible than Feynman, even though the parameter was more obscure, though I don't remember how technical the text was. And it may have been two decades ago (I don't recall the publication date), though I think it was more recent than 1974. — kwami (talk) 02:06, 26 April 2013 (UTC)

The only parameter I can think of that was thought to be 0 and turned out to be 1 was the magnitude of the parity violation in the weak interaction. But the experimental history doesn't match what you described.

"A book in English by a Japanese theoretical physicist" should narrow it down quite a bit already. Was it any of these people? -- BenRG 16:32, 26 April 2013 (UTC)

This reminds me of when I was an undergraduate at Uni. We were required to do a great number of experiments in the Lab - always with the same process: Before coming to the Lab, master the theory and pre-calculate the expected results. On arrival at the Lab, set up the equipment and run the experiment. More often than not, the results did not agree with the pre-calculation. Must be something wrong, so we would all then check apparatus looking for mistakes in assembling and connecting up, maybe bang and wriggle a few connectors in case of bad electrical instrument connections. Of course doing so could just as well make a good connection bad. Run experiment again. Repeat as necessary until results agreed with theory. Overall, a good way to never discover an valid anomally in the theory. Better than having no idea what to expect though. Wickwack 124.182.174.187 (talk) 02:55, 26 April 2013 (UTC)

Except that you aren't in undergraduate lab to test a theory. The point is a) to learn techniques for working with equipment and methodology and b) to physically demonstrate a well-established concept. Those are both valid educational goals. If you don't have those two concepts down, you aren't prepared to make novel discoveries. --Jayron32 04:00, 26 April 2013 (UTC)

True. I wonder, however, if that set up folk for life to just retain that habit. My education stopped after graduating at batchelor level, so I don't know if they teach higher degrees to be more carefull in lab work - but I suspect not. Wickwack 124.182.174.187 (talk) 10:41, 26 April 2013 (UTC)

I heard a story (2nd hand) about a 1st year bio lab where the students were counting the yellow and black corn kernels on cobs of corn to verify Mendelian genetics (not being a biologist myself I could have this wrong). The tutors knew from a cheat sheet what the correct ratio of black kernels to yellow kernels should be, and the students inevitably got the ratio wrong every time for years and years before someone finally noticed that whoever wrote the sheet didn't understand the principles of inheritance themselves and had made a logical error that resulted in the wrong expected ratio. Turns out most of the kids were right after all, had been marked down, and sent away convinced that they had to relearn something they had actually already mastered. 202.155.85.18 (talk) 04:28, 26 April 2013 (UTC)

Instructors being idiots notwithstanding, there is still valid educational value in demonstrating known principles. You can't extrapolate that these type of labs aren't valid from a stupid instructor. --Jayron32 04:34, 26 April 2013 (UTC)

It doesn't sound like the uni I went to anyway. As students we did occaisonally (not very often)get answers different to what teaching staff expected. In any class you get three sorts of student: 1) those who really don't know the subject but may be able to regurgitate; 2) those who when marked down just accept it; and 3) those who have mastered the topic, are confident, and will challenge the lecturer/professor, and show where he/she got it wrong. What impressed me at the time was that teaching staff were quite happy to be challenged, in fact they welcomed it, and would acknowlege they had it wrong. Quite different to school teachers. Wickwack 124.182.174.187 (talk) 10:52, 26 April 2013 (UTC)

Not all school teachers are afraid of challenges or refuse to admit when they are wrong, and some university lecturers get very huffy when challenged! Dbfirs 11:34, 26 April 2013 (UTC)

Anyone who goes into teaching without expecting to learn just as much as they teach, if not more, has no idea of what teaching is about. But if that's the model they got from their "teachers" or "parents", it's probably no wonder. -- Jack of Oz ^[Talk] 12:36, 26 April 2013 (UTC)

I had a friend in college who was training to become a math teacher. He painstakingly 'loaded' about 30 dice by drilling out the middle spot of the '5' and inserting a piece of lead shot into the hole. He gave these to the kids in his class without telling them that they were rigged. So when they did the tedious statistics experiment of graphing the number of 1's, 2's, 3's, 4's, 5's and 6's over dozens of rolls, the answers would show a disproportionately large number of 2's and far too few 5's. He wanted the kids to get results that differed from theory. They were being challenged to actually discover something non-obvious by doing their experiment - and to actively dispute the theoretical results that he'd previously presented in class. As I recall, he said that quite a few of the smarter kids deduced that the dice was loaded - and even figured out how he'd done it - but too many others either fudged the results or were un-curious about the anomalous results. SteveBaker (talk) 14:07, 26 April 2013 (UTC)

This. ^^^^ --Jayron32 14:58, 26 April 2013 (UTC)

So, no idea who the author was? — kwami (talk) 19:10, 26 April 2013 (UTC)

Quoting from Hyperphysics

I suppose many here are familiar with this good academic physics site. I'm trying to find out if the use of the stuff contained therein, including figures, are free to cite and use within the framework of writing a paper. My previous appeals to addresses found there weren't answered as yet. Thank you, BentzyCo (talk) 04:24, 26 April 2013 (UTC)

I suppose that it depends what sort of paper you are writing. Citing small extracts is standard practice as long as the source is acknowledged, but, quoting from the site: "HyperPhysics (©C.R. Nave, 2012) is a continually developing base of instructional material in physics. It is not freeware or shareware. It must not be copied or mirrored without authorization. The author is open to proposals for its use for non-profit instructional purposes. The overall intent has been to develop a wide ranging exploration environment which could be of use to students and teachers." I suggest that you wait for a reply from the author if you want to use a significant proportion of the material. Dbfirs 08:34, 26 April 2013 (UTC)

a. The intention is a scientific-educational paper, to be publicized in a couple of periodicals, e.g., 'The Physics Teacher'.

b. I've already addressed Dr. Nave, more than a month ago, but nada - no reply. What do you suggest ? BentzyCo (talk) 09:04, 26 April 2013 (UTC)

No one can stop you from citing them. But regarding reuse, there is a such a thing as fair use which allows you to republish content without permission, but whether that applies in your case is a legal question we can't provide an answer to. As far as Dr. Nave failing to return your correspondence, well, that's just tough. Copyright holders have no obligation to do anything in response to reuse requests. Someguy1221 (talk) 09:12, 26 April 2013 (UTC)

It's the fact that your paper is going to be published in periodicals that constitutes the problem. If Dr Nave sees exact copies of his diagrams published elsewhere, then he might well have cause to sue for breach of copyright (whether he would do so is another question, and we don't give legal advice). If your publication is not for profit, and you are unable to get permission to reproduce the content, then either make your own diagrams and re-word the text, or acknowledge the source in your paper, then, at least, you cannot be accused of plagiarism. If your publications are for educational purposes then you have a better chance of getting permission to reproduce content (per the cited statement above). If your publication is a purely commercial venture, then don't breach copyright. It could be expensive! Dbfirs 11:23, 26 April 2013 (UTC)

Basically, you do need permission to copy large chunks of this stuff - and if Dr. Nave chooses not to give you that permission - then you can't legally do it. If he doesn't reply - then he didn't give that permission. You might be able to copy some of this material under the "fair use" provisions of copyright law - but we aren't allowed give you the legal advice to tell you the limits on that. It's particularly problematic because "fair use" isn't a "bright line" rule - it's a matter of balancing pro's and con's - and ultimately, unless you have explicit permission, only a court can tell you for absolute certainty whether you copied fairly or illegally. SteveBaker (talk) 13:48, 26 April 2013 (UTC)

physics

what Is difference between electronic filter and electronic circuit ? — Preceding unsigned comment added by Titunsam (talk • contribs) 06:39, 26 April 2013 (UTC)

An electronic filter is one type of electronic circuit. A calculator is an example of an electronic circuit which is not an electronic filter. StuRat (talk) 07:24, 26 April 2013 (UTC)

Completing the previous answer - while circuit refers to the structure, filter refers to its physical function. BentzyCo (talk) 09:07, 26 April 2013 (UTC)

A filter has the specific function of changing one kind of electrical signal into another - typically by reducing the amount of power in some range of frequencies. A good example of that would be the bass and treble controls on a music replay system...when you turn down the treble control, some of the high frequencies in the music are reduced in volume...when you turn down the bass control, some of the low frequencies are reduced.

You can think of it like a coffee filter - which removes the large particles of coffee grounds while allowing the microscopic particles to flow through it...a "bass" audio filter removes the "large" wavelengths while letting the "small" wavelengths flow through it.

An electronic filter can be a very simple circuit - or it may be something complicated and sophisticated - but it is (by definition) an electronic circuit.

Electronic circuits can be almost anything with wires and electronic components like resistors, capacitors, coils, diodes and transistors. The computer you're using to read this text is a gigantic, almost unimaginably complex, electronic circuit - and it almost certainly contains several electronic filters within it. But it's easy to imagine simple electronic circuits that don't contain filters.

SteveBaker (talk) 13:34, 26 April 2013 (UTC)

Gym trainers

What's the difference between a personal trainer and fitness consultant in gyms? Most gyms have both. Clover345 (talk) 11:19, 26 April 2013 (UTC)

A personal trainer is for ill disciplined people who make a mess of their lives and need help to sort them out. Count Iblis (talk) 12:15, 26 April 2013 (UTC)

That statement would require a reference, wouldn't it now. -- Jack of Oz ^[Talk] 12:32, 26 April 2013 (UTC)

In my experience, these internal job descriptions can mean almost anything. What is the difference between a secretary and an office manager? Both are the same, and have different functions at different companies. OsmanRF34 (talk) 17:55, 26 April 2013 (UTC)

frogs eggs in lawn

I live in the fenland area of East Anglia UK.I was mowing my lawn a couple of days ago when a frog jumped out of a clump in my grass.It hopped to the edge of the lawn and hid behind a compost bin.I left a shallow dish of water as there is no pond in my garden.I found it basking in the dish.Each day I have been going out to see if it is still there and I have discovered it has been laying eggs in bald patches of my lawn.I now need to recut my lawn but I do not want to destroy the eggs or harm the frog .How can these eggs surive if not in water and how should I treat them?90.201.129.169 (talk) 11:32, 26 April 2013 (UTC)

What makes you think those are frog eggs? Most types of frog, including the common frog of Europe, lay their eggs in water, where they hatch into tadpoles. Is there something special about this frog? Looie496 (talk) 15:23, 26 April 2013 (UTC)

Place flags or stakes where you find the patches and mow around them. If this is a species of direct-developing frog that lays eggs on land they won't take more than a week or two to develop. You can water them with a watering can if you are afraid they will dry out. If you try picking them up you are likely to disrupt them. It's curious what species this might be, I thought direct-developing frogs were all tropical. μηδείς (talk) 15:28, 26 April 2013 (UTC)

Eternal and infinite time-continuum (Universe/Multiverse/Meta Universe)

In this section I would like not to have a discussion about God or extra-terrestrial life, if possible.

The question is: Are there any reliable sources that propose a hypothesis that our time-continuum (Universe/Multiverse/Meta Universe) is eternal in time and infinite in volume/space?

For the purpose of our question, Universe I have in mind is not something that developed from a Big Bang, and is a current form of our Universe, but that going beyond Big Bang, to what was before, to the earliest possible event or a starting point, or back in eternity. By a Multiverse, I don't mean one of the specific definitions such as a collective name for Universes that are far away from each other and might not be connected. But all 4 names I mean an encompassing definition of everything there is, and it's not of much importance what name the reliable source uses for it. Ryanspir (talk) 11:37, 26 April 2013 (UTC)

Conformal cyclic cosmology for example. Or Eternal inflation. In fact lots of theories but distinguishing between very big and infinite is never going to be fully possible if it is infinite, it'll just be which theories or theories seem to fit the currently known facts the best. Dmcq (talk) 11:44, 26 April 2013 (UTC)

There are plenty of theories - either way - but we don't know for sure, and worse still, there are reasons to believe that we may never know.

• Time: The Big Bang is "known science" - by far the most widely accepted explanation for the beginning of the universe and confirmed by experimental evidence such as careful measurement of the cosmic background radiation. Because the big bang started with a "singularity" (an infinitely small dot), no information from "before" the singularity can ever be passed through to the time period after it. We cannot experimentally measure anything about the "before"...and indeed, a strong possibility is that there was no "before". More importantly, this origination of absolutely everything through this singularity means that even if there was a "before", it cannot possibly have any influence on the "after"...so in a physics sense, it doesn't matter whether there was a "before" or not.

• Space: Again, we don't know whether the universe is infinite or not. The problem is that the speed of light limits the rate at which information can travel across the universe and the rate of expansion of space at larger distances exceeds that. This means that we are at the center of a sphere around 92 billion light years in diameter. Everything outside of that spherical volume of space is forever unknown to us. This means that we cannot directly measure whether the universe is infinite or not - we can only know that it's larger than 92 billion light years.

So from a practical standpoint, the amount of time in the past is finite and the useful, measurable, knowable, "observable" universe is most definitely finite. But from a theoretical standpoint, time in the past may or may not be infinite and the size of the universe may or may not be infinite - and in neither case can we observe anything to prove that.

That leaves one other matter - time extending into the future. Time within the universe may well be finite in the past - but is it finite into the future too? Well, as far as I know, the current lead theories all suggest that time will be infinite into the future - but again, we don't know for sure. However, we can (in principle) figure that out and know for sure at some time in the future.

It's a frustrating answer - we are irrevocably constrained to the finite - and fundamentally prevented from seeing whether there is a greater infinite. The best we can hope is to use evidence from what we can see to extrapolate out to what we cannot see. We'll have theories about what's out there - but we won't know for sure.

SteveBaker (talk) 13:22, 26 April 2013 (UTC)

Steve: "...so in a physics sense, it doesn't matter whether there was a "before" or not" may well be the single most useful, clarifying (to me), whatever-you-calls-it sentence you've ever written here. Thanks!

--DaHorsesMouth (talk) 16:51, 27 April 2013 (UTC)

What you are saying is just a theory, so perhaps it should be treated as such? It hasn't been proved. I propose "my own" theory, and I think there should be sources that express it. It doesn't mean that my theory is right or wrong, what I'm looking is for the sources :).Ryanspir (talk) 17:11, 28 April 2013 (UTC)

Steve, a comment on: ... This means that we cannot directly measure whether the universe is infinite or not - we can only know that it's larger than 92 billion light years, if I may. The issue is more fundamental than that. If it's finite, then we can, at least in theoretical principle (were it not for the practical limitation you describe), measure it. But if it's infinite, then no amount of hypothetical measuring will ever get us to the end, because, well, it's infinite. Except, we'd never know that. All we'd know is that we just keep on measuring and measuring and measuring, and never knowing whether the end is just around the celestial corner or there is literally no end at all. In short, we don't need to come up with any explanations for why we could never measure an infinitely large universe, because the reason is inherent in the meaning of the word "infinite", which would be a reflection of its actual infinitude. -- Jack of Oz ^[Talk] 08:59, 29 April 2013 (UTC)

Yes...but if we were to somehow hand-wave away the issues of the speed of light limiting how far we can see - then perhaps there would be more indirect ways to determine the answer to this question. However, in hand-waving away those limits, you have to imagine a universe without relativity...and when you do that, nothing makes much sense anyway. But you're right - it's possible that if, somehow, we didn't have "observable universe" limitations, you might still not be able to determine whether it's infinite or not.

The idea of a truly infinite universe is a rather disturbing one - if you consider something the size of planet Earth, there are only a finite number of ways of arranging that many protons, neutrons and electrons...hence there must be places in the universe that precisely mimic our earth - were a duplicate-Steve is typing this exact message...and other places where the only difference between that Earth and ours is that in that Earth, I say "screw it, I'm not writing any more Wikipedia answers without a cup of coffee". In fact, there are an infinite number of places where that's happening! Now I need a coffee! SteveBaker (talk) 14:54, 29 April 2013 (UTC)

That's the basis of quantum universe theory. Why would that be disturbing? In fact I find it interesting, it means that all the girls with whom I wasn't successful because I said or did something that lead to the lack of success - in other universes I was successful. :) However for our purpose it's not important. What we want is to assume that the universe is infinite, as its a hypothesis we don't need a prove.

As I understand Big Bang is still though popular yet still a theory. And though some evidence suggests it may be right, and even if it's right, the questions how Big Bang came to existence aren't yet answered.

What's more, if Singularity came from a big Crunch its because of gravitational pull, right? So while the gravitational pull has compressed all the matter and energy into Singularity, I cannot see how it could pull the space itself? Ryanspir (talk) 16:25, 29 April 2013 (UTC)

Well, what's disturbing about that is, among other things ... there are an infinite number of possibilities where you'll be sitting there typing away thinking everything is a-ok, but ... actually there's a piece of you missing ... or added ... and you're about to notice which piece.

However, to be fair, all these substitutions in one universe are still subject to ordinary laws of physics, which makes such events exceedingly unlikely. Likelihood matters; some infinite possibilities are a lot more common than others. And the existing universe is, by all odds, an extremely likely one repeated almost as-is many times over, including mishaps with females alas. Nonetheless, if every possibility can happen, it nonetheless means that every possible configuration of atoms exists, and so I'd think there should be alternate dimensions of time in which you string them together in any given order, using one of any number of different sets of laws of physics... which challenges whether reality as we perceive it is even a real thing, or just a delusion brought on by the belief in some particular set at the moment... (this particularly applies if you consider consciousness to be a thing of finite extent, capable of perceiving the difference between only a limited number of physical scenarios)

I dunno. It seems like a singularity as prone to vapid thoughts as any exercise in multiplying by infinity and dividing by zero. And in the absence of any way to directly test these other places to see if they are real, well... Wnt (talk) 19:09, 29 April 2013 (UTC)

Floss first or brush first?

Has the debate over flossing or brushing first stabilized yet? Is it recommended that people floss and then brush or brush and then floss? My understanding is that there is no agreement in the literature about this. What is the current state of the research? Thanks. Viriditas (talk) 12:25, 26 April 2013 (UTC)

The advantage of flossing first is obvious, the brush bristles can then get between the teeth in places previously occupied by bits of food. But what's the advantage of brushing first ? StuRat (talk) 18:00, 26 April 2013 (UTC)

If I've eaten something that causes to a lot of stuff to stick inbetween my teeth (e.g. fish), then I prefer to first brush my teeth to get rid of quite a large fraction of the stuff, then floss and then brush again. If I try to floss first in such a case, then it takes a lot of time to get everything removed. Count Iblis (talk) 18:18, 26 April 2013 (UTC)

StuRat, I think most medical associations outside the U.S. agree with you. For some strange reason, U.S. dental associations won't take sides. For example, the Canadian Dental Association comes right out and says, "Brush your teeth after you floss - it is a more effective method of preventing tooth decay and gum disease." I would be very interested if you could find a similar statement from a respected U.S. authority. The U.S. medical associations take the position that "it doesn't matter", but that seems to be a strange position to take. Viriditas (talk) 05:48, 27 April 2013 (UTC)

...and "it doesn't matter" is a phrase rarely seen in Wikipedia, and certainly less on the Ref Desk. Richard Avery (talk) 14:40, 27 April 2013 (UTC)

further, you'll note that that phrase occurs in two consecutive posts now. Woohoo...

--DaHorsesMouth (talk) 16:53, 27 April 2013 (UTC)

Postnatal clinic

explaination on postnatal clinic? — Preceding unsigned comment added by 49.138.37.29 (talk) 14:14, 26 April 2013 (UTC)

Start reading the Wikipedia articles titled postnatal and clinic. If you have any more specific questions after reading those articles, we can try to help. --Jayron32 14:56, 26 April 2013 (UTC)

Tilt

Why does the northern hemisphere tilt towards the Sun during the summer? Pass a Method talk 15:37, 26 April 2013 (UTC)

Well, according to common usage, summer is opposite for the northern and southern hemispheres. For each hemisphere, summer is defined as the season during which the Sun is highest in the sky, which means the season during which that hemisphere points toward the Sun. Looie496 (talk) 16:00, 26 April 2013 (UTC)

The axial tilt of the Earth means that days are longer and the sun is higher in the sky for longer periods of time during one half of the year than the other half. Whichever hemisphere you are in, the part of the year with longer days etc. is summer in that hemisphere. See Seasons#Axial tilt for more details. Gandalf61 (talk) 16:00, 26 April 2013 (UTC)

The reason for the axial tilt is more a question of why there is so little tilt! When the earth was formed, it could have been spinning at almost any angle. Venus has a tilt of 177 degrees, Mars 25 degrees, Jupiter just 3 degrees, Pluto 122 degrees and Uranus has 97 degrees. It's a seemingly random thing, so the fact that the earth's tilt is 23 degrees could just be a flook. The axis of rotation would also change over time but it's stabilized by the moon orbiting around us. SteveBaker (talk) 16:12, 26 April 2013 (UTC)

A rare occasion in which Steve is wrong. The solar system emerges from a rotating and contracting cloud of interstellar gas. The Sun AND the planets inherit the rotation of that cloud. so initially, there is no tilt, that is, it is 90 degrees. Collisions with asteroids knock the planets sideways, and so we end up with tilts, and thus, seasons. But I wonder if all the stars of a galaxy have the same orbital inclination for similar reasons. Myles325a (talk) 09:21, 29 April 2013 (UTC)

Sounds like semantics to me - if a planet is being collided with by asteroids/planetesimals hard enough to change its axis of rotation, I'd call that 'planet formation' Wnt (talk) 18:58, 29 April 2013 (UTC)

I should think that when the planets formed, they would have no tilt at all, from the ecliptic, and that tilt was later added due to collisions. However, there may not have been any gap between "formation" and "collisions" (the two might well have overlapped). StuRat (talk) 18:02, 26 April 2013 (UTC)

In my graduate physics study, we actually had a homework problem modeling the probability distribution of planetary axes assuming they formed via the accretion of asteroid sized chunks. One generally assumes the disk of debris from which planets form is relatively flat and on average the angular momentum will be coaligned with the axis of the orbits, which would tend to predict that the planets have axes also aligned with the orbit. However, each accretionary impact event between a protoplanet and an asteroid is a little bit random and can kick the axis of the planet by a little bit. Even though the average impulse is small and effectively neutral, this series of small kicks still results in a random walk that can cause the planetary axis to have large deviations by the time planet formation is essentially finished. For a more interesting question, one might wonder why the sun's axis is tilted 7 degrees relative to the orbital plane. It is actually not entirely clear what formation processes would have the ability to move the sun (or alternatively the planets) by that much. Dragons flight (talk) 18:47, 26 April 2013 (UTC)

Further complicating matters, isn't it the case that the earth has an "overspin", like a top, which rotates once every 23,000 years or so? I have a vague recollection of that from 9th grade science or something. ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:08, 26 April 2013 (UTC)

Every 26,000 years to be precise, but close for a vague recollection. See Axial precession (astronomy) --NorwegianBlue ^talk 23:24, 26 April 2013 (UTC)

Very good. So, if I'm picturing it correctly, 13,000 years from now the seasons will be at opposite times of the year from what they are now? ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:47, 26 April 2013 (UTC)

No, the axis doesn't reverse, it just rotates (the small circle). Dbfirs 07:11, 27 April 2013 (UTC)

In 13000 years, midsummer will occur 180° around the orbit from where it does now. Bazza (talk) 10:30, 27 April 2013 (UTC)

Oh, I see what you mean. That's not the seasons being at opposite times of the year, but the perihelion occurring in July instead of January in 10,500 years. This will make summers in the northern hemisphere slightly warmer, and winters colder. See Milankovitch cycles for various other effects, including this 21,000-year cycle of the aphelion and perihelion against the seasons (actually a combination of two cycles). Dbfirs 12:01, 27 April 2013 (UTC)

I'm not talking about perihelion. I'm talking about the tilt of the earth relative to the sun. As Bazza said. In 13,000 years the earth should be tilted the "opposite" of how it's tilted now, hence the northern hemisphere would have winter weather in July, and summer weather in January. Or maybe I'm not picturing it correctly. ←Baseball Bugs ^{What's up, Doc?} carrots→ 05:01, 28 April 2013 (UTC)

No, you are not picturing it correctly from the point of view of us Earth-dwellers. The year and the calendar are both defined by the tilt, so we will not see this difference, though I agree that astronomers from elsewhere in the solar system might take your point of view, and they would indeed observe the change in tilt over 13,000 years (not the 10,500-year effect on the perihelion). Dbfirs 06:49, 28 April 2013 (UTC)

Yes, I'm talking about the observer sitting at some fixed point in the solar system, watching this process over 13,000 years (he takes a lot of Geritol). If I understand what you're saying, Orion would be a summer constellation instead of a winter constellation, right? Also, we would have to adjust our calendar every once in awhile, to ensure that summer still starts on June 21, right? ←Baseball Bugs ^{What's up, Doc?} carrots→ 07:08, 28 April 2013 (UTC)

We are not really disagreeing here, except on the point of having seasons at the opposite time of the year. They will occur at the opposite position of orbit, but not at a different time of year. This is because we already adjust our calendars to match the mean tropical year (or, more correctly, the mean period between vernal equinoxes which is not quite identical), so we already adjust our year to maintain summers in the same months over thousands of years (see Gregorian calendar). This is what we Earth-dwellers mean by "year". Observers elsewhere in the solar system would see the tilt change over 12,886 years in Earth-time (see Axial precession), as well as observing the slower 112,000 year-Apsidal precession of the orbit. The 10,818-year reversal effect on the perihelion that we observe here on Earth is a combination of the two. Of course, Earth's astronomers observe the two effects independently against the stars. Dbfirs 07:30, 28 April 2013 (UTC)

Abandoned Tower Crane rotation

Tripoli has more abandoned tower cranes than anywhere I've ever seen (and approximately none currently doing anything). Hundreds of construction projects lie in suspended animation at whatever stage they were at when the 2011 revolution shut things down.

Several cranes near where I work normally point north but from time to time I see a couple of them (always the same two) pointing another way. The site is abandoned but anybody could clamber in and probably anybody could climb the crane if they wanted to. But surely they won't have the keys. (Do cranes need keys? Anyway, I doubt these ones have power.) How and why are they sometimes turned around? I wondered if it's the wind (perhaps these two are not locked). What's the most likely explanation?

Hayttom 15:41, 26 April 2013 (UTC) — Preceding unsigned comment added by Hayttom (talk • contribs)

Clearly, these cranes don't have people sitting in them 24/7 the whole time they are out there - so there must be some kind of mechanism to allow them to be safely parked. I guess that the last time the crane operator left to go home, he'd have engaged whatever mechanism there is - just like he'd do every night at the end of his shift. Doing a Google image search on "Tower crane cab" produces photos like this one: http://ic.pics.livejournal.com/nord_operator/16414762/33225/33225_original.jpg that clearly show a key...so it's reasonable to assume they have them. Of course it might be that the crane operators just leave the key up in the cab ready for the start of the next shift...so that's not really proof of anything. It would make sense to point the crane into the prevailing wind at the end of a shift - and according to several sources I looked at, the prevailing winds in Tripoli are from the north...so this is a plausible thing. SteveBaker (talk) 16:28, 26 April 2013 (UTC)

From [1]: "When not in use the rotation / slew brake must be released to allow the crane weather vane thus the wind then poses no danger to the crane". It sounds like the standard procedure is to allow an empty crane to rotate freely in the wind. Whether or not any given crane rotates would then depend on the strength of the wind and the amount of internal friction resisting rotation. Dragons flight (talk) 17:10, 26 April 2013 (UTC)

Resolved

Another crackpot theory question

Hi all, I have another question based on a crackpot theory of mine, so please be kind and moderate, and don't abuse me too much. It's all very sketchy, so someone is going to want to take the mickey, but please go easy. It goes like this: the laws of the universe are time symmetric (or nearly so, but I'll get to that). My theory is that time could just be a kind of disturbance, and the laws are symmetric simply because any function can be resolved into a symmetric component and an antisymmetric component, as in f(x) = (f(x) + f(-x))/2 + (f(x) - f(-x))/2. Now if time is a disturbance, essentially consisting of vibrations forwards then backwards, then the antisymmetric operators would cancel (this is how I picture it) and the symmetric operators would amplify. But every operator (or function) can be reduced to a symmetric component and an antisymmetric component, so any operator would have its antisymmetric component removed.

But: in reality, the universe is governed by CPT symmetry. You have to reverse time, conjugate charges, and reverse polarity, for symmetry to be perfect. Under the covers, mathematically, any matrix can be decomposed into a Hermitian matrix and an anti-Hermitian matrix. My theory is that the transpose operation on a matrix is analogous to changing polarity, and the complex conjugate operation for the matrix is analogous to charge conjugation. Then, the same concept follows: any operator is the sum of a Hermitian operator and an anti-Hermitian one, so if time is some kind of disturbance in a Hermitian space, the CPT-symmetric laws will amplify, and the CPT-antisymmetric laws will cancel.

This depends on a couple of things: firstly, the mathematical operations must be analogous in some way to the physical operations I mentioned, and secondly, the mathematical proof of CPT symmetry (which I do not know) must (presumably) incorporate this analogy in some way. Is there anything that would count for or against this theory? I know it is suitably vague, so if you want an unfalsifiable theory, you've got one, but whilst it may not be strict science, it seems as though it is open to further analysis. And yes, perhaps it is strictly falsifiable, but I just can't show this. IBE (talk) 15:41, 26 April 2013 (UTC)

I can't make any sense of this. If time is a disturbance, what is it a disturbance of? Time is a dimension -- something that can be measured using a numerical scale. How can a disturbance be a dimension? Looie496 (talk) 15:56, 26 April 2013 (UTC)

I take your point, and I admit that it's a problem. I've often wondered that, but the theory seemed so neat when I thought it up, and it connected with things I found out later - things that have been known for ages, but which I myself didn't know. The idea is closest to Max Tegmark's mathematical universe hypothesis, so the universe is just made up of numbers. Essentially it is just a complex number field, or matrix field of some sort, and it consists of spatial dimensions, and an imaginary time dimension. Now what happens with the imaginary dimension? Imaginary numbers cannot be pictured in spatial terms, so let us say the universe has this dimension and doesn't know what to "do" with it. So it oscillates between states - producing time as a by-product. Of course one can imagine a universe that doesn't need to do anything with the imaginary dimension - it just sits there. Such a universe would exist, however, for zero seconds. So the universe exists because it is the thing that doesn't know what to do with the imaginary numbers. I know that is vague, so the theory is intended as a starting point, so people can either refute it, or show how it links to some other theory, and maybe has a grain of truth in it somewhere. IBE (talk) 16:25, 26 April 2013 (UTC)

The problem is that time is the anchor that we use for experience. If time speeded up, slowed down, or even reversed, we'd be completely oblivious of it happening. I don't see what it even means to have time itself change.

Worse still for your theory is that time itself isn't a constantly marching thing. Relativity says that bodies in motion (or inside gravitational fields) experience time dilation...so the rate of passage of time is slightly different for every person, every molecule in the universe...it doesn't seem like your theory works too well with every fundamental particle's experience of time oscillating around separately. The universe doesn't have one single, universal clock...there is no "true" measure of time - it's different for every particle.

For example, an earth-bound fundamental particle called a "muon" has a lifetime of about 2 microseconds before it decays away. But muons created in the upper atmosphere due to cosmic ray interactions move at about 99% of the speed of light - and they decay over about 20 microseconds. Time for a muon creating that way is happening 10 times slower than it does for us. It's clock runs slower than ours does. But if our time and it's time are running at different speeds - what does it mean for time to oscillate? There is no "master clock"...no "true time" for the universe.

The biggest problem is that theories shouldn't come from nowhere. You need to find what's wrong with our current theory - find experimental evidence that it's broken - then propose something new that not only predicts that new evidence - but (most importantly) reproduces everything we already know under the old theory. I don't see that you're doing that here. Occam's razor says that we don't make things any more complicated than we have to - and we simply don't need your hypothesis to explain what we already know.

SteveBaker (talk) 16:53, 26 April 2013 (UTC)

Ok, two main points, both worth noting. Firstly, lack of "true time" or universal time, secondly, lack of a need or basis for the theory. The need is easiest - it is philosophical as much as anything. There isn't a theory it is replacing, and I admitted it is not pure science - it hits philosophy as much as anything. It is an overarching explanation that may add to existing science, and would be judged by its productivity, which we can't decide here - it could, however, be refuted by something specific, or (partially) justified by some specific connection. I'm not sure it comes from nowhere, but I won't go into it. The lack of a cosmic timekeeper is a bigger practical issue - I cannot see why it is a deal-breaker. The variable nature of time places it on a mathematical footing the same as space, and it is always (I think) the imaginary component. Time is not haphazardly variable, but rather, changes according to precise mathematical relationships, worked out by Einstein a century ago. It is not too big a problem, I don't think. IBE (talk) 17:42, 26 April 2013 (UTC)

i think maybe i see your point, may be; it occurred to me way back there in quantum physics class that you could presumably model the universe as a somewhat complex equation, one of the variables being time; and what we see as events unfolding in time is just the values of various other variables at various values of T for time. 206.213.251.31 (talk) 19:31, 26 April 2013 (UTC)

How do I call it?

What is the name for the wheeled cart for a navy gun? -- Toytoy (talk) 16:15, 26 April 2013 (UTC)

Gun carriage. TenOfAllTrades(talk) 16:22, 26 April 2013 (UTC)

Specifically a "garrison carriage" or "ship's carriage" depending on whether you were using it on land (generally in a fortress) or sea.[2] A "field carriage" or "travelling carriage" was a more mobile affair with a pair of large cart wheels, designed to be towed about by a team of horses.[3] Alansplodge (talk) 00:11, 27 April 2013 (UTC)

How do you call it? I'd say "Gun ho!" or "I predict it will be a gun". Plasmic Physics (talk) 02:42, 28 April 2013 (UTC)

We're being a bit mean. For example, in Spanish you ask someone their name by asking como se llama ("how do you call yourself") rather than que es su nombre. Although my first thought was like calling a pet: "Here, cannon, cannon...!" ←Baseball Bugs ^{What's up, Doc?} carrots→ 04:56, 28 April 2013 (UTC)

Well, I guess we differ in opinion about the meaning of 'malice/mean'. In any case, if the OP was offended, then I appologise. Plasmic Physics (talk) 06:55, 28 April 2013 (UTC)

Not very mean, just a bit. He got the question grammatically right, it's just the heading that reads kind of funny to a native English speaker. ←Baseball Bugs ^{What's up, Doc?} carrots→ 07:02, 28 April 2013 (UTC)

The whole thing would normally be called a cannon. Martin Hogbin (talk) 12:43, 28 April 2013 (UTC)

In most military and naval usages it would be a gun. Hence on a warship you would have gun decks, gun crews, gun ports, gun tackles and so on. Sailing warships were rated according to the number of guns that they mounted; see Rating system of the Royal Navy. The OP did ask about "the wheeled cart for a navy gun". Alansplodge (talk) 12:47, 29 April 2013 (UTC)

Holes/tears/cracks in spacetime

Is it actually possible to create a hole/tear/crack in spacetime? If so, what if anything would happen to something that passed through the crack? Whoop whoop pull up ^{Bitching Betty | Averted crashes} 17:01, 26 April 2013 (UTC)

Nobody really knows, but see wormhole. Looie496 (talk) 17:06, 26 April 2013 (UTC)

In either The Elegant Universe or The Fabric of the Cosmos by Brian Greene, the author describes his own mathematical discovery of "space-tearing flop transitions" or something like that, based on some mathematical model of space including Calabi-Yau manifolds. IBE (talk) 17:45, 26 April 2013 (UTC)

According to known physics, there is no such thing as a hole/tear/crack in spacetime. Greene's work on string theory is speculative and has no empirical support. --140.180.240.146 (talk) 19:13, 26 April 2013 (UTC)

Are you referring to the subset of physics which you know yourself, or the totality of physics Brian Greene and his peers know? General relativity was contrary to the "known physics" of the 1920's so far as most physics students and many leading physicists were concerned. Edison (talk) 00:48, 27 April 2013 (UTC)

I am referring to the subset of physics that has been empirically verified and is considered uncontroversial by the scientific community. I don't have the expertise to judge whether string theory, loop quantum gravity, or other speculative physics have any validity, but they certainly have no empirical support, and Greene himself would be the first to admit this. --140.180.240.146 (talk) 09:38, 27 April 2013 (UTC)

There is absolutely nothing out there that suggests that this is remotely possible. There isn't really any clear concept of what those words actually mean. It's a pure science-fiction idea and searching for ways to make science fiction work isn't really science! Even wormholes are entirely speculative. SteveBaker (talk) 19:40, 26 April 2013 (UTC)

Can I call "citation needed" on your claim that it is pure science fiction? It is mathematical physics, which will often mean that it is not easy to test for the time being, but your claim looks pretty strong. IBE (talk) 02:23, 27 April 2013 (UTC)

From the article that you linked: "Andrew Strominger and Edward Witten have shown that the masses of particles depend on the manner of the intersection of the various holes in a Calabi-Yau", so the basic holes in that space are around quark level in size. I haven't investigated ""space-tearing flop transitions", but I think any such claim that Mathematical Physics predicts large-scale tears in the fabric of space-time definitely requires citations. Most scientists regard this idea as "pure science fiction" (it brings to mind the last episode of Star Trek: Voyager), but the best answer is the first one (given by Looie496):"Nobody really knows" the answer to either question. Dbfirs 11:52, 27 April 2013 (UTC)

Most scientists regard this idea as "pure science fiction" (it brings to mind the last episode of Star Trek: Voyager) --Not what I was thinking of; I was thinking of the Doctor Who series 5 finale. Whoop whoop pull up ^{Bitching Betty | Averted crashes} 08:11, 28 April 2013 (UTC)

Wasn't that the collapse of the whole universe? Dbfirs 09:25, 28 April 2013 (UTC)

Ooh, I wouldn't mind a ringside seat to that event. From a safe distance outside, of course. -- Jack of Oz ^[Talk] 10:26, 28 April 2013 (UTC)

Seats are still available at Milliways. Dbfirs 12:36, 28 April 2013 (UTC)

Hmm, I suspect it's going to take me rather a long time to get there. I'd better get started right away. What time's the next train to Clapham Junction? I'm sure I left an internet lying around here some place ... -- Jack of Oz ^[Talk] 08:42, 29 April 2013 (UTC)

Link added, for whoever's convenience. - ¡Ouch! (^{hurt me} / _{more pain}) 11:19, 29 April 2013 (UTC)

If the hole is small, then use several in parallell ? ;) Electron9 (talk) 23:19, 30 April 2013 (UTC)

One could try to squeeze the planck energy inside a volume that is less than the planck length, that is likely to wreck something.. Electron9 (talk) 23:19, 30 April 2013 (UTC)

Do female hormones make a man weaker?

I was just reading this news article, here, which talks about a transsexual man who is fighting in the MMA against women. I was wondering if female hormones injected into a man over the course of a year or so really make a man weaker. I seem to recall reading something about how men have greater grip strength regardless of how well a woman works out or something. ScienceApe (talk) 17:02, 26 April 2013 (UTC)

Nope, because as a teenager he had enough male hormones to build his muscles. Besides that, genetic men competing against genetic women have an advantage due to several reasons. It's not just the muscle, but also the adrenalin, height, weight and size. Any serious sport league should avoid this scenario. OsmanRF34 (talk) 17:30, 26 April 2013 (UTC)

This is pure OR but female hormones do change a man's body. I have a friend who is a MTF transexual, and she told me that her shoe size dropped 2 sizes after she started taking female hormones, with no sign of it stopping (it may have continued dropping in the last year in other words). I'm sure there will be proper references somewhere but I don't have access to the medical sites. --TammyMoet (talk) 18:01, 26 April 2013 (UTC)

I wonder how. Are the bones in her feet actually shrinking ? StuRat (talk) 18:06, 26 April 2013 (UTC)

Yes, that story seems extremely dubious and in contradiction of medical science. If her feet really are shrinking, that could indicate a serious medical condition like neuropathic arthropathy, and she should immediately seek medical advice. --140.180.240.146 (talk) 19:08, 26 April 2013 (UTC)

StuRat: her shoe size has dropped, not the size of her feet. Women will often wear shoes that look good even if they're the wrong size. – b_jonas 20:27, 27 April 2013 (UTC)

my experience is that female hormones definitely make a man weaker, if they are packaged inside a female 206.213.251.31 (talk) 19:34, 26 April 2013 (UTC)

• Insofar as they encourage fat deposition, especially breast enlargement, they won't make him stronger. There was a lot of press about marijuana causing an increase in female hormone in men and the adverse effects about a decade back. I always assumed it was drugwarrior pseudoscience, but it's a place to look. μηδείς (talk) 19:39, 26 April 2013 (UTC)

Male-to-female hormone replacement therapy reduces physical strength, muscle mass, and bone density according to these interviews with physicians who specialize in sex reassignment. This link (scroll halfway down) shows Fallon Fox fighting a cisgender female MMA fighter, Erika Newsome. Fox is on the left, Newsome on the right. Newsome's muscles are huge compared to Fox's. Note that development of muscle mass can vary greatly even among cisgender women. Fighters are paired by weight class. 173.49.177.27 (talk) 21:48, 26 April 2013 (UTC)

The older the male the less will be the effect. Estrogen is an (now obsolete) effective treatment for prostate cancer. I had a friend at work who developed protrate cancer at age 50. They inserted radioactive pellets into his prostate gland tumour to slug it down, and put him on estrogen for 5 years to stop any tumour spread. Apart from growing a nice pair of little schoolgirl tits, there was no real effect on him. He did not change size and he retained full muscle strength. I had an uncle who also was put on estrogen to treat prostate enlargement disease. It had no effect on him physically. His wife claimed it made him a bit silly, but given he was nearly 90 at the time, his sillyness was probably was going to happen anyway. I don't know what the cancer treatment dosage is/was compared with what transvestites take. Wickwack 58.170.141.242 (talk) 03:02, 27 April 2013 (UTC)

Yes, treatment with estrogen for more than a brief period of time will weaken a man's muscle strength. Estrogen will suppress GnRH, which will suppress LH, which will reduce the testosterone produced by the Leydig cells, which will gradually result in some reduction of muscle mass and strength. And, sorry guys, there are errors in almost every single comment so far posted to this question. alteripse (talk) 03:26, 27 April 2013 (UTC)

GMSK modulation vs. 8PSK modulation

Why measure frequency error in GMSK modulation as opposed to EVM? Is EVM not relevant in GMSK modulation? Why measure EVM in 8PSK modulation? Is frequency error not as important? Thanks. — Preceding unsigned comment added by 192.240.14.2 (talk) 17:24, 26 April 2013 (UTC)

(See EVM, Frequency-shift keying and QAM for our articles on the subject). The answer is because those are the relevant parameters for the signals in question. For a GSM signal, you're not particularly interested in the (demodulated) amplitude of the waveform, only the frequency - there isn't a definite "target" point on the polar diagram from which one can measure the error vector, which means that the EVM isn't a useful parameter. On the other hand, for a QAM signal, the frequency error doesn't give you enough information - you need to know how far off the amplitude, as well as the phase, is, in order to characterize the signal quality. Tevildo (talk) 20:06, 26 April 2013 (UTC)

Best office plant for artificial light only

I want to buy a plant as a gift for an office worker whose desk has 8-12 hours a day of artificial light only. Google searches have suggested jade plants. But looking up jade plants direct sun for part of the day is recommended. None of the sources I have found is above the level of a blog. Can anyone provide information about jade plants or other alternatives? Thanks. μηδείς (talk) 20:18, 26 April 2013 (UTC)

I didn't realise these "reference desks" were just a substitute for five seconds on Google!! This is a good start. Depends what sort of office environment your plant will be exposed to. Try this, or this. The Rambling Man (talk) 20:24, 26 April 2013 (UTC)

Jades should survive fine, but they won't grow much. Sansevieria are a common choice, as they are virtually indestructible. They can literally go months with zero light and water, so are great for people who don't know how to care for plants. They can't get too much light in an office, and are also very hard to over water if planted properly. The lists posted above outline many of the common choices. They are also all pretty boring, IMO. Additional good choices are Schefflera, Zamioculcas, Gardenia and Epiphyllum (it might not bloom, but I find them very interesting). Contrary to one of the links above, I'd recommend against trying to grow Azaleas in an office. SemanticMantis (talk) 21:50, 26 April 2013 (UTC)

Why? The Rambling Man (talk) 21:52, 26 April 2013 (UTC)

(assuming you mean "why not grow azaleas as an office plant?") Well, first, I've never seen it done, and I check out interior plants wherever I go. Yet they are a very common landscaping plant across vast swaths of the USA... Secondly (and relatedly), my hunch is that they need a seasonal cycle of dormancy to trigger repeated blooming (e.g. vernalization), but I could be wrong there. Finally, if you google "indoor azalea", you'll find lots of discussion of overwintering them inside, but nobody really talks about keeping them inside. Anyway, that's just my recommendation, take it or leave it. SemanticMantis (talk) 22:06, 26 April 2013 (UTC)

Would you think that it depends where your office is located around the globe? The Rambling Man (talk) 22:09, 26 April 2013 (UTC)

This will be in the Tri-State Area. μηδείς (talk) 17:04, 27 April 2013 (UTC)

Assuming "artificial light only" as medeis has, then no, I assume offices around the world have very little seasonal variation in temp, lighting, or humidity (more precisely, vapor pressure deficit). Sure, there is some variation, but not on the scale that temperate plants are adapted to. It would be a different story if there were no climate control, or if there were a large window. Of course, there are zillions of species, strains, and hybrids of Azaleas, with different cold tolerances and overall habits. I'm sure someone has coaxed some type into long-term cultivation and blooming indoors. But I still wouldn't recommend one as gift. I've seen them sold in small pots with decorative wrapping, presumably for just that purpose. But I suspect most people only keep them inside for a month or so, then either throw them out or plant them outside if they are hardy to that particular zone. Anyway, It's Friday evening here, and I have no time for further refs. For what it's worth, I've been practicing plant husbandry for ~15 years, and currently maintain ~100 plants in ~50 pots, across a few dozen species. Some indoor, some outdoor, some move. SemanticMantis (talk) 22:47, 26 April 2013 (UTC)

The Aspidistra aka: Cast-iron-plant was very popular in Victorian homes. Not only did they have to cope with deep shade but gas lighting fumes and cigar smoke (although, I don't find the latter a problem myself). Failing that, the other popular deep shade plants are the ferns. [4] I have seen these growing deep inside caves (Cheddar Gorge et al.,) with only the electric light bulbs to provide illumination. If you get really desperate, there are all way the pretty Psilocybin mushrooms. They need no light what-so-ever and turn a pleasing blue when crushed... or so I am reliably told.Aspro (talk) 22:50, 26 April 2013 (UTC)

Thanks, all. I think I will stick with a bonsai jade which shouldn't grow too much, prickle, or topple. The plant will be on a large desk manned by multiple people and within reach of the public. μηδείς (talk) 17:14, 27 April 2013 (UTC)

Resolved

Hydrogen manufacture

Can carbon nanotube material be used to filter water into hydrogen and oxygen; with pressure anf/or temperature applied? — Preceding unsigned comment added by 174.100.57.235 (talk) 22:40, 26 April 2013 (UTC)

You can't "filter" hydrogen out of water anymore than you can filter all of the left arms out of a crowd of people. You need to break apart the water molecules to produce hydrogen and oxygen, and that breaking requires a substantial input of energy. Most commonly this energy comes from electricity. See Electrolysis of water. No need for any elaborate nanotubes. A couple of jumper cables and a 12 volt battery will do it. --Jayron32 02:46, 27 April 2013 (UTC)

To be very, very clear about this: Any effort whatever by any means whatever to split water into hydrogen and oxygen will take more energy than you'll get by burning, reacting or otherwise recombining the hydrogen with oxygen again. If anyone tells you otherwise (and especially if they try to sell you a handy gadget that can do this) - kick them hard in the shins. If they specifically claim to be able to power a car with this gizmo - then kick them somewhere where it hurts more than that. You'll be doing the world a favor in the long run. SteveBaker (talk) 03:44, 27 April 2013 (UTC)

Also to be clear, using hydrogen to power a car, in (for example) a fuel cell is not a shin-kickable offense. There are many proposed solutions to generating low cost (and low carbon footprint) hydrogen, but what it boils down to is coming up with a low-carbon-footprint source of electricity to electrolyze the water to produce the hydrogen to put in your fuel cell. The problem with, say, your average "home hydrogen electrolysis kit" (they exist, Honda makes one that runs off your house current to supply hydrogen for its production fuel cell vehicle, the Honda FCX Clarity) is that if you're using your house power supply to make hydrogen, and your house gets its electricity from a dirty coal power plant (and chance are, it does if you're in the U.S. or many other countries), the upside being your supposedly "clean" fuel cell car isn't so clean, as you're essentially burning coal to run your car, and doing it fairly inefficiently in the sense that you're burning coal several hundred miles away to do it. If, perchance, you had a solar-electric system that was generating your hydrogen, and you used that hydrogen to power your car, then, aside from the capital cost in setting the system up, you would be running your car for free. Honda is working on that too, see [5]. However, SteveBaker's caveat is valid: if someone is saying that energetically you can get hydrogen from water for free (that is, in a way that you don't need a massive input of energy in excess of the energy you get from recombining the hydrogen with oxygen in, say, a fuel cell) they deserve a kick in the shins. If, perchance, you could get a reliable source of cheap or free energy (like sunlight) to do so, then it may be cash free, but of course, you still need that energy. --Jayron32 04:04, 27 April 2013 (UTC)

Getting energy by burning coal in a power plant several hundred miles away and then sending it along a line then powering an electric motor is more efficient than burning petrol or diesel locally in a car's petrol or diesel engine, plus it is cleaner overall and needs less processing to produce the fuel. For cars there is also the added advantage that electric motors can get a bit of the energy back going down a hill or braking. However generating hydrogen as an intermediate step wastes quite a bit of the energy and batteries make cars heavy and so use up more energy. Dmcq (talk) 06:13, 27 April 2013 (UTC)

Coal is radioactive, you know? Plasmic Physics (talk) 11:18, 27 April 2013 (UTC)

That was a scare headline for dramatic effect. Nuclear power plants don't have smoke stacks at all and and petrol production does the same thing. Dmcq (talk) 14:40, 27 April 2013 (UTC)

What headline? I have no idea what you're talking about. Plasmic Physics (talk) 15:04, 27 April 2013 (UTC)

Sorry I thought you probably got it from Scientific American Coal Ash Is More Radioactive than Nuclear Waste. Dmcq (talk) 16:44, 27 April 2013 (UTC)

No, that is just ridiculous. At most, coal ash has traces of radioactive material. I was just pointing out that coal, and coal ash is radioactive. Plasmic Physics (talk) 23:43, 27 April 2013 (UTC)

"No" to the article, not to you. Plasmic Physics (talk) 02:29, 28 April 2013 (UTC)

I'm more concerned with the evironmental accumulation of radioactive contaminants from fly ash, than the simple presence of the radioactive contaminants in the fly ash. Plasmic Physics (talk) 02:35, 28 April 2013 (UTC)

From Fly ash 'Studies by the U.S. Geological Survey and others of radioactive elements in coal ash have concluded that fly ash compares with common soils or rocks and should not be the source of alarm.' Big plants are away from the general population and they can fit good industrial scale facilities to clean up their waste. You should be far more concerned about the pollutants spewing out the backs of cars and trucks just beside people. Dmcq (talk) 13:14, 28 April 2013 (UTC)

These are what the molecules involved look like

H-H    O=O     O
              / \
             H   H

When hydrogen is burned with oxygen to form water, two moles (a fixed number of molecules) of hydrogen react with one mole of oxygen, to form two moles of water. More specifically, two hydrogen-hydrogen single bonds are broken, and one oxygen-oxygen double bond is full broken to form free atoms, and then 4 hydrogen-oxygen single bonds are formed.

Bond-dissociation_energy lists the energy required to do this, it is 2 * 104 Joules + 1 * 119 Joules (324 Joules) to split the hydrogen and oxygen, and 4 * 110 Joules (440 Joules) released when the oxygen and hydrogen combine to form water, giving a net surplus of 76 Joules.

To split water back into hydrogen and oxygen, you need to break the water back into free hydrogen (440 Joules required), and then form then into hydrogen and oxygen (releasing 324 Joules), this gives a net deficit of 76 Joules; this needs to be supplied for the reaction to proceed. In practice forcing a reaction against its natural direction generally needs more than the minimum amount of energy; this is wasted as heat.

Also, hydrogen and free oxygen atoms are highly reactive; they will probably react with the carbon nanotubes as well. CS Miller (talk) 11:25, 27 April 2013 (UTC)