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September 10

Average age of first childbirth

In a "natural" human (as in before the Neolithic Revolution), what was the average age at which a woman had her first child? How about a man? --140.180.254.18 (talk) 05:24, 10 September 2012 (UTC)

The Wikipedia article titled Advanced maternal age has some information on recent trends, but I don't see anything from Neolithic times. Still, its a lead and there are references. You could start your research there. --Jayron32 05:55, 10 September 2012 (UTC)

Would that even be possible to determine? I guess women dying during pregnancy leave two skeletons, but apart from that... Ssscienccce (talk) 08:05, 10 September 2012 (UTC)

Actually, it would not be that difficult at all, provided there were enough skeletal remains from the population in question. The pelvic inlet in the skeleton of a woman who has given birth is significantly wider than in the skeleton of a woman who hasn't. Add to that demographic data about fertility, mortality and age structure, and you could get a very good idea about the age of first birth in a given population. All the more so as childbirth itself was one of the leading causes of death among women of that time, especially in young adolescents, so there would be plenty of early adolescent female skeletons that probably died giving birth or shortly thereafter. Dominus Vobisdu (talk) 08:40, 10 September 2012 (UTC)

Are you sure about the "leading cause"? Childbirth complications were a leading cause of death for women in early modern times, but the major risk factor there are infectious diseases like childbed fever. These would be a lot less widespread in hunter/gatherer societies than in more densely populated civilisations. Child birth would still be a significant risk, but possibly less than one would think from simple extrapolation. --Stephan Schulz (talk) 09:46, 10 September 2012 (UTC)

If the average age of first childbirth is hard to determine, how about the average age of childbirth in general? I'd look at the molecular clock to see how many generations had passed between times X and Y, and use radiocarbon dating to determine the absolute time difference. --140.180.247.208 (talk) 17:06, 10 September 2012 (UTC)

Couldn't one assume that the average age is not long after the earliest possible age for reproduction? What would be the factors tending to delay reproduction? The question as originally posed referred to a "natural" human and one before the Neolithic revolution. I can only imagine cultural factors as held by the group of people to which one belonged inhibiting the natural biological urge to reproduce, but I find myself doubting that such cultural constraints would exist as their efficacy in promoting the wellbeing of the group are not obvious to me. Bus stop (talk) 17:12, 10 September 2012 (UTC)

One doesn't 'assume' an average, one measures it. AndyTheGrump (talk) 17:33, 10 September 2012 (UTC)

AndyTheGrump—The original question wonders aloud about the "average age at which a woman had her first child".[1] Whether we "assume" or "measure", we are still addressing the question. I think in some instances assumptions can be worthwhile. The original question was not necessarily concerned with the average age of all births, but rather the average age at which a woman had her first child. Also the original question concerned the average age that a man sired his first child. Yes, measuring for this would be distinctly preferable. But how could one measure for this? Bus stop (talk) 17:49, 10 September 2012 (UTC)

List of youngest birth mothers may be relevant. Bus stop (talk) 18:25, 10 September 2012 (UTC)

Oh, don't be silly Bus stop. All the Grump has to do is go back in his time-machine and select a sufficiently large representative cohort and follow them for a few generations, (as our current-day archaeological evidence of the neolithic period is far to sparse top draw any firm affirmation). However, the earliest time that a female can conceive (whether they primate, meerkat or pussycat) is not a good datum (or posit) (or assumption) as females tend to mate when they are ready. I don't wont to get into the nature/nurture on this, because obviously, they can still be wearingly short-white-ankle-cotton-socks, when they meet some rich guy/pop star/etc., and the money pheromone brings them into heat. --Aspro (talk) 18:57, 10 September 2012 (UTC)

black sunglasses or brown sunglasses for maximum UV protection

I should use black or brown sunglasses for uv protection? how sunglasses work? — Preceding unsigned comment added by 101.63.161.197 (talk) 07:12, 10 September 2012 (UTC)

Blocking UV rays would have no connection with glasses looking brown or black in visible light. It would be an entirely separate attribute of the glasses. HiLo48 (talk) 07:46, 10 September 2012 (UTC)

How do you know that? ←Baseball Bugs ^{What's up, Doc?} carrots→ 07:55, 10 September 2012 (UTC)

As well as the comment below, it's worth pointing out that UV rays are invisible to humans, so the visible colours of glasses are irrelevant to whether they block UV. HiLo48 (talk) 08:37, 10 September 2012 (UTC)

Because it's in our very article, Sunglasses. The color and protection factor can be varied independently. Now to answer the actual question, the original purpose of having a variety of colors for the lenses stems from the fact that sunglasses distort colors and contrast, and can impact your depth perception. The different colors have subtly different effects, and so you may prefer one over the other, although some colors are used purely for their cosmetic appearance. See Sunglasses#Lens for more. For the actual protection offered by the sunglasses, you'll have to look at the labeling on the sunglasses themselves. In the United States, European, and Australia, there are regulations on Sunglasses that require labeling to indicate the level of protection offered, and you can read about those at Sunglasses#Protection. I don't know if such regulations exist in India, to which your IP address belongs, but hopefully you will find some manner of label. Someguy1221 (talk) 08:23, 10 September 2012 (UTC)

That was the right answer. :) ←Baseball Bugs ^{What's up, Doc?} carrots→ 08:45, 10 September 2012 (UTC)

The color of the glass doesn't tell you if it blocks UV. Cheap dark plastic sunglasses may be worse than no sunglasses at all. And what Someguy says... Ssscienccce (talk) 08:30, 10 September 2012 (UTC)

Indeed. Actual real glass (as in silicon dioxide is opaque to UV light. It's why you can't get a sunburn through a window. Nearly all sunglasses have plastic lenses however, and many types of plastic do not offer any protection to UV light. --Jayron32 06:31, 11 September 2012 (UTC)

Not true, not true at all. Silicon dioxide is opaque to ultraviolet light below 200nm, where you'll find germicidal lights emitting, but is almost perfectly transparent to UVA and UVB [2]. Someguy1221 (talk) 07:52, 11 September 2012 (UTC)

I should clarify. The UV blocking ability of "glass" is highly dependent on the manufacturing process: some forms of silicon dioxide are opaque in a broad range of the UV, while other forms are basically transparent to many forms. The article you just cited seems to be dealing with fused silica, which is specifically manufactured for its UV transparency; fused silica cuvettes are used in UV-vis spectroscopy, for example. The introduction of that paper, where it discusses the four types of manufacturing processes used in producing the glass used for their experiemnts, are all used to produce this high purity fused silica. Most common glass isn't made the way they describeHowever, most people don't have fused silica windows, and common glass as is usually found in such uses is often Soda-lime glass, which is far more opaque in the UV. This article quotes a figure of 85% blocking of UVB rays for ordinary window glass, which makes it about SPF30. So, you can get sunburned through glass, but it takes a LOT longer to do so. --Jayron32 12:31, 11 September 2012 (UTC)

Actually, I have a pair of clear safety glasses that are apparently 99.9% protective against UVA and UVB radiation. The reflective tint on the outside surface coating of the glasses go from blue to indigo-violet near the centre, ranging toward orange and yellow near the edges. ~AH1 ^(discuss!) 18:27, 11 September 2012 (UTC)

I don't doubt that, but what makes them so? Does it have to do with the glass manufacture process, a dopant added to the glass, a coating on the glass, etc. etc. There's more than one way to skin a cat. --Jayron32 03:52, 12 September 2012 (UTC)

Allergies

This is not a request for medical advice. I am just wondering, what can cause allergies to spontaneously develop? When I was younger, I never had any allergies at all, but whilst in Japan, after about seven years I started to develop hay fever - or, in this case, specifically an allergy to the pollen of rice plants (which was hell, because I was living in the countryside surrounded by rice fields). Then, after a while I noticed that my stomach just below my navel developed a criminally insane itchy rash, which I later found out was an allergy to nickel (my belt buckle was causing it). My friend told me that in the past five years he has developed an allergy to something or other, but has no idea what. What causes allergies to develop? KägeTorä - (影虎) (TALK) 08:52, 10 September 2012 (UTC)

Some allergies can develop after a certain amount of exposure. For example, this paper starts out with "Beekeepers are strongly exposed to honey bee stings and therefore at an increased risk to develop IgE-mediated allergy to bee venom." [3]. Increased exposure may or may not play a role in the allergies that you describe. My understanding is that allergy development is not well understood in general. (start WP:OR): I was "immune" (not allergic) to poison ivy for years, until I had a very large exposure. Now I am normally susceptible (/end OR). This is not medical advice, but reference information, etc. etc. SemanticMantis (talk) 14:36, 10 September 2012 (UTC)

Very true SemanticMantis. Shortly after leaving school I had a very mild exposer to 'work'. Even now, 50 years on, I'm still trying to recover. The only respite I have found, is to lay down in a darked room and stare at the back of my eyelids. Fortunately, during those episodes (which are frequent) I can often relieve the boredom by turning my monitor's screen brightness level way-down-low and edit Wikipedia. Note: My method of staring at the back of the eyelids is in no way to be taken as medical advice for this very serious condition – consult a specialist.--Aspro (talk) 18:19, 10 September 2012 (UTC)

More on "not well understood in general". This [4] and similar studies have been in the popular science news over the past few years. The authors suggest that increased exposure during childhood may be linked to lower incidence of allergies in rural-raised children, compared to their urban counterparts. So timing and amount of exposure seems to regulate whether the exposure will increase or decrease the likelihood of (some) allergies. In short, there is no simple answer. SemanticMantis (talk) 19:23, 10 September 2012 (UTC)

• All allergies require a period of sensitization to the allergen -- sometimes it is rapid, sometimes slow. Our allergy article describes the process, in the "Pathophysiology" section, but unfortunately in terms that probably only a biologist can make sense of. Looie496 (talk) 19:24, 10 September 2012 (UTC)

• • Also of note is that there is a significant genetic component to allergies, see Allergy#Genetic_basis. This is again one of those nature-nurture unseperatable things: a person may have a genetic predisposition for an allergy, but without preliminary exposure to sensitize them to the allergan, the allergy itself may never fully develop. A person without a genetic predisposition could happily bathe in the same allergan all day and never develop any problems, a person with that predisposition may be fine on a few exposures, but then get "sensitized" and go all anaphalactic without any warning. My father developed a severe bee-venom allergy in his 40s after a being stung by a half-dozen bees; he'd been stung before and never had a bad reaction. Other people can be stung regularly and hundreds of times, and never develop that allergy. It is likely that it was in his genetic makeup to be allergic, but needed a trigger to bring the allergy on. If it wasn't in his genetic make up to have the sensitivity, it may have never developed into a full allergy. --Jayron32 19:31, 10 September 2012 (UTC)

Another component might other changes in the environment. For example, I developed hay fever (to ragweed) about eight years ago (verified by scratch test, etc.). I've lived in the same general area of SW Ontario all my life and never had a problem, but now spend every Aug-Oct period sick as a dog. So, what changed? Did my body require thirty years to gradually build up a sensitivity to ragweed? It seems unlikely, but it's obviously possible. Now, three years ago, we went on a camping vacation to an area about 200 km away. It was the end of August and Murphy's Law required that I forget all the allergy medication at home. However, my whole family (who are also all horribly allergic to ragweed) and I spent the three or four days there with nary a sniffle or itchy eye. So, what caused that? Different species of ragweed? Less irritation from general city air pollution? There are so many different factors that it's hard to say. Maybe your rice allergy developed due to a new strain being planted that changed the pollen just enough to trigger your outbreak? Or perhaps you'd moved during the off season and got exposed to a slightly different kind of pollen. Or maybe some other pollutant acts in concert with the allergen to create the hay fever. It's obviously a complicated bit of biology. Matt Deres (talk) 16:38, 11 September 2012 (UTC)

Anecdotal evidence: it would seem that allergies such as ragweed are more common among those born in an area where the plant pollen is native, though I could be wrong. Also, food "allergies" such as lactose intolerance are a bit more complex: most people who are Han Chinese are not lactose intolerant (citation needed), but the rate of intolerance reaches ~90% among second-to-third-generation populations of Chinese descent in North America. ~AH1 ^(discuss!) 18:23, 11 September 2012 (UTC)

But lactose intolerance isn't an allergy, it's the discomfort of having gut flora digest the lactose, causing gas, rather than enzymatic digestion. If your point is correct, could it not simply be because Han in China don't even carry the gut flora that cause the gas? μηδείς (talk) 02:42, 12 September 2012 (UTC)

Relativity theory: a body moving while emitting an energetic radiation

According to Einstein's formula, when one calculates the current mass of a resting body that has just emitted an energetic radiation $E$ , one should subtract $E/c^{2}$ from the original mass the body had had before it emitted the radiation. Note that Einstein did not have to explain whether the original mass he referred to - is the original total mass - or the original rest mass, because he referred to a resting body, i.e. to a body whose rest mass is equal to its total mass. However, how about calculating the current mass of a moving body that has just emitted an energetic radiation $E$? Should one subtract $E/c^{2}$ from the original total mass - or from the original rest mass? Note that this matters very much, because if one subtracts $E/c^{2}$ from the original rest mass, then one subtracts $\gamma E/c^{2}$ from the original total mass. If you think you know the answer (which I don't), then please explain also the full considerations which could have enabled one to get the answer by oneself. 77.127.218.31 (talk) 09:28, 10 September 2012 (UTC)

I'll give it a try. There are three quantities involved here: energy, momentum and mass. These are related through the four-momentum of a particle. This is a vector with four components (E, p_x c, p_yc, p_zc). The length of this vector is computed as ${\sqrt {E^{2}-p_{x}^{2}c^{2}-p_{y}^{2}c^{2}-p_{z}^{2}c^{2}}}=mc^{2}$, where m is the mass of the particle, this is invariant (what you call "rest mass"). For a particle at rest, the four-momentum is (E_p, 0, 0, 0) = (mc², 0, 0, 0). Let that particle emit a photon of energy E in the positive x direction; the four-momentum of the particle photon is (E, p_xc, 0, 0) = (E, E, 0, 0) (the momentum is equal to the energy because the photon has mass zero. Now apply conservation of energy (for the first - actually zeroth -- component) and momentum (for the other components), to determine the four-momentum of the particle after emission (just the difference between the four-momentum of the particle before emission and the four-momentum of the photon), it is (E_p − E, −E, 0, 0). With E_p = m, you find that the mass of the particle after emission is ${\sqrt {m^{2}-2mE/c^{2}}}$. This shows that the particle after the emission cannot be the same as the one before. Hence, if the particle is an elementary particle it decays to another particle type (examples?). If it is an atom, it changes its internal structure (an electron drops to a lower energy level). If you insist on using "total mass" (but this concept is redundant because it is nothing but the energy), then you can simply say that the photon's energy is subtracted from the total mass, although most physicists today prefer saying that the photon's energy is subtracted from the particle's energy. In terms of rest mass, it is somewhat more complicated, as I've shown. --Wrongfilter (talk) 15:03, 10 September 2012 (UTC)

.

Thank you for your detailed response. I appreciate it. I have two comments, the second one being more important:

.

1. You write: "the four-momentum of the particle is (E, p_xc, 0, 0)", but you probably meant: "the four-momentum of the photon is (E, p_xc, 0, 0)". Later you write: "the momentum is equal to the energy", but you probably meant "the momentum multiplied by the velocity is equal to the energy". Later you write: "With E_p = m", but you probably meant: "With E_p = mc²". Later you write "it is nothing but the energy", but you probably meant "it is nothing but the energy divided by c²". Later you write: "you can simply say that the photon's energy is subtracted from the total mass", but you probably meant: "you can simply say that the photon's energy divided by c² is subtracted from the total mass".

.

2. Let me take your sentence that contains (in my opinion) the most direct answer to my question: "If you insist on using 'total mass'...then you can simply say that [in order to get the mass of the particle after the emission] the photon's energy [divided by c²] is subtracted from the total mass [the particle had before the emission]". Please notice that the total mass of a particle at rest is simply its mass - i.e. its "rest mass" (as I call it), so your sentence mentioned above lets me conclude that: "[in order to get the mass of the resting particle after the emission], the photon's energy [divided by c²] is subtracted from the [rest] mass [the resting particle had before the emission]". However, this conclusion contradicts your (well-reasoned) conclusion that "the mass of the particle after emission is ${\sqrt {m^{2}-2mE/c^{2}}}$ ", doesn't it?

.

By the way, what did you mean by :"I'll give it a try"? Did you mean a try to give the real answer - although you're not sure whether it's the real answer, or a try to explain the answer to me - although you're not sure whether you will succeed to explain it to me?

.

Again, I appreciate very much your detailed response. 77.127.218.31 (talk) 19:15, 10 September 2012 (UTC)

ad 1): "four-momentum of the photon" is what I meant, I've corrected it above. I usually do these things in units where c=1, hence I usually don't write c in any of of these equations. I tried to pay attention to this, but apparently I failed repeatedly... You're right on all accounts. ad 2): I understood "total mass" as being "relativistic mass", i.e. energy divided by c², i.e. the first component of the four-momentum. That's a frame-dependent and redundant quantity, and I very much dislike it. Before emission, the particle's "relativistic mass" is equal to its "rest mass", because we chose to work in a frame where the particle is at rest initially. Incidentally, the equation for the (rest) mass still has the frame-dependent E; in order to make it valid in any frame, E should be divided (right?) by $\gamma$. By "I'll give it a try", I mean "I'll try to remember/figure out how to do this" and "I'll try to explain it in a way that people can understand it". --Wrongfilter (talk) 20:44, 10 September 2012 (UTC)

Unfortunately, I suspect I didn't understand your reply to my second comment: Please note that (in my previous response) I was talking about a particle at rest (in our reference frame), i.e. about a particle whose $\gamma$ equals 1. How can this remove the contradiction I've pointed at? Namely: on one hand you claim that "If you insist on using 'total mass'...then you can simply say that [in order to get the mass of the particle after the emission] the photon's energy [divided by c²] is subtracted from the total mass [the particle had before the emission]". So, in our reference frame - in which the particle's $\gamma$ equals 1, the mass of the particle after the emission must be $m-E/c^{2}$ , where $m$ is the (rest) mass the particle had before the emission; However, on the other hand you claim that "the mass of the particle after emission is ${\sqrt {m^{2}-2mE/c^{2}}}$ ". Can't you see the contradiction (at least in our reference frame in which the particle's $\gamma$ equals 1)? 77.127.218.31 (talk) 21:38, 10 September 2012 (UTC)

The emitting body isn't at rest after the emission—there's a recoil. So $m-E/c^{2}$ is not its final rest mass, though it is its final relativistic mass in the frame where it's initially at rest. -- BenRG (talk) 22:58, 10 September 2012 (UTC)

Yes, thank you. I'd thought $\gamma =1$ because I'd ignored the recoil...

Btw, I think that, when considering the possibility of a mass emitting photons, there is no longer any justification for regarding the amount ${\sqrt {E^{2}-p_{x}^{2}c^{2}-p_{y}^{2}c^{2}-p_{z}^{2}c^{2}}}$ as "invariant" (as Wrongfilter and almost all phisicists regard it).

I also think that, when ignoring the concept of a "relativistic mass" (which Wrongfilter "dislikes" as he explicitly wrote), there's no longer any justification for the equation: ${\sqrt {E^{2}-p_{x}^{2}c^{2}-p_{y}^{2}c^{2}-p_{z}^{2}c^{2}}}=mc^{2}$ when we have no pre-assumptions, except for: logic, mathematics, the newtonian definitions for momentum (namely: the product of mass and velocity) and for energy (namely: the integral of velocity over momentum), and the three invariants: conservation of momentum, conservation of energy, and light speed. 77.127.218.31 (talk) 11:35, 11 September 2012 (UTC)

You're wrong about that. ${\sqrt {E^{2}-p_{x}^{2}c^{2}-p_{y}^{2}c^{2}-p_{z}^{2}c^{2}}}=mc^{2}$ is always an invariant. Note that invariant doesn't mean conserved. An invariant is a quantity that has the same value for all observers. A particle's rest mass is an invariant - that is all observers measure the same value for its rest mass. But when the particle emits a photon, its rest mass changes, so it is not conserved (Though the total rest mass for the particle + photon system is conserved). The new rest mass (after the emission of the photon) is a new invariant - that is all observers measure the same value for it. Dauto (talk) 14:22, 11 September 2012 (UTC)

You probably mean I was wrong about the invariant. Yes, you're right. However, I still think I wasn't wrong regarding my last comment - about the relativistic mass. 77.127.218.31 (talk) 14:51, 11 September 2012 (UTC)

I don't understand what you mean by that last comment. In the particle's rest frame (where its momentum is zero), its energy is E₀ = mc² - Einstein's famous equation, with m the particles (rest) mass. The length of four-momentum is invariant with respect to coordinate transformations (in particular Lorentz transformations), its length is still mc² when viewed in a frame where its momentum is not zero (in fact the vector as a geometrical object is invariant, it's just its coordinate representation that changes). This is a very fundamental equation in special relativity. At this point, you may want to read a book on special relativity — I learned a lot from the first chapters of Bernard Schutz's "A first course in general relativity". --Wrongfilter (talk) 15:23, 11 September 2012 (UTC)

I mean that if you ignore the concept of relativistic mass then you can't deduce (what you call) "Einstein's famous equation" with m being the particle's (rest) mass. However, this is not the case when you do consider the concept of relativistic mass, because then you can mathematically prove Einstein's formula if E refers to the kinetic energy granted to a particle at rest, and m refers to the relativistic mass added to a mass at rest, and then you can generelize Einstein's Formula - also for the energy contained in a rest mass - by a simple induction, which takes into account the conceptual similarity between relativistic mass and rest mass (similarity in the sense of their not being distinguished in terms of conservation of momentum). To sum up, what I'm actually claiming - is that you couldn't have reached Einstein's formula (when referring to a rest mass) if you had ignored the concept of relativistic mass. 77.127.218.31 (talk) 15:45, 11 September 2012 (UTC)

Does anyone know what this plant is?

Hi all,

I have a small bush growing in my yard in Cambridge, MA. Its leaves are smooth and almond-shaped, and they have a marking on the top like someone covered them with a smaller spikier leaf and spray painted over them. The branches end in long tendrils with dozens of tiny tiny red buds on them. It grows out of multiple thin stems, and the whole thing is about two feet tall.

http://img.skitch.com/20120910-jk4qjt9rt6sqkg54casg23mh8a.jpg

(Is there a way to embed non-Wikipedia images?)

Thanks! — Sam 24.128.48.26 (talk) 11:32, 10 September 2012 (UTC)

Looks like this Persicaria. --TammyMoet (talk) 11:47, 10 September 2012 (UTC)

Thanks! The picture linked looked quite different, but looking it up online finds that you're completely right. Persicaria virginiana — Lance Corporal Knotweed. Thank you so much! — Sam 24.128.48.26 (talk) 12:30, 10 September 2012 (UTC)

It's often easier to identify plants once they are in flower. Yours appears to have finished flowering so we can't go much on the stalk that's left. But I've seen that plant quite a bit over the years. Some varieties of Persicaria are edible. --TammyMoet (talk) 18:16, 10 September 2012 (UTC)

Maldives in the Ice Age

Hi. I'm not sure how coral islands work. But I've read in the last ice age, sea levels were about 120 metres lower than now. So were the Maldives then bigger islands? Would they have had land as much as 100 metres above sea level? How big would they have been and where could I find a map. Thank you. — Preceding unsigned comment added by 142.150.38.84 (talk) 13:30, 10 September 2012 (UTC)

Probably not. The way that coral reefs form (see Coral_reef#Formation), they generally stay just ahead of sea-level rise. Coral atolls like the Maldives aren't that old, geologically. Most are likely less than 10,000 years old, according to our article, and thus if the Maldives existed during the lower sea levels of the last ice age, they would have also been comparitively lower in elevation, so they may not have been considerably larger. Atoll#Formation also has some information, but also notes the dynamic nature of atolls. --Jayron32 16:55, 10 September 2012 (UTC)

That makes sense logically, but this study reports finding the remains of a 120-thousand-year old reef only 14 meters below current sea level -- so it seems likely that there would have been about 100 meters of exposure at the LGM. Looie496 (talk) 19:12, 10 September 2012 (UTC)

When another ice age rolls around and the sea level drops again, the reefs will die, but they won't disintegrate, and the Maldives will grow until the sea level starts rising again. Whoop whoop pull up ^{Bitching Betty | Averted crashes} 23:30, 10 September 2012 (UTC)

Eustatic sea level is a tricky subject these days. There is much proxy data but if coral growth is factored in, then paleo-environmental sea level readings are perhaps unreliable in those locations. ~AH1 ^(discuss!) 18:14, 11 September 2012 (UTC)

Cat

Can someone identify the brownish cat in the 21st (or third from bottom) picture in this article for me - [5]? - JuneGloom Talk 19:44, 10 September 2012 (UTC)

It's a caracal. μηδείς (talk) 20:01, 10 September 2012 (UTC)

I just ECed with Medeis, but was about to give the same answer. It looks like a caracal, or perhaps a cross including the caracal, such as the Caraval. --Jayron32 20:06, 10 September 2012 (UTC)

Our article suggests the caraval is spotted. Not to be confused with the also quite handsome Caracalla. μηδείς (talk) 20:43, 10 September 2012 (UTC)

Thank you guys. I couldn't remember what it was, but knew I'd seen one before. I assume the one in the article is domesticated and lives alongside the Cheetah. - JuneGloom Talk 23:07, 10 September 2012 (UTC)

Plate tectonics censored?

(Not sure if this is the right section.) From the agnatology article: "For example, knowledge about plate tectonics was censored and delayed for at least a decade because key evidence was classified military information related to underseas warfare." I can't find any reference to this online, nor am I sure what is meant by 'classified military information related to underseas warfare' in this case. Ratzd'mishukribo (talk) 21:08, 10 September 2012 (UTC)

Well, knowledge of the location of undersea trenches and ridges is important if you want to hide a sub from enemy detection, and those are caused by plate movements. And, when you plot all of those trenches and ridges, it's clear that they are like the seams on a baseball, not just the result of isolated events. StuRat (talk) 21:17, 10 September 2012 (UTC)

Much of the knowledge of the sea floor was gathered through military surveys, and the military's default state is "classify everything we know". It wasn't until the 1950s that a significant civilian effort to map the sea floor was undertaken. However, I don't know that the suppression was all that significant or that it was as widespread and obfuscatory as that statement makes it out to be. Harry Hammond Hess, a U.S. naval sonar operator and later geologist, was key in developing the modern theory of Plate Tectonics. I don't know that he was deliberately censored. --Jayron32 21:35, 10 September 2012 (UTC)

So if that is what was meant, is there indeed any source to the allegation that it was ever censored, or perhaps someone needed an example of subject matter for the the topic "agnatology"? Ratzd'mishukribo (talk) 02:25, 11 September 2012 (UTC)

'Censored' is the wrong word, to quote from a book by Naomi Oreskes "Another group at Lamont had focused on bathymetric data - measurements of the depth of the sea floor - primarily in the Atlantic. These data were highly classified, but Bruce Heezen (1924-1977) and Marie Tharp had found a creative means around security restrictions: a physiographic map, essentially an artist's rendition of the what the sea floor would look like drained of water, based on quantitative measurements, but without actually revealing them."[6]. So even if the data were classified, researchers still had access to them and found ways around the restrictions. Mikenorton (talk) 04:24, 11 September 2012 (UTC)

Is the concept of "hiding submarines among ridges" real or happens only in The Hunt for Red October (film)? I would have thought that mid-ocean ridges are way deeper than where submarines can go (a couple of deep sea research vessels excepted, and assuming they want to come back up.) 88.112.47.131 (talk) 05:50, 11 September 2012 (UTC)

They aren't all deep. Iceland is on top of one. See Iceland#Geology. StuRat (talk) 06:03, 11 September 2012 (UTC)

And do submarines hide there among rocks in real life? 88.112.47.131 (talk) 13:12, 11 September 2012 (UTC)

This article may be of significant yet peripheral interest: Timeline of the development of tectonophysics (after 1952). ~AH1 ^(discuss!) 18:12, 11 September 2012 (UTC)

I wouldn't expect so in peacetime, unless for training purposes, as it's risky. However, in war, when the enemy is out to destroy your sub, that changes the risk/benefit equation. StuRat (talk) 20:45, 11 September 2012 (UTC)

This page mentions wartime secrecy about plate tectonics evidence - essentially as StuRat and Jayron wrote - but also without a source. Ratzd'mishukribo (talk) 02:17, 12 September 2012 (UTC)

September 11

Auric reactivity

What is the reactivity (measured in mol m^-2 s^-1) of gold towards hydrochloric acid, as a function of acid concentration; system pressure, and temperature?

I am planning to run an experiment using thin-film, gold coated glass electrodes. I need to establish the relationship between the film thickness (measured in tens of nanometres) and the usefull lifetime. Plasmic Physics (talk) 10:25, 11 September 2012 (UTC)

Gold should be completely unreactive towards HCl. It will react with aqua regia to produce Chloroauric acid, which can also be formed via electrolysis in the presence of just HCl. But you should be able to safely keep gold in a solution of HCl unperturbed, and you could wait centuries and not notice any change. So, if your gold does react, it will do so because of electrolysis. You can calculate how fast the gold reacts via electolysis using a form of the Nernst equation known as Faraday's law. In that equation, Q is the total electric charge imparted to the gold, which can be calculated if you know the amperage of your input current (amperage = coulombs per second). So there is a direct relationship between amperage and rate of reaction. However, if you just want to know how long a piece of gold will last in an inert solution of HCl, the answer is "about forever". --Jayron32 12:21, 11 September 2012 (UTC)

The soluble equilibrium of gold in water at room temperature/pressure is around 10^-22 kg of gold per kg of H₂O, which is entirely negligible for nearly all purposes. But since you asked about pressure and temperature, it is worth noting that this increases to as much as 10^-7 kg of gold per kg of H₂O at 400 C and 1000 bar. Not sure about the reaction rate. I don't know how to quantify the effect of HCl, though I would expect the solubility to still be negligible near room temperature / pressure. The caveat here, as Jayron32 suggests, is the potential for electrolysis. Au⁺³ will readily react with HCl, and so a current carrying electrode is likely to erode quickly if the current is high enough. Using Faraday's law, referenced above, this comes to roughly 10^-6 mol of gold per second per amp. The compound AuCl₃ remains soluble in solutions up to several percent, so there shouldn't be any problem with the current completely dissolving the gold. Of course, if you are working at low currents it may not matter. Dragons flight (talk) 17:19, 11 September 2012 (UTC)

Good, I will use it only for the cathode then? What should I use for an inert anode? Plasmic Physics (talk) 23:29, 11 September 2012 (UTC)

Before you suggest graphite, it doesn't work very well, the electrode disintegrates rapidly, and poses an explosion hazard at high current. Plasmic Physics (talk) 01:45, 12 September 2012 (UTC)

It might help if you specify the system more completely. In other words, what currents / voltages, what temperature / pressure, and whether any compounds beside HCl and H2O are present and their concentrations. Dragons flight (talk) 01:52, 12 September 2012 (UTC)

Indeed. I should note that the "classic" inert electrode is usually platinum; it is used as the inert electrode for the Standard hydrogen electrode. --Jayron32 03:49, 12 September 2012 (UTC)

Will I not have the same problem with platinum as I will have with gold? Plasmic Physics (talk) 05:56, 12 September 2012 (UTC)

The system involves current and voltage on the dA and dV scale, respectively. Temperature will vary between 25 and 145 degrees Celsius, pressure will remain at ambient levels. Other compounds present are varied metal chlorides, and hydroxides. Plasmic Physics (talk) 05:56, 12 September 2012 (UTC)

Socialization and digestion

Hi. This will be largely based on anecdotal evidence, but is there any actual research to support the idea that socializing in a group actually aids individual digestive ability, say over lunch? I ask this because I'm a slower eater during periods of social withdrawal and potentially a faster eater when socially active. Furthermore, there is also the observation that people are less talkative when hungry - does this mean that eating and social behaviour are intertwined, and if so, could it potentially explain the rise of eating disorders such as anorexia and bulimia across the Western world? Thanks. ~AH1 ^(discuss!) 17:51, 11 September 2012 (UTC)

I think this would be a function of how stressful the social interactions are, since activation of the sympathetic nervous system inhibits digestion whereas activation of the parasympathetic nervous system promotes it. Looie496 (talk) 18:16, 11 September 2012 (UTC)

On the other hand, stress often leads to overeating. StuRat (talk) 18:19, 11 September 2012 (UTC)

Note that eating more quickly in a group may be a psychological leftover of having had to eat before the food was gone, especially if you grew up in a large family, where the slow-pokes often get the less desirable food.

I've also developed the habit of eating quickly, and asking that my food be brought out as soon as possible, as opposed to waiting for everyone else's meals to be ready. This is because I order salads, which take much longer to eat (due to the low calorie-to-volume ratio) than, say, a burger, and I hate being the one holding everyone up while they stare at me trying to finish. StuRat (talk) 18:22, 11 September 2012 (UTC)

Also note that the rate at which you eat isn't necessarily correlated with the rate at which you digest, both because our digestive system can store food for later digestion, and since we can also digest food to a variable degree. StuRat (talk) 18:32, 11 September 2012 (UTC)

There are genetic factors too, some rats have a particular gene eats all the time and become fat while other given the same amount of food but not fat. --RexRowan (talk) 18:54, 11 September 2012 (UTC)

Don't know about digestive ability, but social facilitation claims that people eat more when in company: Furthermore, males ate 36% more food when with other people than when alone, and females ate 40% more food when with other people than when alone. Guess I'm an exception to the rule, as usual. Psychology ... Ssscienccce (talk) 21:16, 11 September 2012 (UTC)

Well, if you wanted to be sssscientific about it, you would have to do a little more then simply "reflect" on your past behaviour.. It's quite plausible that the people who eat more when in company don't actually realize they're doing it, they might even think they're eating less since they are distracted, maybe that's what leads them to eat more. Vespine (talk) 22:14, 11 September 2012 (UTC)

The way to a man's heart is through his stomach. μηδείς (talk) 02:39, 12 September 2012 (UTC)

Does the 'lensmaker's equation apply to non-Gaussian / fast / large-aperture lenses?

According to Gaussian optics, the paraxial approximation is used to derive simple explicit formulate for focal length, magnification and brightness. Is the paraxial approximation used in the lensmaker's equation? 71.207.151.227 (talk) 22:39, 11 September 2012 (UTC)

Any equation that uses focal length is paraxial. Focal length is a paraxial optics property. In real optics, you have to trace the path of each ray separately, and they will come to a focus at different distances depending on where they strike the lens.--Srleffler (talk) 17:15, 12 September 2012 (UTC)

why isn't MSG produced in neutral form?

I need to dissolve MSG in oil, but in order to do that I need to make it neutral by adding vinegar, which is a pain because I then have to decant the oil from the vinegar, and I suspect the oil/water partition coefficient for MSG might favour the vinegar. Which makes me think -- why is MSG produced as a sodium salt anyway? Is it because if its synergistic taste reaction with sodium? What does MSG taste like neutral? 71.207.151.227 (talk) 22:54, 11 September 2012 (UTC)

It's not just MSG, many spices come with sodium/salt, often far more sodium than the actual spice. I suspect that this is because sodium/salt tastes good, and is cheap. Thus, if you sell your glutamate as MSG (with sodium), you will make more money than if you sell it alone (as glutamic acid (flavor)). There's also (self-)deception involved, as people are trying to avoid sodium, but don't think of those other things they are shaking into their food as containing sodium, or at least not much, when they really do have a lot. StuRat (talk) 22:59, 11 September 2012 (UTC)

Curious how you're going to make monosodium glutamate without sodium. He's not saying he doesn't want the sodium. He's saying he wants something acidic included, to balance the alkalinity of the MSG. At least I think that's what he's saying -- original poster, is that right? --Trovatore (talk) 23:02, 11 September 2012 (UTC)

Oh, looks like I didn't read the whole thing. "What does MSG taste like neutral" doesn't really make sense — what you seem to be asking is "what does glutamic acid" taste like. I don't know the answer to that. --Trovatore (talk) 23:07, 11 September 2012 (UTC)

According to the article I linked to above, both give you the glutamate taste, "However, crystalline glutamic acid salts such as monosodium glutamate dissolve much better and faster than crystalline glutamic acid, a property important for use as a flavor enhancer." Presumably this refers to the ability to dissolve it in water. I'm not sure about dissolving it in oil or alcohol. StuRat (talk) 23:10, 11 September 2012 (UTC)

(edit conflict)How is vinegar supposed to neutralise monosodium glutamate which is also a weak acid? Last time I checked you need a base for that. MSG without sodium is just G, or glutamic acid. MSG is not equivalent to G. Plasmic Physics (talk) 23:09, 11 September 2012 (UTC)

It has a free amino group which is just begging to be acidified. Vinegar won't protonate the COO- group, it will protonate the RNH2 group. 71.207.151.227 (talk) 00:03, 12 September 2012 (UTC)

Oh yes, I forgot about that. Plasmic Physics (talk) 01:40, 12 September 2012 (UTC)

• The distinctive flavor of glutamate is called umami -- quite a number of scientists believe that it is a fifth basic taste, in addition to sweet, sour, salt, and bitter. Looie496 (talk) 23:16, 11 September 2012 (UTC)

How is neutralising MSG going to solubilise it? Plasmic Physics (talk) 01:42, 12 September 2012 (UTC)

Fat itself is now believed to be the sixth 'taste', richness. Our articles on these matters are not very comprehensive. μηδείς (talk) 02:38, 12 September 2012 (UTC)

I don't think fat is desired so much for it's flavor as it's smooth texture. StuRat (talk) 04:06, 12 September 2012 (UTC)

I think you left out a comma: "I don't think fat is desired so much, for it is flavor as it is smooth texture." μηδείς (talk) 05:05, 12 September 2012 (UTC)

We have fat receptors. It's not just texture. Anyway, neutral glutamic acid will probably dissolve in oil (just like sugar); but charged salts won't. I'd like to store it oil form because I'm going to prepare an oily condiment mixed with other fat-soluble ingredients. 71.207.151.227 (talk) 04:21, 12 September 2012 (UTC)

Except that neutral glutamic acid isn't neutral, it is zwitterionic. --Jayron32 04:51, 12 September 2012 (UTC)

Doesn't that depend on the conditions such as the nature of the solvent? In either case, both zwitterionic and non-zwitterionic glutamic acid should be poorly soluble in oil on account of its polarity being very much stronger than oil. Plasmic Physics (talk) 04:56, 12 September 2012 (UTC)

Sugar, which is polar, dissolves easily in hot oil. 71.207.151.227 (talk) 04:57, 12 September 2012 (UTC)

It would be considered neutral in the HPLC. For example in my lab, we have an HPLC optimised to detect charged neurotransmitters like dopamine. The non-decarboxylated L-DOPA washes out with the acetonitrile solvent front. The charged amines are retained by the alkylsulfonate solid phase. L-DOPA, which is zwitterionic, is sufficiently neutral to the blood-brain barrier and passes through easily, whereas dopamine doesn't. 71.207.151.227 (talk) 04:57, 12 September 2012 (UTC)

• Alrighty. Lets end this once and for all. This paper is 80 years old, but is highly relevent, being titled "THE SOLUBILITY OF GLUTAMIC ACID IN WATER AND CERTAIN ORGANIC SOLVENTS". Solubility clearly decreases as the size of the hydrophobic chain increases; it is most soluble in water, less soluble in methanol, even less soluble in ethanol, and essentially insoluble in acetone. So no, neutral glutamic acid is highly unlikely to be very soluble in oil, given those trends. --Jayron32 05:04, 12 September 2012 (UTC)

I assume Jayron's source is authoritative, but at least according to this website acetic acid will cause glutamic acid to precipitate from a saturated solution of monosodium glutamate. http://www.microscopy-uk.org.uk/mag/artsep08/rp-glutamic.html μηδείς (talk) 05:16, 12 September 2012 (UTC)

Well, it's all relative. MSG is more soluble in water than glutamic acid, so acidifying a saturated solution of MSG should cause glutamic acid to precipitate. The article gives the solubility figures in grams/50 cc of solvent, but converting to standard units for solubility, which is usually grams/liter, gives values of about 8.8 grams/liter for gram solubility from that article, a figure confirmed by Wikipedia's own article Glutamic acid. Monosodium glutamate is about 9 times as soluble, at 74 grams/liter. --Jayron32 05:36, 12 September 2012 (UTC)

As someone said right off, glutamic acid (flavor) is available. I can't make sense of this site's prices, which are all over the map, but here's an example of lots of people selling it in up to ton quantities. [7] Our article says that stuff like this can be labelled as simply "natural flavor" when it doesn't have the monosodium bit, which would explain how they sweep that one under. It is true that a salt is best suited for a polar solvent i.e. water, and the neutral acid better suited to dissolve in oil. Wnt (talk) 04:27, 13 September 2012 (UTC)

September 12

libya

Moved to Wikipedia:Reference_desk/Miscellaneous#libya - StuRat (talk) 01:14, 12 September 2012 (UTC)

Tootbrush disinfection

I tried bleach on my toothbrush to kill the germs, but, unfortunately, the bristles fell out. I'm considering trying either alcohol or hydrogen peroxide on my new toothbrush. So, will those also destroy my toothbrush ? StuRat (talk) 01:20, 12 September 2012 (UTC)

I've used H2O2 (three percent) and do not recall it having any negative effects on the bristles, at least in a shorter time frame than you're supposed to replace it anyway. Of course I haven't actually tested whether it kills the germs. Also, I'd rinse it well before putting it in your mouth. --Trovatore (talk) 01:28, 12 September 2012 (UTC)

Hydrogen peroxide is not very good as a disinfectant, as I understand it. Alcohol should do the trick, and I doubt it will cause much damage to the bristles, as opposed to bleach, which depending on the concentration is going to be a great deal more corrosive than alcohol. Evanh2008 ^{(talk|contribs)} 01:30, 12 September 2012 (UTC)

Um, why are you worried about germs on your toothbrush? Unless you are putting it to unconventional use, the only place it is likely to get germs from is your mouth, and all you will be doing is putting them back (quite possibly dead) the next time you use it. Still, if you are that worried, according to WikiHow, who cite the American Dental Association, washing it under the tap both before and after use is sufficient most of the time, and you can place it inside a dishwasher occasionally for a deep cleanse: [8] I'd be wary of using a solvent like alcohol, as again the bristles might fall out, and neither bleach nor hydrogen peroxide (in germ-killing concentrations) sound particularly sensible substances to use on anything you intend to put in your mouth, even if they don't damage the toothbrush. AndyTheGrump (talk) 01:37, 12 September 2012 (UTC)

Actually, it's a serious concern, for sort of disgusting reasons, if you keep your toothbrush where most people do (in the bathroom). I'm sure someone will be along to explain any minute now. --Trovatore (talk) 01:40, 12 September 2012 (UTC)

One would suspect that if casual contamination or harm from germs in the bathroom was anywhere near a common occurrence to be a "serious concern" that it would 1. be something the ADA would mention it, 2. someone would sell a product for it, and 3. we'd all know people who routinely were injured in this fashion, probably including ourselves. I'm going to file this under "more things that StuRat worries about that nobody else does" unless there is some real scientific evidence of probability of harm. --Mr.98 (talk) 01:53, 12 September 2012 (UTC)

Harm is not so clear, but contamination, yes. I'm not going to search for a cite from work, but this has been studied. Basically, flushing aerosolizes some of the contents of the toilet. Whether that's a problem depends, I suppose, on who shares the bathroom with you, and what they put in the toilet. --Trovatore (talk) 01:56, 12 September 2012 (UTC)

Closing the lid before you flush should greatly diminish this problem. Lova Falk talk 08:23, 12 September 2012 (UTC)

... or keep your toothbrush in a closed container or small bag, or store it outside of the bathroom altogether. Seems to me there are much simpler and safer solutions to this problem than dipping your toothbrush in bleach etc. Gandalf61 (talk) 14:31, 12 September 2012 (UTC)

On the other hand, the ADA suggests storing it in a closed container may not be good idea [9]. But see below.... Nil Einne (talk) 18:44, 12 September 2012 (UTC)

You can also put your tootbrush in the microwave. Count Iblis (talk) 01:41, 12 September 2012 (UTC)

I'm not sure that would be a good idea. Apparently in some toothbrushes at least, the bristles are held in with tiny metal staples. Then again, I'm not sure that microwaving germs will necessarily kill them anyway - are you proposing to immerse the brush in water as you microwave it? AndyTheGrump (talk) 01:48, 12 September 2012 (UTC)

(ec)Erm, technically this is a medical device, and suggesting methods of modifying it is medical advice. It's conceivably possible that you could leach out the plasticizer and make the bristles harder than the dentist would recommend, or that hydrogen peroxide could react with it to form some carcinogen, etc. Therefore I will say do not rely on any suggestions here for what you do with it. Wnt (talk) 01:51, 12 September 2012 (UTC)

Humm, maybe you're right - though 'wash it with tapwater' is hardly medical advice I'd have thought... AndyTheGrump (talk) 01:54, 12 September 2012 (UTC)

Well, personally I think that answering a medical question with a link to a professional medical organization's statement on the topic should always be permitted, but as we know there's a range of opinion on that around here... Wnt (talk) 02:02, 12 September 2012 (UTC)

And in the best tradition of science reference desk original research, I can confirm that in some toothbrushes at least, a metal doo-dah is indeed used to retain the bristles. I've just sawn part of the head off one (used, needless to say - I'm not made of money ;-) ) and found a tiny sliver of metal, about 2mm x 1.2mm x 0.2mm (roughly - I've not got the tools to measure accurately) at the bottom of the brush-holes. AndyTheGrump (talk) 02:09, 12 September 2012 (UTC)

• This is all ridiculous. The mouth naturally contains a massive contingent of bacteria; a toothbrush can't possibly make a meaningful difference. Looie496 (talk) 02:19, 12 September 2012 (UTC)

Obviously the toothbrush could make a difference. If you were living in Haiti and your toothbrush was five feet from the toilet somebody with cholera just used, you'd be reaching for the bleach! (Nay, come to think of it, in that situation who needs to brush...) Counting total numbers of bacteria is a common fallacy. Wnt (talk) 02:39, 12 September 2012 (UTC)

Right, it's not just how many bacteria, it's which bacteria. Inside the mouth, the bacteria will normally be kept in check, but outside, you have a wet toothbrush with mouth bacteria, bits of food, and various toilet flushings sitting around like a petri dish. God only knows what will grow there. The ADA does suggest replacing brushes constantly, but I'm looking for a cheaper alternative. StuRat (talk) 03:40, 12 September 2012 (UTC)

There appears to be a fair amount of dispute of whether you really need to replace your toothbrush due to bacterial contamination. There appears to be little dispute you probably should do it due to physical degradation. See [10] which while a forum discussion with a fair amount of typical forum nonsense does include some sourced information, include that the CDC say there is no evidence reusing toothbrushes can lead to (re)-infection. Nil Einne (talk) 08:15, 12 September 2012 (UTC)

It's true that there are common soil microbes that can cause disease, like botulism or blastomycosis. But by and large, there is no evolutionary incentive for environmental bacteria to be harmful. So far as I know, most of the common food poisoning bacteria (like Salmonella or EPEC) come from humans or related organisms. And it seems hard to argue with the empirical observation that (unless someone ill is present in the environment) people don't generally seem to get sick from their toothbrushes, not even counting the demonstrable particulates from the toilet. It is possible to argue that theoretically there is some unnoticed rate of illness, but balance that against the hygiene hypothesis of asthma, i.e. that lack of exposure to microbes might be harmful. In the end we live in a world where there's a certain amount of turd in everything, whether it's from dust mites or the Old Faithful toilet. Wnt (talk) 15:14, 12 September 2012 (UTC)

I don't think there could be a cheaper alternative to replacing toothbrushes constantly because dental care costs money too. The bristles also splay out after some use, making them less effective. But shaking out water after use and allowing to dry in circulating air seems like a good idea. There are two-half plastic holders for toothbrushes which have holes in them for air circulation. Sunlight would seem to be a good disinfectant but I haven't tried displaying my toothbrush on the windowsill yet. Apparently ultraviolet light is used for this purpose too. Vinegar as well as "3-percent hydrogen peroxide" are mentioned as possibilities here as well. Bus stop (talk) 15:37, 12 September 2012 (UTC)

BTW I came across this earlier, it didn't seem that useful being a paid Colgate advert, but I now notice it mentions a study suggesting how much the effectiveness of a toothbrush can change over time [11]. Also looking at the ADA's current statement [12], even they don't seem to suggest contamination is a big reason for replacement, they too concentrate on physical degradation. (The replace due to contamination primarily comes in to play after you recover from certain diseases.) A number of suggested methods for cleaning a toothbrush are mentioned in the forum post I linked above including a dishwasher and mouth wash, but personally I'm with the person in this source [13]. Doing anything other then clean with room temperature water runs the risk you're going to damage the toothbrush more. (This includes sunlight/UV light.) And considering we have little real evidence of the risks of contamination, but evidence of the negative effects of physical degradation, there's a good chance you're causing more harm then you're preventing. Nil Einne (talk) 18:44, 12 September 2012 (UTC)

Buy a new toothbrush? 203.112.82.128 (talk) 17:08, 12 September 2012 (UTC)

Soak the bristles of the toothbrush in a small amount of mouthwash. — Preceding unsigned comment added by 148.177.1.210 (talk) 17:19, 12 September 2012 (UTC)

Mouthwash is a good idea, especially since it's designed to kill oral bacteria. I'll try that. StuRat (talk) 20:19, 12 September 2012 (UTC)

I should probably mention the reason I first used the bleach. My toothbrush smelled bad, and I didn't want that smelly thing in my mouth. To me, this was most likely the result of bacterial growth. StuRat (talk) 20:19, 12 September 2012 (UTC)

Medical advice or not, it's safe to say that if your toothbrush smells bad, it's high time to get a new one. ←Baseball Bugs ^{What's up, Doc?} carrots→ 03:59, 13 September 2012 (UTC)

Or disinfect it, hence this Q. StuRat (talk) 04:16, 13 September 2012 (UTC)

Or stop cleaning your teeth, if you are really concerned that the apparently-minor risk of something nasty sneaking in on your toothbrush out-weighs the more obvious risks of not cleaning them - including quite possibly bad breath, which has the advantage that you are less likely to come into close contact with other germ-carrying individuals. Your choice... AndyTheGrump (talk) 04:26, 13 September 2012 (UTC)

Did you read the referenced discussion above? It's not clear how disinfecting it is going to help the fact that by the time it starts to smell bad, it's likely sufficiently degraded to require replacement. If you only brush your teeth because someone will nag you otherwise, then I guess disinfecting it might help since there's a chance after several years there might actually be some risk from contamination. But if you're like most people and brush your teeth to actually clean your teeth and reduce the risk of dental disease, then the merits of disinfecting remain unclear. There's also BusStop's point. Perhaps you have gold plated dental insurance which you don't pay for or something, or you live in some odd corner of the US where toothbrushes cost a few hundred dollars. If not, the cost of a new toothbrush could easily be 2 orders magnitude less then a single visit to the density or extra surgery required. So the cost savings from not replacing your toothbrush every few months are fairly unclear as avoiding even a single extra surgery or visit in 10 years could easily pay for replacing your toothbrush every 2 months in the same timeframe. Nil Einne (talk) 04:36, 13 September 2012 (UTC)

If I wait 2 months, my toothbrush stinks. I will take the advice of soaking it in mouthwash each day. This should cost a penny a day and keep it smelling fresh. I will mark this Q resolved. StuRat (talk) 04:59, 13 September 2012 (UTC)

Where I live, I can get toothbrushes 4 for a dollar at the local dollar store. Certainly, I can pay more, but with the cheapest toothbrushes I can buy, I could replace once a month at a cost of less than a penny per day. At that level, the cost of replacing a stinky toothbrush shouldn't be an issue. --Jayron32 05:02, 13 September 2012 (UTC)

It would still stink by the end of that month, while it won't if I soak it in mouthwash. I also question the quality of a 25 cent toothbrush. StuRat (talk) 05:31, 13 September 2012 (UTC)

Fair enough. But even the fancy high-end toothbrushes with the allleatherinterior and the power moon roof still cost like, what, $5.00? At some point the penny pinches back... --Jayron32 05:36, 13 September 2012 (UTC)

If the toothbrush becomes rancid quickly, the problem might not be the toothbrush. It's time for StuRat to pay a visit to his dentist. ←Baseball Bugs ^{What's up, Doc?} carrots→ 11:26, 13 September 2012 (UTC)

Resolved

Global Ischemia and Death

I'm wondering about this--are people with global ischemia (who get revived) completely dead for several minutes or just don't have a functioning brain (brain activity) for those several minutes while the rest of their body works properly? Also, people with global ischemia can have their whole "mind" (personality, likes, hobbies, etc.) restored if they had global ischemia for several minutes or less, right? Futurist110 (talk) 02:18, 12 September 2012 (UTC)

It helps if you provide a link to what you are talking about. μηδείς (talk) 02:34, 12 September 2012 (UTC)

(ec)Hmmm... brain ischemia#global brain ischemia speaks of restoration after loss of heartbeat. Clearly neurons don't die the instant blood stops pumping - it takes time for the oxygen to run out, among other problems. The death is not a physical law - it can be altered by methods such as treatment with sodium valproate. [14] Deciding when a cell is dead is an example of something that is very, very difficult to define philosophically but all too easy to measure by experiment... Wnt (talk) 02:36, 12 September 2012 (UTC)

• If they come back, their brains did not entirely cease to function. If electrical activity in the brain completely disappears (sometimes known as "flatlining"), the patient is dead, dead, dead, never to return, end of story, dead. Looie496 (talk) 05:29, 12 September 2012 (UTC)

Actually our article on brain death gives the impression that this is not absolutely certain, though with the caveat that the activity might simply not be detectable with the equipment used. It's unsourced there - ought to come up with better references... Anyway, in theory there's no reason why neurons couldn't stop firing entirely for some reason (a drug that blocks the action potential) and yet, if life support can be maintained, return later to normal function. Wnt (talk) 15:35, 12 September 2012 (UTC)

I was under the impression that electrical activity in the brain stopped completely with global ischemia? Am I wrong? Futurist110 (talk) 23:04, 12 September 2012 (UTC)

Late summer sky color

I have often noticed that in the Eastern and Midwest U.S., the sky takes on a hauntingly beautiful shade of blue in the late summer and early fall (this year, starting August 13). The sun takes on a similarly more attractive shade of golden, and together, the impact of the blue and golden on the dark vegetation of summer can be just breathtaking. At night, especially later in the season, the sky can take on a uniquely "electric" indigo shade. What's odd is that it seems so specific to this season, yet it can occur in 90-degree weather or 50-degree weather, in cloudless sky, sky half filled with cumulus clouds, sky marked with the strange swirls of cirrus that came with this week's unusually cold weather. I still do not even know for certain that it is not some purely psychological phenomenon on my part. Is this effect something known? And if so, is it explainable? Wnt (talk) 02:55, 12 September 2012 (UTC)

I don't have a specific answer, but in general the specific color of the sky is highly dependent on the exact angle which the sun's rays are striking the atmosphere, and this angle will be highly dependent on both the time of day and the time of the year. The coloration of the sky is largely due to processes called Rayleigh scattering and Diffuse sky radiation, and accounts for the blue sky at midday and also the red sky which appears just before sunset. So, I don't know exactly what combination of effects causes the specific shade of blue you are noting (though your vivid description brought it to my mind as well!), except to direct you to the general processes that create the sky's color; it is likely just a specific combination of the normal variables that affect the color of the sky in general. --Jayron32 03:47, 12 September 2012 (UTC)

Just a thought: Do you happen to go out at the same time each night, for a walk, say ? If so, then this might be the time of year when your walk time corresponds with sunset. There should be a second time in spring, but maybe it tends to be rainier then. StuRat (talk) 04:03, 12 September 2012 (UTC)

No - the color varies by time of day, but it can reach this unique intensity at any of them - from the deep blue of a bright noonday to the deep indigo of late twilight. Wnt (talk) 05:01, 12 September 2012 (UTC)

Funny, I remarked the exact same thing to myself tonight, and assumed it was a combination of the humidity and the angle of the sun. μηδείς (talk) 04:07, 12 September 2012 (UTC)

Particulates high in the atmosphere can cause pretty sunsets. These are caused by volcanoes, forest fires, and perhaps sandstorms. Volcanoes shouldn't vary with the weather, but fires might, and sandstorms in Africa do sometimes carry particles over to the US. StuRat (talk) 04:22, 12 September 2012 (UTC)

The sky I saw tonight went from a brilliant cyan with magenta clouds just before sunset to a rich emerald and then a deep teal after. I don't think there's anything like a volcanic eruption going on in NJ. I though particulates usually cause red sunsets. Is there some current eruption or fire in the Northern Hemisphere that has been reported as causing brilliant sunsets? My suspicion is what I saw was more due to the clarity of the sky and the angle of the sun, but I have never seen it greener. I also remember the sky on 9/11 being one of the clearest and most brilliant I have ever seen. μηδείς (talk) 05:00, 12 September 2012 (UTC)

"Strike as non-responsive."

You live in New Jersey and need to wonder where particulate matter in the air comes from? It comes from New Jersey. Let me quote George Carlin "If she asks you to kiss her where it smells funny, take her to New Jersey". --Jayron32 05:29, 12 September 2012 (UTC) That comment is as parochial and stupid as not realizing that the sunset from Manhattan lies over New Jersey. μηδείς (talk) 05:41, 12 September 2012 (UTC) Parochial? Yup. Stupid? Yup? Funny? Damn skippy. --Jayron32 05:45, 12 September 2012 (UTC) Frankly, your comments are no different from Polak jokes, the neighbor the Germans like to pick on. Given you've obviously driven through NJ, you tell me how many minutes of Newark refineries you've driven past (with their steam condensors producing not a drop of particulates, BTW) compared to hours of tree-lined greenery along 295 and the Turnpike? Next you'll be joking about backwoods Vermonters and pig-fucking Appalachians, but in small type, thank God. μηδείς (talk) 06:02, 12 September 2012 (UTC) Frankly, it was supposed to be funny and not true. If you want to know, I've spent large parts of my life in many parts of New Jersey, not to live, per se, but also not just driving through. I have vacationed many times on the Jersey Shore, Ocean City and Long Beach Island and Atlantic City and Wildwood, spent some time with friends in the Pine Barrens and Vineland. I have other friends in the Philly suburbs in the southwestern part of the state; My wife went to Washington Township High School, and I've been to Deptford and Cherry Hill several times to meet friends of hers that still live in the area. It was your basic suburb: not offensive, but not all that interesting either. I've also been to Newark, both NJIT a few times, and downtown; it wasn't that bad of a city. As far as driving, I find the traffic on the turnpike maddening and the scenery rather unappealing, so I in the past decade or so I have taken to driving through northwestern New Jersey. The I-78 - I-287 corridor is some of the bost beautiful scenery I've seen, on I-78 the slow decent from the Delaware Valley is nice, but the cuts through the mountains north of the Parsippany-Troy Hills-Morristown area are fantastically awesome; that tight little area on the NY border where I-287 meets up with 17 and the Thruway is also really quite a site to see. All of this is irrelevent to actually constructing a decent joke. Yes, it was a trite and well trodden joke, but saying "You know, New Jersey has some really diverse and nice areas, the beaches are nice, if a bit over developed in places, and the hilly areas of the northwestern part are really gorgeous. But sometimes it smells bad in the refineries and industrial areas of Patterson and Elizabeth" That's not funny. Was the joke rude and inaccurate. Yes. That's what makes most jokes funny, is a little bit of inappropriateness and exageration. If it didn't have those elements, it wouldn't be a joke. So, don't extrapolate someone's life experiences from a joke. The joke is not supposed to be an insight into anything, it's not supposed to be an scrupulously accurate thing. It's supposed to be funny. I also don't necessarily need you to find it funny yourself, humor is personal and I get that. But I would expect an otherwise intelligent person to understand a joke for a joke, and to at least recognize the purpose and proper construction and meaning of a joke, even if they didn't find it personally all that funny. --Jayron32 15:32, 12 September 2012 (UTC)

It's fair humor - I flew out of Newark a couple of times in the 80s and the city deserved the gibes then; maybe they helped prod it to clean up its act (which as I understand, it has done significantly) and if so, by virtue of saving many human lives, these jokes' ethics are beyond reproach. And for our question, artificial sources of particulates certainly should be considered seriously. That said... I'm not sure any industrial pollutant would be worse this particular time of year, though the great decrease in rainfall should reduce the amount that is purged from the atmosphere. My gut feeling, of course, is that pollution couldn't possibly be so pretty, but that's not a scientific argument. :) Wnt (talk) 14:54, 12 September 2012 (UTC)

I think of this as a regular yearly phenomenon (subjectively to me, this effect together with the singing of cicadas and katydids define a season of "latesummer"). So I would discard thoughts of an eruption out of hand. Sahara dust is my leading candidate, but quick searching points to it creating haziness in Florida [15] and being limited to the South [16]. Even so, I suppose that a super fine subpopulation of the dust might make it further north and have these effects? But without seeing proof, I don't really know. Wnt (talk) 05:16, 12 September 2012 (UTC)

Yes, it needs to be a very light layer of particulates, or you just get a dark sky. It's like the difference between light mist causing a rainbow and torrential rains just making the sky dark. StuRat (talk) 06:58, 12 September 2012 (UTC)

Medeis: do you mean the Green flash? Ssscienccce (talk) 16:15, 12 September 2012 (UTC)

No, I couldn't actually see the sun itself when it set, the entire sky was a distinctly greenish shade near sunset, the likes of which I have seen on maybe two or thee other occasions, somewhat like the teal green in our article, but luminous, of course. And I am not sure how true the colors are on my computer, so don't 'quote' me on that. It will be interesting to see if the weather is the same today. μηδείς (talk) 17:16, 12 September 2012 (UTC)

The sky was very similar again last night, with green, orange and grape-soda purple at the horizon about an hour before sunset, depending on which direction you were looking in, although I was too busy to see the color overhead at sunset. Not a single cloud was visible, which makes me think the humidity and sun angle are relevant. μηδείς (talk) 17:53, 13 September 2012 (UTC)

Janet Armstrong

What happened to the first wife of Neil Armstrong after her divorce? I've looked on the internet but she's not been mentioned except in one New York Times article about events in Neil's life where it briefly mentions that she lives in Utah. I'm wondering if she ever re-married and when she ended up moving. If this is in the wrong place, please just move it to the correct area and write where you put it on my talk page; I wasn't sure if this counted as humanities (because it was biographically related to Neil Armstrong's life) or here (because Neil Armstrong was the first man on the moon) so I chose here. --Thebirdlover (talk) 03:30, 12 September 2012 (UTC)

Doing a people search on Janet Armstrong and her maiden name, Janet Shearon, I only found one that seems old enough, a 75 year old Janet Ann Armstrong, who has lived in Pahrump, Henderson, and Las Vegas, NV, as well as Apache Junction, AZ and Stockton, MO. No guarantee this is her, but it's possible. Does anyone know if her middle name was Ann ? StuRat (talk) 03:59, 12 September 2012 (UTC)

Not the right one. His first wife was Janet Elizabeth Shearon, you might need to scroll down a bit. CambridgeBayWeather (talk) 23:58, 12 September 2012 (UTC)

Use of question mark and exclamation mark above equals signs

I have seen the notation ${\stackrel {?}{=}}$ and ${\stackrel {!}{=}}$ used before, and I have forgotten where. (It may have been in a highschool math textbook, maybe)

$X{\stackrel {?}{=}}Y$ : "we would like to show that X equals Y", or "we wonder if X eqals Y"

$X{\stackrel {!}{=}}Y$ : "Yes, X does indeed equal Y, as is now obvious"

These notations are not entirely unknown on the internet, I have found several stack overflow threads in which people ask how to produce them in LaTeX, and other threads in which people ask what they mean. However, it appears they are less well-known than I thought. So, I am looking for actual mathematics/physics papers in which these symbols are used. 81.11.174.45 (talk) 06:19, 12 September 2012 (UTC)

I think that ${\stackrel {?}{=}}$ must be very well-known. I use it in my scratch notes when I'm trying to prove something. However, a search on Wikipedia fails to find any articles here that use it. But I'm sure it's out there somewhere.

As for ${\stackrel {!}{=}}$, I don't recall seeing that before, though its meaning seems obvious. Duoduoduo (talk) 21:22, 12 September 2012 (UTC)

My feeling is that they are primarily used in notes, blackboards, and textbooks; not research papers. Really, starting with the first symbol and terminating with the second is like starting with an ansatz and ending with a Q.E.D.. My point being, researchers are likely to handle these issues with words rather than special symbols whose meanings would have to be carefully defined. SemanticMantis (talk) 21:30, 12 September 2012 (UTC)

This German language mathchat site uses it. I don't read German so I don't know if it might help you find some published papers on the chat topic that use it. Also this one. Duoduoduo (talk) 21:46, 12 September 2012 (UTC)

Wikipedia does have the disambiguation page =?, which mentions a unicode symbol with the question mark on top. Duoduoduo (talk) 22:17, 12 September 2012 (UTC)

It's hard to search for this, but coincidentally a few hours after seeing your question I spotted ${\stackrel {?}{=}}$ in Exploratory Experimentation and Computation by Bailey and Borwein. I think I've never seen ${\stackrel {!}{=}}$, and I would probably guess that it meant "is, surprisingly, equal to", not "has now been shown to equal". -- BenRG (talk) 00:50, 13 September 2012 (UTC)

Can I just say in passing what a joy it is to learn new things on the reference desks. Had I not seen this thread, I could only have interpreted ${\stackrel {?}{=}}$ as an enquiry as to whether I have a predilection for liquorice allsorts. Now I know. AndyTheGrump (talk) 04:07, 13 September 2012 (UTC)

Gravitational pull vs. push

I saw a documentary about exoplanets the other day. They were talking about the way exoplanets make their stars wobble slightly, and that that wobbling can be detected by telescopes, and thus the exoplanets can be detected although they don't reflect enough light to show up themselves.

One of the animations was way off, though. The planet was moving around the star in a circle, and the star was moving more slowly. But the star was always moving in the same direction. Ascii art:

         *    .

  .    *

         *    .

etc.

As far as I can tell, that kind of orbit means that the planet is attracted by the star, but actually repelling it. I think of this motion that it's plain impossible, however I tried to look at the math and find a solution to prove that it's impossible. My best result so far was that, to make that motion plausible, the center of gravity must stay at the same place during the orbit, or move at a constant speed and in a constant direction. If I call the masses involved M and m, and the locations of the bodies X and x, I get an equation like

M X(t) + m x(t) = c,   c being independent of t and of dimension (kg m).

thus if both locations have the same sign, the m asses must have opposite signs, not exactly a common property among celestial bodies.

Which would result in m repelling M, but also in M repelling m.

So, while the negative masses seem to prove the animation wrong, I proved at the same time that there is no stable orbit at all. As sure as I am that the animation is wrong, my calculations look wrong too, but I cannot seem to nail the error.

Is it in assuming M and m independent of t? They should be if they were moving in perfect circles? - ¡Ouch! (^{hurt me} / _{more pain}) 07:54, 12 September 2012 (UTC)

In a 2-body system, both objects should orbit about the barycenter (center of mass) of the system. The barycenter will be much closer to the star, perhaps even inside it, but not at it's center. So, yes, the animation looks off (was this the Discover Channel, by any chance ?). In your equation, it isn't one of the masses which is negative, but rather one of the displacements, where negative means it is in the opposite direction from the barycenter as the other displacement. StuRat (talk) 09:49, 12 September 2012 (UTC)

I know that X(t) and x(t) are the pair which have opposite signs in real life. The point was that I wanted to explore what configuration could exhibit the behavior the animation showed, i.e. the data they fed into the computer in the first place, to produce such a degenerate case of orbit. A sign error looks quite possible, as the bloody comp will process any parameters you feed it no matter how whack-off they are.

If M and m have opposite signs, the barycenter is no longer between the two bodies, though. Effectively, if we put the barycenter into the origin, the equation simplifies to

M X(t) = - m x(t),

which does not explain either what repels the star from the planet.

Sorry, can't say if it was on DC, I was so waisted it could have been on Complete Nonsense Network and I wouldn't remember. Now I'm sober and my brain is still doing barrel rolls trying to find the parameters for that kind of orbit. - ¡Ouch! (^{hurt me} / _{more pain}) 10:51, 12 September 2012 (UTC)

Looking at the real world case, if we start with your equation:

M X(t) + m x(t) = c

But let's just use 0 for the constant:

M X(t) + m x(t) = 0

Now make one of the displacements negative:

M X(t) + m (-x(t)) = 0

We get:

M X(t) - m x(t) = c

Moving it to the other side, we get:

M X(t) = m x(t)

If you're trying to figure out an equation to produce that nonsense animation, that seems like a pointless exercise. I suspect they just entered circular orbits for both into their animation software, around a point not between the two. So, they defined two independent circular orbits, not taking gravitational attraction into account at all. Maybe you can imagine both orbiting about a black hole, to make it all make sense (although the orbital speeds would be different, in this case). They probably weren't using astronomy software at all, but just some general animation program, which is happy to let you define circular motion however you want, with no consideration made for gravity. StuRat (talk) 11:05, 12 September 2012 (UTC)

I think I found the answer myself. Classifying forces as "attracting" and "repelling" is misleading if masses can be negative. Newton's F = m a shows that acceleration changes sign if the mass does. What this means is that a planet will orbit a star in the same way regardless of its mass (assuming it is much less than the star).

Say, the planet is Earth, m = m_E.

Then, gravity is F = G M m_E / (X(t) - x(t))², and a = G M / (X(t) - x(t))².

If the planet's mass is v times the mass of Earth, we get

F = G M v m_E / (X(t) - x(t))², and a = G M / (X(t) - x(t))².

v cancels out, including the case v < 0. Negative inertia is a bit counterintuitive.

However, the impact the planet has on the star does depend on m. And the original solution was correct. I was wrong in thinking that there wouldn't be a stable orbit. The negative inertia fooled me.

The more important point is, however, (as you pointed out, Sturat) that they probly didn't feed any simulation data into the animation, and that their "scientific advisors" were too dazed (even more so than I was???) to catch their mistake.

And thanks for not picking up on "waisted", ;) - ¡Ouch! (^{hurt me} / _{more pain}) 07:01, 13 September 2012 (UTC)

  /
\/  Resolved. </barrel roll>

You're welcome. I'm amazed at how fake many documentaries and science shows are. I once saw one featuring Hero's engine which showed it rotating the wrong way. I can only conclude that they rigged an electric motor to it, to get it to spin. Then I saw a show where they demonstrated "the proper way to build a campfire". They insisted the large logs go on the bottom, then medium sized on top of those, then small twigs, with kindling on the very top. They said this is needed to keep it all stable. Only problem, is, of course, you could never light a fire this way. They were having no luck, until they went to commercial and came back, and it was blazing away like it had just been doused in gasoline. :-)

And here's a proper resolved tag, although yours was a valiant effort. :-) StuRat (talk) 07:20, 13 September 2012 (UTC)

Resolved

sound to electrical energy conversion

sound is a form of energy. each day we speak a lot i.e we waste a lot of energy. i want to use this energy. as well as there is a lot of noise in surrounding. how can we utilize this? atleast we can charge our cell phone. so, how can i convert this energy into electrical energy? — Preceding unsigned comment added by 110.172.159.134 (talk) 12:44, 12 September 2012 (UTC)

Sound isn't very much energy. The I_o of Sound intensity level is 10^-12 w/m². (Compare with the roughly one kilowatt per meter squared of sunlight. I.e. if we could "hear" sunlight it would be at 150 dB, which is well above the threshold of pain. And I don't see very many directly solar powered cars on the road. (Instead they use liquified fossilized sunshine.)) Hcobb (talk) 14:09, 14 September 2012 (UTC)

To elaborate on that, according to the source our article cites at [17], the sound intensity level of 60 dB SIL "conversational speech" is 0.000 001 W/m² Assuming you're in a big cubical room of, oh, 10 meters on a side, a speaker being heard anywhere in the room would need to achieve this over 600 square meters, which would take 0.0006 watts of power. Now note that someone with a basal metabolic rate of 1500 kcal/day = 6300 joules/day = 0.08 joules/second (watts) is expending 1300 times more energy lying in bed than is actually needed for you to hear him in conversation, which tends to sap the drive for improved efficiency. (Hope I didn't foul up the math again...) It would be interesting to make the same calculation for a cicada which puts a higher priority on being heard. Wnt (talk) 15:27, 12 September 2012 (UTC)

There are instruments that convert sound to electrical energy. They are called microphones and your cell phone has one. --Wrongfilter (talk) 15:44, 12 September 2012 (UTC)

The only practical use I know of is a sound-powered telephone using a balanced armature design for efficiency. Ssscienccce (talk) 16:11, 12 September 2012 (UTC)

A seismograph also uses the energy in vibrations, including sound, to drive the needle. StuRat (talk) 20:12, 12 September 2012 (UTC)

Laser Light as Propulsion?

Would it be possible, theoretically, to use the 'pressure' of laser light to move a solar sail through space? Or would Newton's Third Law, (i.e., "every action produces an equal and opposite reaction") prevent this?Honeyman2010 (talk) 17:37, 12 September 2012 (UTC)

We have a Laser propulsion article, which talks mostly about ground-based lasers hitting spacebourne vehicles. -- Finlay McWalterჷTalk 17:40, 12 September 2012 (UTC)

Right. You couldn't use laser light emitted by the vehicle itself to move the vehicle, but you could use laser light emitted by an external source, conceivably very far away. Looie496 (talk) 17:57, 12 September 2012 (UTC)

In terms of the physics of photons and momentum, there's no reason the laser can't be on the craft. "Couldn't" only enters in once you're talking about engineering tradeoffs. — Lomn 18:19, 12 September 2012 (UTC)

I may have been unclear. If the laser is on the craft and you fire it at a sail attached to the craft, you aren't accomplishing anything. You could however fire the laser into empty space, using it as a sort of rocket. That would be a terrible strategy, though, since you would be using a maximum of energy to get a minimum of momentum. Looie496 (talk) 18:56, 12 September 2012 (UTC)

With current technology, that's quite true. However, if you had a nearly infinite source of energy, let's say we get on-board fusion reactors working, and didn't need much thrust, let's just say "station keeping" (maintaining an orbit), then it might be more of a reasonable choice to launch particles at the speed of light. However, there's no need for the particles to be collimated, unless you need the beam to stay together to also drive another vehicle (say when two spacecraft separate). Since very little mass is ejected when you launch particles at the speed of light, you don't need to be worried about running out of material. StuRat (talk) 20:08, 12 September 2012 (UTC)

Regarding a laser-firing-at-its-own-craft's-sail—how (and how well) it works depends on what the sail is made from. If it is a reflective material, then the sail will reflect the beam and give you the same amount of thrust as pointing the beam straight into space, albeit in a different direction. If the sail is nonreflective, then it will absorb the laser light and heat up; eventually it will emit blackbody radiation from its face and produce thrust in that way. It only fails to produce thrust if all of the reflected/emitted light is recaptured by other parts of the spacecraft, or if the sail is both nonreflective and sufficiently thin and thermally conductive that blackbody radiation is emitted equally from both of its faces.

I can see the potential value of a small reflective sail on a photon rocket; the sail would be used to vector thrust without requiring reorientation of the entire laser. TenOfAllTrades(talk) 21:47, 12 September 2012 (UTC)

Lasers are used because they allow a distant sail projectile to be targeted. If you don't need to do this, and you're carrying the light source on-board the vessel, then you don't need to use a laser and you don't need to suffer the laser's inefficiencies. You can use a simpler and more efficient light source. Andy Dingley (talk) 20:28, 12 September 2012 (UTC)

Lasers of sufficient strength are big and heavy and require big and heavy power sources. They also don't let out a huge amount of energy for all of that weight and trouble. Their one good thing is that they can transmit their energy over long distances. So this is why most laser propulsion schemes are about putting the big, heavy, energy-hungry part — the laser — on the ground, and using it to project a high-energy beam of light to something that is lightweight, pushing it along. If you aren't going to do that, then you don't get anything beneficial out of laser propulsion. --Mr.98 (talk) 18:50, 12 September 2012 (UTC)

See also problem 7.5 on page 13 of this set of test problems. Count Iblis (talk) 19:48, 12 September 2012 (UTC)

Also, we have the article photon rocket. Count Iblis (talk) 19:54, 12 September 2012 (UTC)

Also, the Pioneer 10 and 11 spacecraft served as an unintentional tech demonstration of this effect. See: Pioneer anomaly. Though the photon source was a 150W RTG, and the thrust produced "not very much". 8.74±1.33×10⁻¹⁰ m/s² or so. After 40 years running its "photon engine", its now about 1mph slower (because the RTG is pointing outward- in the direction its moving).--Robert Keiden (talk) 00:28, 13 September 2012 (UTC)

Exponents repaired for clarity. - ouch

Laser beams can ping-pong between two mirrors to get more propulsion out of them, too (OR). The beam will red-shift if the moving mirror is moving away or blue-shift if it's moving towards the fixed mirror.

Fusion is not even close to a near-infinite power source BTW. It would be of greater benefit to tap the magnetic bottle for a burst of high-v particles, rather than tapping that kinetic energy to power a light source. The average v of fusion particles is around 0.12c = 36000km/s.

Kinetic energy of 1kg of photons: E = m c² = 9x10¹⁶Ws = 2.5x10¹⁰kWh,

Momentum of 1kg of photons: m c = 3x10⁸kgm/s.

Kinetic energy of 1kg of fusion particles: E = (m/2) v² = 6.5x10¹⁴Ws = 1.8x10⁸kWh,

Momentum of 1kg of fusion particles: m v = 3.6x10⁷kgm/s.

That is, using the kinetic energy of 1kg of fusion particles, you can emit ~7g of light for a momentum of ~2x10⁶kgm/s, or use the particles directly for roughly 18 times that momentum. The savings will only be offset if the power source is fixed (which is feasible only with a laser since you cannot project a focused particle beam into space).

OtoH, the Pioneer anomaly does look like a workable photon propulsion, and it doesn't need a full-scale reactor core either. Once you escape Earth's gravity, it's literally smooth sailing. A parabolic mirror, a lump of radioisotopes at its focus, and a set of attitude control thrusters, and there you go, slowly spiraling outward.

p.s. If the mirror reflects (or merely catches) neutrons, it's even better, because their momentum-to-energy ratio is superior to photons, either. - ¡Ouch! (^{hurt me} / _{more pain}) 07:57, 13 September 2012 (UTC)

Interesting calcs, but what are your assumptions ? Are we talking about fusing normal hydrogen into normal helium ? How about if the reactor continues until you get iron ? You should be able to get more energy out of fusion that way. Also, you can eject the reaction products at high speed, too.

Or, if that's still not enough, we can go with the old favorite sci-fi matter-antimatter reactor, with the anti-matter being generated prior to launch, and the matter just being waste products generated by the ship and crew (broken machinery, etc.). StuRat (talk) 08:28, 13 September 2012 (UTC)

If you leave the laser here on earth (or preferably, on the moon or in orbit someplace), there is even a way to have the spacecraft decelerate and eventually fly back towards the laser for the 'return trip'. Have the craft carry two sails - deploy one in the conventional manner on the outward journey - then when you need to decelerate, you cut the lines holding it and deploy a second sail pointing in the opposite direction - laser light reflects off of the detached sail (which continues to accelerate away) and onto the newly deployed sail and the craft will be able to decelerate, do whatever needs to be done - and eventually return home. The real problem is the power of the laser required to do this. Even if it's left back here on earth, keeping the beam sufficiently well aimed and focussed that enough of it's light is 'caught' by the sail is a tough problem that gets worse and worse the further the craft gets - the increasing time delay for the craft to report back with laser realignment instructions would also get seriously problematic as distances increase. SteveBaker (talk) 13:29, 14 September 2012 (UTC)

Seeing Our Own Creation

Telescopes are making huge leaps in their ability to look further and further back in time, and to see greater detail than ever before. I believe astronomers have even captured images of protostars and proto solar systems, and imaging actual new planets is not too far in the future. Therefore, since we can see the creation of other solar systems, isn't it accurate to say that every event since the creation of our solar system is currently being carried by photons created at the time and now a maximum of 4.7 billion light years away from us? The creation of the moon, the extinction of the dinosaurs, etc., all luminous events could be entering some distant civilization's telescopes at this moment.

If this is the case, and if there is really no 'center' to the universe but rather we are all on this ever expanding balloon of space-time, is there any scenario where we could even capture those photons which would show us our own creation?Honeyman2010 (talk) 22:44, 12 September 2012 (UTC)

It would seem unlikely. They're speeding away from us at the speed of light. We'd have to go faster than the speed of light to be able to interact with them so as to gather their information payload. Faster than light is, I think, forbidden in this universe. --Tagishsimon (talk) 22:49, 12 September 2012 (UTC)

We could certainly never catch up with those photons, but they may be returned to us by a couple of means. Theoretically, a very strong gravitational lens might return light from the early solar system back to us, although it may be dim and distorted beyond all recognition. Furthermore, if the universe is finite in extent but sill centerless (as in a [hyper]spherical universe), that light will eventually return to us regardless, but that may be a very long time in the future, which would mean it's very dim, and we'd need a freakin' huge telescope to view it. Someguy1221 (talk) 22:51, 12 September 2012 (UTC)

We've detected extrasolar planets up to 50,000 light years away, which is an impressive half of our galaxy away. In contrast, Andromeda galaxy is one of the closest galaxies to us and it is 2.5 MILLION light years away. You are asking about planetary sized objects 4.5 BILLION light years away? I don't know if I'd go so far as to say it's "impossible", but I won't be holding my breath.. Vespine (talk) 23:36, 12 September 2012 (UTC)

3C295 (Cl 1409+524) is one of the most distant galaxy clusters observed by X-ray telescopes.

At 4.5 Gly, the Milky Way, Andromeda and Triangulum galaxies would all blur together. This is what a cluster of 100 galaxies looks like, from about the same distance: --Robert Keiden (talk) 00:56, 13 September 2012 (UTC)

It's really just a technical limitation. A sufficiently large telescope could view the pimples on your face from 4.5Gly away, although that telescope may itself be light years across. I can't remember how to calculate the limits. Someguy1221 (talk) 01:23, 13 September 2012 (UTC)

No, it is more than a technical limitation, it is a physical limitation as well. In order to view your pimple, the telescope would have to capture some number of photons from that pimple, and at certain distances, you just can't capture enough photons. See Shot noise for a discussion of some of the problems. --Jayron32 01:31, 13 September 2012 (UTC)

Nevertheless, Hubble Ultra-Deep Field managing to image objects as they were ~13 billion years ago is pretty cool. Sean.hoyland - talk 03:40, 13 September 2012 (UTC)

Sure, but the objects it is seeing are considerably larger than the pimples on my face. --Jayron32 03:43, 13 September 2012 (UTC)

So here's a factual question, assuming we have unlimited resources to build arrays of interfering telescopes, at what distance do certain things become unobservable? Individual stars, Earth-like planets, cities, people on those planets, etc. My claim came from the recollection of an astronomy lecture years ago that discussed the theoretical construction of a telescope array that could resolve to one pixel a person's head on a planet 50 light years away. It hadn't occurred to me that noise would put a limit on how far something could be seen even with an arbitrary telescope. So, back to my question, assuming a sun-like star and an Earth-like planet in orbit, typical interference from intergalactic/interstellar gas, at what distance does noise preclude any observation? Someguy1221 (talk) 06:58, 13 September 2012 (UTC)

There's possibly no limit to the math involved, I suppose, but you start to get rediculous when it comes to actually constructing a telescope. For example, if you build a lens so large that it conatains all the mass of the known universe, or you build one so large that light could not travel from one end of the lens to the other fast enough to get there before the heat death of the universe, or the telescope is so large that its own mass causes it to collapse into a black hole, or any of a number of other ludicrous propositions, well, I don't think we're dealing with mere "technical limitations" anymore. That is, if you narrow yourself to just those equations that relate the size of a lens to its resolution, you could build a telescope of any size and have nearly infinite resolution. But that is only if you ignore all of the other laws of physics that come into play, and if you're going to ignore some set of the laws of physics, you're doing no better than invoking "magic". The article, by the way, that will answer the main question about how "far" we can see is Optical resolution, and sure, using just those equations, you could construct a telescope of any size which could resolve to any arbitrary accuracy at any arbitrary distance. But there are other considerations when constructing such a telescope, and there are limits to the size a telescope can get before other laws of physics start to make infinite resolution at any distance impossible. --Jayron32 13:10, 13 September 2012 (UTC)

WMAP image of the CMB temperature anisotropy.

This question has a rather unambiguous answer. The furthest back in time we can see is the cosmic microwave background radiation, before that the universe was "opaque"; i.e., a glowing cloud we can't see through. Here is an image. μηδείς (talk) 17:47, 13 September 2012 (UTC)

September 13

Storm chasing from the sky

Here in the Oklahoma City television market the local news stations will in times of tornadic thunderstorms send up their news helicopters to chase the storms and any tornadoes they produce. Strangely, despite flying in close proximity to large supercell thunderstorms, there seems to be very little turbulence (or at least the cameras on the helicopters holds a very steady picture). Why is this? Also, what distance might the helicopters have to stay away to avoid hazards from the thunderstorm, such as the tornado, hail, etc? Ks0stm ^{(T•C•G•E)} 03:10, 13 September 2012 (UTC)

I know you asked specifically about supercells and tornados, but you may find 53d Weather Reconnaissance Squadron and Hurricane Hunters interesting, as it has some information on arial reconnaissance of major storms. --Jayron32 03:25, 13 September 2012 (UTC)

Cells that produce tornadoes can be highly localized, so the weather may be fine where the chopper is filming. This is in contrast, say, with a hurricane. StuRat (talk) 04:14, 13 September 2012 (UTC)

Also, most the cameras in most television station helicopters are mounted with gyroscopic stabilizers.    → Michael J Ⓣ Ⓒ Ⓜ 08:59, 13 September 2012 (UTC)

Gyro stabilizers prevent the camera from being rotated by the motion of the aircraft - but they can't prevent positional motion from producing problems. It may be that they also record a larger image than is displayed and use motion tracking (eg Digital image correlation) to crop each frame to eliminate the motion. That is the basis of the Image stabilization provided by many modern digital cameras. In this case, since the camera and the subject are many hundreds of feet apart, a foot or more of helicopter motion would only produce a few pixels of image motion between frames - and that can easily be compensated for in this way. SteveBaker (talk) 13:31, 13 September 2012 (UTC)

I guess they would keep enough distance to be able to get away if it came towards them... Thunderstorms are steered by the winds in the area, maybe they preferably film it from the side, outside of it's predicted path. The FAA advices to stay at least 20 miles away from bad weather, but that's general advice for all aviation. Looking for info I came across a law firm page that from the looks of it specialised in "news chopper related injuries to employees" cases. Not sure if that's an indication of the risks they take, or if the firm has similar pages for every potentially lucrative law suit they could think of.

Keep in mind that those crews have quality equipment with powerful zoom lenses, so it may look closer than it really is. Ssscienccce (talk) 19:21, 13 September 2012 (UTC)

How accurate are bathroom scales?

Seems like a simple question. How accurate are digital bathroom scales? My weight has been being shown as either 222.2lbs or 220.0lbs all through this week...never any other number. I suspect that it's internally measuring to the nearest kilogram - but displaying in pounds with a readout that's showing tenths of a pound. Just to check, this morning I was able to weigh myself then pick up two soda cans (12oz each...so 1.5lbs) and see no change whatever in the readout - so clearly there is an internal error that's at least ten to twenty times the displayed precision...and I also kinda suspect that the machine is remembering the last weight it displayed and showing the exact same number again if you weigh yourself again and the results are similar enough.

But Wikipedia has no specific article about these machines - and the general Weighing scale article doesn't mention them beyond a mention in the intro that basically says that they exist! Surfing around the web for a while, reviewers who claimed to have done careful testing gave only vague results ("Very accurate", "Accurate", "Not very accurate") with no actual numbers. Manufacturers and retailers said things like "Display is accurate to 0.2lbs"...which anyone with an ounce of critical thinking skills can parse to mean "We put a ridiculously accurate LCD display on a crappy weighing machine and we're trying to pursuade you that this is an OK thing to do"...which is evidently what happens here.

I'm trying to diet - but any reasonably sustainable diet only drops my weight by a pound or two per week...this horrifying inaccuracy in weighing myself means that I can't track my progress at any kind of reasonable rate...even if I'm careful to weigh myself right after peeing & pooping in an effort to eliminate that source of noise in the measurements - and I average my readings over a week to track progress.

Obviously I don't care much about my absolute weight so much as the rate of change - so I don't care if the machine has my weight off by a few percent, so long as it's consistent from one day to the next and able to show variations of better than 1%.

SteveBaker (talk) 13:23, 13 September 2012 (UTC)

Interesting I had noticed what seemed to be a memory effect on scales. However please clarify the obvious: that the scales are on a hard surface in the same place, with your feet in the same place and your centre of mass similar in all cases? The old scales with counterweights seem able to spot a few grams but modern digital ones I am less convinced about. Good luck with the diet. --BozMo talk 13:33, 13 September 2012 (UTC)

Yeah - I'm very careful to place them in the same spot on my hard bathroom floor - and to stand reasonably consistently...although shifting my position by an inch or two or leaning my weight onto one foot or the other doesn't change the readout. The "memory effect" could well be a simple software hack to make the scales SEEM more reliable by simply repeating the last readout if the measured weight is within the internal error tolerance...but I've tried weighing myself, then putting one foot on the scale and pushing down until it reads some highish number...then weighing myself again...and the results don't change. So either they are remembering the last N weighings in order to defeat my test - or the weighing mechanism is indeed heavily quantized. The problem here is that not knowing how it works makes experiments to determine it's accuracy quite challenging! SteveBaker (talk) 14:10, 13 September 2012 (UTC)

Is your weight consistant to within 2 pounds over the course of any arbitrary length of time? It may be that the scale is accurate enough, but your weight isn't consistent, given the amount you have eaten and drunk, whether you just took your morning dump or not, how much sweating you've done, etc. Your weight is basically fluctuating +/- 1 percent and I don't know that that is outside of the normal range for what a person will do on a given day or week. The way to test this would be to get an object whose weight you know isn't changing, and then weigh it at various times. Your body is far too dynamic and unpredictable a thing to test the reliability of a bathroom scale. --Jayron32 13:47, 13 September 2012 (UTC)

Sure, there is indeed no way that my weight could be that consistent...if I drink a can of (diet!) soda - that's 12 ounces...add a light lunch and I could easily gain a pound or more - and lose it again when I next pee/pooh. So of course it's necessary to weigh oneself after a morning trip to the toilet - and to average over a week or so...all of which I'm careful to do. But I'd hope that this would get the error bars down to within a pound or two...which is what I need to know in order to figure out whether my diet is still keeping on track. Should I walk further next week? Can I let up on the semi-starvation a bit? I know that dieting too fast is a bad idea because it can put your body into crisis/starvation mode and drop your metabolic rate to the point where your diet hits a plateau. The best advice seems to be to try to stick between one and two pounds per week of loss. That's 1% for chrissakes...I really ought to be able to measure that as an average over a week and see if I actually did lose somewhere between one and two pounds over that time.

Bottom line is that if this machine I have is really quantizing in ~1kg increments internally - and maybe isn't even accurate to 1kg - I could easily be getting a 2% error in my measurements - and that's equivalent to an entire month of dieting - and I can't track whether my dieting behavior is excessive or inadequate. If that's the case then I need to invest in a better machine...if such a thing exists for the bathroom at a reasonable price. It's frustrating that I can't find out how accurate these things actually are in order to make an intelligent choice over whether to upgrade my bathroom scales!

SteveBaker (talk) 14:04, 13 September 2012 (UTC)

As a seasoned dieter, I can only repeat the advice every diet programme I've ever seen over the years gives: Only weigh yourself once a week. Resist the temptation to weigh yourself every day. Obviously this advice has the raison d'etre that (a) your weight will fluctuate naturally from day to day, and (b) the scales may not be as well calibrated as you'd like. So same time, same place every week to weigh yourself. Regardless of whether your scales are that accurate, you will at least get an idea of the weekly result and be able to compare it over time. --TammyMoet (talk) 14:55, 13 September 2012 (UTC)

The problem with the weigh-once-a-week and the weigh-daily-and-compare-with-yesterday approaches is that both give too much 'weight' to a single measurement. One's measured weight can swing through two or three pounds based purely on minor changes in the amount of water in one's body and the fullness of one's bowels and GI tract. Daily measurements prompt both the "Sweet - I lost two pounds yesterday!" delusion as well as the "Oh my God - I was fasting and still gained half a pound!" panic. Individual weekly measurements can give similarly misleading impressions about the overall success of a diet. (If I lost two pounds in week one, and no pounds in week two, was there really a thousand-calorie-per day difference in my eating between the two weeks, or am I just seeing noisy data?)

Far better to accept that one's measured weight varies naturally from day to day and hour to hour, and apply some sort of smoothing or averaging to one's measurement history. The Hacker's Diet provides instructions and spreadsheets for doing this; it plots a trendline based on weighted averages of preceding days measurements, to give a plausible impression of one's 'real' weight without the superimposed day-to-day noise. Even a straight three-, five-, or seven-day moving average is going to be more resistant to irrelevant fluctuations than pure daily or weekly measuring. TenOfAllTrades(talk) 15:32, 13 September 2012 (UTC)

Yeah - I think the advice to not weigh yourself every day is for the benefit of stupid people. Intelligent, math-aware, scientific thinkers (which I aspire to be!) can only gain from having more information - providing they are smart about using it. So I gather data by averaging over a week - and look at the trend of averages to give myself some idea of whether I'm doing OK or not. However, not having a decent measurement device to collect that data - one that induces error comparable to several weeks of expected weight loss isn't making life any easier! So the question remains...how accurate are bathroom scales? SteveBaker (talk) 16:58, 13 September 2012 (UTC)

As someone once said, I resemble that remark... I guess there's no accounting for those who would obsess over their weight. If you're not losing weight, don't blame the scales! --TammyMoet (talk) 17:39, 13 September 2012 (UTC)

OK, it's time we stopped guessing. If you have access to Consumer Reports magazine, they have done the testing. You can't read the results of the test online unless you have a subscription, but here is their methodology for studying the accuracy of bathroom scales. For any sort of consumer product testing, that's usually where I go first; they have a reputation for being thorough and impartial. If you don't have or don't want to pay for a subscription online just to view this one article, the magazine is widely availible in many libraries, so perhaps you can get it that way. Good luck! --Jayron32 17:09, 13 September 2012 (UTC)

Well, I don't have a subscription, so I'm going to guess some more. Steve, I think you should dump the digital scales. I have done some of the same style of testing as you, and I hypothesize that they try to make themselves look more consistent than they are, by having logic that attempts to figure out if the person on them is the same one as a few minutes ago, and if so, give the same reading.

Analog scales have limitations — the spring moves more when you step on it than the pressure sensor in a digital scale, so you might think it would soften faster. I don't know if that's true or not. But at least they're too stupid to be trying to fool you.

Best of all should be a real doctors'-office-style balance with the moving counterweights. A little more money but might be worth it if you're looking at this level of detail. --Trovatore (talk) 17:30, 13 September 2012 (UTC)

This would likely depend significantly on both scales. My last 2 analog scales would change about 5-10 kg depending on which way you were leaning and precisely where you were standing on the scale, even when on a flat hard surface. Nil Einne (talk) 09:44, 14 September 2012 (UTC)

That's a .... pretty bad scale. But — at least you could tell how bad they were. I assume neither was of the doctors'-office sliding-counterweight variety? --Trovatore (talk) 09:59, 14 September 2012 (UTC)

My small digital scales (500 gr and 10 gr max) weigh continuously, i.e. the readout changes when the applied weight changes. My bathroom scale gives only one reading, for a second one it has to turn off and on again. The difference is most likely because people move, so it has to average the weight over a longer timeframe. If your's is like that, picking up two soda cans will have no effect. The minimum difference of 2.2 lbs you get may be because they use a non-linear strain gauge to provide the same relative accuracy over the whole range. So a weight of 20lbs would be within 0.2lbs, but 300lbs would only be accurate within 3lbs. It would allow them to use a cheaper (lower resolution) A/D converter. But that's pure speculation on my part. Ssscienccce (talk) 21:30, 13 September 2012 (UTC)

My girlfriend has been weighing herself every morning for about 14 weeks, while losing 0.95 lbs / week. The data actually plot pretty smoothly on a line. At 1-sigma, the typical daily difference from her measurements and the line is +/- 0.7 lbs. So, I would infer that our scale is at least that reproducible, and probably more so since her daily values should also fluctuate based on variations in the daily routine from day to day. Of course your mileage might vary, but the digital scale we have seems to do a pretty good job of tracking relative changes. Dragons flight (talk) 21:37, 13 September 2012 (UTC)

We had a discussion of this very topic maybe a month ago here. Yes, cheap digital scales do have logic in them to provide false accuracy by not changing the reading if it's similar to a recent one. How recent and how much change it will mask may vary by model. To overcome this:

A) Use a known weight which is over the range your scale masks. 10 pounds should do it. So, get on the scale holding the ten pound weight, and you should get your actual weight plus ten pounds. Just subtract. The next day, weigh yourself without the weight, and you should get your true weight. Just alternate days with and without the weight.

B) Pull the battery out between uses. I bet the previous reading is in volatile memory, and will go bye-bye.

As I mentioned in the previous discussion on this topic, the most annoying thing is that they don't tell you prior to purchase that this is what it's doing, you only figure it out later. We need some consumer protection laws here. StuRat (talk) 04:06, 14 September 2012 (UTC)

Ideally, digital bathroom scales should behave like Ssscienccce's 500g scale — the reading stays on all the time you stand on it, and changes to reflect the weight it senses at that moment. Then you could stand there statue-like and wait for it to stop changing, and then you'd know all the transients had damped out. If they did that, I would withdraw my objection in favor of analog scales.

Analog balances, with the movable weights, would still be better, of course. --Trovatore (talk) 04:16, 14 September 2012 (UTC)

(Although, those actually measure your mass...not your weight...but that's probably what you ought to care about anyway! Gravity can vary by as much as half a percent depending on where you are on the earth's surface. Fortunately, I'm only planning on weighing myself (er "massing myself") in my bathroom...so it's OK.) SteveBaker (talk) 12:59, 14 September 2012 (UTC)

Maybe digital scale manufacturers figure (or focus groups have shown) that a dancing digital number (like many a digital multimeter measuring a jittery voltage source) that never stops due to the user's shifting his/her weight, which frequently tops 0.1lb on the sensor, frustrates the user to the point where they return the scale and the manufacturer doesn't make money. 20.137.18.53 (talk) 14:45, 14 September 2012 (UTC)

Merck Manual/s

Is/are Merck Manual/s worth it/are the buy and etc? Or if not what is/are worth and etc?

Basically, I was looking at it earlier and it seems its worth the buy and etc. In the end didn't get it because I'm still totally unsure about it and etc. — Preceding unsigned comment added by Mybodymyself (talk • contribs) 22:32, 13 September 2012 (UTC)

I couldn't say, but keep in mind that a number of Merck's products have had to be yanked from the market, so use your best judgment. ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:58, 13 September 2012 (UTC)

I don't see any direct connection between a Merck drug being pulled, and the quality of The Merck Manuals. They have a long and well reputed history. And they're worth it if you like that sort of thing. It's not a question, OP, we can answer without knowing more about what it is that you're looking for in a medical handbook. --Tagishsimon (talk) 23:05, 13 September 2012 (UTC)

I second that. Ssscienccce (talk) 23:12, 13 September 2012 (UTC)

The manual is aimed at health care professionals and medical students. Still makes a good read imo, if you like that sort of thing. Don't expect layman terms, it mostly stuff like: "Immature WBCs and RBC precursors are found in the peripheral blood, and marked anisocytosis and poikilocytosis , with microcytes, elliptocytes and teardrop-shaped cells develop." (first random page I picked) If you can find the older centennial version, you get a copy of the 1899 first edition as well. Ssscienccce (talk) 23:12, 13 September 2012 (UTC)

Also, not sure which Merck Manual the OP is interested in, but all Chemists sleep with two books on their nightstand: the Merck Index and the CRC Handbook of Chemistry and Physics, both of which are fantastic and useful reference books. From a chemists perspective, the Merck Index is worth it. Again, though, not sure which Merck publication you are interested it. --Jayron32 00:59, 14 September 2012 (UTC)

You can buy old editions rather cheap, I have bought or seen them at library sales, and you can get them at online used book sellers like Amazon and, in the US, abebooks.com μηδείς (talk) 03:14, 14 September 2012 (UTC)

September 14

Removing color from/clarifying orange juice

I know it doesn't make much sense, but I am trying to make a clear green drink that contains a decent amount of orange juice. Using yellow 5 and blue 1 I can get the right color of green, but it is darker than I would like, presumably because of the orange color in the base. Diluting the juice to 10% also does nothing to eliminate the opacity - I would prefer it to be clear. Even if it stays orange-colored, I think I can fix it with food coloring as long as it is clear. Pulp-free orange juice just clogs up a coffee filter, and I'm not really sure what else to try to clear it up. Any ideas? 108.194.140.240 (talk) 02:00, 14 September 2012 (UTC)

Have you tried to use orange extract instead of juice? --Jayron32 02:05, 14 September 2012 (UTC)

I've found a few answers to my own question, after realizing I hadn't thought to use "clarifying" as a keyword until I wrote here. I'll have to look into extracts, and see what I think of them. I just found some suggestions to mix in a small amount of gelatin powder, freeze, and let it thaw through a strainer. The other option I've seen that is apparently used by some chefs is to use a centrifuge - but I don't have access to anything like that. I won't mark this resolved just yet, maybe someone else will have some ideas or experience with this. 108.194.140.240 (talk) 02:15, 14 September 2012 (UTC)

Both of the techniques you note sound a lot like Molecular gastronomy which has been all the rage among the avant garde fringe of the culinary world for the past few years. --Jayron32 02:23, 14 September 2012 (UTC)

I think this may boil down (sorry:) to what you mean by "contains orange juice", or more specifically what your intended result (rather than method to getting it) is. For example, taking orange juice and trying to clarify it (remove the suspended and emulsified...whatevers) is one of many ways of getting to a clear liquid that tastes like orange. DMacks (talk) 02:42, 14 September 2012 (UTC)

I just found this [[18]], and also someone in the comments mentions that pectinase works on OJ. Pectinase means I can just stir and refrigerate, which is easier than any other method out there. I think I've seen enough ideas to be happy with them. Sorry I bothered the reference desk - I just was doing a bad job searching until after I posted! 108.194.140.240 (talk) 03:06, 14 September 2012 (UTC)

Resolved

Pointless referencing in science articles

Whilst at University, the first thing we were told was not to reference wikipedia, but to use the journals. And wikipedia editors are somewhat insistent on journal references.

I wonder how many realise how much those journal entries cost a member of the public to view and who is taking the cash.

And I wonder how many editors are aware that some of the big name distributors now don't even offer public accounts. I believe Science Direct is one such group. What possible use could a list of $25 to $35 a view entries, inaccessible to the public, be to the public? To access the evidence, one must give up a week or twos worth of wages to check the references. — Preceding unsigned comment added by 86.150.236.65 (talk) 02:30, 14 September 2012 (UTC)

If you wish to check the references to Journals which cost a lot of money, Wikipedia offers a service where someone with a subscription will check them for you, for free, and provide you with a copy or with relevent info as you need. See WP:REX. --Jayron32 02:39, 14 September 2012 (UTC)

The abstracts are usually free, so you can at least get a sense of what the article is about and some of the the main conclusions. That might be enough to see whether the cited statement is already supported (or sounds likely enough that you don't need to check the nitty-gritty details) without having to spend money. Or at least help you decide which of the many cited refs you actually do want to check in detail. DMacks (talk) 02:45, 14 September 2012 (UTC)

• The reason we require references is to validate the material in the articles. It is nice if the references are easily accessible, but that isn't the essential thing. We actually don't require that references be available online at all -- many book references are not. Looie496 (talk) 03:44, 14 September 2012 (UTC)

In addition, the full text of papers accepted for publication since April 2008 and supported by funds from the U.S. National Institutes of Health are, as a rule, freely accessible to the public under the NIH Public Access Policy. -- Scray (talk) 04:17, 14 September 2012 (UTC)

• In Australia, many university libraries allow visitors to access their online journals if they log on in person at the library. The Western Australian library service provides online access from home to thousands of textbooks via eBook Library. --Anthonyhcole (talk) 04:18, 14 September 2012 (UTC)

I guess I'm not really adding much, but an inaccessible reference is better than no reference. The situation with availability online is changing, because most of the research is supported by universities, and they know they are getting gipped if they have to pay again for access to knowledge they probably funded in the first place. Even for my PhD, I much prefer refs that are available full text online, as a courtesy to others, and because I hate having to bother logging in. I think most people do the same, but not all knowledge is freely available. IBE (talk) 05:07, 14 September 2012 (UTC)

I agree. Whenever two sources are equally reliable but one is free, I always cite the free one. --Anthonyhcole (talk) 05:15, 14 September 2012 (UTC)

Unless the free one is a direct, verbatim copy of the other, the better action may be to cite them both. More source material is always more better. --Jayron32 05:17, 14 September 2012 (UTC)

There is really no problem getting access to articles, and no reason not to cite almost any orginal source as a reference. In Western Australia, if you are a student, you can get almost anything journal-wise for free, though I assume if you abuse the privilige by say requesting lots of papers on the sexual perversions of eskimos when you are an engineering student, you may be spoken to. As Anthonyhcole has said, Universities allow free access to all printed journals held, and most online journals they subscribe to, to any member of the public who visits in person. If you are a member of your local (shire) library, you can ask them to email you a scan of anything held by any public or university library in Australia. That includes just about anything - I have succesfully obtained articles that appeared in Russian journals. The only downside is that the service is very slow, typically taking several weeks. But the service is completely free! Anyone in Australia can request almost anything from the Australian National Libray - they can, via the US Library of Congress and the British Library access anything, and the service is very fast - 2 days or so. But they charge about the same as commercial services. Ratbone120.145.29.139 (talk) 11:05, 14 September 2012 (UTC)

The thing is, Wikipedia could be set up to have articles that are no more than a list of references - no text whatever. But we don't - and the reason is that most people can't/don't-want-to get a hold of all of those referenced documents and read them all. The entire point of an encyclopedia is to provide a useful distillation of all of those core documents. You shouldn't reference Wikipedia because a better reference will always be to the things that Wikipedia references...but that doesn't make Wikipedia useless - very often you only need a summary and a reasonable presumption that the summary really does match the references it points to. But if it's a critical matter - then you're just going to have to get those source documents - no matter the cost or difficulty.

You ask whether Wikipedia editors are aware of this - and since they presumably had to read the referenced articles first before they could choose them to back up facts in the article, I'd have to say "Yes". Your underlying question is presumably something like: "Why don't we simply shun references that are costly/difficult to read?" - but if we didn't do that, then you wouldn't even be able to find our "summary" information for a large fraction of human knowledge...and you wouldn't know when (or where) to go with large piles of cash to find out whatever is missing. So it's important that we reference those documents - especially when they are hard to find or expensive to read. SteveBaker (talk) 13:12, 14 September 2012 (UTC)

Dihydrogen vs. dihydrido

Can dihydrogen and dihydrido complexes be considered true tautomers? Plasmic Physics (talk) 06:31, 14 September 2012 (UTC)

Probably not, except by extension of the definition in some way. A true tautomer is almost always an organic compound that has two stable forms that are seperated by a low energy barrier, almost always involving the migration of a hydrogen atom and the shifting of a pi-bond. The kind of isomerism you note 1) doesn't involve organic compounds 2) doesn't involve the same sort of hydrogen atom shifting and 3) doesn't involve the movement of a pi bond. Rather, I think the better term for this may be Resonance in the sense that there aren't two forms of the complex (which is what tautomerism would imply: that there were two distinct forms actually found), but rather the difference between drawing the "Dihydrogen" model and the "dihydro" model doesn't represent a real difference, rather more a shortcoming in the lewis diagram model. --Jayron32 12:50, 14 September 2012 (UTC)

From what I gather, the article on dihydrogen complexes indicate that they are distinct with a low energy barrier. Plasmic Physics (talk) 13:28, 14 September 2012 (UTC)

Fine then, they are, but it still isn't tautomerism, which describes a specific type of single proton transfer within a specific set of molecules. Metal-ligand coordination complexes are not the right sort of molecules, and the way in which hydrogen "moves" is not the right sort of movement. Other than "involves hydrogen in some way", there's nothing about the reaction you describe which qualifies it as "tautomerism". --Jayron32 14:10, 14 September 2012 (UTC)

Entropy of a fan

Let's say we have a fan connected to an electric motor. The motor is switched on, and work is performed on the fan. Does the entropy of just the fan increase or decrease as a result of the motor being activated? Why?

I'm curious because, as I understand it, entropy is defined as heat flow divided by absolute temperature. Here, though, we have energy transferred from the motor to the fan in the form of work, not heat. I think.

(You may be able to tell that I've never studied physics formally.) 75.60.184.181 (talk) 14:03, 14 September 2012 (UTC)

Friction converts work to heat, though. --Jayron32 14:04, 14 September 2012 (UTC)

Thanks. So heat is transferred from the fan to the surrounding air? I thought that might be the answer, but then I wondered what would happen if the temperature of the air were greater than the temperature of the fan. How could heat then flow from the fan to the air? 75.60.184.181 (talk) 14:26, 14 September 2012 (UTC)

Friction always increases the ambient temperature of what is being, well, frictioned (is that even a word? I don't know, but go with me on this). That is, whatever tempertaure it was before, the temperature will always increase as a result of friction because friction adds heat to the surroundings. So, the ambient temperature of the air and the fan is unimportant to the role of friction: friction warms the fan up, regardless of what its prior temperature was, and it will continue to do so until the relative temperature of the fan is greater than the air, at which point the heat will begin to transfer to the air. --Jayron32 14:30, 14 September 2012 (UTC)

I see. Am I correct, then, that the entropy of the fan will begin to decrease once it begins to transfer heat to its surroundings? Also, during the time between the activation of the motor and the point at which the fan's temperature overtakes the air's temperature, is there any entropy change? Thanks for your help. 75.60.184.181 (talk) 14:46, 14 September 2012 (UTC)