# Wikipedia:Reference desk/Archives/Science/2009 January 6

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# January 6

## Looking back in time through space

In terms of looking into space, does anyone know, in light years, what is the furthest back in time man has peered? Are we talking thousands or millions of years here? 79.75.238.142 (talk) 02:37, 6 January 2009 (UTC)

More like billions: [1]. StuRat (talk) 02:43, 6 January 2009 (UTC)
13,699,600,000 years ago!
It's a substantial fraction of the time since the big bang. We've observed and mapped the "cosmic background radiation" - which according to our article was just 400,000 years old at the time. We believe the universe is 13.7 billion years old - so the answer is something like 13,699,600,000 years. (OK - we should be rounding that to 13.7 billion). (That's "years" not "lightyears" - a light year is a measure of distance - not time). In some sense, it's not possible to look further back in time than that because there was nothing like atoms for photons to bounce off of - so we've pretty much seen as far back as it's possible to see. SteveBaker (talk) 05:15, 6 January 2009 (UTC)
If you consider detecting microwaves to be a method of "peering", the cosmic microwave background radiation has been travelling in space since the universe first became transparent to light 13.7 billion years ago, 400 000 years after the Big Bang. It isn't possible to detect light from any earlier time because earlier photons were continuously being emitted and scattered before travelling any appreciable distance.--Bowlhover (talk) 05:11, 6 January 2009 (UTC)
There are two potential ways to "look" even further back in time. One is the detection of the cosmic neutrino background: neutrinos decoupled from the rest earlier than the electromagnetic radiation of the cosmic microwave background. The other method would be to look for gravitational waves from the earliest epochs of the universe. Alas, we do not have the technology for detecting either of these backgrounds yet. --Wrongfilter (talk) 13:42, 6 January 2009 (UTC)
Yes, agreed. Although detecting neutronsneutrinos and gravitational waves stretches the definition of the word "see" further than I'm comfortable with! I'd prefer "infer from". I believe the CMB is the first detectable electromagnetic radiation (although you could probably argue with using the word "see" for microwaves too!). SteveBaker (talk) 15:07, 6 January 2009 (UTC)
Careful - neutrinos, not neutrons. --Tango (talk) 01:28, 8 January 2009 (UTC)
oops! all better now! good catch. Thanks. SteveBaker (talk) 16:41, 8 January 2009 (UTC)
Considering the observations of specific objects see the Hubble Ultra Deep Field. --mikeu talk 17:15, 7 January 2009 (UTC)

## Potassium supplement dosage

I was at the health food store this weekend and looked at Potassium supplements. I was surprised that they were all 99mg and only supplied 3% of your RDA. Any one know why they are all capped at 99mg? A person would have to take 33 tablets to get the full recommended daily allowance! --71.158.216.23 (talk) 03:00, 6 January 2009 (UTC)

The reason is that an overdose of potassium can kill you, so they don't want to take any chances that the supplement, along with your normal diet, will do that. The 3% is just so they can claim their supplement has a valuable nutrient in it. Potassium is actually what they use in lethal injections (in much higher dosages, of course). See hyperkalemia. StuRat (talk) 04:53, 6 January 2009 (UTC)
That explains the reason why there would be a limit. If all the tablets are exactly 99 mg, then a likely reason for that particular size is that someone wrote the law or regulation so as to say "any tablet containing 100 mg or more of potassium requires a prescription" rather than "any tablet containing more than 100 mg of potassium requires a prescription". Perhaps at the time the next-largest size below 100 mg was 50 or 75 mg, and they assumed it would continue to be, but manufacturers saw a loophole and created 99 mg tablets in order to gain a competitive advantage and stay within the law.
In that paragraph I'm just guessing, but I do know about a similar occurrence in the field of railroads. In 1922 the Interstate Commerce Commission in the US was trying to reduce the number of train crashes due to signals being passed at danger, so they mandated the installation of measures such as automatic train stops on all railways that allowed trains to run at 80 mph or more. And the result is that to this day a large number of main US rail lines have a speed limit of 79 mph. --Anonymous, 07:56 UTC, January 6, 2009.

## The Universe

Could it be possible that the universe is spherical and when one looks through a telescope in any direction they could see all the way around the universe back to ones position at earth in the future,assuming light would bend around the universe and also assuming one had a telescope that powerfull.Grimmbender (talk) 03:58, 6 January 2009 (UTC)

Sounds like you mean the universe being on a spherical surface not being a sphere itself. Consider standing on Earth: if you go forward a long distance along the surface you come back to where you are. But if you are underground and you move in a straight line (cartesian, not spherical) you wind up bursting through the surface and heading out into space. The only way "universe is a sphere" would lead to "seeing forward back to behind you" is if light somehow bounced (or tunneled, or whatever) around when it got to the edge. DMacks (talk) 04:21, 6 January 2009 (UTC)
A sphere is a surface. The solid that you're thinking of is a ball. Technically, he should have said 3-sphere. I think it should actually be 3-spherical cone, to account for space expanding through time. Anyway, I think we all know what each other mean. — DanielLC 21:06, 6 January 2009 (UTC)
It is possible that the universe wraps around itself like that (I think we'd be talking about a hypersphere or something) - we don't know for sure - but if it does, we'll never be able to do the experiment you're thinking about because the 'observable' universe appears to be smaller than the entire universe. Because the speed of light is the universal speed limit - we can only ever see or know about parts of the universe that are close enough for light to have travelled from there to here in less than the time since the big bang. Anything further away than that (including, perhaps, the back of your own head) is forever too far away to ever be visible. SteveBaker (talk) 05:05, 6 January 2009 (UTC)
"The back of one's head is inches from the eyes but too far away to be seen." DMacks sends contents of coffee mug out for tox-screen. DMacks (talk) 05:17, 6 January 2009 (UTC)
I haven't seen anything later than this and this. It looks like the Universe is a closed Poincaré dodecahedral space, it is a "small universe" with positive curvature, and yes, we can see all the way around it (same glowing spots from the CMB in different parts of the sky). This is difficult stuff, I can put the cites here or send copies of various papers and reviews to anyone interested. Franamax (talk) 07:41, 6 January 2009 (UTC)
"same glowing spots from the CMB in different parts of the sky" -- sweet Christ that's weird! --Sean 13:04, 6 January 2009 (UTC)
Wow! That is REALLY cool! But doesn't that just prove that back at 400,000 years after the big bang (which is when the CMB was 'formed') - it was all close enough to be within the observable universe - but the expansion of space (which happens faster than the speed of light at large distances) has since made everything so much larger that now we can't see it all anymore? This is tricky stuff - and the theories are changing rapidly - so it's hard to know what is considered to be "true" on any given day! SteveBaker (talk) 15:02, 6 January 2009 (UTC)
Almost no cosmologist accepts the conclusion of those papers currently. I think they blame the apparent effect on cosmic variance. Here's a slightly more recent analysis that reaches an opposite conclusion. The best bet right now is that the universe doesn't wrap around at the scale we can see, as SteveBaker said.
Practically everyone believes that the part of the CMB that we can see was all in causal contact at one time, since it's pretty hard to understand the uniform temperature otherwise. But according to current dogma that time was before the inflationary epoch, while the fluctuations in the uniform temperature arose during the inflationary epoch, at which point the different parts of the CMB sphere had lost contact with each other. Even if that's wrong, it's hard to imagine what mechanism (other than a wraparound universe) could produce correlated fluctuations of the specific kind you expect to see in a wraparound universe. (As the paper I linked explains, you can expect to see correlated circles, regardless of the details of the wraparound.) -- BenRG (talk) 17:48, 6 January 2009 (UTC)
Hmm, interesting stuff. The paper you link doesn't completely falsify Luminet, but it looks like Cornish et al bought a bigger computer and kept at it. Here they devote a paper to trashing all notion of a dodecahedron. So we still don't know the shape of the Universe and the LHC is still busted. C'mon scientists, get it together! :) Franamax (talk) 19:21, 6 January 2009 (UTC)

## Four fundamental interactions as four formulas

It's my understanding that the inverse square law applies to gravitation for most intents and purposes, but with famous exceptions such as the Mercury anomaly. I'm trying to understand (1) why exactly the relativistic understanding of gravity changed the actual calculations of orbit, and (2) whether all the fundamental interactions can be expressed as simply as gravitation (either Newtonian or Einsteinian) can. I know this is a "big" question, so feel free to contribute whatever you can — don't feel pressured to answer the "whole thing"! Also, let me know exactly how I am thinking about this incorrectly, as is usually the case with me and quantum physics.${\displaystyle \sim }$ Lenoxus " * " 05:01, 6 January 2009 (UTC)

It's not quantum physics here, but general relativity. Mercury does obey the inverse square law for gravity, but space is behaving weirdly. I think Kepler problem in general relativity will answer most of your question (if you get the maths, which I don't just now ;-). --Stephan Schulz (talk) 09:11, 6 January 2009 (UTC)
There are four known fundamental interactions: gravity, electromagnetism, strong nuclear and weak nuclear. Gravity, at least the Newtonian model of it, is pretty simple to understand. Electromagnetism isn't too bad, that's just Maxwell's Equations. It doesn't get it down to one equation but close. I haven't seen a simple expression for either of the nuclear forces. DrAstro (talk) 21:25, 6 January 2009 (UTC)
There are only three! Electroweak theory (unifying what was known as electromagnetic interaction and weak interaction) is well-established, and I think it's the one that's best understood. Gravity is hard to test experimentally because it's so weak, and the strong interaction is hard to compute because it's so strong. Icek (talk) 23:50, 6 January 2009 (UTC)

Thanks for all your responses! They've all helped a lot with my understanding of the subject. I think I'll rephrase my question as follows: Has all of physics been "in principle" narrowed down to the consequences of the fundamental forces, and if so, does that mean that everything in the universe can be understood as a consequence of three or four equations and the wave/particles they affect? (Obviously, the whole deal is a heck of a lot more complicated than that, but it's just that I figured that physicists wouldn't keep saying "just four/three forces!" if something like, say, momentum or thermodynamics was really just as fundamentally "mysterious" as gravitation.) ${\displaystyle \sim }$ Lenoxus " * " 20:56, 8 January 2009 (UTC)

All three theories are still subject to experimental tests (this is science!). And furthermore, not everything in the universe can be understood exactly because there are 3 theories and not just 1 (there are situations where all interactions are important, and the predictions are different depending on how you combine results from different theories). Attempts to get 1 theory for electroweak and strong interactions are called Grand Unification Theories (GUTs). Most attempts to get 1 theory for all three interactions are called quantum gravity as they are quantum mechanical in nature (Roger Penrose thinks that it should be non-quantum mechanical).
The equations are:
The latter 2 are quantum field theories, while general relativity (the theory of gravitation) is not quantum mechanical.
Icek (talk) 00:19, 9 January 2009 (UTC)

So to re-re clarify, there are formulas for each force, but not just one per interaction. Even though I understood that the different models/formulas/understandings conflict with one another in various ways, I guess I'd always assumed that each basic interaction got its own formula that could be described in concise language, the way each of Newton's laws can — and that everything beyond that involved the "mixture" of the formulas, something complex enough it couldn't all be done a priori. To put it all another way, why does an interaction that requires multiple formulas get to be considered "one" interaction, instead of the Standard Model saying, "Here are the ten fundamental forces that together produce gravitation, and here are the four that produce electroweak", ect? Thanks again for keeping up!${\displaystyle \sim }$ Lenoxus " * " 01:12, 9 January 2009 (UTC)

(Clarification: I know the Standard Model doesn't actually include gravity, but you know what I mean ${\displaystyle \sim }$ Lenoxus " * " 03:45, 9 January 2009 (UTC))
Maybe I can illustrate the reason using a simple example in electromagnetism - why e. g. Coulomb interaction and magnetic interaction are not considered separate interactions (I will neglect the weak interaction as well as quantum mechanics).
Consider 2 balls, each having a charge of +q. Now these balls are initially at distance x from each other and are traveling at the same velocity v (parallel to each other). The Coulomb interaction now says that they will repel each other. The magnetic interaction says they will attract each other because each moving charge is also an electric current. You can add the forces (you will always get a net repulsion, but of course smaller than the Coulomb repulsion).
Now consider the same problem in the inertial frame moving also at velocity v - the balls are initially stationary in this frame. They only repel each other by Coulomb's law. You might think this is a contradiction, as the repulsion is larger in one frame than another one, but it isn't, when viewed in the context of special relativity. Time dilation makes everything consistent (if the acceleration caused by the forces calculated for one frame is Lorentz-transformed to the other frame, the acceleration is the same as the one calculated from electromagnetism in the other frame).
My point is that a purely electrostatic phenomenon in one inertial frame has magnetic components in other inertial frames.
Icek (talk) 11:14, 9 January 2009 (UTC)

All right, I think that covers everything I was wondering to the best of my understanding. Thanks again to everyone, especially Icek! ${\displaystyle \sim }$ Lenoxus " * " 20:50, 11 January 2009 (UTC)

## mech engineering related(new idea)

i want to do project on solar pumps,in a new way.i want that hand-pump must be operated automatically. for that my idea is that like in IC as connecting rod connects piston and crank, in this my idea is to connect piston(some long rod may be connected along its axis)and handle of hand pump by a connecting rod, so that linear motion of piston causes handle to move so taht we can get water w/o human. my question is whether we can achieve this.pls answer its urgent —Preceding unsigned comment added by 210.212.223.138 (talk) 05:55, 6 January 2009 (UTC)

See Newcomen steam engine from 1712. Substitute solar energy for coal fired boiler. Edison (talk) 15:32, 6 January 2009 (UTC)
It doesn't sound very efficient to me, as there is an unnecessary conversion from rotational motion to linear mechanical motion. Instead, I suggest a small submersible pump, which pumps water into an above-ground tank when the Sun shines, to be poured out of the tap as needed. It would need a shut-off valve that triggers when the tank is full. This allows water to be had at night as well as the day, and doesn't waste solar energy when nobody wants water but the Sun is out. If in a cold climate at winter, you would need to protect against freezing by insulating the tank and/or placing it partially underground or inside a heated building (or, a more fun option is to add grain alcohol to the tank). StuRat (talk) 21:44, 6 January 2009 (UTC)

## Ice in craters of Mercury

There may be ice in craters at the North Polar region of Mercury. How many creaters with possible ice in them? What is the diameter and how deep? —Preceding unsigned comment added by Johnz Johnz (talkcontribs) 09:29, 6 January 2009 (UTC)

At first I thought this might be similar to the problem of measuring a coastline, that it's always longer if you look for smaller details. However, craters below a certain size probably can't support any ice, because the sunlight hitting the lip would warm the inside of the crater enough to melt any ice. This, of course, wouldn't apply to craters inside a larger crater, and thus in permanent shade. BTW, why not any at the South Polar region ? StuRat (talk) 21:30, 6 January 2009 (UTC)
Ice In An Unlikely Place: Mercury states that there are "about 20 circular areas" and there is a radar image/map at The Discovery of Water Ice on Mercury --mikeu talk 14:14, 8 January 2009 (UTC)

## Atomic masses

which element is chosen as a standard for measuring the atomic masses?why? —Preceding unsigned comment added by 59.103.70.116 (talk) 09:32, 6 January 2009 (UTC)

According to atomic weight the standard is 1/12 of the mass of an atom of carbon-12. I think the isotope is chosen because it has the same number of protons and neutrons. A historical account can be found in [2]. EverGreg (talk) 10:16, 6 January 2009 (UTC)
If you look at that "historical account", it specifically says that the carbon-12 scale was suggested because "because of carbon's use as a secondary standard in mass spectrometry" and because it gave numerical values close to the old scale used by chemists (which was based on a natural mixture of oxygen isotopes). Also, I remember reading in one of Isaac Asimov's monthly science essays that carbon-12 was adopted because it was particularly easy to measure and for the same reason regarding the numerical values. This is likely an interpretation of the same facts: if it was used as a "secondary standard" that was probably because it was easy to measure. --Anonymous, 19:34 UTC, January 6, 2009.
And don't copy the above word-for-word, everyone will know :) hydnjo talk 04:44, 7 January 2009 (UTC)

## chemicals for surgical gloves

i am small scale manufacterer of surgical gloves. i want to improve my product. so, i want a chemical composition for (natural latex)surgical gloves. Thanking you.Arijitkm (talk) 10:08, 6 January 2009 (UTC)

Natural latex is a polymer of isoprene. Graeme Bartlett (talk) 10:58, 6 January 2009 (UTC)
Makes me wonder where my hospital gets its surgical gloves :( hydnjo talk 04:41, 7 January 2009 (UTC)

## Definition of "Life"

How do we define "Life"? I have already looked up in a number of books. One said "Life is a set of characteristics which distinguish living organisms from non-living objects" but I want a definition without relativity with non-living things. Please note this is not a homework. Many thanks. —Preceding unsigned comment added by 59.103.70.227 (talk) 12:31, 6 January 2009 (UTC)

There is no hard, fast, agreed-upon definition for life. As our article notes, "to define life in unequivocal terms is still a challenge for scientists, and when derived from an analysis of known organisms, life is usually defined at the cellular level." Further, we note that life exhibits all or most of a given (and itself flexible) set of criteria. Our article contains many further attempts to phrase a concise definition. — Lomn 14:04, 6 January 2009 (UTC)
Well, "life is the process that organisms go through during their lifetime", perhaps? I'm not sure it's reasonable to demand a definition that doesn't relate to non-living things, though, simply because I'd say a big part of the definition is precisely that it's not a part of the non-living things. Not that you can't define it otherwise, but it might not be a very good or informative definition. -- Captain Disdain (talk) 14:06, 6 January 2009 (UTC)

See Life. It has a section on "definitions." -Arch dude (talk) 14:37, 6 January 2009 (UTC)

I've heard there are two kinds of people: These who divide things into distinct categories, and those who don't. What would be the criteria for determining whether a potato is "dead" or "alive" when it is on a shelf in my kitchen? It arrives full of vitality, and eventually sprouts. If planted, it would produce roots and leaves and more potatoes. Seems to be alive. But if I remove the "eyes" when it arrives, surely that does not mean it is suddenly dead, any more than a steer is dead. If I boil it long enough, that would seem to make it dead. If it rots, that would also seem to make it dead, although full of bacteria. Edison (talk) 15:22, 6 January 2009 (UTC)
Indeed - "There are 10 kinds of people in the world - those who understand binary numbers and those that don't.".
The example you give exhibits the same problem as the guy at the back of my high school biology class - we aren't defining whether some particular instance of a thing is "alive" at this precise instant - we're asking whether an entire class of things can be considered to be "life". Reproduction does indeed seem to be an important feature of life (rocks don't make copies of themselves - bacteria do - but viruses can only do so with help from something that is in itself able to reproduce) - but an individual creature may be considered to be alive even if it has not nor ever will reproduce. Indeed, creatures who are born with some congenital defect that prevents them from reproducing are not pronounced "dead". That's because we're talking about a general property of all such creatures - not of one individual. So the definition of "life" doesn't really encompass the issues of whether your potato is alive or not right now. That is the subject of an entirely different fuzzy term: "death" - which doctors and medical ethicists struggle with all the time...which is a similar debate - but over an entirely different word. Coming back to the "10 kinds of people" joke - the problem is that when you try to treat words like "planet", "life" and "death" as binary states, you get into a lot of trouble - and the two kinds of people you were talking about overlap fairly strongly with the 10 types I'm talking about! SteveBaker (talk) 16:03, 6 January 2009 (UTC)
The Stewart Test is probably the best you can do outside of a given context. --Sean 16:13, 6 January 2009 (UTC)
I remember reading a Lyall Watson book on the subject of "what happens after death", which was similarly ambiguous about the definition of "death". --TammyMoet (talk) 19:15, 6 January 2009 (UTC)
Entropy and life#What is life?. Schrödinger and Lehninger both state that life can be said to be anything which feeds off of negentropy. I think Gibbs free energy also has something to do with it. --Mark PEA (talk) 20:44, 6 January 2009 (UTC)
The trouble with negative entropy is that all sorts of things that are very clearly not 'alive' (refrigerators, for example) exhibit that. It's very easy to show it by (for example) programming a robot to sort a pile of randomly colored lego bricks into piles of red, blue, green and yellow. The entropy of the blocks after sorting is less than when they were randomized beforehand - so the robot is producing negative entropy...do we want to call a really simple 'bot qualify as 'alive'? I don't think so. SteveBaker (talk) 05:11, 7 January 2009 (UTC)
I've seen arguments, based on some definitions of life, that black holes are actually alive. They slowly move towards food sources. They maintain a high gravity within themselves which is the only thing keeping them from exploding. And there were a bunch of other arguments to attempt to satisfy the various established conditions of life. Anythingapplied (talk) 21:52, 6 January 2009 (UTC)
Again - if you make a bad enough definition - you'll either let in a lot of silly things like refrigerators and black holes - and you'll exclude a bunch of things like celibate biology teachers. I do think that (the capability of) reproduction is one of the touch-stones of life - so refrigerators and blackholes should certainly be excluded for that reason alone. I can't think of anything that I'd call "alive" that doesn't have the capability to reproduce (even if only at the cellular level). You need other caveats to avoid things like fires from being declared 'alive' (a fire creates more fire - it consumes food and produces waste - it moves away from places where there is no food and into places where there IS food - it is born and eventually dies - it breathes oxygen and expels CO2 - it consumes animals and plant material for nutrition). To exclude fire - you have to require inheritance - that some attribute of the parent is handed on to the offspring. But a definition that excludes 'artificial life' computer software (like Conway's Game of Life for example) is exceptionally tricky.
A LOT of the definitions to "Life" come down like Oliver Wendell Holmes, Jr.'s definition of obscenity, to paraphrase, "I may not be able to define it, but I know it when I see it". The problem is that any basic definition of life either excludes things which should probably be considered alive (like viruses and prions.) or includes things which should probably not be considered alive (like computers and fire and black holes and all sorts of weird things). --Jayron32.talk.contribs 13:23, 7 January 2009 (UTC)

## Whistle in water

When dip into water I hear (as everybody, hopefully) a sort continuous whistle, or hiss. I wonder what it is, and where it's coming from. Is it just my eardrums under the effect of the pressure, or is it a kind of ground noise that propagates particularly well into the water? --PMajer (talk) 13:27, 6 January 2009 (UTC)

You are hearing the underworld. Try to talk into the water... Do you hear replies? —Preceding unsigned comment added by 94.27.209.85 (talk) 13:52, 6 January 2009 (UTC)

no but I can read it --PMajer (talk) 14:44, 6 January 2009 (UTC)
I think it's like when you hold a seashell to your ear (actually, pretty much any concave object - a teacup for example - will do) and "hear the sounds of the ocean". What you're hearing is the blood flowing through your head that's being reflected back into your ear because of the unusual acoustics. SteveBaker (talk) 14:38, 6 January 2009 (UTC)
If blood makes a faint noise as it circulates, a microphone should pick it up and it could be amplified and heard. Furthermore, the microphone could be placed on any part of the body. I have never read that to be the case except for an audio stethoscope placed over the heart. Just what does a stethoscope pick up anyway? Blood rushing through heart valves? And what is a heart murmur? And just what is heard when a sea shell is held near the ear - is it resonance to ambient noise? No, a sea shell is too small to resonate to audible sound. Also, if two persons place their ears together, they will not hear anything, so why does a sea shell pick up sound emanating from the ear, and reflect it back? –- GlowWorm —Preceding unsigned comment added by 174.130.253.174 (talk) 23:43, 6 January 2009 (UTC)
There are lots of conflicting explanations around - and many pooh-pooh the old blood-flow explanation. However, putting a seashell over a microphone doesn't produce any sound at all - so THAT explanation is bogus. I wouldn't expect a microphone held over your body to produce a sound - the concavity of the seashell is acting like a 'retroreflector' - sending all of the blood supply sounds precisely back into your ear...that has the effect of amplifying that very subtle sound to the point where you can hear it. You can actually use your hand in place of the seashell - and that lets you adjust the shape. A flat palm doesn't produce anywhere near as much white noise as a cupped palm. The 'resonance' theory fails miserably here because that soft bag of water that is your hand can't possibly resonate in the same way as a seashell or a teacup - so how come it sounds the same? My 'focussing' explanation also explains why two people's ears put close together don't do it - the ear is carefully evolved to absorb the energy from feint sounds and translate them into nerve impulses - so it's not gonna reflect ANYTHING. Sure - this isn't a 100% accepted explanation - but it does fit the facts better than the others. SteveBaker (talk) 04:52, 7 January 2009 (UTC)
@GlowWorm: Your questions about what the stethoscope picks up, are answered in the articles Heart sounds and Heart murmur. --NorwegianBlue talk 15:29, 7 January 2009 (UTC)

## Charges on a sphere

Imagine positive charges confined to the surface of a sphere. If there are two positive charges then they will move apart until they are "poles apart".

But I have difficulties imagining three positive charges. If the location of one of the charges, is defined as position A, where will the other two charges be located relative to position A? 122.107.203.230 (talk) 13:40, 6 January 2009 (UTC)

While I'm not certain that charges would behave in this fashion, three equidistant and maximally distant points on a sphere are on a plane that bisects the sphere, 120° apart from each other. — Lomn 14:01, 6 January 2009 (UTC)
...and 4 would be located on the points of a regular tetraeder that is circumscribed by the sphere. Both solutions are non-unique unless you fix at least two of the points. --Stephan Schulz (talk) 14:10, 6 January 2009 (UTC)
• For greater clarity, in English that's a tetrahedron. --Anon, 19:36 UTC, January 6, 2009.
Thanks! --Stephan Schulz (talk) 22:32, 6 January 2009 (UTC)
I would expect a charged conductive sphere to have evenly distributed charges, in the absence of an external field. The "charges" would not generally isolate themselves to a small number of distinct locations. Do you consider each of your "positive charges" to be one atom which has given up an electron? What is the experiment, or thought experiment, exactly?Edison (talk) 15:15, 6 January 2009 (UTC)
The total energy of 3 unit point charges constrained on the unit sphere, ${\displaystyle \scriptstyle {\frac {1}{2}}\sum _{i\neq j}|x_{i}-x_{j}|^{-1}}$, is certainly minimized exactly when the charges are in the positions described by Lomn (equilateral triangle on a maximal circle). This is easy to see by direct comparison (one first proves that they have to stay on a maximal circle, then that they have to be equidistant). The general geometrical problem of the minimal energy position of n unit charges on the unit sphere is called Thomson problem (yes, we have it!!); here there are some nice pictures [3]. Note that the problem is not even obvious in the unit disk version; in particular for ${\displaystyle \scriptstyle n\geq 12}$ the configuration of minimal energy is not on the boundary of the disk, a 1985 result by A.A.Berezin [4]. --PMajer (talk) 16:17, 6 January 2009 (UTC)
Well, for 2, 3, and 4, it is easy, because they correspond to the 1-, 2-, and 3-simplices, respectively, because they all fit in 3 or less dimensions. However, I wonder if there is an easy systematic way to compute the optimal solution for higher numbers; say, if you had 17 charges. I wonder if the solution is even unique (up to rotations and reflections of course) in that case. --Spoon! (talk) 05:35, 7 January 2009 (UTC)
well, no... 4 is not that immediate, still it's true that it is the regular tetrahedron. In the reference above you can find a list of the numerical solutions up to n=400 and also interesting links --84.221.209.108 (talk) 09:44, 7 January 2009 (UTC)
By the way, I see in the wikipedia article that the solution for n=24 is still unique up to rotation and reflections, but not up to just rotations (it is not congruent by rotations to its mirror reflection, like a hand). It is given by the vertices of the snub cube, a chiral polyedron--PMajer (talk) 16:16, 7 January 2009 (UTC).

## Breathing in cold weather

When I want to go to sleep, I unconsciously breathing using my mouth. Is it normal? When I remember I try to breath using my nose, but in a while it's just getting back using mouth. Is it a phenomenon in cold temperature or is it just me? Second question, when breathing in cold weather, the air we breath in should be converted to our body temperature, right? How cold it is that human will feel uncomfortable breathing in cold temperature? What about people who is standing in an -20 degree Celcius environment, is the air temperature would change from -20 to 37 degree just within a few seconds? Thanks for the response. roscoe_x (talk) 15:55, 6 January 2009 (UTC)

See, believe it or not, "Mouth breathing". The nose warms air before it gets to the lungs more efficiently than the mouth does, so I would expect nose-breathing to be a better cold-weather strategy. As for the temperature that would cause discomfort in breathing, I suppose that that depends on the person. The coldest I've been out in is minus 5 Farenheit, about...let's see, 32 is zero, 1.8 per degree, 5 below is 32 + 5, 37 divided by 1.8 is times 5/9, roughly 20 below, Centigrade, I mean Celsius. I don't remember having trouble breathing, but that's me. --Milkbreath (talk) 20:54, 6 January 2009 (UTC)
I sometimes feel a burning in my lungs when breathing extremely dry, cold air. It starts somewhere around 14°F, or -10°C, but, as I said, the humidity may also play a part. When it gets colder than that, I try to breathe through a scarf, as that gives the air more time to warm up. For someone at the South Pole at it's coldest, I'd expect they'd want to use a tube to breathe through, wrapped several times around their torso, so it warms up a lot before hitting their lungs. StuRat (talk) 21:22, 6 January 2009 (UTC)
I've never heard of someone using a tube for breathing like that in Antarctica, but then again, I'm sure that people improvise a lot down there. It kinda makes sense to me, but also potentially very, very inconvenient, not least so because when the cold gets really extreme, most flexible things -- like long plastic tubes -- tend to become rigid and brittle. Do they really do that? Man, it's a fascinating place.
Anyway, as for breathing in cold temperatures, I've been at and around -30 degrees Celsius, which isn't a lot of fun, but I don't recall having much trouble breathing, at least when compared to how my nose and cheeks felt at the time. That said, if you run around or do something else that makes you breathe heavily, you'll definitely feel it in your lungs. It's certainly not very gentle weather, especially if it's windy (though make no mistake, it's almost balmy compared to the South Pole during the cold season). -- Captain Disdain (talk) 23:21, 6 January 2009 (UTC)
I've been out in -40C (also -40F) a few times and didn't have much trouble breathing. However, I noticed a strange sensation in my nose that I think was the water vapor in my breath freezing on the hairs in my nose. After about 5-10 minutes my sinuses started to hurt and they cooled down. After that if I took a deep breath my throat hurt, but I never noticed any problem with my lungs. Of course they are in your chest and should be at core body temp, which should stay high unless you're getting ready to freeze to death.Tobyc75 (talk) 03:39, 8 January 2009 (UTC)
How much the air warms up before it hits the inside of the lungs depends on how much, and how fast, you are breathing. If you are running or shoveling snow vigorously, it won't have much time to warm up. StuRat (talk) 20:25, 9 January 2009 (UTC)

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## Gaining immunity to the common cold?

Hopefully, this question isn't considered medical advice. Per the Wikipedia article, "Common colds are most often caused by infection by one of the more than 100 serotypes of rhinovirus, a type of picornavirus. Other viruses causing colds are coronavirus, human parainfluenza viruses, human respiratory syncytial virus, adenoviruses, enteroviruses, or metapneumovirus. Due to the many different types of viruses, it is not possible to gain complete immunity to the common cold."

I have 3 questions:

1) Does that mean that each time someone catches the common cold, he or she builds an immunity to that particular virus? (I'm pretty sure the answer is yes, but I want to confirm.)

2) As a person gets older (ignoring the effects of aging) is he/she less likely to get a cold since he/she may have immunities to dozens of cold viruses?

3) Does the immune system 'forget' how to fight a virus if it's been a long time since it encountered a virus? For example, I were 100 years old, would my immune system remember how to kill that cold virus I had when I was 20?

216.239.234.196 (talk) 16:32, 6 January 2009 (UTC)

The problem with immunity to viral diseases is that viruses mutate so quickly. You catch this year's strain of cold - now you're immune so you probably don't get another cold this year - but as the virus makes its way around the world and comes back the following season - it's mutated to the point where your immunity stops working and you catch it all over again. That's why (for example) influenza shots have to be given each year - and the manufacturers have to guess which mutant of the 'flu virus will strike this time around.
SteveBaker (talk) 17:58, 6 January 2009 (UTC)
1) Yes.
2) Not really, as the strains to which they have developed an immunity no longer exist, having died out after a significant portion of the population became immune to them. Entirely new strains replace them.
3) Yes, loss of immunity does occur over time. First there is a period of partial immunity, then eventually none at all. StuRat (talk) 21:11, 6 January 2009 (UTC)
2) Yes to some extent. The Spanish flu is supposed to have affected younger people disproportionately because of this. Dmcq (talk) 08:35, 7 January 2009 (UTC) Sorry I see the latest theory is that people who had a good immune system died because of a cytokine storm and the old with weaker immune systems livd through that. Dmcq (talk) 08:52, 7 January 2009 (UTC)

## Fighter plane firing a bullet

What is the effect on the speed of a fighter plane chasing another when it opens fire? What happens to the speed of pursued plane when it returns the fire? Please explain. Also note, it's not a homework. Many thanks. —Preceding unsigned comment added by 59.103.69.24 (talk) 20:33, 6 January 2009 (UTC)

I think recoil is a good place to start. Also see Newton's laws of motion- in particular, the third law. Friday (talk) 20:36, 6 January 2009 (UTC)
And yes, there are extreme instances of this. The A-10 Warthog's main gun has a recoil on par with the thrust of one of its two engines (see the GAU-8 article). — Lomn 21:36, 6 January 2009 (UTC)

## Shape of Planets

What is the reason for all the planet has spherical shaped? 91.140.217.144 (talk) 21:42, 6 January 2009 (UTC)

Because they're massive enough that their gravity is strong enough to pull them into a hydrostatic equilibrium. In fact this property is now part of the definition of a planet. Algebraist 21:45, 6 January 2009 (UTC)
Note that they aren't perfect spheres, however. They are all somewhat elliptical due to centripetal force, and can have some "lumps" as well. StuRat (talk) 01:04, 7 January 2009 (UTC)
Oblate spheroid is the actual term for their shape, in case you were wondering.-RunningOnBrains 01:15, 7 January 2009 (UTC)
A simple way to think about this is that gravity is pulling everything as close together as they can get. Any bits of the planet that were a lot further away from the center than the rest would tend to move to be closer to the center...this results in everything ending up at the same distance - and the geometric shape that has everything at the same distance is a sphere.
Think of it another way - if the planet was (say) a cube - then there would be eight amazingly tall mountains (one at each corner) - these would be so massive that they would tend to slump and collapse under their own weight - so gradually, the planet would turn back into a sphere again.
The reason planets are slightly 'oblate' (squashed at the poles and fatter around the equator) is because they are spinning and at the equator (where they are spinning faster) the centrifugal force slightly opposes gravity - that "weaker" gravity results in that bulge.
Only 'lumps' that are small enough and strong enough to stay put under their own weight can withstand that tendancy to becoming spherical - hence mountains here on earth don't get bigger than Mt.Everest. On lower gravity worlds (Mars, for example) there are bigger mountains - "Olympus Mons" is truly, stunningly gigantic!
SteveBaker (talk) 04:33, 7 January 2009 (UTC)
See geoid and equipotential surface. If a planet's surface were covered by liquid then (in the absence of tides) the liquid would assume a shape such that the local apparent gravity vector (true gravity vector - centripetal force vector) was everywhere perpendicular to its surface - this shape is a geoid. Rock, unlike liquid, is able to resist tangential forces, so it can form mountains and valleys that deviate above and below the geoid surface. Gandalf61 (talk) 11:32, 7 January 2009 (UTC)

## Deliriants vs. Hallucinogens

Hi, just a quick question. What is the difference between a hallucinogen and a deliriant? I have heard the 2 terms and don't know the difference.Cssiitcic (talk) 23:08, 6 January 2009 (UTC)

Delirium says: "While the common non-medical view of a delirious patient is one who is hallucinating, most people who are medically delirious do not have either hallucinations or delusions". --Sean 00:10, 7 January 2009 (UTC)
Hallucination is the sensing of objects which are not in the real world. There are halucinations of every sense (visual, auditory, tactile, etc.) but the common thread is that our brains tell us something is there which is not. Thus, a hallucinogen causes that reaction in the brain. One can be perfectly lucid and still be having a hallucination. Delirium is a loss of lucidity, that is one becomes less than perfectly responsive to the outside world. Its not that you see stuff thats not there, its that your brain does not react "appropriately" to stuff that IS there, so you don't recognize the real world for what it is. It is also entirely possible to be delirious without hallucinating; and there are of course situations where both are occuring... --Jayron32.talk.contribs 13:11, 7 January 2009 (UTC)
To confuse matters Delirium tremens does cause hallucination as one of its symptoms. 76.97.245.5 (talk) 16:21, 7 January 2009 (UTC)
I think the general view is that hallucinogens produce hallucinations which the person is aware are hallucinations. Where as deliriants produce more vivid hallucinations (like those in dreams) which the person is not aware are hallucinations. --82.21.25.219 (talk) 17:51, 7 January 2009 (UTC)
The above was me by the way. I don't like to recommend anecdotal evidence, but trip reports are one of the only sources of evidence for this kind of thing (due to obvious legal and ethical reasons). If you are further interested in this kind of thing, on Erowid compare the trips of say magic mushrooms ([5]) with belladonna ([6]). My judgement is that being on a typical anticholinergic delirant is like dreaming whilst being awake, which could cause some large issues such as walking into roads or cutting your self completely unknowingly. --Mark PEA (talk) 18:10, 7 January 2009 (UTC)