Wikipedia:Reference desk/Archives/Science/2011 February 21

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February 21

Conservation of (angular) momentum

I remember an argument Feynman used to show that the conservation of angular momentum implies the conservation of linear momentum. I've never seen this anywhere else, though, so is it possible that he was mistaken? The proof goes something like this:

Suppose we have a system of particles. Conservation of L doesn't depend on where the we choose the axis, so we can choose an axis that is very far from the all the particles. Then, all the particles will have the same position vector r relative to this axis, which doesn't vary with time. Then ${\frac {d}{dt}}\mathbf {L_{TOT}} ={\frac {d}{dt}}\sum \mathbf {L_{i}} ={\frac {d}{dt}}\sum \mathbf {r} \times \mathbf {p_{i}} =\mathbf {r} \times {\frac {d}{dt}}\sum \mathbf {p_{i}} =\mathbf {0}$ implies ${\frac {d}{dt}}\sum \mathbf {p_{i}} =\mathbf {0}$.

The only flaw with this argument that I can see is that r will be infinite, but I don't see how that invalidates the proof. Any insight? 74.15.137.130 (talk) 04:59, 21 February 2011 (UTC)

So basically he views the conservation of linear momentum as a special case of the conservation of angular momentum, where the radius is infinite. Sounds reasonable, to me. StuRat (talk) 05:20, 21 February 2011 (UTC)

Okay, after a little more thought, I realized that the reason that conservation of L w/ respect to one axis --> conservation of L w/ respect to all axes requires that the center of mass' velocity be constant, which is true iff momentum is conserved. So it's not all that surprising after all. 74.15.137.130 (talk) 06:42, 21 February 2011 (UTC)

If our choice of axes is one very far from all the particles, then r will be very large but it won't be infinite. It would be possible to do this with full mathematical rigor by taking the limit as r increases without bound. Some people would summarise this by saying the limit as r approaches infinity but increasing without bound is more rigorous than approaches infinity.

There is much in common between these two conservation laws. I assume Feynman was saying that if we have established the truth of one of these conservation laws then a consequence is that the other is also true. It doesn't matter whether we start by establishing the truth of the conservation of linear momentum, or angular momentum. Either way, it follows that the other is also true. Dolphin (t) 11:23, 21 February 2011 (UTC)

I don't think that angular momentum conservation follows from linear momentum conservation. For linear momentum conservation you just need "for every action there is an equal and opposite reaction", but for angular momentum conservation you also need to know that the action and reaction act along the same line. In terms of Noether's theorem, you can imagine a space that's translationally but not rotationally invariant (fill it with a constant vector or tensor field), but not the other way around. -- BenRG (talk) 11:37, 22 February 2011 (UTC)

Noether's theorem connects symmetries to conservation laws. I'm just parroting the article here, but a rotationally invariant/symmetric Lagrangian implies conservation of angular momentum, while symmetry under a continuous translation in space implies conservation of linear momentum. I guess that would be a good starting point if you want to derive this more rigorously. EverGreg (talk) 12:45, 21 February 2011 (UTC)

I think this is very easy: any translation is a composition of two rotations. -- BenRG (talk) 11:37, 22 February 2011 (UTC)

Infinite universe and probabilities

My friend told me a theory which he had heard from someone who had read it from somewhere. It's been bugging me for a week now: I believe it must be false, but I just can't tell him exactly why (except that the "from someone/where" part is pure snopes.com material). The theory goes something like this: if the universe is infinite, then there are infinite possibilities how matter is organized. Thus, the probability that in some place else in the universe the matter is organized exactly like here is 1. This would mean that there are at least two copies of me around, sitting in this universe typing this post, although the other guy might have some atoms missing.

My answer to him was that at least the amount of matter in the universe can't be infinite. But is there a more accurate / scientific answer available? --Albval (talk) 07:37, 21 February 2011 (UTC)

Yes, and there are several variations on the theory. See multiverse.--Shantavira|^{feed me} 08:00, 21 February 2011 (UTC)

This isn't actually about the multiverse - it applies equally well to a single universe operating under known laws of physics. Back to the question, while the amount of mass in the visible universe must be finite, there is no theoretical limit on the total mass of the entire Universe; it may well be infinite (see shape of the universe for speculation that touches on the potential size of the universe). Someguy1221 (talk) 08:34, 21 February 2011 (UTC)

The probability of something happening may be 0, (impossible) in which case it still does not happen even if there is an infinite chance of it. Graeme Bartlett (talk) 09:48, 21 February 2011 (UTC)

Graeme, I was just about to respond to Albval on this point; I thought you would know better. Probability zero does not mean impossible, and probability one does not mean certain. See our almost surely article. --Trovatore (talk) 09:50, 21 February 2011 (UTC)

Forgive me for being uncivil, but the "almost surely" article is patent nonsense. An event with probability exactly equal to 1 is identical to an event with probability $0.{\overline {9}}$ and is exactly guaranteed to occur. The "corner-cases" mentioned in the "almost surely" article deal exclusively with numerical models of statistical systems. I think some statisticians need a formal retraining in measure theory, basic mathematical limits, and the finite precision capabilities of numerical computers. The example case of the "dart" landing on an abstract concept (a "line") is merely a restatement of Zeno's paradox (something to the effect of never being able to get arbitrarily close to the abstraction of the imaginary line). See also, treatment of infinity in computers. This is a matter of axiomatic definition: an event with probability zero will not happen. Nimur (talk) 20:32, 21 February 2011 (UTC)

No, you're just wrong. Consider an infinite sequence of fair-coin flips. For any fixed ω-sequence of heads and tails, the probability that the actual sequence will match it is zero (so the probability that it will not match it is one). However, if every actual sequence were impossible, then it would be impossible for the sequence to come out any way at all, which is nonsense.

It is true that these concepts don't correspond to physical experiments that are easy to design, but they are absolutely fundamental to probability theory as it is modernly conceived. There are people who don't like them, but they have basically lost. You're the one who should learn measure theory. --Trovatore (talk) 22:58, 21 February 2011 (UTC)

I think the theory is correct. --Gr8xoz (talk) 12:27, 21 February 2011 (UTC)

The source: This exact theory, which relates to our universe under the assumption of inflation in an infinite universe, was put forward in 2001 by Alexander Vilenkin of Tufts University in Medford, Massachusetts and Jaume Garriga of the Independent University of Barcelona. Inflation guarantees that the different regions are isolated. The time elapsed since the big bang determine the size of each region (how far we can see in the universe).

As for "exactly like here", Vilenkin and Garriga argued that there are only a limited number of distinct universe configurations in the infinite universe. While say the distance between two atoms may be a continuous variable, 'quantum blurriness' means that two distances in two worlds can't be told apart if they'r too small. Vilenkin and Garriga estimated that there were only 10^10150 distinct histories in the universe.

Just because something is infinite, doesn't necessarily mean that every possible configuration has to happen in that infinity. A simple example is the decimal expansion of π which is known to be infinite, but it is not known whether it contains seven consecutive sevens (for example). See Trovatore's link above. Dbfirs 14:36, 21 February 2011 (UTC)

The decimal expansion of π is infinite, but it is not a random sequence of digits (because it can be encoded in a finite number of symbols, a series or 4 atan(1)). Whether that is true of the Universe is an interesting question but I guess it is metaphysical rather than physical. The probability of finding a copy of OP in the observable universe is 0 or virtually 0. --Wrongfilter (talk) 15:23, 21 February 2011 (UTC)

Er, Random#In mathematics says it's likely that π is random in some senses. I think you're taking an information-theory approach, not a math/statistics approach? But also, how would one write a finite expression to generate the numerical represenatation of pi? To get the digits, you have to start doing actual iterations of the expansion (and an infinite number of them to get the infinite-decimal-places string), not just say "in the theoretical limit, it is symbolically exactly equal to pi" I think the best you can say is that the whole value is something like non-arbitrary or that it's reproducible (I'm trying to pick words that don't have subtly different technical meanings than in lay-language), not that the string is a "non-random sequence of digits". DMacks (talk) 17:19, 21 February 2011 (UTC)

The digits of pi are absolutely not random. For example, the first one is always a 3. When people say "the digits are random", they (if you really pin them down) usually mean that pi is a normal number, which is only a conjecture. Staecker (talk) 01:10, 22 February 2011 (UTC)

It's "only a conjecture", but it would be astonishing if it were false. Mathematicians are extremely careful to separate what has actually been proved from what has not, which is a useful trait, but it sometimes causes outsiders to think that there is reasonable doubt about propositions when there really isn't. --Trovatore (talk) 01:59, 22 February 2011 (UTC)

In an generic infinite universe, not only would there be an infinite number of copies of you (actually of the entire visible part of the universe), but you also cannot identify yourself as any single copy. This is due to the fact that you have a limited amount of information about yourself. So, e.g., if you are not bald and assuming that you haven't counted exactly how many hairs you have on your head, there will be different versions of you, each with a different amount of hair and they all share the same personal identity. This despite the fact that the distance between the copies is astronomical, as Tegmark points out here. Count Iblis (talk) 16:35, 21 February 2011 (UTC)

One thought experiment that might help to understand this is "if there were an infinite number of monkeys randomly typing away on typewriters, they would eventually reproduce all the works of Shakespeare". You might think this is impossible too, but here would could actually calculate how many it would take. Let's say we just want to see the word "HI" (ignoring case) and that there are 100 keys on the typewriter (a typewriter with fewer extra keys would obviously give better odds, though). For two letters, it should take, on average, 100² or 10,000 monkeys (or one monkey with that many trials). For a 50 letter sentence, it would take 100⁵⁰ monkeys. If we assume Shakespeare's entire work is 10 million letters long (if anybody has a better figure than this guess, please let me know), that should take 100^10,000,000 monkeys. That's a very large number, but definitely less than infinity. StuRat (talk) 19:27, 21 February 2011 (UTC)

If anyone want to try this at home I really recommend implementing the standard RFC 2795. When running large experiments it is important to follow relevant open standards to avoid vendor lock in. I also think it would be a good idea to run it on RFC 2549 in in order to increase the biological diversity. Unfortunately RFC 2460 is way to limited to be used by it self. (Why do they never learn that you should not build standards with arbitrary limits) --Gr8xoz (talk) 21:35, 21 February 2011 (UTC)

I take it that site is just an elaborate hoax ? StuRat (talk) 22:35, 21 February 2011 (UTC)

*blink* Um, IETF? No, they are the standards-body for pretty much everything that lets your computer interact with wikipedia's servers. DMacks (talk) 22:41, 21 February 2011 (UTC)

Well, not the entire works but you need look no further then the Infinite monkeys article. Even if the observable universe were filled with monkeys typing from now until the heat death of the universe, their total probability to produce a single instance of Hamlet would still be less than one in 10^183,800.... but not zero! lol. Vespine (talk) 22:59, 21 February 2011 (UTC)

Ah, but that said the "observable universe", which is infinitely smaller than an infinite universe or infinite multi-verse. StuRat (talk) 07:57, 22 February 2011 (UTC)

You might want to take note of the publication dates for RFC 2795 and RFC 2549. Red Act (talk) 23:17, 21 February 2011 (UTC)

As far as I know, though, there's no RFC for IP over demographics. --Trovatore (talk) 23:27, 21 February 2011 (UTC)

No, they are actually the main standardization organisation for the internet protocols but they actually have humour, it is tradition to release a joke standard on the Aprils fools day. This is the case for RFC 2795 and RFC 2549. We even has an article about this tradition April Fools' Day RFC This started when the internet was very much smaller than to day, the very first joke standard was written when there was less than 300 computers on the Internet. RFC 2460 is a real serious standard for IPv6 the internet protocol that will replace IPv4. IPv4 has only 2^32=4.3*10^9 addresses and that will not be enough for all our devices in the future so IPv6 has improved this to 2^128=3.4*10^38. It is often joked about how huge this address space is and that it should be enough for all civilisations in the universe. --Gr8xoz (talk) 23:45, 21 February 2011 (UTC)

Thank you everyone for answers! Hopefully, somewhere in this universe there is a copy of me who understood them all:-) Albval (talk) 19:37, 24 February 2011 (UTC)

Diving: max. depth without minding

How deep can you dive without caring about your security? —Preceding unsigned comment added by 212.169.187.5 (talk) 13:44, 21 February 2011 (UTC)

Just to clarify, do you mean how deep should the water be in a swimming pool before you should dive into it, or how deep can you go when SCUBA diving? Googlemeister (talk) 14:19, 21 February 2011 (UTC)

Either way, the answer to the question depends on how stupid the diver is. Dauto (talk) 14:23, 21 February 2011 (UTC)

I meant SCUBA + free-diving. And no, the answer do not depend on how stupid the diver is. The question is how stupid the diver can be, without consequences. 212.169.177.33 (talk) 14:31, 21 February 2011 (UTC)

May be that's what he wanted to ask. But that's not what he asked. Dauto (talk) 14:51, 21 February 2011 (UTC)

Our article on Deep diving has some recommendations. Dbfirs 14:43, 21 February 2011 (UTC)

It's not just the depth, but how fast you reach this depth, how long you remain there and how fast you come back to the surface. The type of breathing gas you use is also relevant here. The answer for SCUBA is simply that you'll go through some certification program and keep the rules that you learned there. Quest09 (talk) 15:03, 21 February 2011 (UTC)

Free-diving is not limited by risk of bends (there is none) or breathing gas. Not sure what it is limited by, other than the ability to hold ones breath, but the no limits world record is 214 metres according to our article. SpinningSpark 18:09, 21 February 2011 (UTC)

As stated above, free-divers don't get bent. In SCUBA diving, this question is not answerable as asked, because you could get bent, theoretically, even diving to a 10 foot depth if you stay down long enough. Looking at the dive tables isn't actually helpful; the NAUI dive table doesn't start until 40 feet, but this is because people don't really do 20-foot dives, not because you're immune to the bends if you only do 20-foot dives. The question could be answerable if you included a time limit. Comet Tuttle (talk) 18:33, 21 February 2011 (UTC)

There have been cases as low as 6' deep getting the bends if the diver is there for hours and the dive was at a mountain lake. Googlemeister (talk) 22:15, 21 February 2011 (UTC)

The bends is not the only risk. Even in a short dive to 20 feet, it would be easy for a novice diver to accidentally inhale some seawater, panic, and shoot to the surface without remembering to breath out, which can do significant damage to the ears or lungs. There is really no type of scuba dive where the diver can be stupid without consequences. Looie496 (talk) 19:14, 21 February 2011 (UTC)

Agreed; I am assuming the original poster is asking about decompression sickness. Obviously a 1 inch dive could be fatal if you stupidly breathe in all the water possible. Comet Tuttle (talk) 19:43, 21 February 2011 (UTC)

"The bends" is one of many scuba-diving hazards. In addition to "the bends," decompression sickness covers more of the hazards related to re-gassification of dissolved nitrogen. Other risks for ordinary-air SCUBA diving include oxygen toxicity, and nitrogen narcosis - both of which can occur while still under water. Proper SCUBA training will teach you how to correctly account for each of these risks. As has been mentioned, the "maximum depth" is a complicated factor - it depends on how long you stay at each depth, and how long since your previous decompression or surface trip. Divers use a dive table or dive computer to assist them in calculating safe dive depths and dive profiles - because "safety" is not described by one single depth value. If you breathe a gas other than air, such as nitrox or trimix, or if you are a technical diver and breathe multi-gas out of various cylinders in sequence, other hazards can exist. If you are an underwater construction expert, you may dive a profile that is considered "unsafe" by recreational standards. If you are a Navy Seal, you may be ordered (or choose) to use a dive-profile that could be fatal (but Navy SEALs understand that risk of fatality is a part of their job). If you dive at altitude, such as in Lake Titicaca, you run additional interesting risks related to pressurization; divers typically undergo a specific training for "altitude diving" such as the NAUI Recreational Altitude Diver course. If you will be diving out of a submarine or hyperbaric chamber, your safe-dive profile is very different than a surface scuba dive. Nimur (talk) 20:08, 21 February 2011 (UTC)

In fact, we have an entire article: Maximum operating depth. Needless to say, "don't try this at home" - of course, technical dives require special training and equipment. Nimur (talk) 20:11, 21 February 2011 (UTC)

Has anyone tried to find a way to allow exchange of oxygen while free-diving, so that it would have the advantages of scuba? I mean, I think in theory you should be able to stick a pair of tubes down into someone's lungs, or even (maybe) provide oxygen via someone's dialysis shunt? A way to provide only the truly necessary amount of oxygen while leaving the lungs collapsed? Wnt (talk) 21:01, 21 February 2011 (UTC)

No, I'm not aware of any such technology. It would be incredibly hazardous to attempt to exchange gas at below the ambient water pressure. (Gas will not flow against the pressure gradient; you have to depressurize the lungs below ambient pressure to "suck" air in. That depressurization, which we normally call "inhaling", would be the same as "crushing" if ambient water pressure was very high; alternatively, your hose can "squirt" over-pressured air into the lungs, but the risk of hyperinflation is incredibly hazardous and defeats the point of a free-dive. For these reasons, a SCUBA regulator regulates the breathing gas pressure to ambient water-pressure. If for any reason you do not want the human to be exposed to ambient water pressure (such as when you dive to great depths), you use a pressure hull to isolate their atmosphere. A rigid diving suit can do this - but no such suit can withstand great depths. Most good pressure hulls look more like a submarine - sort of spherical or cylindrical, so there are no regions where force is concentrated (so the material won't crumple). Then, the "ambient" pressure inside the hull can be maintained far below ambient water-pressure. Submarine atmospheres are often kept higher than 1 atm, but still at a safer level than the water pressure. Take a look, for example, at bathyscaphe or bathysphere. I believe the crew of ALVIN breathes at atmospheric pressure - but they have two inches of solid titanium to keep the water out. Nimur (talk) 21:25, 21 February 2011 (UTC)

I don't know if anything like what I'm suggesting has been attempted, but I don't think it's that impossible. If you have a sack of oxygen in equilibrium with the external water pressure, then just a slight extra pressure should send it into the lungs; likewise the return tube should carry out air with just a slight pressure. Of course, tubes to the lungs (at any pressure) might have safety issues of their own, but I don't see why there has to be a hazardous amount of crushing or exploding involved. (The 'sack' might still have some minimum pressure it holds at the start, and use a regulator at such low pressures, to limit its buoyancy near the surface). Wnt (talk) 01:49, 22 February 2011 (UTC)

If you regulate the pressure, you have SCUBA! And if you don't regulate the pressure, it won't work. You might be underestimating the crushing-force that 10 meters of water can exert; you physically will not be able to inhale unpressurized air. It will be prohibitively difficult for you to "breathe in" air that is pressurized at 1 atm when your body is surrounded by 2 atm of pressure - your diaphragm musculature just isn't strong enough to work across that pressure gradient. (In other words - as you try to "breathe in" by expanding your thoracic cavity with your diaphragm muscle, the water will literally crush your lungs and your thoracic cavity back to the original shape). This is exactly the reason why you can't get away with a super-long snorkel: you need the air to be pressurized very close to ambient water-pressure so that the air pressure inside your lungs helps your diaphragm work, pushing back against the crushing-force of the water pressure. (Here's a "How Stuff Works" post explaining in more detail). Let me give you another example of exactly how much crushing you are subject to while diving: when you're down at even mild depth, say 50 feet, your weight belt (which was tightly fitted at the surface) will be very loose and often requires re-adjustment; and if you tighten it at the bottom, you'll find it uncomfortably tightens and squeezes your waistline pretty hard when you re-ascend. The weight-belt, usually made of nylon, barely changes size at all - but both your wetsuit and your body have been significantly and measurably "shrunk" by the massive compressive force of ambient water pressure. Nimur (talk) 03:26, 22 February 2011 (UTC)

I was talking about a flexible "sack" of air at the same level as your body, subject to the same force, under the same pressure. Wnt (talk) 19:21, 22 February 2011 (UTC)

If you could keep it from floating away, it might work; it'd be like SCUBA, but with an inconvenient air-tank. You would still have the exact same risks of SCUBA, though - you'd be breathing pressurized air, which means that if your downtime was long, nitrogen could supersaturate in your bloodstream, and you could get the bends; and you'd still have to worry about oxygen toxicity and nitrogen narcosis, just like regular compressed air dives. Nimur (talk) 21:34, 22 February 2011 (UTC)

Hmmm... if you don't add nitrogen, there should be no nitrogen narcosis. Having the air in the lungs be pure oxygen might be a problem, yet I'd think that with the lungs collapsed that less oxygen would be entering the bloodstream than with them expanded. And I wonder if the amount of nitrogen can be optimized more precisely. Wnt (talk) 02:48, 25 February 2011 (UTC)

I was snorkeling once and saw a shell, what must have only been 3 meters below me. I'm a pretty good swimmer but I've never had any diving training and never been deeper then the bottom of a pool. It took about 5 attempts to get down that deep and I regretted it when I got back up, my ears hurt quite badly for about an hour. It was a lot more taxing and dangerous then I suspected, so I don't recommend it unless you have an experienced diver give you some training. Vespine (talk) 22:03, 21 February 2011 (UTC)

On the ear thing, the Valsalva maneuver is what divers use as they descend in order to prevent their ears from feeling like someone is stabbing knives into them. I'm a little surprised to read that article talks about blowing the air with "moderate" pressure; I would never do it with anything stronger than "gentle" pressure. Comet Tuttle (talk) 18:41, 25 February 2011 (UTC)

Do not hyperventillate, even in shallow water, as it may lead to a shallow water blackout. ~AH1^(TCU) 21:26, 26 February 2011 (UTC)

DNA and RNA

Would the complimentary messenger RNA strand of a coding DNA strand, be an exact copy of the coding DNA strand, only with the execption of replacing T(thymine) with U(uracil)? Thanks.

for example

ATCGAATT dna coding strand

AUCGAAUU RNA messenger —Preceding unsigned comment added by 99.146.124.35 (talk) 14:58, 21 February 2011 (UTC)

Yes, as explained in Coding strand. Assuming no mutations occur, the RNA messenger will be the same except for the Uracil/Thymine. Chipmunkdavis (talk) 15:06, 21 February 2011 (UTC)

If an extra A is added after ATC in the DNA messenger strand, It would change many of the animo acids in the sequence. What exact processes would cause this mutation to happen? —Preceding unsigned comment added by 99.146.124.35 (talk) 15:26, 21 February 2011 (UTC)

Non sense mutation

Would

ATC-GAA-TTC-CGA-CCA-TGC... (non mutated dna)

ATC-AGA-ATT-CCG-ACC-ATG-C... (mutated added extra A)

Would it be consitered a nonsense mutation, Frameshift mutation or a missense mutation? —Preceding unsigned comment added by 99.146.124.35 (talk) 16:25, 21 February 2011 (UTC)

The example given is a frameshift mutation. You could get a frameshift in the RNA relative to the DNA by translational frameshift or RNA editing. Wnt (talk) 17:27, 21 February 2011 (UTC)

how much of an effect do daily multivitamins really have?

how much of an effect does taking a daily multivitamin supplement (like Centrum or whatever) every day really have on one's health? How would it compare to, say, cutting out one bad thing you do (go to mcdonald's, or not exercise enough). I mean, is it really a drastic effect, if you take those properly for the rest of your life, or more like accredited homeopathy. nobody really knows if it does anything, but why not? 109.128.213.73 (talk) 19:18, 21 February 2011 (UTC)

It's not correct to characterize homeopathy as "nobody really knows if it does anything". Homeopathy is known scientifically to be useless, and there are a lot of references in the homeopathy article to back that statement up. People buy that stuff because of cultural, psychological and marketing reasons, not because there's any scientific uncertainty about it being nonsense. Red Act (talk) 20:54, 21 February 2011 (UTC)

Forget something? Thank you for understanding my analogy. I've marked where you forgot to answer my question by applying the analogy to what I'm asking about. 109.128.213.73 (talk) 21:35, 21 February 2011 (UTC)

It depends drastically on the individual. There's no question that there are people with serious vitamin deficiencies, and also no question that many people have none at all. Wnt (talk) 19:25, 21 February 2011 (UTC)

Though the benefits of daily multi-vitamin pills can be (and are often) exaggerated, please do not confuse it with homeopathy. Unlike the homeopathy, the effects of vitamins, particularly vitamin_deficiency are well documented by the scientific community. SemanticMantis (talk) 19:40, 21 February 2011 (UTC)

Well, see our Multivitamin article; there are "evidence for" and "evidence against" sections. Comet Tuttle (talk) 19:42, 21 February 2011 (UTC)

My understanding is that the multi billion dollar "supplement" industry is mostly, quite literally "pissed" away (excuse the profanity). Unless you have a pregnant or have a deficiency, which most people in the developed world don't, there is not much evidence that taking supplements has any benefit. In fact, there is some evidence to suggest some supplements, specifically anti-oxidants, might actually have a negative effect. Vespine (talk) 22:04, 21 February 2011 (UTC)

A multi-vitamin is no substitute for a healthy diet. They only contain a portion of the healthy nutrients which we know about.

Also, some of the nutrients they contain may be slightly different formulations which may not be bioavailable, or may need to be taken in conjunction with certain foods to be absorbed. Furthermore, you can potentially overdose on some vitamins, and having them in pill form can make that easier to do.

Finally, if you use your multi-vitamin as a justification for continuing to eat junk food, keep in mind that the pills do nothing about the excess calories, sugars, animal fats, trans fats, bad cholesterol, and sodium you are getting.

So, eating a good diet will provide you with all the nutrients you need, in a form you can use, including things like fiber, which are difficult to give in pill form, due to their bulk. StuRat (talk) 22:23, 21 February 2011 (UTC)

Some dietary supplements are critical for optimum health. Count Iblis (talk) 00:18, 22 February 2011 (UTC)

I'm not sure how you reach that conclusion from the linked reference. Vespine (talk) 00:26, 22 February 2011 (UTC)

Last line of first paragraph:

As director of LPI, I am often asked what supplements I take—after all, thinking about and researching micronutrients every day, I should know what dietary supplements are most important. While I think eating a healthy diet, exercising regularly, maintaining a healthy body weight, and avoiding tobacco are of utmost importance to maintain good health, I also think that some dietary supplements are critical for optimum health.

Count Iblis (talk) 00:46, 22 February 2011 (UTC)

Sorry, are you kidding or? Just because someone thinks it, doesn't make it so. Linus Pauling is well known for his controversial views on vitamins and megadosing so you might want to read the claims coming from his eponymous institute with a low dose of sodium. Vespine (talk) 01:34, 22 February 2011 (UTC)

Linus Pauling is long dead, the LPI is named in his honor. The statement is made by the current director of the LPI: Balz Frei, Ph.D.

LPI Director and Endowed Chair Distinguished Professor of Biochemistry and Biophysics. Count Iblis (talk) 14:00, 22 February 2011 (UTC)

Taking regular exercise and avoiding McDonalds style junk food would have a much bigger effect than taking a multi-vitamin. I used to take multi-vitamins, but after reading scientific research about it I concluded that it was better to get your vitmains and minerals from a healthy diet including plenty of fruit and veg. Although I do take Vitamin D in the winter, as I live in the north. 92.15.8.100 (talk) 12:40, 22 February 2011 (UTC)

I also only take vitamin D supplements and I'm thinking of taking fish oil supplements. Count Iblis (talk) 14:03, 22 February 2011 (UTC)

I prefer eating sardines to taking fish-oil supplements. Being young fish, sardines have not accumulated any significant mercury as other fish do. They will contain many more nutrients than fish oil pills, including for example high levels of B12. I also eat tinned wild Alaskan salmon, which has plenty of fish oil in it. 92.28.241.15 (talk) 16:07, 22 February 2011 (UTC)

I also preferred these things ... but the damned gout says otherwise. Whether it's just the purines or the heavy metals also, fish oil offers an alternative for odd situations. Not that I ever remember to take it... Wnt (talk) 19:24, 22 February 2011 (UTC)

Cold

Cold slows down chemical processes, so why would that not apply to my own body? Even if it slows down only some of my cells some of the time, I'm sure that adds up to save my body some body life-cycle units (I'm not a biologist) in the long term. I don't know how it works, sorry, that is why I'm asking. I looked up stats and northern countries seem to have longer average life spans, and northern US states seem to have longer average lifespans. I realize there are many factors, such as availability and quality of health-care to be factored in, so those numbers alone are not good enough. —Preceding unsigned comment added by 88.160.92.233 (talk) 21:34, 21 February 2011 (UTC)

Your body is very good at regulating core temperature. In most of the places where it counts (i.e. internal organs, brain) your temperature does not vary much at all (just a few degrees) due to external temperature.Vespine (talk) 21:54, 21 February 2011 (UTC)

More like a few tenths of a degree. StuRat (talk) 22:08, 21 February 2011 (UTC)

If you could keep someone alive while their core body temperature is lowered, then you could theoretically extend life. (However, when you hit freezing, their cells all explode, and this certainly doesn't help extend life. ) This might actually be a reasonable medical treatment at some point in the future, if we can figure out how to make people hibernate, like many other mammals. This could keep someone alive, say, until a cadaver heart could be delivered from a pro-democracy demonstrator in China. :-) StuRat (talk) 22:08, 21 February 2011 (UTC)

Hypothermic people have been able to in some cases go without breathing for much, much longer then usual, our article suggests as long as 1 hour, but as the mortality from that particular situation runs 38-75%, I wouldn't want to try it. Googlemeister (talk) 22:12, 21 February 2011 (UTC)

Our Normal human body temperature article actually states In adult men and women the normal range for oral temperature is 33.2–38.2 °C so normal variation is about 5 degrees, but yes in a individual the normal range appears to be about 0.7 of a degree. I'm not sure if at all that correlates with the climate a person lives in. Actually, that reminded me that this "slowing down" is done during some surgeries, like Cardiopulmonary bypass surgery: hypothermia is maintained; body temperature is usually kept at 28ºC to 32ºC (82.4–89.6ºF). This gives the surgeons a lot longer to perform the surgery while lowering the risk of damage to other organs due to lower blood pressure or oxygen availability. But it's certainly not a state you would like to be in while conscious. Vespine (talk) 22:50, 21 February 2011 (UTC)

I can say from personal experience that this concept works well in nematodes (cold-blooded, if you count them as having blood at all); their life span extends significantly from lowering the ambient temperature from, say, 15^oC to 4^oC. I believe the same is true of flies (certainly, their life span takes longer), but I haven't bothered to deconvolute their longer lifespan from the increased availability of food (they eat less at low temperatures).

However, the important distinction humans have in this case is we're warm blooded. We've evolved as warm blooded for tens of millions of years, and as such, everything in our bodies is designed to operate at ~37^oC. Lower that, and things shut down. Maybe you could keep someone alive on life support at relatively low temperatures, but you're not going to eek out a meaningful existence at a persistent state of hypothermia. Nematodes and flies are used to massive fluctuations in their body temperature, so they can handle it. Someguy1221 (talk) 21:04, 22 February 2011 (UTC)

"Personal experience" ? So you're a nematode ? Welcome to Wikipedia, you may well be our first invertebrate member ! :-) StuRat (talk) 22:29, 22 February 2011 (UTC)

Something else to note is that even if we could get people to survive at lower body temperatures, and thus extend their life, and we were able to have them conscious, as well, it still wouldn't seem any longer, since their brain processes would be slowed down along with the rest. Everyone else might appear to go zipping by and speak too fast for them to understand. StuRat (talk) 22:18, 22 February 2011 (UTC)

This effect can be quite dramatic in terms of developmental biology - to quote some unpublished data, if you take buckeye butterfly pupae and inject them with certain drugs you can change the pattern that forms on their wings. If you put them at 4 C for a week and inject them just before or just afterward, you get the same patterns as if they had not developed more than a few hours in the whole time. How this works I have no idea, though I'm suspicious that c-myc is involved. Wnt (talk) 06:39, 23 February 2011 (UTC)

See also North-South divide and multiple sclerosis. Life expectancy is dependant on many demographic factors and not likely directly on average temperature and climate. ~AH1^(TCU) 21:18, 26 February 2011 (UTC)

Plants

Are there any plants that photosynthesis in moonlight, or entirely in moonlight and never sunlight? — Preceding unsigned comment added by K4t84g (talk • contribs) 21:37, 21 February 2011 (UTC)

I doubt it. First, the visible light from the Moon is orders of magnitude less. But, more importantly Also, there is virtually no proportionately less UV in moonlight, which is what some plants tend to may also use for photosynthesis. StuRat (talk) 22:03, 21 February 2011 (UTC)

Plants don't tend to use UV for photosynthesis, see for example this graph and Simarouba amara#Physiology where there is some data on the wavelengths that chlorophyll and leaves absorb - most of it is the visible spectrum. CAM plants (like cacti) are as close as you'll get to photosynthesis in the dark, but they only temporary fix CO₂ to use in photosynthesis the next day. SmartSE (talk) 00:48, 22 February 2011 (UTC)

While there is an absorption tail on many of the photosynthetic pigments that extends into the ultraviolet, absorption at wavelengths shorter than 400 nm accounts for a very small fraction of total photosynthesis. (See also photosynthesis, action spectrum, absorption spectra of plant pigments.) Elevated UVB exposure – caused by the ozone hole over Antarctic waters – has been shown to decrease the activity of marine phytoplankton; such exposure has also been shown to have variable but generally negative effects on most plants tested: [1]. It's also worth bearing in mind that many plants are grown quite successfully in greenhouses, whose glass panes reduce UV exposure. TenOfAllTrades(talk) 14:57, 22 February 2011 (UTC)

One more point—while the moon's reflectivity is lower at near-ultraviolet wavelengths, it's substantially better than zero. This report (figure 4.2, middle row) shows spectra of sunlight (left) and reflected moonlight (right). While the UV 'shoulder' below 400 nm is somewhat smaller in moonlight (relative to the visible portion of the spectrum) than it is in sunlight, it's definitely present in respectable relative amounts. It is the deficiency in absolute quantity of light that is most problematic here, not a spectral issue. TenOfAllTrades(talk) 15:10, 22 February 2011 (UTC)

OK, I modified my answer to de-emphasize UV light. StuRat (talk) 20:56, 22 February 2011 (UTC)

According to Howard Griffiths of Cambridge University, "probably no". In fact, he states that plants avoid moonlight, speculating that it might disrupt their circadian rhythm. Clarityfiend (talk) 22:08, 21 February 2011 (UTC)

Deeply ingrained superstitions...

I'm sure that I'm not the only person who finds himself in this situation sometimes - that is, knowing in my head that a certain superstitious behaviour/reaction is pure bunk, but still getting a deep-seated feeling of 'wrongness' in the gut if one does not act in the accepted manner in a situation where folklore suggests that bad luck may result.

For example. I see one magpie, I look around for another one. If there is only one magpie present, I salute the bird and verbally acknowledge its presence.

Or never, ever, under any circumstances walking under a ladder. Even if it means going out of my way.

Or feeling slightly uneasy if the numbers 13 or 666 come up in random situations and taking steps to avoid prolonged exposure to the number.

Just wondering if there is a name for this conflicted feeling? Thanks. --95.148.108.189 (talk) 21:56, 21 February 2011 (UTC)

If you want fancy words to describe this scenario, you might say that you "disbelieve the superstitions, but the prevalence of these cultural presuppositions amongst your peer group has had a normative social influence on you, and you willingly suspend rational analysis in certain circumstances." You could also use the term "cognitive dissonance" to describe the conflict between rational and irrational thought. Nimur (talk) 22:02, 21 February 2011 (UTC)

Sometimes conscious understanding of an illusion "breaks the spell" but there are lot's of illusions that are not dissipated with conscious knowledge. Sometimes you have to try really long and hard to break the habits you've been raised with and sometimes you will never truly rid yourself of them. I heard once that there have been "superstition parties" where people go to break mirrors and walk under ladders etc.. to try to clear them selves of silly beliefs / habits, but i can't find any reference off hand. Vespine (talk) 22:39, 21 February 2011 (UTC)

Having been brought up without these superstitions, I have no problems with 13 or broken mirrors or ladders (though I do check that nothing is likely to fall on my head before I walk underneath). In the past, I have had to fight (mentally) to avoid being "infected" with these superstitions from friends who do seem to believe in them. The cultural transfer seems to be surprisingly strong. Dbfirs 00:12, 22 February 2011 (UTC)

Could this be because the alleged 'punishment' for not warding off the superstition (be it death, a curse, eternal damnation, etc.) sounds so much worse than the minor little act (such as politely greeting a magpie - I know a lot of otherwise rational people who are terrified of a lone magpie) required to ward it off? Sort of a 'just in case I'm being too smart for my own good and it turns out to be real' thing... --Kurt Shaped Box (talk) 14:03, 22 February 2011 (UTC)

There actually is a word for it: compulsive. In obsessive–compulsive disorder these feelings become so strong that they can make life miserable, but a great many people show milder forms of obsession or compulsion. Looie496 (talk) 00:52, 22 February 2011 (UTC)

• This brings to mind Niels Bohr's (alleged) response to being asked if he really believed a horseshoe over his door would bring him luck - "Of course not ... but I am told it works even if you don't believe in it". DuncanHill (talk) 18:07, 22 February 2011 (UTC)

Could someone expand on the references above to saluting/greeting a lone magpie? The article doesn't seem to have anything and I've never heard of such a thing. Matt Deres (talk) 20:05, 22 February 2011 (UTC)

There's some information at European Magpie#Magpie in culture, referenced to Brewer's Dictionary of Phrase and Fable. Karenjc 20:15, 22 February 2011 (UTC)

Magpies seem to have evolved a powerful survival mechanism in relation to humans. Much like dogs and cats. 20:19, 22 February 2011 (UTC) — Preceding unsigned comment added by Bus stop (talk • contribs)

The name for this is "being human". thx1138 (talk) 22:46, 22 February 2011 (UTC)

Don't you mean "being unknowingly conditioned by the culture in which you live or were raised"? Dbfirs 09:36, 24 February 2011 (UTC)

Gasoline Octane - can you FEEL the difference when you drive?

So my buddy fills up the tank of his crappy, old Chinese-brand automobile yesterday and promptly declares that switching from 94 to 97 octane fuel has made a tremendous difference in the way it runs. I find this hard to believe and suggested it was a bit of bias due to the higher price he paid. He insisted he was correct. He's agreed to blind test octanes over several tanks of gas, but that's going to take awhile. In the meantime, I'm asking here: can you feel the difference between octanes when you drive, particularly between such a small change as 94 to 97? I hypothesized that a high performance vehicle might run differently, but a beater like his wouldn't know 97 octane if it bit it in the bumper. There's just too many other inefficiencies in the engine for that to bubble to the top as a defining quality. What says the RefDesk? The Masked Booby (talk) 22:05, 21 February 2011 (UTC)

Higher octane is not "better" fuel - it is "chemically different fuel that burns with different parameters." If your engine is not designed for high octane, you will obtain worse performance by filling up with it. A common symptom is engine knocking. See our article's section: effects of octane rating. Nimur (talk) 22:07, 21 February 2011 (UTC)

You won't get any knocking by using higher octane gasoline. It is just a waste of money. Dauto (talk) 22:25, 21 February 2011 (UTC)

High octane gasoline typically has a lower calorific value than the normal stuff. So it is possible the he would see an unmeasurably slight deterioration in performance and/or economy. However there are all sorts of possibilities. If he's happy paying for 97 octane, why not? In my own car using premium gets rid of a slight knock on acceleration, makin the engine seem smoother. Frankly i am not prepared to pay extra get rid of it. Greglocock (talk) 22:37, 21 February 2011 (UTC)

I once fell victim to the claim that high octane gasoline "helps clean out an engine". What it actually does is make the CHECK ENGINE light come on. Wnt (talk) 22:53, 21 February 2011 (UTC)

The octane rating has to do with the requirements of high-performance, high-compression engines or turbocharged engines to fuel that isn't subject to pre-detonation ("knock") in the compression stroke, which can damage the engine. The fuel is less volatile (in a different sense from its tendency to evaporate) under compression than "regular" fuel, and, as pointed out, can have less energy content. It has no value at all in an engine that doesn't specifically require the higher rating. It isn't better, regardless of the silly "premium" marketing, it's just suited for specific engines that need it to perform their best. If the engine isn't designed to demand high octane ratings, you're wasting money and buying lower-performing fuel at a higer price. Acroterion _(talk) 02:08, 22 February 2011 (UTC)

One kind of obvious point that no one seems to have mentioned is that it could be that your friend's car pings at 94 octane, but not at 97. Yes, he would notice that difference, for sure. It seems a little unlikely because it's usually very high-compression engines that want the very high octanes. On the other hand, I believe that old, dirty engines are more prone to pinging, so it's not totally beyond the realm of belief. --Trovatore (talk) 02:14, 22 February 2011 (UTC)

Here's a fun summary : The Straight Dope : What's the difference between premium and regular gas?

APL (talk) 06:00, 22 February 2011 (UTC)