Wikipedia:Reference desk/Archives/Science/2009 June 7

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June 7

bound photons

Spontaneous emission of bound photons from relativistic free electrons. What is a "bound photon"? Please provide an answer that can be understood by a biologist. --JWSchmidt (talk) 01:57, 7 June 2009 (UTC)

From a really quick glance at the referenced paper it looks like the "bound photon" is a photon in one of the cavity modes, that is, a photon emitted in a discrete rather than a continuous spectrum. Cavity modes must obey the boundary conditions imposed by the cavity walls, and therefore may not, generally speaking, have just any wavelength (and any energy), but only one of the discrete -- but infinite -- list of allowed wavelengths (and thus allowed energies). Generally, a particle is said to be bound when it cannot escape to infinity in any direction. An energy spectrum of such particle is discrete, at least as long as the potential it is bound in is not changing too fast or the lifetime of the particle is not too short. --Dr Dima (talk) 02:25, 7 June 2009 (UTC)

Thanks. Cavity quantum electrodynamics talks about cavity photons functioning as part of a quantum computer. Would such photons be "bound photons"? --JWSchmidt (talk) 02:45, 7 June 2009 (UTC)

Yes, I think they would be. --Dr Dima (talk) 03:56, 8 June 2009 (UTC)

Many-worlds Question

Is it possible that beings from another universe can come into ours and interact with us? Or if quantum immortality is true, then won't a person visit us, or has already visited us (even me personally)? —Preceding unsigned comment added by 66.66.149.43 (talk) 04:41, 7 June 2009 (UTC)

No. Nimur (talk) 05:00, 7 June 2009 (UTC)

There is nothing in physics which would prevent this from happening, because what you are describing is beyond the purview of physics (which only deals with our universe, even if other universes exist). It is on the same level as asking "is it possible that God exists?" Of course it's possible, but physics has nothing to say on the matter.-RunningOnBrains^{(talk page)} 05:07, 7 June 2009 (UTC)

Mind me asking what the heck is quntum imortality? Dauto (talk) 06:19, 7 June 2009 (UTC)

Why not read our article Quantum immortality? Nil Einne (talk) 08:36, 7 June 2009 (UTC)

"Quantum immortality" is an incorrect and backwards reading of the anthropic principle. Its premise is something like this - "if a being is conscious, then all previous incidents which might have rendered it no longer conscious did not occur." For some reason, which is the really illogical part, it then concludes, "no future event will occur that will render this being unconscious (dead)." Somehow, this is linked into the idea of a quantum non-deterministic universe, by way of superposition of all possible outcomes of every possible event - and self-selection (by anthropic principle) of only those events which maintain the being as conscious (living). This doesn't really have anything to do with "quantum" anything. I'm not even really sure why this theory gets propagated - it's sort of a half-baked, nonsensical idea. But, like perpetual motion, immortality seems to spark some kind of fury in non-scientists, who hope to cherry-pick the parts of science that will somehow support their wishful thinking. Nimur (talk) 14:41, 7 June 2009 (UTC)

I think you are the one misreading it. The theory is more like: If the "many worlds" interpretation of quantum mechanics is true - then every quantum-mechanical event that can turn out in more than one way results in multiple universes in each of which the event happens differently. Since all events are ultimately quantum mechanical in nature - there is a universe in which you personally died because you were killed in a car wreck that you narrowly avoided in this universe. There is another in which you died due to a childhood disease. There are in fact an infinite number of universes in which every possible fate that could befall you has indeed befallen you.

Now - marry that (highly theoretical, and possibly dubious) hypothesis with the "anthropic principle". The 'strong' anthropic principle says that the universe is conducive to your existance BECAUSE if it was not, you would not be here to observe it. So, (for instance) it has been remarked that if the earth did not have a large moon - or if the charge on the electron differed by 1% or if water didn't expand a little as it froze - then life on Earth would be impossible. Why are we so amazingly lucky in all these respects? Well, the anthropic principle points out that in all existances in which there is intelligent life that is able to observe these conditions - the conditions must be conducive to that life.

OK - so put the many worlds and anthropic principles together - and you say that since (by definition) the only universes in this infinite number of universes in which you are able to note that you are still alive are the ones where you happened (by sheer chance) to have had all of the prior quantum events come out in favor of you living. Taken to an extreme - you cannot die because there will always be a universe in which by an AMAZING sequence of lucky breaks and one in a quadrillion chances - you happen to survive. Hence, you PERSONALLY will be immortal - although those around you will not (although they'll all be immortal in some other universe).

However, those who think this is "wishful thinking" need to carefully consider what this really implies. Suppose you are really sick - something painful and incurable. With this combination of hypotheses, you can't die. No matter what - you'll be in a universe where (though in continuous and nearly unbearable pain) - you can't commit suicide. You put a gun to your head and pull the trigger to try to end your misery...the gun will jam - or a freak gust of wind will blow the gun out of your hand. No matter what you do, there is no way out. Given you'll live forever - it's certain that you'll lose limbs, become blind and deaf - be in continuous anguish...for all eternity. If ever there was something close to old-testament hell - then Quantum Immortality is it.

(Going off-topic here.) Steve, I thought you said you worked your way through the whole Bible once? Where exactly did you find this "old-testament Hell"? There are a few lines in the Book of Daniel, but other than that Hell seems to be almost entirely a new-testament concept, as far as I can tell. The OT God might have done a bit of smiting, but once the smitten were dead, the narrative appeared to stop. --Trovatore (talk) 21:11, 7 June 2009 (UTC)

I don't remember much of whatever I read of the bible but based on the behaviour of God in the OT I would be just as worried about the OT heaven as I would be the (largely undisscussed?) hell. Let's face it, that God was psychotic.Nil Einne (talk) 19:55, 9 June 2009 (UTC)

I sure hope there is an error in the theory somewhere!

SteveBaker (talk) 17:10, 7 June 2009 (UTC)

Also, not every divergence into "many worlds" will result in an explicit separation between "living" and "dead" - there are hundreds of trillions of quantum "decision points" which occur all the time, and most of them have no impact on whether or not you will be conscious and alive at the next "time step." (Think about every photon of sunlight colliding with every molecule of nitrogen on its way down to earth - most of the time, whichever way the stuff scatters out, you live anyway). But, the many-worlds interpretation suggests that the universe necessarily schisms at each of those. So, there should be exponentially increasing numbers of universes, most of them pretty similar, with an exponentially schisming "thread" of consciousness. Which one is the "real" you? This whole infinite-universe-schisming methodology is sort of silly, especially since it yields absolutely nothing predictive or otherwise scientifically helpful in understanding our universe. Nimur (talk) 18:30, 7 June 2009 (UTC)

And it also does not follow from logic or basic understanding of quantum mechanics. Quantum mechanics describes statistical probabilities for certain state transitions, rather than deterministic behavior. However, the common interpretation that there is a "small but nonzero chance" that anything can happen is simply incorrect. There is a probability density function for any physical observable, and that probability can and is often zero for certain outcomes. So, "immortality" implies that at no time will you ever encounter any situation which has a "dead-end" on the constantly schisming set of outcomes - which is sort of silly also, given that we observe living creatures dying all the time. Changing the quantum state of a bunch of atoms must still follow rules. Nimur (talk) 18:36, 7 June 2009 (UTC)

I think you really need to start by defining "universe". I would (roughly) define "universe" to mean a collection of causally connected things (you may not have everything causally connected to everything else, but there needs to be at least a connection via other things). By that definition, any being that can enter our universe is part of our universe. So, by definition, the answer to your question is "no". --Tango (talk) 16:17, 7 June 2009 (UTC)

Thanks for the explanations. I can't help but think that calling that wishful thinking is too generous. Hogwash comes to my mind. Dauto (talk) 22:29, 7 June 2009 (UTC)

OK, great points all around first of all. To me, quantum immortality makes sense. But because I can personally theoretically live forever, then that means that everything works like that, right? Since everything is made up of quantum particles then everything should have "quantum immortality" in the sense that anything can happen to it, it's just that in this universe we're observing it doesn't. But wouldn't it follow that if anything is possible then wouldn't someone from another universe be able to travel to this universe and say "hey! I'm from another universe!"? Because it's a possibility. So if the theory is true, then how come this hasn't happened OR is it completely probable that it can happen, it just hasn't happened yet? —Preceding unsigned comment added by 66.66.149.43 (talk) 00:22, 8 June 2009 (UTC)

A possible answer is that it is physically impossible to travel between universes. Everything which can physically happen, in some universe somewhere, does; but not anything else. A more gratifying answer is that it is physically impossible unless a time machine exists at the destination. (That is to say an inter-universe travel machine, universes and moments in time being the same thing.) 213.122.32.39 (talk) 03:52, 8 June 2009 (UTC)

Another thought which just occurred to me is that the emergence of a traveler into just one universe, and not any of the very similar alternate versions of that universe, is by definition an extremely unlikely event. ("The same across universes" is the definition of likely.) So if time machines have perfect accuracy, then you'd pretty much never see anybody arrive, just because of probability. Then again I suppose you have to multiply that tiny probability by the number of possible visitors, which is very large since it includes everybody in all possible futures (even though most of them wouldn't want to come here), so in conclusion, meh. 213.122.32.39 (talk) 04:22, 8 June 2009 (UTC)

Speeding up formal education

What research has been done into increasing the rate at which students can complete their formal education (e.g. decreasing sleep and leisure requirements to allow a greater course load, increasing retention from lectures, increasing comprehension-adjusted reading speed, increasing learning ROI on practical exercises)? Is this likely to become more of a priority if the demand for highly skilled workers continues to increase? NeonMerlin 04:42, 7 June 2009 (UTC)

I think a lot of research has been done on time-to-productivity. In my fields, there is an always-raging debate between the need for advanced degrees versus simple bachelors' degrees - in other words, which is more productive, two years of on-the-job training, or two years of post-graduate study towards a Master's Degree ( ... or six years of post-graduate training for a Ph.D.)? There's a lot of academic debate about this as well. Most academic research into education is not about "speedup" as much as it is about improved retention and comprehension. There's a lot of research on "force-multipliers" - such as using technology to allow one teacher to impact more students. I think this is the most practical framework you will find for "speeding up" formal education. Also, the military has developed a huge body of rapid training - take a look at AKO or NKO. I hear that the language courses for many overseas deployments claim to teach "basic fluency" from zero prior knowledge in the span of six or eight weeks (though this may be a dubious claim). Nimur (talk) 04:59, 7 June 2009 (UTC)

The question that comes to mind is "..and then what?" Formal education should not be a goal to be completed in and of itself. Used to be a time where it was thought to prepare you for a career in your later life. Standard testing (which works for some) has created a slew of students who are not prepared for anything but solving standard tests. There are "robot hack" jobs that require nothing else. The worrysome thing is that ingenious and creative minds that industry would need fall through the hiring cracks. As a consequence technological advances and inventions needed to stay ahead of the curve aren't happening. (And we'll all go wondering why the next economic crash happened.) If you don't spend time-off from studying for "leisure requirements" how do you ever build up a social network. Your course results will only get your resume past the software keyword engine in hiring. If you don't show sufficient social skills during the interview you won't get hired. (OR: Just last week my hubby's group turned down the more qualified applicant because they all agreed he appeared to be hard to work with.) Word of mouth is a far more effective tool for getting contracts or getting hired than any other I know of. Most (better) universities check applications for extracurricular activities. 71.236.26.74 (talk) 19:26, 8 June 2009 (UTC)

Magnetic force based "black holes"

Is there a "Black hole" based on the force of magnetism rather than gravity? -- Taxa (talk) 04:58, 7 June 2009 (UTC)

No. Nimur (talk) 04:59, 7 June 2009 (UTC)

Black holes form by a runaway collapse of matter, i.e. increasing density causes further increase in density ad infinitum. Since there is no phenomenon in physics by which a magnetic field could intensify itself, a "magentic hole" is impossible. -RunningOnBrains^{(talk page)} 05:03, 7 June 2009 (UTC)

but the magnetic force is much stronger than the gravitational force (brian greene: "a puny magnet can pick up a paper clip, overcoming all of the massive Earth's gravity" [paraphrased]. In fact gravity is some e-42 as strong as magnetic force!).

So it is a valid question, could we collapse a black hole by magnetic compression? 79.122.74.190 (talk) 07:35, 7 June 2009 (UTC)

No, as Runningforbrains said, the issue is not the strength of the force, but the feedback loop that is set up. Gravity results in an increase in density, causing increase in gravity. There is no mechanism by which the magnetic force could alter its magnetic domain and cause an increase in force, without the external application of energy.YobMod 08:16, 7 June 2009 (UTC)

My question is whether a traditional, gravity-based black hole, could be created through the initial compression of the proper mass down to the proper size not due to Gravity, but through a magnetic force? The point is that without the magnetic compression, the mass would not be critical enough (small enough volume) to turn into a black hole, but with the magnetic compression, the mass becomes small enough to pass the critical point and become a blcak hole. Would this be possible? 79.122.44.114 (talk) 10:23, 7 June 2009 (UTC)

Sure. It doesn't matter how the necessary density is achieved, just that you get enough mass into a sufficiently small volume. Some physics models predict the possibility that we will even be able to create micro black holes using the Large Hadron Collider. (In the LHC, very large amounts of energy – effectively mass, by E=mc² – get packed into a very tiny space when highly energetic particles collide. There's an outside chance that sufficient density can be achieved to make tiny, temporary black holes in this way.) TenOfAllTrades(talk) 15:38, 7 June 2009 (UTC)

Why would the black holes from the LHC be temporary? Once they are formed, doesn't anything that come within their event horizon just increase their mass (remaining in a point volume), making them ever bigger and bigger black holes? If not, then what's to stop whatever process would end these black holes from ending (on a larger scale) normal black holes? Thanks! 94.27.225.206 (talk) 15:53, 7 June 2009 (UTC)

See Hawking radiation. In theory, the smaller the black hole, the shorter its lifespan. A black hole created by the LHC would likely evaporate before it even gobbled up a single atom. This is why actual physicists are not concerned by the possibility of mini-black holes, even if a few botanists and biochemists are. -RunningOnBrains^{(talk page)} 16:29, 7 June 2009 (UTC)

The magnetic force is not stronger than the gravitational force. They have different units. It's like saying a pound is heavier than a foot is long. The closest you could get is using Planck units, in which case the gravity and the electromagnetic force both have a strength of one. — DanielLC 15:29, 7 June 2009 (UTC)

Daniel, you should be more careful with what you say. I can't think of a single way in which your statement that the electromagnetic force may have a strength of one would make any sense. The most natural way to measure the strength of the electromagnetic force is through its coupling constant which is adimensional. Dauto (talk) 17:53, 7 June 2009 (UTC)

Muscles + Oxygen = Energy

Would any of you know of an exercise or a few types of exercises to make your muscles require less oxygen?

Sleep. Cuddlyable3 (talk) 09:58, 7 June 2009 (UTC)

I don't think there's anything that makes them require less oxygen, but I know anaerobic exercise makes it so you can get oxygen there faster. — DanielLC 15:24, 7 June 2009 (UTC)

Intelligence as absence of stupidity

It is often said that "intelligence has its limits, but stupidity knows no bounds." This suggests that intelligence is merely the absence of stupidity, just as cold and darkness are the absence of light and heat (hence there is a minimum possible temperature but no maximum possible temperature), and that as a perfect vacuum would be perfectly dark, so too would it be perfectly intelligent. Is there a theoretical elementary particle that carries stupidity? Is it possible that some or all Standard Model particles carry it? NeonMerlin 07:07, 7 June 2009 (UTC)

Um what? Particles carrying stupidity??? Intelligence is present in a vacuum??? How do you even define intelligence and stupidity let alone carry them in particles or observe them in vacuum? I think your confusing a cute, quaint or fun comment with some sort of deep scientific statement. I doubt even many philosophers will make such a suggestion Nil Einne (talk) 07:14, 7 June 2009 (UTC)

It's also said, among other things, that a stich in time saves nine, but to hear that and start wondering what kind of a garment the stitches are on is to miss the point of the proverb. These aren't some kind of universal and literal hard facts, you know. -- Captain Disdain (talk) 07:28, 7 June 2009 (UTC)

Sayings do hold deep truths but may have to be slightly adjusted to be exactly right. For instance one can't have one's cake and eat it, unless one does them in that order (and I'm not going to think of the other alternative). A bird in the hand is worth two in the bush, unless you're talking about women. If pigs could fly a lot of biology and physics would have to be rethought but I'm sure we'll manage it one day with genetic engineering. I think in this case the saying is referring to the fact that for instance an intelligent person would say something intelligent and then stop whereas an idiot could go on wittering for ever. And furthermore.... :) Dmcq (talk) 07:49, 7 June 2009 (UTC)

It simply means that stupidity is not a conserved quantity. SpinningSpark 09:26, 7 June 2009 (UTC)

Intelligence is an attribute that can be defined and tested in various ways. There is no consensus on its limits. The word stupidity is not as suggested the opposite of intelligence but a disparaging term for negligent thinking. Persons that have high intelligence by some criteria, e.g. IQ score, are not immune to stupid actions. The OP offers hypotheses about a particle nature of stupidity that fail the scientific method because they lack falsifiability or a useful prediction. Cuddlyable3 (talk) 09:53, 7 June 2009 (UTC)

I'm sure I saw an advertisement that said there was a clever-dumb balance so it must be a conserved quantity. Advertisements are covered by the advertising standards agency and must be truthful. Dmcq (talk) 14:23, 7 June 2009 (UTC)

I presume you are referring to the toothless Advertising Standards Authority (United Kingdom) created by the advertising industry themselves whose idea of "self-regulation" is the occasional meaningless ticking off. If you believe anything they say . . . Stupidity behaves rather more like the (unconserved) entropy equation;

$\Delta S=\int {\frac {-dQ}{T}}$

Where Q is intelligent actions and T is the "intelligence temperature" - average intelligence per individual. SpinningSpark 13:18, 8 June 2009 (UTC)

Does anyone else see how ironic this question is? --Shaggorama (talk) 22:52, 7 June 2009 (UTC)

I do. There's even a related old saying, "It is better to remain silent and be thought a fool, than to open your mouth and remove all doubt". I think next week I'll post a topic here that says that I heard that all people who post here are actually dimwits who are in denial. We'll count how many responses come back as verbose versions of "No we're not!". They probably won't 'get' that either.

Heat death, temperature and entropy

The article about Heat death says "The heat death is a possible final state of the universe", and that is a state in which "it has reached maximum entropy" and 'In a "heat death", the temperature of the entire universe would be very close to absolute zero'. The article about entropy says "When heat is added to a system at high temperature, the increase in entropy is small. When heat is added to a system at low temperature, the increase in entropy is great". So In heat death, the entropy becomes maximum and temperature is the lowest, but if heat is added to a system (I think that means a rise in temperature), the entropy increases. Then what is the high entropy in the heat death comprised of? I have been guessing that high entropy usually means high temperture. Like sushi (talk) 10:37, 7 June 2009 (UTC)

You can think of it this way: when you transfer some heat from a high-temperature system to a low-temperature system, the high-temperature system loses a relatively small amount of entropy while the low-temperature system gains a relatively large amount, so overall entropy increases. The entropy is (locally) maximum when the temperatures are equal. Adding heat would increase the entropy, but the heat has to come from somewhere (first law). If the whole universe is at the same temperature then the loss in entropy where the heat was removed would exceed the gain in entropy where it was added, which is forbidden by the second law. -- BenRG (talk) 14:20, 7 June 2009 (UTC)

Wet matches

I left a box of matches outside last night and they got soaked in the rain. When I try to strike them, the head of the match just crumbles into a wet mush. If I leave them and the box out in the sun today, might they dry out and be useable again? Frank Bruno's Laugh (talk) 11:18, 7 June 2009 (UTC)

Sure, why not. Try it and let us know tomorrow. Avoid the temptation of putting them in the oven on a low setting though (for obvious reasons!) 94.27.225.206 (talk) 12:41, 7 June 2009 (UTC)

Yep, that worked fine. The charcoal that go wet is still a bit mushy! :) I did consider putting the matches in the electric oven on a low setting (starting with one), thinking that it wouldn't have been hot enough to ignite them, but I guess that wouldn't have been a good idea? Might it work with the charcoal? Frank Bruno's Laugh (talk) 15:01, 7 June 2009 (UTC)

I'd be surprised if they still worked. In my experience regular matches don't have a lot of tolerance for weird environments (and degrade rather quickly with age or humidity). That being said, matches are pretty cheap, so it's not exactly the end of the world. --98.217.14.211 (talk) 14:59, 7 June 2009 (UTC)

They actually seem in pretty good shape, so I guess I got lucky (they did get a real soaking) Frank Bruno's Laugh (talk) 15:05, 7 June 2009 (UTC)

If you're taking some matches somewhere they run the risk of getting wet (e.g. a camping trip), a tip I once read is to dip them in melted candle wax. I can't say I've ever tried it, and I imagine a cheap lighter would be a better bet. 87.81.230.195 (talk) 23:58, 7 June 2009 (UTC)

Yes, yes. Before use, just melt the wax off with a lighter. 79.122.119.18 (talk) 06:21, 8 June 2009 (UTC)

I see what you did there. Tempshill (talk) 17:41, 8 June 2009 (UTC)

I have dipped old-style kitchen matches (the old-fashioned strike-anywhere kind) into wax and that made them completely waterproof and they would still light just fine without having to melt the wax away or anything. I haven't tried it with safety matches. 207.241.239.70 (talk) 19:12, 8 June 2009 (UTC)

Human blood

What are the risks with drinking (fresh) human blood? Vimescarrot (talk) 14:01, 7 June 2009 (UTC)

Oh, and the possible (dietary) health benefits? Vimescarrot (talk) 14:07, 7 June 2009 (UTC)

Maybe I'm stating the obvious but a dietary benefit would be increased iron intake. I'm guessing that there are no immunoreactive issues as the antigens should be metabolized and/or not absorbed by the gastrointestinal tract. --Mark PEA (talk) 15:32, 7 June 2009 (UTC)

It might be worth pointing out here that increased iron intake is not always a benefit. Iron is a necessary nutrient, but it's also somewhat toxic, and there is apparently no dedicated mechanism for getting rid of the excess. There is some information at our articles on human iron metabolism and iron overload.

A few years ago there was a minor splash made by the observation that many of the risk factors for heart disease have in common that they are associated with higher levels of iron in the body, and it was hypothesized that iron was a contributing cause to heart disease. I can't find an article about that in WP, and I don't really know how the idea has fared, though I imagine we'd have heard more about it if there were good evidence for it. --Trovatore (talk) 20:01, 8 June 2009 (UTC)

There are obvious risks of the transmission of disease. I think that's probably about it. You could overdose on iron (and maybe other constituents) if you drank too much of it, but that basic principle applies to anything. --Tango (talk) 16:35, 7 June 2009 (UTC)

There's also likely a risk of criminal charges, depending on who's blood it was and how you acquired it :-D -RunningOnBrains^{(talk page)} 17:03, 7 June 2009 (UTC)

You're not planning to eat your bone tumor, are you? Nimur (talk) 18:20, 7 June 2009 (UTC)

Sorry, but I must note that this line made me laugh for no less than 20 seconds. Tempshill (talk) 04:29, 8 June 2009 (UTC)

Bone tumours aren't made of blood. On a slightly related note...What about the, uhh...various components of what comes out of a female during menstruation? (I'm given to understand that it isn't just blood...) Is that any more risky? Also, what about eating a tumourous bone? That last one is just for laughs, really. The first one, I'm specifically thinking of oral sex. Vimescarrot (talk) 19:16, 7 June 2009 (UTC)

What are you, 12? Menstrual fluid doesn't get ingested during oral sex, mmmmkay? 94.27.225.206 (talk) 20:12, 7 June 2009 (UTC)

I see no reason why that isn't possible. Vimescarrot (talk) 20:33, 7 June 2009 (UTC)

If you give oral sex to a woman that is menstruating, I would expect some of the menstrual fluid to be ingested. --Tango (talk) 21:04, 7 June 2009 (UTC)

The amount would be so small though, would it really matter? —Preceding unsigned comment added by 82.43.88.87 (talk) 22:26, 7 June 2009 (UTC)

That's exactly what I'm asking. Vimescarrot (talk) 22:34, 7 June 2009 (UTC)

According to a (purportedly non-fiction) book I read some years ago, committing cunnilingus on a menstruating woman (with or without her consent) was an act for which US Hell's Angels could earn a Red Wings badge, or Black Wings if she were black. I mention this purely in the spirit of encyclopaedic completeness, not because I endorse the custom or attitudes involved in any way. 87.81.230.195 (talk) 23:55, 7 June 2009 (UTC)

This is a reference desk. Is it too much to ask for a few references, especially when dealing with matters of health? According to Aidsmap, a project of NAM (National AIDS Manual, UK registered charity, founded 1987):

The levels of HIV in vaginal fluid vary. They are likely to be highest around the time of menstruation (having your period), when HIV-bearing cells shed from the cervix are most likely to be found in vaginal fluid, along with blood. Oral sex will therefore be more risky around the time of menstruation. Factsheet here

According to The Body, a service of Body Health Resources Corporation:

It is important that you do not perform unprotected oral sex on a woman when she is menstruating (blood has a much higher concentration of HIV), and if you are experiencing any problems with your oral health (sores, abrasions, inflammation). If you would like to decrease the risk of transmission even further, you can use a latex barrier--like a dental dam or a condom cut into a square--between your partner's vagina and your mouth. Explanation here

Sources please! BrainyBabe (talk) 12:06, 8 June 2009 (UTC)

vampirism? —Preceding unsigned comment added by 82.43.88.87 (talk) 19:59, 7 June 2009 (UTC)

Not exactly human blood but animal blood (eg. pig blood) is a delicacy in Southern China; very smooth and tender, with a jello-like texture and a bit of iron after-taste. --antilived^{T | C | G} 23:06, 7 June 2009 (UTC)

I believe there is less risk from animal blood than human blood, since the diseases present in animal blood are less likely to be infectious to humans. --Tango (talk) 00:45, 8 June 2009 (UTC)

I seem to remember reading of a nomad culture where cattle are regularly bled for drinking; but maybe that was fictional. —Tamfang (talk) 04:43, 8 June 2009 (UTC)

The Maasai of East Africa certainly do that - they mix cow urine into a mixture of cow blood and milk to make it clot. According to our article, these days they only do this on ceremonial occasions or when someone is sick. When we lived in Kenya in the early 1970's, my parents bought a rather beautiful beaded gourd from a Maasai tribesman - sadly, they assumed it was a tourist trinket when in fact it was all-too authentic and still contained the guy's lunch! Despite a lot of cleaning, and soaking the insides with bleach, the...erm..."distinctive" odor of the thing never quite went away! SteveBaker (talk) 10:53, 8 June 2009 (UTC)

Well, since we are digressing, see Blood as food (alas, not a great article). Note that the consumption of raw duck and chicken blood in dishes that are delicacies in Vietnam is one of the few ways in which humans can catch so-called "bird flu" (H5N1) from birds, see BBC article. BrainyBabe (talk) 08:11, 8 June 2009 (UTC)

I believe there is less risk from animal blood than human blood, since the diseases present in animal blood are less likely to be infectious to humans. That is a rather risky belief given the number of counterexamples of diseases transferred from animals. BTW. Off hand I could probably come up with a shorter list that can not be transferred than ones that can. Just for tip of the iceberg examples see Rabies, Trichinella, Apicomplexa, vCJD, Cryptosporidiosis. Some of these get most commonly transferred by other means than consumption of blood, but that doesn't mean it can't be transmitted that way. Even intestinal parasites are often present in an infected individual's blood. Some allergologists think that humans need a certain exposure to parasites and infectious agents to prevent allergies. If only 1 in X gets infected it that figure won't make you happy if you turn out to be the one. 71.236.26.74 (talk) 15:59, 8 June 2009 (UTC)

See selection bias. Most diseases cannot be transferred from one species to another. The only animal diseases you have probably heard of (unless you are a vet) are ones that can be transferred to humans since those are the ones that make the news (or ones your pets have caught, I suppose). There is certainly some risk, but it is far lower than the risk of drinking human blood. Pretty much any human disease can be transferred by drinking human blood (the risk is rather low for some since stomach acid, etc., are quite good at killing ingested contagions, but there is still a significant risk). --Tango (talk) 16:54, 8 June 2009 (UTC)

We have an article, Zoonosis, but it doesn't seem to make comparisons to human-to-human infections. --Tango (talk) 16:58, 8 June 2009 (UTC)

Interesting discussion is interesting! I love digression - it's a great way to learn. Vimescarrot (talk) 17:14, 8 June 2009 (UTC)

You might e right about selection bias (although I do know plenty of diseased that affect only non-humans - and I'm still not a vet). There is however the factor of immunity that makes diseases that "jump species" worse than those within the same. Trusting expert opinion reported in the media in this matter our immune system is just having a lot harder time dealing with pathogens introduced via animal vectors than human to human. So, you could catch the flu by drinking human blood, but would your immune system be more likely to deal with that than a bird flue virus you ingested with chicken blood? 71.236.26.74 (talk) 19:48, 8 June 2009 (UTC)

Yes, I think that is true. Most human-to-human diseases are fairly minor (like the common cold), whereas animal-to-human diseases, while very rare, are often quite a lot worse. That isn't just due to immunity, it is due to evolution of the disease as well. Killing your host is not generally a good way to reproduce, so serious diseases in humans tend to die out in time, but if they were less serious in a given animal they could live on in that animal and only be serious when they get transmitted to humans (usually it would stay in the original human, diseases of animal origin that can be transmitted from human to human are very rare indeed - that's the big fear about bird flu, that it could mutate into a form that can be transmitted between humans, at the moment it can't so just kills a handful of people that have close contact with birds). --Tango (talk) 17:05, 9 June 2009 (UTC)

Surgical memento

If surgeons are removing something from you, are you allowed to keep it? Vimescarrot (talk) 14:03, 7 June 2009 (UTC)

Let me guess. No? Bus stop (talk) 14:06, 7 June 2009 (UTC)

Why not? And if that's a guess, why would you think that? Vimescarrot (talk) 14:08, 7 June 2009 (UTC)

I don't know. I'm just joking. I really don't know. But since intuitively I would assume one should have the right to take possession of a part of one's body that has been surgically removed from one's body, it probably is the case that it is the opposite. There is probably some reason why removed body parts become the property of the medical facility in which the surgery took place. But I really don't know. Bus stop (talk) 14:51, 7 June 2009 (UTC)

Human tissue is generally classified as biohazardous material, and has to be properly disposed of in a high temperature medical incinerator. So the hospital isn't allowed just to give you a decaying necrotic kidney home in a plastic bag. They'll give you an inanimate calculus like a gallstone, because there's no risk of disease. In theory they could preserve stuff in formaldehyde, but there are considerable additional compliance costs to do with human anatomy regulations and biomedical waste; it's not the hospital's core business, so they're unlikely to want to do all that stuff. Hopper Mine (talk) 15:45, 7 June 2009 (UTC)

Is bone an "inanimate calculus"? Vimescarrot (talk) 15:57, 7 June 2009 (UTC)

Very much not; it's a complex matrix full of living cells (it couldn't very well get cancer if it was inanimate). It is possible for a lab to sterilise bone (this is done to cadaverous bone allowing it to be safe for transplant), leaving only the inorganic hydroxylapatite matrix, but this is an involved and expensive process - it's not something they'd do for souvenirs. Hopper Mine (talk) 17:57, 7 June 2009 (UTC)

I guess it depends on what they are removing. My friend kept his gall bladder stones, but some things might be toxic?91.111.74.247 (talk) 14:55, 7 June 2009 (UTC)

A bone tumour. Vimescarrot (talk) 15:01, 7 June 2009 (UTC)

note that this is by the OP. So, OP, why would you want to keep a bone tumor??? 94.27.225.206 (talk) 15:47, 7 June 2009 (UTC)

Well, why do people keep teeth? Vimescarrot (talk) 15:55, 7 June 2009 (UTC)

To get money from the tooth fairies, obviously. Now, everyone knows that the bone cancer fairies are just a myth. Frank Bruno's Laugh (talk) 18:51, 7 June 2009 (UTC)

Good thing I don't have bone cancer, then...Vimescarrot (talk) 19:13, 7 June 2009 (UTC)

This may be of some help to you. Frank Bruno's Laugh (talk) 15:10, 7 June 2009 (UTC)

Maybe the OP was talking about surgeons removing a foreign object. If it's not dangerous, I see no reason why you couldn't have it. Of course, if it was a bullet, knife, or some other weapon, they might be obliged by law to hand it over to the police for forensic investigation. Astronaut (talk) 17:35, 7 June 2009 (UTC)

-just re-read that ... a bone tumour? Are you serious? Astronaut (talk) 17:36, 7 June 2009 (UTC)

Why would I not be serious? Vimescarrot (talk) 17:47, 7 June 2009 (UTC)

A cancer is no more hazardous to others than regular tissue of the same type (barring use in transplants, obviously, and maybe cancers associated with viruses). It's just a minor programming error, not some primal nuncio of feculence; it's not contagious. That said, all living human tissue is thick with a wide range of pathogens, and it's all considered hazardous for good reason. Hopper Mine (talk) 18:05, 7 June 2009 (UTC)

If you're just thinking "ew gross", it's surprisingly common for people to feel sentimentally attached to what is, after all, part of themselves. This is true even for tumours; some people seem to think "that's the little bastard that caused me so much trouble", and want to keep it as a trophy. Hopper Mine (talk) 18:14, 7 June 2009 (UTC)

It sounds like Hopper and Frank answered my question, although if anyone disagree they're welcome to say so. Thanks. Vimescarrot (talk) 19:13, 7 June 2009 (UTC)

Famous persons could sell their excised body parts for considerable sums. These would pass from hand to hand as collector's items or investments. Soft tissue would be in a sealed glass tube. There would be a registry of ownership to ensure against fraud. If authenticity is still questioned, DNA tests could be made. – GlowWorm. —Preceding unsigned comment added by 98.21.108.254 (talk) 03:39, 8 June 2009 (UTC)

In some cases where something is removed the surgeon may actually be required by the insurance company to keep what was removed or even send it (preserved) to the insurance company as proof the surgery was actually conducted. I don't know how common that requirement is. Someguy1221 (talk) 03:47, 8 June 2009 (UTC)

— all due respect, but I think you must have just made that up. I imagine an insurance adjuster in his cubicle taking 50 FedEx shipments per day that are all dripping pus and blood. Tempshill (talk) 04:27, 8 June 2009 (UTC)

Or rather, I said that because a close relative was unable to keep a bone that was removed for precisely that reason. I do think your image is pretty amusing, and is why I mentioned I don't know how common that is. The incident I mention was the first time I'd ever heard of it being a practice. Someguy1221 (talk) 02:33, 9 June 2009 (UTC)

Just last week, my wife came home from the hospital with two screws that were removed from her leg. (They had, of course, been installed in a previous surgery.) The screws were clean and shiny, and had been autoclaved after removal. -- Coneslayer (talk) 16:23, 8 June 2009 (UTC)

Not supposed to give legal opinion, but I've heard that in Australia you retain ownership of removed body parts, but not your dead body. Polypipe Wrangler (talk) 05:31, 12 June 2009 (UTC)

Inertia on the bus(non-technical explanation preferred)

When riding the bus, passengers experience inertia when it accelerates(they move backwards relative to the bus) and when it decelerates(they keep moving forwards). However, why does there seem to be less inertia(and consequently less risk of getting tossed around and crashing into things/people) when you're seated than when you're standing? I don't have much background in physics, so I'd like a non-technical answer. 69.224.113.202 (talk) 18:15, 7 June 2009 (UTC)

You have exactly the same inertia, whether you are seated or standing, because inertia only depends on your mass. While standing, though, your center of mass is higher, and you are generally in a less stable position, so you might feel the sways and even stumble around a bit as the bus accelerates. This is strictly due to how stable your stance is, not how much inertia you have. Nimur (talk) 18:18, 7 June 2009 (UTC)

To expand a little on what Nimur said: when you're sitting down and typically leaning back against the seat, that's a very stable position. Not only are you braced against the seat, your butt and the backs of your thighs against the seat afford you plenty of friction that tends to hold you on the seat, especially if the seat is made of something relatively coarse, like fabric, instead of a more slippery material. If I tried to yank you away from the seat, I'd probably have to use quite a bit of force. But if I wanted to do the same to you when you're standing, it'd be a lot easier, because not only would your center of mass be higher so it'd be much easier to tip you over, there wouldn't be a lot of friction to hold you in place. Unless you were prepared for me and had strongly braced yourself, I could probably yank you off your feet without any real trouble. Another way to look at this might be to imagine a rolled-up carpet in a bus. If you stand it up, it tips over very easily, but if you lay it down, it'll most likely pretty much stay put. If the roll starts to tip over, you're going to have to use quite a bit of strength to get it back upright, because at that point you'll be working against its own mass. That's what happens when you stand up and have to work to stay on your feet: once your balance starts to go, it's your own mass that's making you fall down, more than anything else. You need to work to re-balance the load that consists of your body. -- Captain Disdain (talk) 23:33, 7 June 2009 (UTC)

Similarly, notice that when you're standing, you get thrown around most if you don't use a handhold, less if you hold on with one hand, still less if you hold on with two. The force on you is the same, but the more you're braced, the more easily you can resist it. Sitting down braces you even more, and sitting down with a hand against the seat in front (if possible) does still better. --Anonymous, 3:55 UTC, June 8, 2009.

When a person is standing in a bus, the body's center of mass is at the end of a longer leverage arm than when the person is seated. Specifically, the axis of turning is at the feet rather than the buttocks. Thus when seated, the center of mass is only slightly above the center of turning.
Also, when a bus is decelerating, the abdominal muscles of a seated person resist the inertial effect rather than the weaker ankle muscles of a standing person. Specifically, when a person is seated, holding the abdominal muscles and knees rigid and bracing the feet on the floor opposes the turning inertia of the upper part of the body. When the bus is accelerating, the person is simply pressed back in his seat. Furthermore, the mass of the legs of a seated person is below the axis of turning, reducing the body mass above that point.
All this assumes there is no slippage on the seat or floor. Inertia thus results in a turning effect. - GloWorm. —Preceding unsigned comment added by 98.21.108.254 (talk) 03:59, 8 June 2009 (UTC)

Twins conceived on successive months

Further to the excellent sourced answer above, about the time between the delivery of twins (or other multiple births), I remembered a friend-of-a-friend story. It seemed improbable, but then, so did the idea of delivering one baby and then keeping the other in utero for more than a few minutes. Here is the idea -- twins are born: one is substantially bigger than the other, and hits all the developmental milestones a full month or more before the tinier sibling. What is the probability of the bigger being conceived one month, and the smaller the next, because ovulation for some strange reason was not suppressed? In other words, not "can you get pregnant if you do it standing up?" but "can you get pregnant if you are already pregnant?". BrainyBabe (talk) 20:19, 7 June 2009 (UTC)

I don't know the answer, but I think it is quite unlikely to happen - you don't just need ovulation to take place, but the embryo to successfully implant in the uterus lining. I know some contraceptive pills don't reliably prevent ovulation, but do prevent implantation, so I would imagine pregnancy could be similar (although ovulation is even more unlikely when pregnant than when on the mini-pill). --Tango (talk) 21:02, 7 June 2009 (UTC)

This being an encyclopedia, we have an article on this rare phenomenon: see superfetation. --Cookatoo.ergo.ZooM (talk) 21:03, 7 June 2009 (UTC)

Well, I knew it was quite unlikely to happen; I wanted to know if it was impossible. I am grateful for the pointer to the article, which I never would have found on my own! BrainyBabe (talk) 08:17, 8 June 2009 (UTC)

Submersed in Water

Is there a length of time after which it would become dangerous for a person to remain in water? Assuming access to a toilet and the water is freshly running would a person eventually get sick? I'm thinking there is some sort of limit as your fingers prune after too long. The Aquatic ape hypothesis, which albeit mostly unsupported, provides some possible insight, but I can't seem to find conclusive proof of a danger from extended exposure. TheFutureAwaits (talk) 21:15, 7 June 2009 (UTC)

Incidentally, why do fingers prune? And why don't toes prune? Does pruning take place in other land-dwelling animals after prolonged water exposure? —Preceding unsigned comment added by 98.21.108.254 (talk) 02:42, 8 June 2009 (UTC)

Toes do prune. Try it and see. As for the causes there is a section dedicated to this. Sifaka ^talk 03:13, 8 June 2009 (UTC)

OK. Toes prune too. It's been a long time since I took a bag lunch and spent the whole day at the swimming pool with my pals. (What fun we had !) - GlowWorm.

You might like to look at David Blaine - see Drowned Alive. DJ Clayworth (talk) 16:58, 9 June 2009 (UTC)

Or you might realise that David Blaine is a magician and is therefore irrelevant to this discussion. SteveBaker (talk) 03:27, 10 June 2009 (UTC)

Interesting, I'm sure I've ever heard anyone allege that the "Drowned Alive" stunt was an illusion. What would be the point? It seems like it would be entirely possible to do it straight, and rather difficult to fake? APL (talk) 04:07, 10 June 2009 (UTC)

Oh, or did you simply mean that the accounts of his injuries may be exaggerated (or censored) for dramatic publicity reasons, and therefore unreliable? APL (talk) 04:09, 10 June 2009 (UTC)

Most people spend 9 months fully immersed in an aquatic environment! The main risks with extended water immersion are hypothermia or salt exposure. Rob Hewitt spent 4 days lost at sea with only his wetsuit. Other people have swum the Atlantic or the Mississippi taking several months, at 8 hours per day in the water. ~~ Ropata (talk) 09:03, 12 June 2009 (UTC)

The need to urinate

Why is it that if I already need to urinate, the closer i get to a toilet the worse it gets? Like, for instance, if I'm driving home I may notice that I need to urinate, but have no trouble getting it, but once I get to the door of my house I start dancing around, and then it rapidly accelerates into a mad dash to the bathroom! Is it psychological? positional? --96.231.171.95 (talk) 22:46, 7 June 2009 (UTC)

If it's any comfort, I experience the very same thing. I've always assumed that it's an example of Pavlovian conditioning, and it's interesting that this still works even when one is consciously aware of it. 87.81.230.195 (talk) 23:42, 7 June 2009 (UTC)

Yep, it happens frequently enough to wonder just how Pablovian could it be if we are so concious of it happening and still can't control it? hydnjo (talk) 00:47, 8 June 2009 (UTC)

If this were an instance of conditioning, then we would expect the urge to pee to arise whenever a suitably coniditioned individual was near a bathroom, but this is clearly not the case. --Shaggorama (talk) 04:06, 8 June 2009 (UTC)

I disagree. I've gone into a bathroom to wash my hands or do something else that is only going to the bathroom in the literal sense, and felt the need to do it in the figurative sense. 67.182.169.172 (talk) —Preceding undated comment added 23:06, 9 June 2009 (UTC).

Something autonomic, I bet. Getting out of the car means you relax, relaxation makes you liable to pee. Once you've actually released the flood gates it's difficult to stop mid-flow, so I guess there's some kind of preparatory stage, a pre-pee process if you will, which is likewise difficult to stop. 213.122.32.39 (talk) 05:08, 8 June 2009 (UTC)

I suspect it's just that you have mentally 'released', you've made it to your goal (home) and then like 213.122 says once you've released it becomes much harder to stop so makes it more rushed to get there. I know that's how I explain it to myself. 194.221.133.226 (talk) 07:44, 8 June 2009 (UTC)

I think it's definitely psychological. If you suddenly get distracted by something that's important enough, the sensation will cease -- or at least that's how it goes for me. Sure, it takes a moment, but I've had a phone call knock me out of that "I gotta go right now" mode, for example. -- Captain Disdain (talk) 08:39, 8 June 2009 (UTC)

Pareto principle definitely. Jay (talk) 18:01, 9 June 2009 (UTC)

electrical generator

How does a generator work?

In particularly, I'm wondering how a wind turbine generates electricity? —Preceding unsigned comment added by 69.144.23.204 (talk) 23:23, 7 June 2009 (UTC)

See Electrical generator or more specifically, Electromagnetic induction. In short, voltage is produced across a conductor situated in a changing magnetic field or a conductor moving through a stationary magnetic field. -- Tcncv (talk) 00:00, 8 June 2009 (UTC)

As for wind turbine specifics, you may find the Wind turbine design article worth reading as well and many of the other articles linked from the "Wind Power" navigation box at the botton of that page. The conversion of wind energy to rotating mechanical energy and the conversion of rotating mechanical energy to electrical energy are best though of as two separate processes. -- Tcncv (talk) 00:15, 8 June 2009 (UTC)

In brief - when a magnetic field is moved past a wire, it generates a tiny amount of electricity. Spinning a magnet inside coils of wire (or spinning coils of wire inside some magnets) makes the magnetic field move past LOTS of wires - which makes a useful amount of electricity. The wind spins a shaft which spins the magnet (or the coil). It's basically an electric motor working 'backwards' - instead of putting electricity in and getting mechanical rotation out - you put mechanical rotation in and get electricity out. You can prove this yourself: If you take a small battery-operated electric motor (eg from a kid's toy) and connect a flashlight bulb across its terminals - you can spin the motor with your fingers and light the bulb. If you connect some fan blades to your motor and let the wind make it spin - then you've made your own personal wind turbine! SteveBaker (talk) 01:08, 8 June 2009 (UTC)

Are you sure that's true - I though they were wired in a different way. If it is true, then it would be easy to make a home wind turbine using an old washing-machine motor, although the electricity out of it would fluctuate greatly in its voltage, current, and Hz. 78.149.238.54 (talk) 19:38, 8 June 2009 (UTC)

It really has to be a DC motor. Washing machine motors run on alternating current - so probably this trick won't work (it depends on the design). But a direct current (DC) motor - pretty much any kids' toy or battery operated electric drill motor - will work wonderfully. And yes - of course you can make your own home wind turbine from bits and pieces you have lying around someplace...you can make nearly anything if you put your mind to it! A few years ago I made a computer controlled milling machine from an old router and some rollerskate bearings (and my current project is to re-make it with more power and precision). However - whether your home wind turbine would be efficient enough to be "useful" - is one of those devils that will be hiding in the details. SteveBaker (talk) 12:33, 9 June 2009 (UTC)

Ah ha! Now you are saying it won't work. "this trick won't work (it depends on the design)" - in other words the wiring is different as I wrote earlier. 78.147.57.78 (talk) 20:41, 9 June 2009 (UTC)

No - I'm saying that you have to read what I actually wrote:

"If you take a small battery-operated electric motor (eg from a kid's toy)..."

At no point did I say "It would be easy to make a home wind turbine using an old washing-machine motor"...those were your words. DC motors are NOT wired differently from DC generators - do the experiment it works just fine. AC motors may or may not be - it depends on the design...there are synchronous motors, squirrel-cage motors, induction motors, DC motors with AC rectification...some designs could work as generators - but others do not. It's the design of the motor that matters - not that "they were wired in a different way" as you said. AC motors are totally different beasts from DC motors. So I'm not changing my story - and I'm certainly not agreeing that you're right - I'm telling you to read what I wrote - which was correct. SteveBaker (talk) 03:25, 10 June 2009 (UTC)

It's absolutely true. A fun thing to do is to wire two small electrical motors to each other and turn the shaft on motor A and watch the shaft of motor B move. (Or maybe I'm just easily amused.) APL (talk) 00:41, 9 June 2009 (UTC)

Would that work? If you had two toy electric motors for example, I presume the output of one would be AC, yet the required input of the other would be DC. I'm still sceptical that electric motors would work in reverse as a dynamo without changing the wiring. Are there any YouTube videos of this? 89.242.125.32 (talk) 11:04, 9 June 2009 (UTC)

Yes - it works - providing you have two DC motors of similar size/voltage/etc. You can do it easily - just connect the wires from one motor onto the back of the other - turn one motor and the other one will turn as well. I've done this a gazillion times to demonstrate the effect - mostly using Lego Technic's motors (because you can connect them back-to-back very easily using the standard Lego connectors). Obviously there are losses in the system - so the second motor turns a bit more slowly than the first. But "Proof by YouTube" is a really REALLY bad idea because a solid 90% of the "science" videos on YouTube are faked. SteveBaker (talk) 12:33, 9 June 2009 (UTC)

Like SteveBaker said, A YouTube proof is worse than no proof at all. But this is a trick you can try yourself really easily. Get yourself two of these and wire them together. You don't even need to solder them, get some wires with alligator clips on them. I've done this many times, (Like I said, I'm easily amused), I promise that it works. APL (talk) 04:23, 10 June 2009 (UTC)

Yes, it really works. BTW it works for both AC and DC motors. They will give you respectively AC and DC currents. Dauto (talk) 18:28, 10 June 2009 (UTC)

A toy DC motor has to spinn pretty fast to make enough voltage to light a bulb or to spin another motor. A drill atttached to the shaft of one motor will spin it fast enough to power another motor. A DC motor is easier to use as a generator than an AC motor, where you will likely have to control the excitation voltage and strictly control the speed. There are mechanical variable speed drives which have been used in AC home hydro plants of 10 kilowatt output. Edison (talk) 21:33, 10 June 2009 (UTC)

"A toy DC motor has to spinn pretty fast to make enough voltage...to spin another motor." Did you TRY it?

Ok, you want a video? Here you go!.

I can't believe I spent time on this, but any excuse to get out my Legos is a good excuse. (Sorry the video isn't that great. I really need to get a tripod.) APL (talk) 01:07, 11 June 2009 (UTC)