Wikipedia:Reference desk/Archives/Science/2009 November 14

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November 14[edit]

Blood Type Compatibility[edit]

Just forestalling any possible misunderstandings, this IS NOT a request for medical advice. And now that that's out of the way, the question: Is it possible (I have heard stories places) to determine whether two people are compatible to donate blood to each other by having each prick their finger and mixing their blood on something like a microscope slide, then watching to see if it clots like crazy? Ks0stm (TCG) 03:53, 14 November 2009 (UTC)

No, it's not that simple. For one thing, clotting, or the production of a blood clot, is distinct from the agglutination reaction that indicates incompatibility. Second, while agglutination is a clear indication of incompatibility (for one or both potential recipients), more sophisticated tests are needed to ensure compatibility. -- Scray (talk) 04:31, 14 November 2009 (UTC)

Schrödinger's cat - can you actually stop observing it?[edit]

With respect to the Schrödinger's cat thought experiment, is it possible, even in theory, to set up a situation where a cat is not observable?

As a thought experiment, the Schrödinger's cat scenario is not constrained by practical limitations, but does it even make sense to have a cat so completely cut off from the rest of the universe that it is not observable, even in principle? I am curious specifically about the fact that a live cat, in contrast to a dead cat, has a metabolism generating heat, has a heartbeat, has a nervous system with electrial currents, etc., and must therefore be continuously interacting with the universe at large. I realize that the point of the thought experiment is to demonstrate the paradox, but discussions always gloss over this point. Peter Grey (talk) 06:05, 14 November 2009 (UTC)

Different assumptions would make it a different scenario. -- Scray (talk) 06:12, 14 November 2009 (UTC)
It's not necessary for the cat to be completely isolated from you, only that the effect it has on you is something that could have been a consequence of either state. I think enough insulation, or just being far away from the box would make it work. Even with full knowledge of the initial state of the system, \there will be a wide range of possible observable outcomes for each the live and dead cat. If what you observe is in the intersection of those two sets of possibilities, then the cat is to you still in a superposition of both alive and dead. Once you observe something that couldn't be caused by one of the fates of the cat (or is extremely unlikely to be caused by one of them) then you're entangled with the cat's alive/dead-ness and to you the cat is definitely alive or definitely dead. Rckrone (talk) 09:32, 14 November 2009 (UTC)
It'd pretty difficult with an object the size of a cat, never mind the ethical issues. There are proposals though to do the equivalent with a virus. Not quite so impressive having a 'Schrödinger's virus' but few people will worry about its fate. Dmcq (talk) 12:03, 14 November 2009 (UTC)
Paper here. Apparently they propose to maglev a small object like a virus and cool it down to the ground state of quantum harmonic oscillation, then put it into a superposition of the ground state and the first excited state. They don't create a superposition of internal states of the virus, much less a difference as complicated as "alive" and "dead" (whatever that would mean for a virus). This experiment is only possible because viruses are inert hunks of matter when they're not infecting a cell; you couldn't do it with a living cell because of all the chemical activity going on inside (I think). Also, a superposition of ground and first excited states is only a "superposition" in the sense that it's not an energy eigenstate. But complicated systems are never in energy eigenstates anyway. The cooling to the ground state seems more interesting than the subsequent excitation, which just makes it slightly more normal again. Nevertheless, if this experiment is performed, it will of course be reported by the media as "an experimental realization of Schrödinger's cat". -- BenRG (talk) 13:15, 14 November 2009 (UTC)
In practice it is absolutely impossible to shield something of the size and temperature of a cat from the outside world. All that's necessary for the system to become classical is that the environment contain enough information to distinguish the states in principle—whether or not that information is accessible in practice—and that's inevitable when enough particles are involved. The heat radiating from your computer case contains information about the calculations being performed by the CPU. If nothing else, the cat's gravitational field will give it away through any amount of insulation, I think. Even if the system's state remains unknown to you, you still have to model it as a classical mixed state (1/2 chance of live cat, 1/2 chance of dead cat) and not as a quantum superposition (1/√2 alive + 1/√2 dead) to reflect the fact that it has objectively collapsed by environmental decoherence. -- BenRG (talk) 13:02, 14 November 2009 (UTC)
Maybe you could set up noise around the cat, like randomly moving weights, sounds, etc, determined by a lot of different quantum measurements. Would it be possible to create enough noise so that from your perspective it would be impossible to distinguish the two cat fates even in theory from the observables available to you? Rckrone (talk) 17:23, 14 November 2009 (UTC)
Unless I'm missing something, that's impossible. Because of unitarity, the only way to eliminate the information from the environment is by, in effect, undoing the process that produced it. If that was a thermodynamic process (like blackbody radiation) then you can't undo it because of the second law. -- BenRG (talk) 18:00, 14 November 2009 (UTC)
The wave function is fully determined, but when you take a measurement, you aren't finding the full wave function. Two different wave functions can produce the same measurement. That's the idea behind interference. Rckrone (talk) 18:42, 14 November 2009 (UTC)
Noise doesn't do it. It may make it harder, or perhaps impossible to determine which information comes from the cat, and which information comes from the noise, but the information is still leaving the box. APL (talk) 21:23, 14 November 2009 (UTC)
Right, but it's not enough for information to be leaving the box, it has to be information that is determined by the fate of the cat. If the same measurement could as likely come out of the box with a live cat in it as the box with a dead cat, then making that measurement doesn't lead you be entangled with the cat's alive/dead state. BenRG is right that the measurement you're making includes any way that the box affects you which includes quite a lot of information, but I'm wondering if it wouldn't still be realistically possible to set up the box so that the measurement isn't necessarily indicative of the cat's state, at least for some short amount of time after the event. Rckrone (talk) 22:02, 14 November 2009 (UTC)
In principle it's very simple: you put the cat (in a box) in the middle of interstellar space, with the closest observer a billion miles away. The cat is then unobservable over the time course of the experiment simply because of the finite speed of light. Looie496 (talk) 18:38, 14 November 2009 (UTC)
Ahhh....but who put the cat there? Myles325a (talk) 02:21, 17 November 2009 (UTC)


No. Because ANY interaction with the universe is deemed an 'observation'. So as soon as anything observes it (or interacts with it ), the WF collapses into a definite state.--79.75.63.71 (talk) 00:50, 18 November 2009 (UTC)

origin of a cyclone[edit]

formation of cyclones —Preceding unsigned comment added by 124.43.90.158 (talk) 08:32, 14 November 2009 (UTC)

Are you asking, "How does a cyclone form?" If so, see Cyclone#Formation.--CurtisSwain (talk) 08:59, 14 November 2009 (UTC)

Ferrofluid buoyancy?[edit]

Supposing a powerful (perhaps superconducting) permanent magnet was placed over a tube of ferrofluid. Would pressure still increase with depth, and what would happen to an object of neutral buoyancy in such a liquid at different depths?Trevor Loughlin (talk) 14:19, 14 November 2009 (UTC)

It would depend on if the magnetic "up" force more than counteracted the gravity "down" force. Pressure in a liquid (condensed fluid) is just controlled by the forces acting on the fluid (gas pressure works differently). Under normal circumstances, gravity is the only force acting on it. If you had a stronger force pulling straight up, such that the net force vector on the fluid was up rather than down, I would expect the pressure gradient to act differently. Of course, since the magnet will lose strength with distance, this effect could result in some unusual pressure gradients. --Jayron32 16:05, 14 November 2009 (UTC)
Having neutral buoyancy by definition means an object neither rises nor falls from its position in a presure gradient. Cuddlyable3 (talk) 16:36, 14 November 2009 (UTC)

Supposing the magnet was strong enough to completely reverse the pressure gradient. Would an object which would normally float upward float downward? And if it was moved sideways out of the gradient (since the float is not magnetic itself this takes little energy) when it reached the bottom, would it float up again? And if it was moved sideways back into the reverse gradient when it reached the top, would it sink again, creating a mechanical over-unity device?Trevor Loughlin (talk) 12:37, 15 November 2009 (UTC)

Moving it sideways into or out of the gradient takes energy -- specifically, exactly as much energy as you get from it moving along the gradient. --Carnildo (talk) 01:34, 18 November 2009 (UTC)

Hanging[edit]

The OP is asked not to edit responses to their question. Cuddlyable3 (talk) 23:25, 15 November 2009 (UTC)

how come profesional hangings the victom goes out quick even if the neck is not broke iv watched 30 iran and german hangings they are all instent alothough they are all suspension. but suicide hangings iv seen take at least 10 secs to pass out why?

examples: http://video.google.com/videoplay?docid=7181755851454128977#


http://www.liveleak.com/view?i=317_1218265064


there are many others many in which the are slowly raised by hand as well . the result is the same. why? —Preceding unsigned comment added by 74.65.3.30 (talk) 16:34, 14 November 2009 (UTC)

There are different ways of hanging a man by the neck until he is dead. The long drop, favoured by English hangmen, produced death by breaking the neck. You may also wish to consider whether censorship was applied to the video clips you have been watching: was a long struggle cut from the end result so that it could be distributed? --TammyMoet (talk) 17:33, 14 November 2009 (UTC)
Breaking the neck does not, of course, produce instant death. What it produces is instant paralysis, sparing the feelings of the onlookers; the interested party stops moving, because he can no longer send signals to any muscles below the neck, and usually his face is covered. --Trovatore (talk) 20:53, 14 November 2009 (UTC)
The article Death erection cites from a RS that one in three hanging men have an erection. That statistic means the OP may have watched 10 males with erections but no associated enjoyment. Unless any were female or very very peculiar. Cuddlyable3 (talk) 22:54, 14 November 2009 (UTC)

The OP is asked not to edit responses to their question. Cuddlyable3 (talk) 23:25, 15 November 2009 (UTC)

bipolar disorder[edit]

percentage of people affected by bipolar disorder globally and age wise .. —Preceding unsigned comment added by Arun vvv (talkcontribs) 17:29, 14 November 2009 (UTC)

Is the information on epidemiology in our article on bipolar disorder helpful? TenOfAllTrades(talk) 18:53, 14 November 2009 (UTC)

homeostasis during illness[edit]

Hello! I've been asked to look at the role of homeostasis in the human body during health and illness. As for health, I thought this seemed quite simple - homeostatic mechanisms such as regulation of blood glucose concentration, blood pressure, serum potassium concentration, osmoregulation etc. help maitain our health and without them we would obviously quickly die. When it comes to the role of homeostasis in illness I got stuck and I need someone to point me in the right direction. What examples of homeostasis occur during illness? Would the mechanism of a fever be an example? Please help! RichYPE (talk) 19:33, 14 November 2009 (UTC)

You might look at CO2 levels, pulseox and respiratory volume when O2 is administered. You might look at the blood sugar levels in diabetecs when they have an infection. Edison (talk) 20:18, 14 November 2009 (UTC)
Additional pointers:
Hope this helps. -- Scray (talk) 05:46, 15 November 2009 (UTC)
I think perhaps the article on Stress (biology) is the best place to begin. It will point you to other relevant articles, such as HPA axis. Looie496 (talk) 18:15, 15 November 2009 (UTC)

Many thanks for the contributions everyone. Scray provided a good example of what I am looking for by mentioning reticulocytosis. I'm just basically looking for some examples of homeostatic mechanisms that occur during illness. Any more ideas about the kind of thing I need to look for will be great. Thanks all! RichYPE (talk) 23:07, 16 November 2009 (UTC)

LHC beam dump[edit]

The LHC has around 8000 miles of rock and metal around and under it. The beam dump of the LHC has to absorb high particle energies, and needs to be several meters long to do so.

Why isn't the earth itself suitable for absorbing the energies of the beam? It's less absorbent than custom materials (graphite, etc) but there's miles of rock compared to meters of custom material, and a less efficient but longer absorber would presumably spread the heat dissipation over a larger volume. Heat dissipation is its main purpose, and presumably nobody really cares how warm a chunk of off-site rock at the end of a separate tunnel 50 - 175 m underground gets.

FT2 (Talk | email) 21:00, 14 November 2009 (UTC)

They may care how radioactive it gets. It could well become a safety issue. Groundwater can move radioactive isotopes to unwanted places. Graeme Bartlett (talk) 21:28, 14 November 2009 (UTC)

Rising longevity and the remaining number of years of expected survival[edit]

Its 2009, I'm X years old and I've got a 50% chance of surviving for another Y years. Going back 100 years say with a much lower longevity for the population, then to have the same chance of surviving another Y years I would have to be very much younger - I would need to be Z years old. Is there anywhere on the internet where I can see my Z-age (to coin a term) during the 20th and earlier centuries? Or get the data to estimate it? 78.147.25.95 (talk) 21:50, 14 November 2009 (UTC)

I don't know where to point you for the specific data — I could Google it but so could you. But when you find it, I think you may find that the differences are not as dramatic as you think. As I understand it the life expectancy of adults has not really risen that awfully much. The huge increases in life expectancy at birth are largely due to the fact that far fewer people die in childhood than they used to (at least in the industrialized world). --Trovatore (talk) 22:01, 14 November 2009 (UTC)
I've read that idea old chestnut several times here, but I think its not particularly true. My grandfathers for example died in their sixties probably due to unwittingly consuming too much saturated fat and alcohol and not taking much exercise. In the early 20th. century at least even people who survived childood still died much younger than currently. 84.13.173.43 (talk) 11:53, 15 November 2009 (UTC)
There certainly has been an increase in how long people that reach adulthood live, but it is small compared to the increases in life expectancy at birth. According to Life expectancy#Lifespan variation over time, the average life expectancy at birth in mediaeval Britain was 20-30 years. It is now about 80. That is an increase of at least 50 years. I can't find figures for adult life expectancy, but I think we can be sure it has increased by significantly less than 50 years. --Tango (talk) 12:13, 15 November 2009 (UTC)
Nevertheless, the lifespan for people who have for example reached the age of twenty has still been rising considerably. The number of people over 100 has risen greatly. 92.27.154.139 (talk) 13:55, 18 November 2009 (UTC)
Is X=Y in the question or is it a two parameter question? i.e. do you just want the age at which you have a 50% chance of living as long again? By the way of course the chance of you surviving for another Y years is technically unknown since we don't know what will happen to the curve of longevity in the future (obviously). Incidentally an appreciable fraction of all the people who have ever lived are still alive (bout 10-15% I think) which does not imply you have a 10-15% chance of not dying but does illustrate the growth in the population. --BozMo talk 22:09, 14 November 2009 (UTC)
X=Y?? You mean if I'm two years old I'm going to only survive another two years? You've misunderstood the question. 84.13.173.43 (talk) 11:56, 15 November 2009 (UTC)
That last statistic depends heavily on what you mean by "people". You have to draw the line somewhere, but where you draw it can make a big difference (population numbers were low in proto-humans, but they were around for so long that the total numbers are high). We have some discussion of this topic at World population#Number of humans who have ever lived. --Tango (talk) 22:27, 14 November 2009 (UTC)
The questions seems to be about the estimated lifetime remaining, conditional upon reaching a given age. Insurance and census data would be a major source, it would also depend upon other factors such as locale, health, employment, family/medical backgrounds, and so on. The question can be paraphrased (as I understand it) this way:
Holding all other matters constant: - "For any given age of a person (A) and calendar year (B), there will be an age (C) such that a person who is A years old in year B, has a 50% chance of living to C years old. Where can I find a table of (A, B) -> C?"
Or more generally: - "For any given age of a person (A) and calendar year (B), there will be a distribution (C) for eventual age at death vs. probability of dying at that age. Where can I find a table of (A, B) -> C?"
FT2 (Talk | email) 22:33, 14 November 2009 (UTC)
The article Life expectancy will help. Life insurance companies keep data for life expectancy estimates when selling a Life annuity which is a form of Longevity insurance. Cuddlyable3 (talk) 22:39, 14 November 2009 (UTC)
Life tables typically give you A->C for fixed B. What the OP wants is B->A for fixed C. I don't think that exists ready made. --Tango (talk) 23:39, 14 November 2009 (UTC)

What I'm effectively looking for are life tables for various times in the past. 84.13.173.43 (talk) 12:02, 15 November 2009 (UTC)