Wikipedia:Reference desk/Archives/Science/2009 January 15

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January 15

Why don't mountain climbers' eyeballs freeze?

I've seen documentaries on TV about how cold some mountain climbs are. Exposed flesh will get frostbite in a matter of seconds. Why then don't eyeballs freeze? I would expect the very least that the moisture on the surface of the eye would either evaporate quickly or freeze. --71.158.216.23 (talk) 00:39, 15 January 2009 (UTC)

Exposed extremities freeze pretty quickly because they have a large surface area and are a long way from the warmth of the body's core. That doesn't really apply to eyeballs. People get frostbitten ears and noses, but the rest of the head is probably pretty difficult to freeze because it is basically one big sphere, which has the lowest surface area to volume ratio possible. For the eyeball to freeze, the rest of the head would have to get pretty cold, and blindness would be the least of the climber's worries. --Tango (talk) 01:10, 15 January 2009 (UTC)

In addition, vasoconstriction often reduces perfusion of the extremities, but would not cut off circulation to the head (and the brain therein). --Scray (talk) 01:59, 15 January 2009 (UTC)

Frostbite of exposed cheeks is fairly common, so these explanations don't seem to work. Algebraist 02:06, 15 January 2009 (UTC)

Just speculating here, but it could be due to their being sunken within the head. I know when it's cold and a wind is biting at my face I squint my eyes, and it seems to keep them warmer.-RunningOnBrains 02:14, 15 January 2009 (UTC)

Cheeks, or cheek bones? The cheek bones stick out quite a bit. --Tango (talk) 02:30, 15 January 2009 (UTC)

They wear mountain climbing goggles: [1]. StuRat (talk) 04:15, 15 January 2009 (UTC)

That's more to protect against wind, snow and bright light. I've seen photos of people on top of Everest that have taken their goggles off for the photo, their eyes didn't seem to be in any great discomfort. --Tango (talk) 05:43, 15 January 2009 (UTC)

The wind, snow, and dry air (causing evaporative cooling) will chill the eyes quickly, perhaps quickly enough for frostbite. On the other hand, on a clear, wind-free day, the goggles can probably be removed long enough to take a few pics safely. StuRat (talk) 06:20, 15 January 2009 (UTC)

(after ec) : People can, and do, get cornea frostbite (freezing keratitis). We have it mentioned in Doug Swingley article. Goggles, indeed, protect your corneas from both frostbite and UV exposure, see snow blindness --Dr Dima (talk) 05:47, 15 January 2009 (UTC)

Is it possible Swingley got this frostbite because he was travelling very quickly in his sled and his goggles slipped? With massive wind chill, I wouldn't be surprised if any part of the body could get frostbitten within seconds. --Tango (talk) 05:56, 15 January 2009 (UTC)

I would think it's because there's a lot of blood vessels in your eyes (as shown when they go bloodshot) and they probably have a relatively large amount of blood flowing through them. This would definitely keep them above freezing, but I'm not at all sure this is correct. Just a theory. -Pete5x5 (talk) 06:15, 15 January 2009 (UTC)

That's true of the white parts of your eyes - but the corneas have special issues because they have to remain transparent. Since you can't have all of that red stuff pumping across your field of view, there are no blood vessels in the cornea. So if it really was "cornea frostbite" rather than the entire eyeball - then blood flow is not relevent. SteveBaker (talk) 13:41, 15 January 2009 (UTC)

As long as the blood goes reasonably near by (which it does), the thermal conductivity of the various tissues would probably be enough to keep the cornea warm under most conditions. --Tango (talk) 17:45, 15 January 2009 (UTC)

work and momentum

Okay,so let's say I want to open a door 90 degrees as quickly as possible. Naturally, my first inclination is apply a force as far away from the axis of rotation as possible, as this will increase the distance I apply the force for the 90 degree angular displacement, thereby increasing the angular acceleration. Clearly this is the best solution. If I were to apply the force closer to the axis of rotation, I would be applying the force for a longer period of time, right(hence is beneficial not to push close to the axis of rotation). But since Im appplying the force for longer, would the momentum of the door be greater if I were to push closer to the axis of rotation? I know this can't be, because that would mean it would be better to push closer to the axis of rotation, but I can't see a reason why it wouldn't be true. And also, if you bring up the difference bewteen angular momentum and linear momentum, well angular momentum is really just a bunch of different linear momenta with a centripetal force to change it's direction... —Preceding unsigned comment added by 65.92.7.221 (talk) 06:34, 15 January 2009 (UTC)

I assume you mean that you have a constant maximum force that you can apply no matter where you push on the door or how fast that point is already moving while you are pushing on it. (In many real machines, the force available becomes less as the speed increses, because they are limited by power, which equals work over time, which is force times distance over time, which is force times speed.)

In that case, for any particular place where you push, the angular acceleration will be constant. The greater the angular acceleration, the greater the final angular velocity and therefore the greater the final angular momentum. So pushing with the same constant force at the point farthest from the hinge will give the greatest angular momentum. (Which makes sense because that situation involes the greatest power.)

--Anonymous, 12:50 UTC, January 15, 2009.

The problem with that answer is that it neglects the biomechanical limitations of the human body. Sure it may require more force/work/energy to open the door by pushing on it's center point than it does at the outer edge - but there are other limitations at work here. For example - no matter how little the load is, you can't physically move your hand faster than (let's say) the speed of a baseball pitcher's pitch (100mph?) - even with close to zero load. So to get maximum angular velocity on the door - you might have to push further in than the outside edge of the door simply in order to have enough speed - even though it requires more effort to do so. I don't know whether that's truly the case that it's better - but this consideration certainly has to be examined before coming to a conclusion. SteveBaker (talk) 13:36, 15 January 2009 (UTC)

• If the question is meant as I interpreted it, that's a problem with the question, not the answer... as I said, in effect. --Anonymous, 20:41 UTC, January 15, 2009.

It would require more force to open the door from the center point than the outer edge, but the work and energy will remain constant no matter where you push it from. All assuming speed, door size, etc. remain constant, of course. — DanielLC 17:37, 15 January 2009 (UTC)

With some common basic assumptions (i.e. friction is independent of velocity) it is clear that it recquires the same amount of energy to open the door; wherever you push it for a given final velocity. This is because work done is ʃFdx not ʃFdt as was implied by the question. This means that however long you apply a force for is irrelevant, and only how far you push it determines your energy cossumption. —Preceding unsigned comment added by 129.67.39.140 (talk) 21:36, 15 January 2009 (UTC)

Fate of feral pigeons, feral gulls and feral starlings if humans became extinct?

Would they all die off, or would some of them manage to learn to survive in the 'real wild'? Supposing that the event that precipitated human extinction left the ecosystem mostly as it is now, I mean. --90.240.126.109 (talk) 08:47, 15 January 2009 (UTC)

Humans as a race has still quite a good survivability, so an extinction should have been caused by very drastic events. Or, I suppose, you were asking about the hypothetical situation where all humans suddenly and magically disappear? --131.188.3.20 (talk) 09:20, 15 January 2009 (UTC)

In the very unlikely event of humans suddenly disappearing from the earth the animals (in its widest sense) that have become partially or wholly dependent on us would have two alternatives, in survival terms. They would have to adapt to the new environment and find new sources of food and/or breeding environments or they become extinct. A certain Charles Darwin proposed a similar idea very much earlier, he thought that those that were more adaptable would stand a better chance of surviving. I won't bore you further with the details but you might find the Theory of Evolution interesting. Richard Avery (talk) 10:47, 15 January 2009 (UTC)

I think the questioner knows that. That's why he asked. All you've done is restate his question. The questioner wants to know which of the animals he mentioned are irrevocably dependent on humans and which are not. APL (talk) 17:14, 15 January 2009 (UTC)

Hmm, if the OP thinks seriously that any of the birds he names are dependant on humans then he may need a wider view on the subject. Richard Avery (talk) 23:09, 15 January 2009 (UTC)

The more domestic of the domesticated animals, like domesticated turkeys, would perish. The more feral, like wild turkeys, would survive. Some dogs and cats are much wilder, smarter, and better able to survive without human assistance than others. I have even seen a few feral chickens which would fly from the chicken yard to a tall tree in the woods at night. If all humans disappeared and all domestic animals were set free, the 1% or .001% with the best survival skills would be the genesis of the next generation, after the great starvation and the great devastation by predators. I expect that starlings and gulls would get along fine without humans, and that some small number of pigeons would have the appropriate survival skills/instincts and be near enough a niche environment in which they could find shelter (like a cliff edge) and naturally occuring/wild food. Some pigeons could adapt to the habitat of their wild ancestors, rather than waddling around on the sidewalk eating stale bread someone throws to them. The extinction of humans would not necessarily remove all the cornices pigeons presently use for nesting away from rats/cats/snakes. Edison (talk) 20:01, 15 January 2009 (UTC)

So what exactly is a feral gull and a feral starling. My understanding is that both these species are wild birds which have very little reliance on humans for their existence. They both may casually feed in urban areas but that does not define them as feral. The possibly feral pigeons that exist in cities would certainly continue to forage in urban gardens or leave for the more rural parts. I think the OP needs to brush up on his biology a bit. —Preceding unsigned comment added by 86.4.182.202 (talk) 22:56, 15 January 2009 (UTC)

I saw a doco recently called life after people which explored just that question. It basically said most domestic animals would be the 1st to die off, cats would fare better then dogs, (unless they were locked inside) but many dogs would also survive, the toy and exotic breeds not doing as well as the more "traditional" dog. Rodent populations would explode over a short term, things like supermarket cereal isles and houses would provide a heaven for them, eventually tho those resources would run out and the populations would dramatically decrease again. Similarly with birds, junkyards and stuff which have sustained massive populations of birds would eventually "dry up" and with no one to feed the pigeons in the town square, those populations would also see a decline, sometimes dramatic, but the majority of those populations are nested in a niche which reaches further then total reliance on human waste, so they would survive.. Vespine (talk) 23:52, 15 January 2009 (UTC)

Was that the one that mentioned that the descendants of domestic cats might eventually evolve the ability to glide from skyscraper to skyscraper in the urban wastes, a-la flying squirrels? That was an interesting premise. IIRC, it also said that gulls would (eventually) thrive in a human-free world - due to the seas recovering to their pre-human state after a few hundred years and flourishing with edibles again. --Kurt Shaped Box (talk) 00:37, 16 January 2009 (UTC)

For cats to evolve wings, it would need many thousands of years at least. Skyscrapers won't last that long without maintenance. --131.188.3.20 (talk) 20:26, 24 January 2009 (UTC)

For the record, Life After People. You could try thinking about life after Maccas. Terry Irwin in the short term suggests they'll adapt much as humans would if fast food chains started closing down. She left the conclusion open. Not betting either way, Julia Rossi (talk) 03:04, 16 January 2009 (UTC)

The Macca's analogy is a good one. Remember also that the feral population would have less competition from us as well- while the pelicans might not have us to feed them, they wouldn't have to compete with fishermen so much either. The grass seed eating pigeons might not have us to feed them, but they'd have less competition from lawnmowers and combine harvesters. WotherspoonSmith (talk) 06:06, 16 January 2009 (UTC)

Well perhaps here in Sydney, a vast number of seagulls might get their first view of the sea, as opposed to the malls outside of Maccas outlets 20 miles inland, where they live on discarded French Fries. I've read accounts that suggest that deserted cities would, even many hundreds of years later, contain rich though remnant populations of flora and fauna previously cultivated in gardens, homes and zoos. The film I Am legend has evocative sequences of lions and deer running through a post-apocalyptic New York, and the original Planet of the Apes featured places where such remnants could be housed in almost perfect museum conditions: in the vast underground rail tunnels beneath the city. Once again, it was NYC. (Doesn't anything happen anywhere else? I've just seen Klaatu plunk himself down in Central Park - from The Day the Earth Stood Still (2008)). Myles325a (talk) 05:57, 17 January 2009 (UTC)

Neutron star with event horizon

My question is, is it possible to exist a neutron star with radius less than its Schwarzschild radius i.e. is there a neutron star covered by an event horizon. It is clear that if the Pauli exclusion cannot stop the collapse, then the star becomes a black hole, I mean at the and its state satisfies the Schwarzschild vacuum-solution. But is it possible to exist a singularity at r_s without the singularity at r=0. Thanks! Mozó (talk) 09:48, 15 January 2009 (UTC)

Standard physics has it that if something is compressed beyond its Schwarzschild radius, it will collapse or has collapsed to a black hole. Two arguments can be raised for this. First, that time and space switches place inside the black hole, so that anything right under the horizon must fall in to the singularity as surely as it must move forward in time. Secondly, what's "really" beyond the horizon is regarded by some as an immaterial question, it's a black hole for all intents and purposes. EverGreg (talk) 11:44, 15 January 2009 (UTC)

Thanks a lot EverGreg! So can we say that, we do not have (or probably don't have) stabile solution concerning a body compressed beyond its Schwarzschild radius? Regards Mozó (talk) 16:44, 15 January 2009 (UTC)

Correct. Anything inside the event horizon of a black hole must inevitably reach the point r=0 within finite (proper) time (this is due to what EverGreg was saying about time and space being reversed, although that's a rather confusing way to put it - technically speaking, the radial direction of space becomes "timelike" and time becomes "spacelike", but all that really means is that some minus and plus signs have been swapped around in the equations). Once an object collapses beneath its Schwarzchild radius it is a black hole and it indistinguishable (from the outside) from any other black hole with the same mass, charge and angular momentum. --Tango (talk) 17:55, 15 January 2009 (UTC)

Hold on - is that right? We can distinguish a rotating from a non-rotating black hole - right? As our neutron star shrinks - won't it start spinning faster and faster? So even if it started off rotating infinitesimally slowly - when it reaches a singularity, even that most minute rotation becomes infinitely fast...right? Wouldn't that allow us to tell what the fate of the collapsing star was? (Probably not - I just like to understand why not!) SteveBaker (talk) 01:37, 16 January 2009 (UTC)

The key thing isn't rate of rotation, but angular momentum. While rate of rotation increases as the star collapses (to infinity, which is your first clue that something isn't right), the angular momentum remains constant. Remember, a black hole (after sufficient time has passed) is just a singularity, there is nothing actually there to rotate. We call it angular momentum, but really it's just a constant of integration that appears when you solve the differential equations and happens to behave like angular momentum (the same way the spin of an electron behaves like angular momentum despite it being either a point particle or a probability wavefunction, depending on how you look at it, neither of which have a meaningful definition of rotation). I don't honestly know how astronomers measure the angular momentum of a black hole, but it's not by picking a point on the surface and seeing how long it takes to come back round. --Tango (talk) 02:44, 16 January 2009 (UTC)

As an apropos, the Kerr metric predicts that a flattening of the black hole as well as a frame dragging effect will be indications of a rotating black hole. But i guess astronomers will only see indirect hints, like a high angular momentum of gas going into the black hole, which must then absorb this momentum and "rotate". EverGreg (talk) 10:28, 16 January 2009 (UTC)

Indeed, but the frame dragging is going to depend on angular momentum, not rate of rotation. --Tango (talk) 23:32, 16 January 2009 (UTC)

Thanks Tango, you explain the argument in a mathematician-wise way which is close to me :) Mozó (talk) 08:36, 16 January 2009 (UTC)

Actually, in a black hole, there are only three laws of physics that aren't nessecarily broken: the law of gravitation, electromagnetism, and rotation. So, time probably will not follow its usual mechanism in a black hole, as time would be bent along with space. There's even the concept, as partially suggested by João Magueijo, that nothing can ever enter a black hole because time stops. As for neutron stars, a marshmellow dropped onto its surface would release energy equivalent to roughly 50 Hiroshima bombs, so they too have very strong gravitation. ~AH1^(TCU) 23:49, 16 January 2009 (UTC)

Is there a center of the universe?

I was reading this thread here [2] where it was stated that there is no center of the universe. Couldn't the location of the big bang be considered the center of the universe? Everything is moving out in a 360 degree sphere from this point, correct? A Quest For Knowledge (talk) 13:30, 15 January 2009 (UTC)

I'll take a shot at this one: in brief, there is no point where the big bang took place, because everything was "there". If the material that makes up Alpha Centauri, and the material that makes up my pocketwatch, were both in the same place at one time, we can't really say that one or the other is moving away from the center of the universe. Be careful not to think of the big bang as a concentration of material in empty space -- it IS that space; there was, at the time, no universe outside of that big bang. jeffjon (talk) 13:47, 15 January 2009 (UTC)

We've answered this one before - and it comes about because of our inability as humans to properly visualise the expansion of space. It's tempting to think of the big bang as if there were this vast, utterly empty universe - with a tiny dot someplace - which then explodes, scattering stuff out into the inky void. But that's WRONG. The big bang produced the space itself. So at the instant of the big bang, space was infinitely scrunched up - and space itself expanded, along with the matter within it...as it continues to do today. So even though it was an infinitesimal 'dot' at the outset, that dot completely filled the infinitesimal "space" that comprised all of existence. So we have to go back to the balloon analogy. The surface of a deflated balloon is space. Dots drawn onto that balloon are "matter" (galaxies maybe). You have to imagine the balloon deflated down to a dot (not a wobbly lump of rubber like a real deflated balloon!).

Balloon analogy	Real universe
The deflated balloon is very tiny.	The universe at the time of the big bang is infinitesimal.
The dots drawn on the balloon are so close together, they almost touch.	The matter in the universe is scrunched together into a 'dot'.
As the balloon is blown up...	As the big bang explodes...
the surface area of the balloon grows rapidly.	the volume of space itself expands rapidly.
Yet there is no 'edge' to the surface of the balloon.	Yet there is no edge to the universe.
As the balloon inflates...	As space expands...
so the dots on the balloon move further apart.	so the stars and galaxies move furhter apart.
No point on the surface of the balloon could be called "the center".	No point within all of space could be called "the center".

SteveBaker (talk) 14:51, 15 January 2009 (UTC)

I think it's important to clarify - you are describing a "closed" (ie. finite) universe. If the universe is open (and infinite) now, then it was infinite to start with (although the observable universe was infinitesimal). Don't try and visualise the expansion of already infinite space, you'll just give yourself a headache! (Now, where did I put that aspirin...) --Tango (talk) 18:02, 15 January 2009 (UTC)

To make sure I understand the analogy, the universe is the surface of the balloon and not the air inside the balloon? A Quest For Knowledge (talk) 19:06, 15 January 2009 (UTC)

Yes, except there is no balloon. —Preceding unsigned comment added by 82.124.85.178 (talk) 19:13, 15 January 2009 (UTC)

Yes, it's a 2D analogy of our 3D universe, the rubber of the balloon represents "the fabric of spacetime" (just be careful with the inverse tachyon emitter!). --Tango (talk) 19:53, 15 January 2009 (UTC)

To do the best justice to the balloon analogy - try to forget that the balloon is round - look at one small, nearly flat patch of the balloon. Then, the balloon can be considered to be a two-dimensional analog of the universe...except that the rubber of the balloon is 'space-time'...but it's still stretchy! Now that we're looking at a flattish local patch of the balloon - we can cautiously extend our analogy to imagine that it has other 'shapes' - maybe it's infinite and flat...maybe it does curve back on itself - maybe it's "saddle-shaped"...but it still expanded from an infinitesimal 'dot'. But as Tango says - just don't let your poor overtaxed brain try to envisage an infinite universe that's scrunched into zero volume...yet is still infinite. Because...OW! (Tango - could you just pass me over some of those Asprin?) SteveBaker (talk) 01:34, 16 January 2009 (UTC)

I never did find them... Perhaps if I make your head hurt enough it will just go numb: The infinite universe isn't scrunched into zero volume, it's still perfectly infinite, it just also has infinite density. Now try and imagine how much matter/energy that is! --Tango (talk) 02:52, 16 January 2009 (UTC)

This is a very complex concept. If there was a "starting point" for the Universe, would that "point" be in the same location in spacetime now as it was when it was created? No. Even at the "center" of the universe, if it still exists, space is expanding at the same rate as the rest of the universe, and there would be no redshift anomaly if an observer was looking at the "origin". The Universe is considered "flat", meaning it is of infinite size, and the "surface" of the universe's exterior, which doesn't exist, would be "flat", and not "round". There's the concept that a spaceship launched fron Earth would arrive back at Earth googols of years later. This probably doesn't work in an infinite universe, and besides the "location" of Earth in the present universe would have moved "away" by then. Also with the balloon analogy, if you pop the ballon, the "universe" explodes. This probably doesn't work in real life, but there is the Big Rip scenario hypothesis. ~AH1^(TCU) 23:38, 16 January 2009 (UTC)

Power adapter

I am searching a power adapter for a laptop and found one with an output of 15/16/18/19/20V (3,5 A) or 24V (3A). However, the original adapter had an output of 19V (3.42A). Does this adapter fit the laptop? Should I use 18V instead of 19V to compensate for the extra 0.08A? Can this adapter burn the laptop? --Mr.K. (talk) 15:06, 15 January 2009 (UTC)

Short answer: yes, if the polarity is right and the plug fits; no; and no. Longer answer: When you look at a power supply, think of the amps figure as a maximum capacity; in other words, the adapter you found can provide any amount of current up to 3.5 A before the rated voltage starts to fall off (or the magic smoke is released). Your laptop draws no more than 3.42 A—very little in hibernate mode, more when a drive fires up, but always less than 3.42—so you're fine as far as that goes with 3.5. (The adapter could be rated at 3.5 million amps, and that would be fine, too.) As for the voltage, a volt or two either way often doesn't make any difference to an electrical device, but to be absolutely safe set it to the voltage of the original adapter. --Milkbreath (talk) 16:31, 15 January 2009 (UTC)

Set it to 19V. Think of voltage as flow and current (amps) as capacity; the new adapter has a bit more capacity. --—— Gadget850 (Ed) ^talk - 20:43, 15 January 2009 (UTC)

It's essential to get the voltage right - you could easily damage the laptop by giving it the wrong voltage. The current is merely a capability of the power supply. We assume from the old powersupply that the laptop never pulls as much as 3.42Amps - probably quite a bit less. The new supply (at 19volts) should be able to supply the most the laptop will ever demand - with a bit to spare. It's a good idea to have plenty to spare because power supplies get hot when they are operating near to their limits - and that prolonged heating shortens the life of the power supply. The plug shape actually tells you almost nothing. Sadly there are at least a dozen very similar connectors and little or no control over which ones relate to which voltages and currents. But you do need to get the polarity right...again, there is a risk of damaging the laptop if you get it wrong. SteveBaker (talk) 01:26, 16 January 2009 (UTC)

The universal adapter for laptops had no way of changing the polarity. I suppose that all laptops have the same combination. Anyway, it has worked fine, but what would have happened if the polarity was wrong?

Well, the laptop MIGHT have internal protection against that - in which case it would simply not work. Failing that, you'd fry some chips - and it would almost certainly kill the laptop. I havn't yet done that to a laptop - but I messed up my music synthesiser that way - but I got lucky and replacing the power transistor (which was the first thing I tried) fixed it perfectly. SteveBaker (talk) 00:05, 17 January 2009 (UTC)

No sleep

Respected

Well i would kindly request you to give approximate numbers on how much a 22 year old should sleep for a healthy life. the next doubt is what and the problems that one will face in present or in future for spending sleepless nights.

THANK YOU —Preceding unsigned comment added by 220.227.68.5 (talk) 15:17, 15 January 2009 (UTC)

The Ref Desk cannot and will not provide specific medical advice for you. However, I can point out that our sleep article addresses the idea of optimal sleep for humans. To summarize what's there, it varies both by how long you sleep and when you sleep. A quick overview of the studies we reference suggests that something in the 6-8 hour range during nighttime is common. — Lomn 15:51, 15 January 2009 (UTC)

You might also want to read out article on insomnia. We cannot give you medical advice, but there are many on-line tips for falling asleep if you have insomnia. ~AH1^(TCU) 23:26, 16 January 2009 (UTC)

menstrual cycles

so girls who spend a lot of time together menstruate all at once.

my question is could the evolutionary reason for this be that it means they will also be their friskiest and most promiscuous (ovulating) all at once... setting up the perfect storm for dorm orgies, banging sisters or best friends, hell the whole cheerleading squad why not, it happens???!!

Because if it is the reason, that is just sweet. —Preceding unsigned comment added by 82.124.85.178 (talk) 16:27, 15 January 2009 (UTC)

Evolution doesn't have reason. Its entirely random. If you make up your own reason, it will be just as good as any that someone here could give you. --Jayron32.talk.contribs 16:53, 15 January 2009 (UTC)

God does not play dice! —Preceding unsigned comment added by 82.124.85.178 (talk) 18:03, 15 January 2009 (UTC)

W-e-e-l-l-l-l... Evolution doesn't have an "ultimate" reason or a target it's working toward, but adaptations surely have a reason - they allowed the relevant population to differentially outbreed populations with different adaptations. I don't have any idea why women tend to synchronize their cycles, but I'm sure there have been theories put forward. I would bet (were I a betting man) that there is some kind of tie-in to the "hidden" ovulation humans have. Many/most mammals come into "heat" or oestrus, where people and a few others don't. It would be interesting to see if those other hidden ovulators also get synchronized menstrual cycles. Matt Deres (talk) 17:54, 15 January 2009 (UTC)

A bit of elaboration. Humans are strange, reproductively speaking, in two ways. First, they have hidden ovulations: neither the woman nor her partner know exactly when she’s capable of being fertilized. Second, they have sex much more frequently than other mammals. Most mammals only mate when there is a chance of the female to get pregnant. Researchers such as Jared Diamond (among others) have theorized that there is a connection between these two strange adaptations. The theory goes something like this: human children are a huge time and resource investment to parents, which leads to a problem: men aren’t going to want to invest their time/energy is raising a child that’s not theirs while women have a huge incentive to make sure some man – any man – helps her raise the child (she is, after all, sure of maternity). Women who obscured their ovulation would have an adaptive advantage because her potential partners would have no idea when the best time would be to mate, making it advantageous for them to stick around long term, so they can be sure to mate with her at the right time. He "knows" the child is his while she gets help bringing up baby. Everyone wins, at least according to this theory.

The reason I suggested the synchronized ovulation might be tied to this is that a population of women who ovulated at the same time would have an easier time tying down a specific partner. While he might be tempted to stray, he runs into two problems: first, he’s not sure who is ovulating, so he’s not sure who to try to mate with. Second, with everyone ovulating at the same time, he probably doesn’t want to be away chasing "the other woman" at the very time his partner is also ovulating. That’s just asking for some other philanderer to sneak in while he’s away. Matt Deres (talk) 19:00, 15 January 2009 (UTC)

We have an article McClintock effect. In particular, note that it's by no means established that this effect occurs at all. Algebraist 18:00, 15 January 2009 (UTC)

Cecil Adams covers it here, with some speculative reasons. Short answer: nobody even knows if it really happens, much less why. Whenever you hear someone explain some trait of humanity with "well, in prehistoric times ..." your bullshit detector should start tingling immediately. --Sean 18:24, 15 January 2009 (UTC)

It doesn't particularly sound like the OP is thinking of prehistoric times Nil Einne (talk) 19:32, 15 January 2009 (UTC)

You mean they didn't have cheerleaders or dorm orgies in prehistoric times? :) A Quest For Knowledge (talk) 19:43, 15 January 2009 (UTC)

They didn't have them in the 1990's when I was in college, or atleast I never got invited to those kinds of parties. Not sure if that qualifies as "Prehistoric". --Jayron32.talk.contribs 00:39, 16 January 2009 (UTC)

I have another theory: Not only would the women ovulate together - but they'd also enter that god-awful bitchy, naggy, grumpy phase of the month simultaneously. If the all did this at different times - the man who's suffering this particular day may well go look for fun with other women in the tribe. But if they all do it at once - the men simply have to leave the cave for a few days and go do "guy stuff" - hunting mastodons for example. SteveBaker (talk) 01:16, 16 January 2009 (UTC)

Synchrony of estrus is a common theme in mammalian reproduction, though its best studied in mice. There are two major and contrasting factors the modulate estrus. Firstly there is the Whitten effect: contact with adult males, or their pheromones, cause the acceleration of estrus. This makes a lot of evolutionary sense from the male perspective, as it is to his reproductive advantage to have fertile females around. A version of this is also called the Vandenbergh effect, which is when exposure to male pheromones causes the acceleration of puberty of young female mice (the advantage of that for the male is obvious too). In contrast to that is the Lee-Boot effect: when female mice are housed together they suppress each other's estrus cycles, and their pheromones delay puberty in young females. The dual effect of these conflicting pressures tend to result in synchrony.

The best argument for the existence of human pheromones lies with the McClintock effect. Though its by no means been proven, and the statistical validity of the data has been questioned, it does seem to fit with what we see in other mammals. The major problem with it is the organ the detects these pheromones in other animals is not present and/or functioning in humans. In an excellent textbook on pheromone communication (Wyatt, Tristram D. (2003). Pheromones and Animal Behaviour: Communication by Smell and Taste. Cambridge: Cambridge University Press. ISBN 0-521-48526-6.) the evolutionary aspect of esrus synchrony in social animals is discussed on p285. The major hypotheses are:

• It helps regulate seasonal breeding (and all the advantages that has for survival)
• It helps combat predation pressure (a troop with many young will move slowly, but a lone mother may get left behind)
• It helps to equalize the operational sex ratio, bringing it closer to 1:1, which increases paternal investment by creating a situation that favors pair mating.
• It promotes communual rearing (which benefits the mothers and the young).

The relevance of these to modern humans is questionable, but its entirely possible that the McClintock effect (if it actually happens) is an evolutionary relic from our primate ancestors. As an aside, a student in my lab is currently working on identifying the specific molecule that drives the Whitten and Vandenbergh effects. Once she characterizes it, we may have more evidence for (or against) the same effect in humans. Rockpocket 01:21, 16 January 2009 (UTC)

Meniscus Lens Ray Diagrams

Hello. How do I draw a ray diagram for a positive meniscus, thin lens since there are two focal points on the "concave" side? Thanks in advance. --Mayfare (talk) 20:53, 15 January 2009 (UTC)