Wikipedia:Reference desk archive/Science/2006 September 22

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September 22[edit]

Animal Respotory System[edit]

What animal breaths through its mouth and its anus? Amber Could it be the sea cucumber? I don't know if they breathe through their mouth, but they do have organs inside their anus to extract oxygen from the water. Gary 03:26, 22 September 2006 (UTC)

some primitive animals have only one hole that acts as both the mouth and the anus. and i guess they're probably able to breath through it. Yaksha 03:33, 22 September 2006 (UTC)
One-holers generally have a high enough surface area and a low enough oxygen need that they absorb oxygen by diffusion through the skin. --Serie 20:40, 22 September 2006 (UTC)
I know some people who talk through their anus, and you can't talk without breathing, can you? DirkvdM 09:19, 22 September 2006 (UTC)
I don't know. Maybe you should try. See how long you can talk for without breathing. (disclaimer: i take no responsibility regarding what happens when people follow my suggestions.) --Yaksha 12:32, 22 September 2006 (UTC)


What happens if you cut a live worm in half?--Light current 02:11, 22 September 2006 (UTC)

It depends on the worm. The group of animals called "worm" is polyphyletic. A lot of the best known worms will live (both halves) and regenerate the lost body parts (like the earthworm and at least some flatworms), but I doubt all do. --Allen 02:39, 22 September 2006 (UTC)
I think Light current means earthworms. In that case, and if you live in southern Ontario, nearly all of the worms you see crawling out of the ground will live if you cut them in half. (Trust me, I've done this before many times!) --Bowlhover 03:22, 22 September 2006 (UTC)

Yeah Im certainly not talking about tapeworms 8-)--Light current 17:03, 22 September 2006 (UTC)

and i guess if you're really sadistic, get some earthworms, and start cutting. And see how big a 'piece' has to be before it becomes unable to regenerate into a full earth worm. IIRC, for worms which can regenerate, they only will if you cut between certain parts. There are some bits where if you cut, you don't get two worms...just one new worm and a dead bit. Yaksha 03:29, 22 September 2006 (UTC)
If you cut an earthworm in half, only one side has a clitellum, so doesn't that factor in? At the very least, it certainly wouldn't be able to reproduce anymore.  freshofftheufoΓΛĿЌ  05:48, 22 September 2006 (UTC)
Wouldn't it just grow a new one? I've seen photos of people who've cut up worms (not sure what type though) into many pieces, and later photos showing all the pieces growing bigger. I don't know...go try XP --Yaksha 12:16, 22 September 2006 (UTC)
Here is our article about regeneration in general, though it doesn't talk about earthworms specifically. --Allen 05:49, 22 September 2006 (UTC)
...and here is the answer given when a similar question was asked a few months back.--Shantavira 07:37, 22 September 2006 (UTC)
...which I have therefore added to the reference desk faq. DirkvdM 10:01, 22 September 2006 (UTC)
Hey thats great Dirk. Thanks for doing the 'FAQ' thing 8-)--Light current 17:00, 22 September 2006 (UTC)
Ah, it looks like I was wrong about earthworms, then. I'm not so sure about that poster who said that the flatworm cut has to be lengthwise, though; I'm pretty sure you can separate the tail from the head and both halves will fully regenerate. --Allen 16:42, 22 September 2006 (UTC)
No, only half wrong after all. Sorry I didn't look that up before. --Allen 16:47, 22 September 2006 (UTC)
I know some people who talk through their anus, and you can't talk without breathing, can you? DirkvdM 10:01, 22 September 2006 (UTC)
The connection is not immediately obvious to me. In any case, maybe you should seek new friends. --LambiamTalk 16:14, 22 September 2006 (UTC)
Ooops, I dont know how this got here. It was meant for one thread up. I like lateral thinking, but this one went off the scale in that respect. :) DirkvdM 18:25, 22 September 2006 (UTC)
Yes, when thinking laterally, one has to be careful that the ideas do not come out of ones ears and hit people in the wrong place 8-)--Light current 20:07, 22 September 2006 (UTC)

No, although worms have the ability to regenerate, and move about for a while afterwards (in an attempt to confuse any predators), almost always both halves die. Philc TECI 17:34, 22 September 2006 (UTC)

OMG another classic! Can you imagine trying to confuse your predators by wriggling about if you had just been cut in half. I mean whats the point? LOL! 8-)--Light current 19:59, 22 September 2006 (UTC)
Maybe you should read the response I gave Allen, who also didnt understand what I meant, though he expressed this in a much more polite manner. Philc TECI 20:48, 22 September 2006 (UTC)
I am being polite. I thought your answer was very funny . THats all. No offence!--Light current 21:24, 22 September 2006 (UTC)
OK it wriggles about to deter predators just in case it manages to regenerate? Is that what you mean?--Light current 21:29, 22 September 2006 (UTC)
Ok, sory, if you meant no harm, the I've just misenterprated what you said. All is cool.. And yeh, thats the jist of what I mean, except one half always dies, and its this half that wriggles, in the hope that the half which can survive will get away as the predator is distracted. Much like some salamanders and their tails. Philc TECI 21:39, 22 September 2006 (UTC)
I don't understand. They regenerate and then die? And what's the evolutionary advantage to confusing predators if you can't live long enough to reproduce? --Allen 17:49, 22 September 2006 (UTC)
No, they can regenerate, but its not a 100% success rate, if you cut a worm in half you cannot count on it regenerating. It will often die before it manages. The advantage of continued movement after seperation is that if one half is to small to survive it will continue to move, thus distracting a predator from the other half that may survive. Philc TECI 18:19, 22 September 2006 (UTC)
I see; thanks. --Allen 19:34, 22 September 2006 (UTC)

Does it?[edit]

Does white absorb more heat than black?

No--Light current 02:34, 22 September 2006 (UTC)
Things appear "black" during the night because they don't reflect a lot of light. Black objects appear black during the day, also because they don't reflect a lot of light. And if light is not reflected, it has to be absorbed. Therefore, black objects absorb more light than objects with any other colour. --Bowlhover 03:16, 22 September 2006 (UTC)
which means it's black that absorbs more heat than white. a "experiment" teachers often show students in primary school is that they put many piles of ice outdoors where the sun can shine on them. Then they cover the piles of ice with different colored pieces of cloth. And the pile under the black cloth generally melts the quickest, and the white one the slowest. Yaksha 03:26, 22 September 2006 (UTC)
Yes. No. Maybe. The question is ill-posed. Everything being equal (and this is saying a lot, as we will see), the answer is a qualified no.
Given an infinite amount of time, any two object, regardless of their color or other physical properties, exposed to the same radiant source will achieve approximately the same temperature -- the temperature at which the black-body radiation of the objects matches the incoming radiation. These temperatures will not be the same because real, physical objects do not emit exactly the black-body spectrum. See the discussion at Color temperature about assigning correlated color temperature to fluorescent lights. The black-body radiation article shows a plot of the emission spectrum of an incandescent bulb, which shows slight deviations from the ideal spectrum. The solar spectrum also shows deviations from the ideal spectrum due to absorbances caused by the atoms out of which it is made. (Helium was discovered by noting details of these departues from ideality in the sun's spectrum.) Consequently, the total emission energy of two distinct objects at the same temperature can differ, sometimes significantly, therefore they will achieve different asymptotic temperatures when exposed to the same source.
When a painted, asphalt street is heated in the summer, one may note that the black asphalt is too hot to walk on, but the painted lines are cool enough to walk on. This is sometimes taken to imply that the black absorbed the heat more than the reflective/white/yellow paint did. This implication is incorrect. The black asphalt has a vastly higher heat capacity and thermal transport coefficient (in contact with your foot) than does the paint. In short, the paint is the same temperature as the asphalt, but when it starts transferring thermal energy to your foot, it runs out of energy far faster than the equivalent volume of asphalt would. The same effect can be seen when bare-handed handling the space shuttle heat tiles. The tiles don't contain much energy when they are heated (difference between temperature and heat capacity) and second, they conduct heat very poorly internally, so you can easily deplete excess heat where you are touching the material, but thermal energy a millimeter away takes a very long time to transfer to the cool spot and continue heating your hand. Thus, material heat capacity and internal heat transport properties are significant when evaluating the heat absorbed by objects when you intend to contact them.
If an illuminated object doesn't have a mechanism to convert electronic transitions into rotational, vibrational, or translational modes ("kinetic modes"), then a white object will absorb energy faster than a black object but will not express that energy as heat. The reason is that the "white molecules" absorb twice as much recoil momentum as the "black molecules", so the white molecules are made to translate more rapidly than the black molecules. Raman scattering is a mechanism for converting photon scattering events into kinetic modes. The zero-phonon line and phonon sideband represent an additional mechanism which is a consequence of collective electronic modes in a material. Brillouin scattering is a more directly coupled mechanism for transferring scattered photonic energy into collective motions of the molecules. Most real materials have several mechanisms to convert scattered photons and electronic modes into kinetic modes and thus, unlike the description at the front of the paragraph, real black materials do not store the absorbed energy in modes that don't produce heat. Real black materials absorb the energy and turn it into vast amounts of heat. In comparison, the momentum recoil is tiny. For example, a 400 nm photon contains enough energy to cause a water molecule to recoil at 5800 m/s; it's momentum would only make that water molecule recoil at 0.055 m/s. Consequently, if there is no mechanism to convert photon scattering or electronic excitation in kinetic modes, then the white molecules recoil with twice the momentum of the black molecules (and the black molecules store huge amounts of energy in non-inetic modes, i.e. not as heat). If there is/are such mechanisms, then the black molecules express huge quantities of absorbed energy as heat. Note that the final temperatures are not affected by these considerations; the asymptotic temperatures are unchanged, but we expect black objects to reach those temperatures vastly faster. Also, some materials may exhibit some consequences related to this feature so that they heat unusually slowly -- rarefied noble gases do not have rotational or vibrational modes, and do not easily exhibit collective modes, and therefore only very slowly convert electronic excitations into heat.
So, the answer to your question is: "it depends". Depending on how you're going to measure hot, the color may be correlated with other physical properties like heat capacity or thermal conductivity which could limit the transfer of heat to the measurement device (perhaps a thermometer, hand, or foot). The black object and white object may have sufficiently diverse graybody spectra that their asymptotic temperatures under identical illumination are different. Unusual properties of the black material may make it difficult for the material to express internal energy as heat and so it may take vastly longer for the black object to catch up to the white objects temperature.
Nevertheless, in the absence of exceptional behaviours such as those I've listed, you may assume that (1) the black object gets hot much faster than the white object, (2) they are both trying to reach the same temperature, and (3) either one may feel hotter due to details of heat transfer through and out of the materials. -- Fuzzyeric 03:50, 22 September 2006 (UTC)

Black absorbs more light energy than white, and radiates this as heat. So although there is a minor misunderstanding in this, black objects will get hotter in the sun. Philc TECI 17:31, 22 September 2006 (UTC)

So its 'no' then? THought so! 8-)--Light current 20:00, 22 September 2006 (UTC)
Yeh, the answer is no, but the difference is I thought he might like to know the origins of the myth, and that its not complete rubbish, its just misquoted. Philc TECI 20:50, 22 September 2006 (UTC)
Ah well Ive not heard of the myth. So...--Light current 21:26, 22 September 2006 (UTC)
Fair enough, its just things like people often say they dont want a black car, because it "absorbs" more heat, but it doesnt, it absorbs more energy, and radiates it as heat, a lot of which is trapped inside the car. Philc TECI 21:30, 22 September 2006 (UTC)
If its trapped within the car, how does it radiate it?--Light current 21:32, 22 September 2006 (UTC)
You have to remeber in this case the car is not treated as a single object, black objects, such as bashboard, or black paint radiate heat, some of this goes into other objects within the car, or the air in the care, some of it is radiated out of the 'car' system completely. So, you see, parts of the car are radiating heat everywhere, not the whole car radiating heat out of itself. Maybe my initial explanataion was a bit to simplified to exaplain that point completely, sorry. Hope you understand now! Philc TECI 13:29, 23 September 2006 (UTC)
P.S. Fuzzyeric, has posted a more indepth reply to your question below, as he seems to be more learned on this subject. hehe. Philc TECI 13:30, 23 September 2006 (UTC)
Nope. The black object's temperature increases faster in the sun, but it does not reach a higher final temperature than a white object.
(Why do I know this? I and a friend I met in college performed a sequence of experiments to settle this. This was after five years of collecting evidence about which way this should work, which is why I am able to explain the theory at some length. The least confounding experiment involved several thermocouples, a half dozen hotdogs painted white, and a half dozen hot dogs painted black. The paint was selected to have equivalent (to within 5%) heat capacity and thermal conductivity. The black painted hot dogs's temperature rose more rapidly than the white painted hot dogs (initially). However, the asymptotic temperatures of all the hot dogs were identical. No hot dog showed thermal overshoot.)
The black object does not "get hotter in the sun". The black object is initially hotter in the sun and the difference in the temperatures may increase briefly before going to zero. Unavoidably, the difference goes to zero.
And to Light current: The black paint absorbs the energy, transfers it to the metal body through conduction, which then provides the heat to the plastic and glass portions of the vehicle, also by conduction. The glass radiates the energy both into and out of the cabin and the metal and plastic surfaces in the cabin will radiate heat into the cabin. (They also radiate heat to the outside of the car, but most of that escapes to the ground, trees, buildings, et al. On an especially hot day, you can see air, heated by conduction with the car's surfaces, convectively rising from the car.) However, the glass in the cabin is an excellent infrared reflector, so that heat does not escape through the windshield. More and more energy enters the cabin (by conduction from the rest of the car's body and by directly shining through the windows), but most of that energy is trapped until the interior is so hot that the windows and car's body are no longer an effective insulator (like a thermos) and the excess heat leaks away by conduction. -- Fuzzyeric 04:02, 23 September 2006 (UTC)
I dont doubt the results of your experiments and these results are interesting. THe only thing you may have overlooked here is the emissivity of the paints. Im assuming the emissivity of the black dogs was near to unity whereas the white ones would have lower emissivity (how much lower is hard to say). If both sets of dogs reached a steady equal temperature, then it is obvious that no net energy was flowing into or out of the dogs. But, one would expect the black dogs to be radiating more heat energy due to increased emissivity and therefore taking in more heat energy. THe temperature of each set of dogs (neglecting convection, conduction) will be given by solving the eqaution for zero net energy loss/gain. So i dont get it, unless the 2 sets of dogs had equal emissivities.!--Light current 13:43, 23 September 2006 (UTC)
The difference in kT is small in experiments like the car or the hot dog. Thus, the difference in blackbody spectrum is small. These two objects are not blackbodies and do have some spectral content at various frequencies. For blackbodies, the peak emission wavelengths are 9.7 um (1030 cm^-1) at 300 K (80 F) and 9.1 um (1100 cm^-1) at 320 K (116 F). The total emitted power increases from 460 W/m^2 (300 K) to 590 W/m^2 (320 K). The point is that very little new spectrum is relevant going from the lower to the upper temperature, although the change in power is huge. So, we may expect that paints using similar formulations will have similar mid-/long- IR spectra and should have equivalent emissivities for the range of temperatures tested. Also, empirically, the near constancy of emissivity of paints is observed. Q.v. 1 (black paints: 0.88, white paints: 0.87-0.92). The differences, ~ 4% will result in ~4% change in emitted power at the same temperature. However, the full change in emitted power is +28% for +20 K (from the numbers above) so the difference in temperature will be < 3 K. (Quite a bit less since the emitted power is not a linear function of the delta-T.) Remember also that the visible spectrum of materials is not particularly related to its IR spectrum -- glass is trasparent in visible and reflective in the room temperature band (near 1000 cm^-1) ref -- so the visible blackness or whiteness of a material has very little relation to its IR reflectance.
Of course, this only takes account of the paint. The hot dog material is the same in both cases. Since the paint thickness is about the wavelength of the light, it's semitransparent for most of the emission spectrum. So any difference in the paint spectra will be averaged out a bit by the hot dog spectra. It is thus feasible to expect < 1 K differences in the final temperatures of the two hot dogs. Larger fluctuations can be driven by differential convection (if there's any wind at all) and so this difference is insignificant in practice. -- Fuzzyeric 20:04, 23 September 2006 (UTC)
Thanks for that exhaustive explanation of the theory. So it seems your experimental results confirm the theory. Im not too concerned now, since you have indicated the very similar emissivities of both paints and maybe the lack of relevance of the paints anyway due to their thinness cf wavelength of the IR. So basically you showed that similat bodies (as far as IR is concerned) heated up the same amount? But for some reason the black one got there more quickly. --Light current 20:15, 23 September 2006 (UTC)
Yes, the similar bodies race to the same temperature. The black body gets there faster because it retains more of the incident energy as heat. The white reflects more of the incident energy as light, so gets there slower. -- Fuzzyeric 05:57, 24 September 2006 (UTC)
Well sunlight is of course a relatively broadband source of electromagnetic radiation.--Light current 04:02, 25 September 2006 (UTC)
Black absorbs faster, I believe. --Proficient 06:04, 25 September 2006 (UTC)

Do Bugs "Hybernate"?[edit]

Up here in the great white north (Canada), it's just starting to get a bit nippy. Of course we'll have some "Indian Summer" when it'll warm up again for some reason for a couple of weeks (curious! but that's a different issue).

In any case, the mosquitos are mostly gone by now. But they'll of course be back in full form annoying the hell out of us next spring.

I know certain birds, like robins, can't take our winters and so they "fly south", and certain others seem to be more hardy about the whole thing, like sparrows, crows, pigeons etc.

But what about the bugs? The mosquitos, the spiders, the houseflies, the earthworms, the moths? Certainly an earthworm doesn't "crawl south" for the winter!

Seriously though, each species must somehow survive our incredibly long winter, by laying dormant in some sense. What do the worms do? What about mosquitos and the houseflies? Do the adults somehow deposit their larvae or eggs or whatever in some reasonably protected, reasonably warm spot and then die, only to have their offspring hatch next spring? I can't see how the adults can survive the entire winter "hybernating", but I may be wrong.

Another odd thing is that while during the summer, all the bugs mentioned tend to annoy us and somehow manage to creep into our homes, curiously, over the winter, the only pests seem to be the spiders. It could be the dead of winter, -30°C, far too cold for a weakling mosquito to handle, but somehow a spider will manage to appear crawling on the ceiling. How do they do it? Loomis 02:38, 22 September 2006 (UTC)

I'm no entomologist, but I read that some larvae can live much longer than adults can. I guess some adults hybernate, some eggs wait over the winter, and some insects wait as larvae.
PS. Spiders aren't usually pests. :) --Kjoonlee 03:02, 22 September 2006 (UTC)
Some bugs go into tons or tonnes or something like that, like wooly bears. — X [Mac Davis] (SUPERDESK|Help me improve)03:04, 22 September 2006 (UTC)
I don't have an exact answer for your question, but i do know that insects do not hibernate. Hibernation is something that warm-blooded animals do. The entire point of hibernation is that in winter, it becomes simply absurd to maintain the normal body temperature (as warm blooded animals do), so they hibernate. Animals that never maintain their own body temperature has no need to hibernate.
as for what insects do, i guess they just find a place warm and stay relatively dormant. which is why you don't see them around in winter. Some insects can keep their core temperature sort of warm by vibrating their flight muscles. A lot of insects do this when they need to fly in winter - they vibrate flight muscles to warm up before taking off. Bees do this collectively - so all the bees in a hive will do this at the same time, which is used by bees to keep their hive at a decent temperature during winters so all the individual bees don't get frozen to death.
i'm not sure what happens to other insects, but insects (and spiders) really don't have a very high energy requirement. For a warm-blooded animal, the vast majority of what we eat goes into keeping us warm, staying warm is VERY tiring. For something like an insect, the only time it really needs much energy is when it's growing, or when it's producing young, which both happen in the warmer seasons. When an insect just needs to stay alive, it really doesn't need to eat a lot. If you've ever kept something like a cricket or a spider as a pet, you'll know you really don't have to feed it an awful lot to keep it alive.
as for the spider --> spiders come indoors in winter because they're looking for a warmer place. Some are probably able to be active in very cold temperatures. Generally, cold-ness is more dangerous to warm-blooded animals than cold-blooded ones. Cold-ness kills animals because it causes the water in their bodies to freeze (very bad), or the proteins that are needed to push forward vital chemical reactions and drive metabolism stops working (also very bad.) Aniamsl adapted to the cold have chemicals that prevent water from freezing, or adaptations so that the important parts don't freeze, and specialized proteins that can still carry out their functions even when it's very cold. How do you think fish survive in very cold waters?
about larve. Eggs and larve and stuff are generally very resilient. Similar to plant seeds. As i said - cold is dangerous because it freezes water and stops metabolic reactions. If something doesn't have much metabolism in the first place, they can stay dormant very easily in very cold temperatures. Yaksha 03:13, 22 September 2006 (UTC)
  • I'm positive they don't "hybernate". Whether they hibernate is another question altogether.- Mgm|(talk) 12:27, 22 September 2006 (UTC)
  • Hybernate = To hibernate in Hyderabad. :-) StuRat 14:45, 22 September 2006 (UTC)
  • I scrolled past quickly and saw a "" in bold and thought someone asked a question about me. Stupid spelling mistakes. :( Hyenaste (tell) 18:29, 22 September 2006 (UTC)

I found the book I had read six years ago, and will now correct, clarify, and elaborate.

Water bears are also known as tardigrades, a name that means "slow seppers." They are very small—from one to 20 could fit on a pinhead. Some live in the sea, between particles of sand and mud. Others dwell in fresh water. But most live in places that are sometimes wet and at other times dry—like muddy puddles, insdie a moss plant, or in the rainwater gutters of buildings. They can survive here because, as the dampness dries out, the water bear turns into a special resting stage called a tun. It simply dries out, shrivels up to about one tenth its size, and becomes inactive, almsot like an egg. This tun stage can surivve for many months, even yeras. When the moisture return, the water bear "hatches" out of its tun stage, and carries on living!"

The book was Animals of the World. I loved that book when I was eight! — X [Mac Davis] (SUPERDESK|Help me improve)18:05, 22 September 2006 (UTC)

Alright everyone. Mark this in one in your calendar. On September 22, 2006, Loomis actually made a spelling mistake! I actually spelled hibernate as hybernate! Remember where you were on this day, just as people always remember where they were when they found out JFK was shot. Consider yourselves to be privileged to have witnessed a truly remarkable event in the history of mankind, for, on September 22, 2006, Loomis actually made a spelling mistake! :--) Loomis 01:12, 26 September 2006 (UTC)

M>S in Engineering from your University[edit]

Sir My daughter is doing B.E in Electronics&Telecomunication from Pune University [India] with 60% marks.She wants to do M.S from your University.We want to know what are the procedures for admission. Will you help us. Thanking You Tanveer Asma Trading Corp Yavatmal[M.S] India 445001

Our website is not a university. It's an encyclopedia, where you look up information if you're curious about something. --Bowlhover 03:31, 22 September 2006 (UTC)
the university of wikipedia? its got majors in edit warfare, seagull science and bagelometry Xcomradex 03:34, 22 September 2006 (UTC)
There is something called Wikiversity. I'm not really familiar with it, but it aims to have some things in common with a university. You can't earn a degree from it, though. --Allen 03:39, 22 September 2006 (UTC)
I think Wikiversity is simply a collection of information that's more organized than Wikipedia. --Bowlhover 04:11, 22 September 2006 (UTC)

60% doesn't sound very good. There is a lot of competition for most university slots, and I doubt if those grades would be sufficient. Universities will sometimes overlook poor grades for those who live in the area or have relatives who attended or who contributed large amounts of money to the university. Unfortunately, it doesn't sound like any of these apply here. Also, consider that, if she is struggling where she is, that will likely only get worse if she studies abroad, and then has language and culture issues to deal with, too. My suggestion is that she focus on improving her study skills and grades where she is, before attempting a move. StuRat 03:45, 22 September 2006 (UTC)

Wikiversity is different. It aims to provide learning materials. Tests, class ideas and the like. It probably seems more organized because it's new, smaller, and subject to a lot less rules because of it. If you take some time to learn the basics, Wikipedia isn't all that complicated either.- Mgm|(talk) 04:58, 22 September 2006 (UTC)
I don't believe the questioner is asking about Wikiversity. They are obviously under the assumption that since Wikipedia is an American company, most of the users would be too, and thus "your Universities" means "American Universities". If your daughters grades can't get her easily into a good international university, consider looking into international exchange programs at her school. It seems there is some sort of sister-schoolship with the University of Calgary, though I'm not sure if they have any courses for your daughter. You should check at the university international student center for options.  freshofftheufoΓΛĿЌ  05:40, 22 September 2006 (UTC)
Of course, Calgary is in Canada, not the US. StuRat 06:16, 22 September 2006 (UTC)
The user may also have searched for a particular university on a search engine, found the Wikipedia article on that university, and mistaken Wikipedia for the university's website. Apparently this happens from time to time. -- SCZenz 07:45, 22 September 2006 (UTC)
Come on guys, you really think india's grading system is exactly the same as Americans? One of my good friends went to a Mexican high school and the grading system is completely different there. The grades were much less inflated than in America


Which relationship is genetically stronger? with a sibling(eg;sister-sister) or with a parent (eg; mother-daughter)?

In either case the two individuals share basically 50% of the genome. I'm assuming "full sibling" here. See our article Heritability (although not what I'd call a Good Article). --LambiamTalk 09:13, 22 September 2006 (UTC)
That's not entirely accurate. The parent-child relationship is guaranteed (barring any extremely rare abnormalities) to share 50% of their genome. Siblings however, have no such guarantee. It's possible (but unlikely) that two siblings will share no DNA -- that they have each inherited a different half of their parents genome. It's similarly possible that they will have identical DNA (identical twins). It's almost always somewhere in the middle, and I bet it's usually pretty dang close to 50%. -- Plutortalkcontribs 10:34, 22 September 2006 (UTC)
Identical twins happen fairly often (1 in 150 births), while identical non-twins should only happen 1 in every 2^46 births, or 1 in every 70 trillion sets of siblings. This means it's probably never happened yet. Genetically unrelated siblings should also happen in around 1 in every 70 trillion sets of siblings. So, these are unlikely enough that we can ignore them. Identical twins don't occur by random distribution of chromosomes that happen to match, however, but when a single embryo splits into two. So, I would conclude that siblings, with parents who are unrelated, would be slightly more genetically related, on average, due to the portion which are identical twins, than parent and child, assuming the child's parents are unrelated. StuRat 14:37, 22 September 2006 (UTC)
It's also worth noting that male siblings share a Y chromosome precisely (up to mutation), and female siblings share one X chromosome precisely. (A brother and a sister have no such special correspondence.) Furthermore, there is the subject of mitochondrial DNA, which ties you more closely to your mother and siblings (and equally so), but not to your father. I wonder if the insanely unlikely "completely different siblings" case (aside from sex chromosomes and mitochondria) would be more likely to result from pairing of two pairs of gametes that were formed in the same meiosis than from lucky genetic recombination? --Tardis 15:14, 22 September 2006 (UTC)
And let's not forget if we're talking about real world human relationships, things may not be as clear cut as you believe. I've heard some fairly outrageous statistics but in any case, it's fair to say without DNA test results, you shouldn't assume your genetic relationship with your father is definitely something special :-P

What is the word that means 'linking a memory to a smell'?[edit]

I used to know the word (and have forgotten what it is!) so when I see it again, I'd know what it is.

I know it is not 'Proust Effect' nor 'Olfactory'.

So I want to get as many options as I can here - so the way I remember it, the word describes how a memory is triggered by a smell. I used to use it to describe how walking past men who were Hugo Boss's Innovate would give me flashbacks of my ex.

Thanks for your help ! Mavis ----09:10, 22 September 2006 (UTC)----

There is the term "involuntary memory" (Proust's mémoire involontaire). Although not explicitly confined to smell, the vividness of memories evoked by scent stands out; see also Limbic system#Practical application (sic!). --LambiamTalk 09:57, 22 September 2006 (UTC)
Sometimes it's called the Madeleine effect, again after Proust.--Rallette 10:00, 22 September 2006 (UTC)
And I meant madeleine with a lower case m, of course, a pastry not a person.--Rallette 10:07, 22 September 2006 (UTC)
A pastry or woman can also have a strong smell that triggers memories..."That's right, I was supposed to go buy deodorant for Madeleine". :-) StuRat 14:25, 22 September 2006 (UTC)
Why do you associate pastries with women. Are you still a mummy's boy? 8-)--Light current 05:58, 23 September 2006 (UTC)
A more general term is context dependent memory; could that be what you're looking for?
It looks like someone forgot to write that article. :-) StuRat 05:55, 23 September 2006 (UTC)

Hmm .. still one of the above. ARGH. the sense of frustration is creeping in .. it's like the feeling of having something at the tip of your tongue but not being able to utter it .. Mavis

(Semi-) Random relevant phrases: "The primary olfactory cortex, which receives information about smells from nerves in the nose, links directly to the amygdala, which controls expression and experience of emotion, and the hippocampus, which controls the consolidation of memories." "Odor-evoked memories" "an ambient odor would trigger a better memory" "demonstrated the relational memory organization through appropriate transitive inferences" "a general declarative memory capacity" "recognition of the odor in the olfactory process will simultaneously evoke the correlated memory" -- Fuzzyeric 05:52, 24 September 2006 (UTC)


I have two thermometers here, neither of which type is listed in our thermometer article. The first is a bath thermometer, with a glass tube part filled with a bright red liquid, so it is obviously not mercury. What is it? The other thermometer is a Toblerone-shaped piece of polished slate, with a plastic strip along one side. There is a numbered scale along the plastic strip, but only one number is visible at any time. Is there are name for this type of thermometer and how does it work?--Shantavira 12:40, 22 September 2006 (UTC)

The red stuff is alcohol, this is mentioned in the Mercury article at least. MeltBanana 12:57, 22 September 2006 (UTC)
The second sounds like a liquid crystal thermometer. For the second, could you describe the strip more clearly? If you hold your finger on the strip where you expect the human body temperature to be, does it light up in addition to the current room temperature? JBKramer 12:58, 22 September 2006 (UTC)
I agree that #2 is definitely a Liquid crystal thermometer. The fact that it's mounted on a triangular prism of slate is weird, though. -- Plutortalkcontribs 13:36, 22 September 2006 (UTC)
Yes, you're right. Thanks. I guess a liquid crystal thermometer can be mounted on anything. I figured the stone was intended to stabilize it, but it's mainly decorative. I shall add the thing about alcohol to the thermometer article later on.--Shantavira 15:31, 22 September 2006 (UTC)

Is fire plasma?[edit]

Is fire an example of a plasma? My understanding is that it is not, since the gas would need to be ionized, and ordinary fire is not nearly hot enough to accomplish that: but there are conflicting sources on the web: for example, yes 1) [1], no: 2) [2] and [3]. (There's a corresponding disagreement on the fire article.) Antandrus (talk) 14:58, 22 September 2006 (UTC)

Flame is not a plasma. Electrons remain trapped in their orbatal. The color of fire comes, if I remember correctly, from the black body radiation of the heated particles. JBKramer 15:11, 22 September 2006 (UTC)
I think you may have remembered the last bit incorrectly. The colour of a flame is caused by excited electrons falling back to lower energy states, thereby emitting photons. That's why a flame goes yellow when you sprinkle some kitchen salt on it: it is the colour of the sodium D emission lines. Of course, as the temperature goes up and the electrons get really excited, the average energy of the photons also goes up, just as it would for black-body radiation. --LambiamTalk 16:54, 22 September 2006 (UTC)
both wrong. see flame.--Deglr6328 00:14, 24 September 2006 (UTC)
That makes sense. So what leads to the common misconception? --Allen 16:36, 22 September 2006 (UTC)
There is some degree of ionization in a fire, so it all depends on how much ionization one requires to classify a phenomenon as a plasma. It's similar to trying to decide whether Pluto is a planet or not. StuRat 16:53, 22 September 2006 (UTC)
Dwarf plasma? --LambiamTalk 16:55, 22 September 2006 (UTC)
Interesting. --Proficient 06:06, 25 September 2006 (UTC)

Some flames do qualify as plasmas, though they are at fairly low percentages of ionization. For example, the most common detector for gas chromatographs are Flame Ionization Detectors (FID). When organic compounds are burned in a hydrogen + oxygen flame the flame has significant electrical conductivity. I can not think of any mechanism that would make the flame electrically conductive without the presence of ions. Hence, the flame is a plasma. In fact, FID works so well that very few other detector methods can match FID's sensitivity or dynamic range. How this applies to say a candle flame is a more difficult question. A hydrogen / oxygen flame is pretty hot, much hotter than a candle flame. In both the candle and FID cases organic compounds are being burned. Is a candle flame hot enough to produce enough ions to qualify as a plasma? 22:34, 9 January 2007 (UTC)


After ovulation, how long is the unfertilized ovum responsive to fertilization before it breaks up and becomes a part of the uterine lining?

Apologies for previous wrong answer. Implamentation must take place within 6-12 days from ovulation. Still working on fertalization timing. PMID 10362823 JBKramer 15:15, 22 September 2006 (UTC)

I can find no good studies relating to the actual fertalization of the egg, as opposed to timing for intercourse/implamentation. Intercourse is most effective 6-1 days before ovulation, while implmentation is most effective 6-12 days after ovulation. JBKramer 15:20, 22 September 2006 (UTC)

By what measure is "intercourse" "effective"? probability of fertilization? And what is "implamentation"? did you mean implantation? From what I know, a sperm lasts about 4 days in the accomodating fluids of the uterus and fallopian tubes. An egg that has been released from the ovary takes about 6 days (often longer, usually not shorter) to travel to the uterus. Once the egg reaches the uterus, it is reabsorbed, causing a hormonal cascade that leads to the shedding of the uterine lining. Fertilization must take place while the egg is in the fallopian tube (but not implantation, that would lead to an ectopic pregnancy). Thus the window of conception-producing sex is about 4 days prior to ovulation to about 6 days after ovulation: a 10-day window.Tuckerekcut 15:47, 22 September 2006 (UTC)
PMID 7477165, widely cited, disagrees with your 10-day window. m:dick disagrees with your spelling (f)lame. JBKramer 16:05, 22 September 2006 (UTC)
6-1 ? StuRat 15:48, 22 September 2006 (UTC)
Six to one.
That would normally be written as 1-6, so I'm wondering if they meant something else, like 6-10, with a missing 0. StuRat 16:50, 22 September 2006 (UTC)
JBKramer: Referencing my Guyton & Hall Textbook of Medical Physiology, Tenth Ed. (Saunders 2001, Page 941), "The ovum remains viable...after it is expelled from the ovary for no longer than 24 hours. Therefore sperm must be available soon after ovulation...intercourse must occur sometime between 4 and 5 days before ovulation and up to [a day] after ovulation..." It seems that my measure of 6 days for the travel down the fallopian tubes (gleened from my Ob/Gyn course notes) was inflated and incorrect. With this new information it seems that a 4-6 day window is more likely. However your discussion of implamentation still doesn't make any sense to me. I still don't know if you mean the impl[e]mentation of something (perhaps implementation of an IVF protocol?) or implantation. I pretty much assumed that you meant implantation, but I was unsure because you used the word three times. That's why I asked. By the way, the article you mentioned, it is only cited 20 times on PMC.Tuckerekcut 19:02, 22 September 2006 (UTC)
Stu, the 6 to 1 thing sounds remarkably familiar. If scientists find it acceptable to express it that way, it must be right. :) Loomis 13:07, 27 September 2006 (UTC)

ID Card size2[edit]

I am posting this question again because i could not get any satisfactory answer. What is the logic behind such an odd size od Id-1 cards that have a size of 85.6x53.98mm?

Maybe because it is small enough to fit nicely in most people's hands and still large enough to hold a fair amount of information? Possible scenario: somebody (probably English) issues a popular ID card (3 3/8" x 2 1/8"); others like it and start imitating it. Logic? What's logic got to do with it? Why do people drive on one side of the road? Somebody started it and then everybody else went along.
Here's another thought. Maybe somebody used the other side of playing cards, back when only one side was printed on? The sizes are pretty close (3 1/2" x 2 1/2"). Or it could just be a form of Parallel evolution - it's just a pleasing size. Clarityfiend 17:01, 22 September 2006 (UTC)
The previous time was not the first time the question was asked: Wikipedia:Reference desk archive/Mathematics/April 2006#ID Card size. The reason you can't get a satisfactory answer is because we don't have one. We are flummoxed. Rarely has the reference desk been so perplexed as by this question. We'd love to be plussed, but the fact is, we're all nonplussed. Sorry. The members of the ISO 7810 committee who might have known the rationale all died mysteriously within days of each other through a variety of accidents and natural causes briefly after the standard had been adopted. --LambiamTalk 17:22, 22 September 2006 (UTC)
Plussed, eh? Does that mean, if we were minussed, then we'd be nonplussed? Wait - what happens with the zero? Can we be both plussed and nonplussed at the same time? Clarityfiend 19:57, 22 September 2006 (UTC)
I suspect it has to do with a hard conversion vs. a soft conversion between Imperial measurements and Metric measurements. With a hard conversion, the original (in this case Imperial) measurement is exactly multiplied by a very precise conversion factor, and the number of significant digits is important. So you get a seemingly precise answer such as 3-1/2" --> 85.6mm. With a soft conversion, the answer need not be so exact, the conversion factor has fewer significant digits, and so 86mm would suffice. Whoever opted to convert the Imperial measurements of an ID card to the metric system opted to use a hard conversion. As to why, that's the unknown. Often in fields like Land Surveying a hard conversion with many significant digits is necessary, so that the survey is accurate to tenths of a foot over an area of many miles.
A different example would be, say, electrical conduit. A 12mm conduit is approximately the same as a half-inch conduit. If an electrical job was being converted from english to metric, I wouldn't take the half-inch measurement, multiply it by 25.4, get 12.7mm, and thus say 12.7mm conduit would be used; there's no such standard size available.
This is not a satisfactory solution unless you can give plausible imperial measures whose hard conversion results in these values. Personally I tend to think 337/100 inch and 3451/1024 inch are not plausible, but maybe you can make a case for them. Another reason why imperial measures are less likely is that this is an ISO standard. ISO standards are based on the SI system. For example, the ID-2 card has A7 format: 2−13/4 m × 2−15/4 m, and the ID-3 card has B7 format: 2−3 m × 2−7/2 m. They could easily have picked A8 format, which is 74 mm × 52 mm (2.913 inch × 2.047 inch). --LambiamTalk 21:00, 22 September 2006 (UTC)
This is interesting. The dimensions are very close to the Golden ratio. Clarityfiend 20:02, 22 September 2006 (UTC)
The golden ratio is about 1.618034. The aspect ratio of the ID-1 card is 1.585773. Not that close. If they had wanted to achieve the golden ratio, they could have gone for 83.62 mm by 51.68 mm, with an aspect ratio of 1.618034. The aspect ratio of the other cards (ID-2 and ID-3) is the square root of 2: 1.4142. --LambiamTalk 21:21, 22 September 2006 (UTC)
Well, the ISO 7810 constellation of standards appear to have been standardized in the past few years. (Irritatingly, the article doesn't give a history of the standard.) The standard appears to have codified the general sameness of the dimensions of credit cards which have definitely varied very little since the Diner's Club card in the 1950s (personal experience). ... which were roughly sized to match business cards and trade cards. ... which were the same size as the pockets in wallets. ... which were designed to carry calling cards. Wallet directly asserts this history for dimensioning to calling cards and then dimension standardization following the release of credit cards in the early '50s. "Rules of Etiquette & Home Culture", 1882 (quotation) suggests 1.25" x 3" (76.2 x 31.75 mm) as a small card, so it is not inconceivable that a "medium size" could be close to ID-1 dimensions.
Nevertheless, the answer is, "because those are the dimensions of the Diner's Club card." And there may not be a much better documentable answer. -- Fuzzyeric 03:44, 23 September 2006 (UTC)

Submarine Periscopes[edit]

Ive often wondered how the periscope is designed to slide up and down without letting any water in into the sub, especially at great depths. Any answers?--Light current 17:16, 22 September 2006 (UTC)

At great depths you don't slide your periscope up and down. You only do that at depths up to the length of the periscope. --LambiamTalk 17:26, 22 September 2006 (UTC)
[after ed con]The periscope is only used at very shallow depths, so there isn't much of a problem with water pressure. There is a close fitting seal (and I assume quite a lot of grease) to prevent water penetration. When the sub is at any depth, the water pressure will actually help seal it.--Shantavira 17:27, 22 September 2006 (UTC)

[edit conflict]Yeah but the waters still trying to get in even if the scopes down?--Light current 17:29, 22 September 2006 (UTC)

Rotating face mechanical seal

Speaking of periscopes, did you know that it was once proposed, as a countermeasure against submarines, to teach seagulls to defecate on raised periscopes? [It's true!] (and I can't believe I added a seagull-related answer to the Ref Desk!)

You did and thats minus 20 points to you! Congrats! Anyway thats a load of bird crap!--Light current 19:27, 22 September 2006 (UTC)
Yes, and now I feel bad. But another example came to me: Think hydraulic cylinder, like you see on bulldozers, backhoes, and so on. There is a simple o-ring style seal on the cylinder, keeping the hydraulic fluid in. Hydraulic systems operate at very high pressures, up to 2,000psi. So they keep the fluid inside, whereas a periscope seal keeps the fluid outside. But both depend on a smooth machined surface sealing with a flexible o-ring type seal.
Is there a g**l conspiracy? There's another one just a few questions down. Was Alfred Hitchcock onto something? Clarityfiend 20:08, 22 September 2006 (UTC)
There is an infestation that needs to be eradicated!--Light current 20:10, 22 September 2006 (UTC)
Who'll clean up the chips? Mattopaedia 08:34, 28 September 2006 (UTC)
O yes. That rings a bell. 8-)--Light current 19:55, 22 September 2006 (UTC)

Involuntary/Voluntary memory[edit]

I am a bit flummoxed by the whole part. Can somebody explain to me the differences and frequencies further? Are involuntary memories as common in the normal person as I? Every second of life for me is filled with involuntary memories. — X [Mac Davis] (SUPERDESK|Help me improve)18:14, 22 September 2006 (UTC)

Your question is not well phrased. Could you clarify?--Light current 19:30, 22 September 2006 (UTC)
Try not dipping your madeleines in tea. While there is a large variety between individuals (and for a given individual between different periods of life) in the incidence of involuntary memories, every second sounds rather high. You may be talking about something else. An involuntary recollection intrudes with great strength and usually vividness upon your consciousness. It's more than "Oh, that reminds me of ...", it is almost like re-experiencing. Some people never have this, some regularly, but not more than a few times a day. --LambiamTalk 20:25, 22 September 2006 (UTC)

Cognitive psychology research has shown that memory is less like a playback of a recording, and more reconstructive. People can be quite certain of details of an incident or a person's description which were not there in the original incident. So one could have involuntary "recall" of memories all the time, with much of the detail invented. Example: One recalls how his family had a cow on the farm when he was little, and remembers seeing the big black and white Holstein cow getting milked in her stall. Then a parent corrects him to the effect that it was a Jersey, and now he can remember seeing the little brown and white cow getting milked. Attention and effort, with such techniques as repetition, the Method of Loci or other mnemonic,techniques are useful for retaining memories.Edison 21:15, 22 September 2006 (UTC)

How do you remember? Do you have to try? With almost every word I type I associate it with a past experience or objects, events, or trivia. — X [Mac Davis] (SUPERDESK|Help me improve)03:08, 23 September 2006 (UTC)
Sounds like it might not make for the greatest productivity; more like daydreaming. Have ADHD?Edison 15:13, 23 September 2006 (UTC)

Blood Physiology[edit]

Naqa inc 18:20, 22 September 2006 (UTC)×'''Blood Physiology'''What would be the simplest answer to the question : "Why is it necessary to control blood sugar ?" The answer should not mention diabetes mellitus or any diabetes. Assume its a 40 marks question.this was our assignment but we cannot seem to agree on hypo-/hper-glycemia.

That's a big question, and it would help to know what grade (or level, or whatever the gradations are called in your region) you are in. Perhaps you could write out your answers, and I/we can tell you where you are wrong or need elaboration. That said, we aren't going to do your homework for you, so dont expect something you can copy and give to your teacher.Tuckerekcut 19:08, 22 September 2006 (UTC)
The question you are asking can be read from several angles. From a physiologic standpoint, the evolutionary benefit from maintaining a tight blood sugar range can be approached from a conservation of energy standpoint. When the blood sugar gets too high, it is freely filtered in the urine and provides an osmotic load which keeps water from being reabsorbed by the renal tubules. A person can urinate a significant portion of their calories, often resulting in weight loss and wasting. Dehydration is also common due to water loss. If blood glucose is too low, there are significant consequences to the brain, which uses glucose preferentially as a fuel source. Too little glucose leads to seizures and coma.
From a molecular biology standpoint, elevated blood glucose leads to glycosylation of plasma proteins, resulting in the multiple consequences which are typically associated with diabetes mellitus. There are many other answers to the question from a variety of standpoints and it will depend what you are looking for. InvictaHOG 22:26, 22 September 2006 (UTC)

Do gulls swim?[edit]

I've seen them floating on top of the water but they seem to end up going wherever the current takes them. Can they actually 'swim' as such? -- 19:13, 22 September 2006 (UTC)

They have webbed feet... that certainly suggests that they swim in a duck-like fashion. – ClockworkSoul 21:31, 22 September 2006 (UTC)
They have webbed feet but it looks like they can only just paddle and tread water a bit, unless the water is completely still. I wouldn't really call it 'swimming'. -- 23:11, 22 September 2006 (UTC)
They can swim if necessary, but as they are a bird of the open sea and need to cover large distances there is little incentive to swim anywhere when they can fly. It's a matter of energy use. If I am in my local park and throw some bread, ducks will fly over if they're any distance away, but swim if it's only a matter of yards. Swans and geese, being quite heavy, will always swim over. If there is a gull around it will invariably fly, in fact it prefers to catch bread on the wing rather than come down for it. Coots use an interesting hybrid tactic: they run.--Shantavira 06:46, 23 September 2006 (UTC)

AIDS after WWII in Europe[edit]

Hello, I read in a book called The Tipping Point about how after WWII in German mining towns it was noticed that a lot of children were dying of an immune deficiency disease, and that people now speculate that it may have been a precursor to the AIDS virus. I wanted to find more background on this but haven't found much information regarding this occurence, so I was hoping that someone else knows what I'm referring to. Thanks! Andromeda321 19:28, 22 September 2006 (UTC)

Starvation, overwork, stress in general, and extreme cold also hamper proper immune system functioning.Edison 21:18, 22 September 2006 (UTC) this may help. BTW I knew nothing about it but search inside a book at amazon is a very useful for this kind of research. MeltBanana 21:30, 22 September 2006 (UTC) Actually it probably will not help much as it barely mentions it, you will probably have to buy his book. MeltBanana 21:34, 22 September 2006 (UTC)

There are many conditions which cause immunodeficiency, so it's not a good idea to call them all "AIDS", which is a specific type of immunodeficiency caused by the HIV virus. StuRat 03:17, 23 September 2006 (UTC)

Suppose another acute one comes into existence, what would we have to call that then? AIDS 2.0? The sequel? TNG? DirkvdM 05:22, 23 September 2006 (UTC)
The "A" does not stand for "acute" (as in SARS) but for "acquired". I think, though, that any disease not caused by a close relative of HIV can no longer be called "AIDS", even if it matches the clinical description (AIDS was originally defined by its symptoms, not its cause, since its cause was unknown). I think that now that the cause is known, it no longer makes much sense to keep around a term defined by a clinical history, and if language were rational (or, perhaps less flatteringly, "rationalistic") the term "AIDS" would simply disappear, to be replaced by "HIV disease" or some such. But language isn't rationalistic, and attempts to impose rationalism on it rarely have happy outcomes. --Trovatore 20:14, 23 September 2006 (UTC)

Battery life[edit]

About 20 yrs ago I bough a (Philips CD 650) CD player that is still working. With the player came a remote control powered by Philips type 035 (alakaline?) batteries. Altho I dont use the remote a lot, I'm pleasantly surprised that the batteries are still powering the remote OK. Is this some sort of record?--Light current 19:51, 22 September 2006 (UTC)

Probably close, although a Google search for "battery record" gives at least one result of someone claiming longer (34 years). The alkaline battery was only invented 47 years ago, and this is far longer than the life of any standard cell battery I've ever heard of. You might want to toss them, though, especially if you have kids. Old batteries corrode and leak, and before 1996, alkaline batteries had Mercury in them. -- Plutortalkcontribs 11:32, 23 September 2006 (UTC)
They are quite safe, not leaking, and still operating the remote! I dont intend eating them either, but thanks for the answer.--Light current 11:50, 23 September 2006 (UTC)
If I wrote to Philips with this wonderful news, do you think they would give me a new CD player?--Light current 14:56, 23 September 2006 (UTC)
Much like the guy who got the magic carburetor from Ford (and quickly had the car revoked for a normal one) they will likely abduct the battery since it is in their best interest to keep a lid on the technology that allows you to never need to buy replacements. Good luck! (yes i am kidding) --Jmeden2000 14:53, 26 September 2006 (UTC)

light acting as waves[edit]

dear sir / mam

i just wanted to know that if light acts like particles, and as waves than why doesnt it collide like particles. for example, when you shoot two laser beams across each other , it never deflects or even collide, where as waves do collide and particles also collide. so ya, i am confused. your help will be highly appreciated.thank you shaneel deo165.196.194.137 20:16, 22 September 2006 (UTC)

First of all, you might find our article wave-particle duality helpful. When you shoot two laser beams across each other, I think some interference would take place, but maybe not enough to notice, but don't take my word on that. --Allen 20:24, 22 September 2006 (UTC)
Interesting question! What happens if two photons collide! --Light current 20:28, 22 September 2006 (UTC)
I think when two photons interact, that interaction is going to look like wave interference, not particle collision. Photons don't have size or exact location, so I don't see how they could ever collide in a billiard ball sort of way. --Allen 20:33, 22 September 2006 (UTC)
Look up scatttering of light by light. I don't know much about it, but I think it could be naively described as "photons colliding". --Trovatore 20:39, 22 September 2006 (UTC)
In the standard model, in particular the subtheory of quantum electrodynamics, photons only interact with electrically charged particles such as electrons. Why nature behaves like she does we can't know; all we can do is make the simplest models we can that conform to our observations. In the best model we currently have, photons do not interact, and such interaction has not been observed. --LambiamTalk 23:01, 22 September 2006 (UTC)
Yeah, now that you say that I feel like I should have known. What about interference, though, like in the double-slit experiment? I guess that's not considered interaction? --Allen 00:32, 23 September 2006 (UTC)
Right; that is simply the result of adding the amplitudes, and the wave equation is linear, so if there is no quantum collapse or other interaction the photons proceed as if they had never encountered another photon. --LambiamTalk 01:52, 23 September 2006 (UTC)
Hey, hang on! Bare photons may not be able to directly interact with each other, but in QED, photons are constantly producing virtual electron pairs and re-annihilating. One photon can strike an electron from another photon and get re-emitted in a different direction -- not as if the two photons had never encountered. Melchoir 08:27, 23 September 2006 (UTC)
Can you sketch a Feynman diagram of this? I don't understand what it means for a photon to be re-emitted by an electron. --LambiamTalk 14:53, 23 September 2006 (UTC)
Well... I can describe one pretty easily. Electron loop in the middle, two photons in and two photons out. Melchoir 19:18, 23 September 2006 (UTC)

I fear there is a misunderstanding in the original question, specifically the part stating "waves do collide". When two or more waves "collide" they simply pass right over one another. During the overlap they may reinforce or cancel one another, but once each wave moves beyond the others, it reforms it's original shape once again. StuRat 03:13, 23 September 2006 (UTC)

I see such interaction regularly in the canal I live at. When a boat passes by and the bow waves bounce off the walls, the original and reflected waves interact to form checkered paterns. Some nice examples can be seen at Ripple tank. DirkvdM 05:39, 23 September 2006 (UTC)

Limestone vs Siltstone for aggregate[edit]


I was wondering if anyone could tell me if siltstone is suitable for aggregate. And if it is and you had Limestone to choose from as well, which is better and why?

Thanks in advance.

Limestone is commonly used for roads etc as hardcore but I dont know if its better that siltstone. Usually gravel is used as aggregate--Light current 21:45, 22 September 2006 (UTC)

Is it possible for a man to have multiple penises?[edit]

Is this a real condition that some men have or not?-- 22:04, 22 September 2006 (UTC)

Yes it is. See diphallia. Hyenaste (tell) 22:06, 22 September 2006 (UTC)
(edit conflict) It has been documented, and if I recall correctley their is a caste made of one such occurance of this should you wish to see it. The owner claimed both functioned well, and he was happilly married. Hehe. Philc TECI 22:08, 22 September 2006 (UTC)