Wikipedia:Reference desk/Archives/Science/2009 January 24

= January 24 =

Non-teratogenic mutagens
Are there any human mutagens which are not human teratogens? Neon Merlin  02:38, 24 January 2009 (UTC)


 * Look at Mutagen and Teratology. Human cells contain DNA. A substance or process that will alter the DNA is called mutagen.  It can, e.g. lead to cancer.  If the DNA that gets damaged is in an cell that will develop into a baby's cell then a birth defect may result. 76.97.245.5 (talk) 02:56, 24 January 2009 (UTC)
 * How about UV light? lots of skin cancer but no birth defects.--Digrpat (talk) 14:39, 24 January 2009 (UTC)
 * Thats only because embryos tend to be protected from UV exposure during development. If you expose embryos to UV light, you see teratogenic effects . Rockpock  e  t  19:05, 24 January 2009 (UTC)

Space tube: Sucking or Blowing?
If I created a hollow tube to space, like a giant earth straw, would it have any movement of air at the bottom? If I covered the bottom with my hand, would I feel a slight sucking or blowing? Would there be any forces at play that might suck air constantly through it like a siphon? (such as air pressure differences, time dilation, movement of the end of the straw through space). Or might it blow out air? ever so slowly? Would the answer changed if the tube was slowly getting narrower as it went up or wider? I know it wouldn't act as a straw in the same sense because the vacuum of space should be almost exactly countered by the weight of the air, but is this changed at all by putting a tube around it? Thanks. Anythingapplied (talk) 07:12, 24 January 2009 (UTC)


 * No, I really don't think so. Assuming that such a tube could really exist, the reason the atmosphere (mostly) stays where it is is that gravity keeps it where it is, just like it keeps us from flying off. Some of that gas would be inside the tube, but I don't see what difference that would make: gravity would still hold it down. I guess if the tube conducted heat really ridiculously well, it might cool way down in the vacuum when it was in the dark and heat up when it was in direct sunlight, and that might have some small effect on some of the gas inside... but, really, no. (And if for some reason I'm wrong, that's great, because in that case the real answer can only be interesting as hell... but I'd be very surprised to hear that.) -- Captain Disdain (talk) 09:00, 24 January 2009 (UTC)


 * If the air was moving one would able to extract energy from it; which would break conservation of energy. So one can assume that there would be no air flow in the tube. —Preceding unsigned comment added by 129.67.37.225 (talk) 13:44, 24 January 2009 (UTC)


 * I agree - nothing special happens unkess the tube itself is doing something (which, in this kind of thought experiment, it doesn't!) SteveBaker (talk) 14:30, 24 January 2009 (UTC)


 * I have a way to convince the OP. Consider a straw in a glass of water.  The pressure at the bottom of the glass is higher than the pressure at the surface - just as the pressure at the top of your giant atmosphere straw is greater at the bottom of the atmosphere than at the top.  The liquid level in the straw stays level with the top of the liquid in the glass...same deal with the atmosphere.  QED.  (Oh - if you try to do the experiment with a real straw in a real glass of water - the level in the straw will actually be a millimeter or two higher than in the glass - but that's because of surface tension...if you used a glass of mercury (please don't!) then the level in the straw would be lower than in the glass.) SteveBaker (talk) 15:00, 24 January 2009 (UTC)


 * You wouldn't get any net movement of air, but you would get temporary winds blowing in or out when the barometric pressure of the atmosphere changed. Specifically, when the barometric pressure goes up, wind would blow into the straw, and when it went down (which usually means bad weather is on the way), wind would blow out of the straw.  This assumes that the straw is perfectly rigid, so it's sides can't expand or contract to compensate for the pressure.  Note that this air-blowing-in-or-out-to-equalize-pressure effect doesn't have to do with the straw being vertical, or extending into space, but applies to any large volume of air with a narrow opening to the atmosphere, such as certain caves. StuRat (talk) 15:23, 24 January 2009 (UTC)

Space/matter
In what sense does matter "create" space? Does it mean that without matter there is no space? Is the quality of space somehow dependent on the amount and distribution of matter? 196.2.124.248 (talk) 08:02, 24 January 2009 (UTC)
 * As far as I'm aware, there is no sense in which matter creates space. Where have you encountered the idea? Algebraist 13:36, 24 January 2009 (UTC)


 * This might be a reference to the metric expansion of space which is affected by the amount of mass in the universe. Spacetime could exist wothout matter and in that case would be perfectly euclidean.  Sp in ni  ng  Spark  13:55, 24 January 2009 (UTC)


 * There is unfortunately some religious arguments from some fairly ignorant people on how the universe was clearly created by a deistic entity. They refer to space as 'the distance between obejcts' as one of their steps in the argument. But on the flip side space is to an extent defined by objects; as with a zero or one object universe, space does not exist. —Preceding unsigned comment added by 129.67.37.225 (talk) 13:50, 24 January 2009 (UTC)
 * Now now, let's not go ripping on religious people here. You can't disprove God scientifically.- Running On  Brains  20:28, 24 January 2009 (UTC)
 * That's one of those "If a tree falls in a forest and there is no-one there to hear it - does it make a sound?" questions - to which the answer is an unambiguous "Well, DUH!". If there are no particles around for the space to be measured against then does it exist?  This is the kind of thing that gets philosophers very excited.  But in truth it's a rather bogus argument because we have virtual particles causing quantum foam - so if there is space-time then there is matter (at least briefly) - and it is sometimes argued that the existence of that foam is what causes space-time to exist.  Which means that even in a universe that's completely devoid of galaxies, gas clouds, stars, planets, grand pianos and pink aardvarqs - there would still be virtual particles and hence matter.  I don't think we know the answer to our OP's question at the quantum level.  At the macro level, it's hard to argue that without at least one object there could be no space - we don't know that and because a universe with no matter in it could not be measured or probed in any way - we can't tell for sure.  If you can't put a tape-recorder into the forest then it's hard to demonstrate that the falling tree makes a sound...although all that we know says that theoretically, it must.  SteveBaker (talk) 14:26, 24 January 2009 (UTC)

If matter then has very little to do with space, what is meant by a "finite" universe - is it the matter which is finite (which one can understand even if it is unlikely) or is it the space which is finite (difficult to understand and even more unlikely) or is it both (extremely unlikely and impossible to understand)? 196.2.124.248 (talk) 22:24, 24 January 2009 (UTC)
 * I think it usually refers to space, ie. the volume of the universe is finite. I don't see anything particularly difficult to understand about that - it's just like saying the surface area of Earth is finite. --Tango (talk) 22:46, 24 January 2009 (UTC)

Well now, I think one immediately gets into deep water there. If the volume is finite then what would the bounding surface consist of? Obviously not a rigid structure and also not open space, since that would just be more space - space would have to extend infinitely in all directions holding an infinite amount of matter - see Cosmological principle. 196.2.124.248 (talk) 07:47, 25 January 2009 (UTC)
 * You don't need a boundary. Go down a dimension and think of the surface of the Earth. The Earth's surface area is definitely finite, but there is no edge. --Tango (talk) 03:40, 26 January 2009 (UTC)

No - that argument is totally specious. You wrote "the volume of the universe is finite" which directly implies that it is bounded. There is no need to "go down a dimension". Speak to a mathematician. 196.2.124.248 (talk) 18:59, 26 January 2009 (UTC)

Compressible liquids
its afact that liquid get heavier as its gets colder, which means that liquid mass increase as the temperature decrease. and recalling that mass dont change, so to have the same mass the volume must get smaller. which lead to the idea that liquid could be compressed. and as you know liquid couldnt be compressed. —Preceding unsigned comment added by 86.108.61.209 (talk) 15:21, 24 January 2009 (UTC)


 * It's almost a fact that liquids get 'denser' (not 'heavier') as they get colder. It's not true of water close to freezing point.  So a plastic bucket of water won't get any heavier as it cools (it's mass won't change either) - the level of the liquid in the bucket will get lower as the water shrinks (until close to freezing - when it'll get a little bigger).  It's almost a fact that liquids are incompressible.  But they aren't UTTERLY incompressible.  Our article on water explains that water at the bottom of the deep oceans is compressed by about 1.8% compared to water on the surface at the same temperature...not much - but it IS compressible.  So, sorry but you're, wrong! SteveBaker (talk)


 * Many of the "rules of thumb", like that liquids are incompressible, are only convenient approximations. The reality is that they are, but for most casual computations, the amount of compression is low enough so that it may be ignored. StuRat (talk) 20:18, 24 January 2009 (UTC)

Second moment of area
what is the second moment of area ... i need a text meaning, please no math expressions. thank you ...? —Preceding unsigned comment added by 86.108.61.209 (talk) 15:30, 24 January 2009 (UTC)
 * Sadly - there are some things for which you really NEED math - and this is one of them. This is a means to calculate the likely stiffness of a beam by reference to the cross-sectional shape.  It's sometimes called "The area moment of inertia".  But the property depends on where the beam is being bent so it's not as simple as a number that tells you how stiff something is.  You need the math...sorry! SteveBaker (talk) 16:52, 24 January 2009 (UTC)
 * Second moment of area will tell you a lot, but again it is a bit math-heavy (calculus included). The lead paragraph of that article gives a pretty good description of "what it is", but there is no way to find its numerical value aside from the math on the page.- Running On  Brains  17:53, 24 January 2009 (UTC)

treeful of goats??
despite the protestions of this article, it's a hoax, right? —Preceding unsigned comment added by 82.120.227.136 (talk) 15:42, 24 January 2009 (UTC)
 * We have a picture of goats in an Argan tree at our article. DuncanHill (talk) 15:53, 24 January 2009 (UTC)


 * I thought encyclopedia dramatica links were forbidden on Wikipedia?? —Preceding unsigned comment added by 82.120.227.136 (talk) 16:09, 24 January 2009 (UTC)


 * The photo in our article really doesn't look like a photoshop job (although it has been processed by the Mac version of photoshop in some way - that's probably just a color tweak and crop). The lighting is perfect and the interaction of the fluffy fur and the complicated leaves doesn't have the signs of the more common tweaks people do when faking photos. The picture in the OP's article is pretty low-rez so it's hard to tell.  So no - I think it's real.  Mountain goats are insanely nimble - and those trees are pretty gnarled and have plenty of footholds.  Our article makes it clear that there is fruit on the trees - and in that barren landscape, the goats would have plenty of incentive to try. SteveBaker (talk) 16:43, 24 January 2009 (UTC)


 * There is (or was recently) an advert on British TV which included a treeful of goats - I forget what the advert was for (trees, possibly) but you saw the goats in the tree and their owners saying something about what trees meant to them. DuncanHill (talk) 16:50, 24 January 2009 (UTC)
 * Wasn't it one of the HSBC's "we're the world's local bank" adverts? --Tango (talk) 20:33, 24 January 2009 (UTC)
 * Could have been - I must admit that trees (and goats) are rarely advertised on British television. DuncanHill (talk) 20:40, 24 January 2009 (UTC)


 * See lots more goats in trees, and a video, at . DuncanHill (talk) 16:53, 24 January 2009 (UTC)


 * Some of my extended family have goats, and I've seen them climb trees, on top of sheds, even on top of other animals (the stupid sheep, usually). They're pretty nimble, and these are just the domesticated goats.  --  JSBillings  17:41, 24 January 2009 (UTC)
 * All sorts of kids apparently like to climb trees.Edison (talk) 20:40, 24 January 2009 (UTC)


 * There's a picture very similar to this one (7 goats in an argan tree) in the Photo Journal section of the Feb 2009 issue of National Geographic. Seems legit there.  --Scray (talk) 22:34, 24 January 2009 (UTC)
 * Both the Argan and Argan oil mention goats climb the trees. While the supporting refs I've seen aren't great (one is dead, the other one is not necessarily a reliable source) the argan tree and oil definitely exists as supported by multiple refs and I suspect a number of the refs in the argan article probably mention the goats Nil Einne (talk) 06:34, 25 January 2009 (UTC)


 * 'Tis moi belief that they're nestin'. : ) 87.81.230.195 (talk) 06:44, 15 February 2009 (UTC)

What kind of bacterium does this?
Okay, so this might sound like a medical question, which of course is not allowed, but before you delete it, please read the following. The object of my question happened over a decade ago, I am completely healthy and I am not asking for medical advice. It's only for curiosity.

When I was two years old, I was operated of a hernia in the groin. It seems the doctors forgot a couple of sutures inside, which became infected. For eight years, this infection gave no symptoms, until one day I felt a hard thing in my groin, went to the doctor and was operated straight away. It seems a bag of pus had been growing in my groin for eight years, and it was about to burst (and maybe cause sepsis). The operation and subsequent course of antibiotics eliminated all bacteria, so I recovered pretty quickly.

So, I have two questions which, as I said above, are just for curiosity and are in no way a request for medical advice:


 * 1) How come it took eight full years for the bag of pus to develop? Don't most infections spread quickly?
 * 2) What kinds of bacterium create bags of pus within the body? I read Staphylococcus aureus does, but surely there must be others?

Thanks for your help! —Preceding unsigned comment added by 83.34.49.91 (talk) 16:23, 24 January 2009 (UTC)


 * I'd say there are at least a couple of scenarios to explain the late appearance of the abscess. First it is possible that the infecting organism had been there all along but something changed after 8 years, like an alteration in you immune system, that caused it to suddenly reproduce itself in greater quantity and cause the sign you mention. The other possibility is that the suture caused some minor damage to your internal tissues at some point which was then infected by a passing organism. I guess it is possible that the abscess slowly developed over 8 years but I think you might have discovered it before. Not all infections spread quickly. The tubercle bacillus is an increasingly common example of an organism that can live within the body for months or years not showing signs and undetected. There are a number of bacteria that might cause this type of abscess and with out culturing a swab from the site we can only guess. Richard Avery (talk) 16:55, 24 January 2009 (UTC)


 * It most likely didn't take eight years to develop, rather, the suture that was left in place became a focus of infection years later. It was "seeded" by a transient bacteremia, say from a dental procedure or other instrumentation, or from local infection.
 * Staph is the most likely candidate. Though other organisms cause abscesses, Staph is the most common cause, and is ubiquitous. Tubercular groin abscess would be distinctly unusual - the microbiology of abscesses vary with setting and location. In one study of cutaneous abscesses, 66% of all positive cultures were staph. Streptococcus comes in second; this is what you'd expect, as they're skin organisms. Some abscesses are polymicrobial; others are sterile; almost every organism has been reported to cause abscesses somewhere. You don't mention the size of the abscess, but obviously to acquire an impressive size, the abscess must occur in a place where a small abscess will not cause pain, and where there is sufficient leeway to grow - thus some places are more common than others.  - Nunh-huh 03:00, 25 January 2009 (UTC)


 * This certainly may have taken years to develop. Small colony variant staphylococci are one example of organisms that can cause occult infections that, even without treatment, can lay dormant for decades.  If this was a staph infection, and I agree that's likely, then hematogenous seeding from a dental procedure is exceedingly unlikely since staph are a tiny proportion of dental bacteria.  Another explanation that has not been discussed is that the suture may have been infected but walled off in fibrous tissue that was not conducive for bacterial growth; then, some sort of local trauma (stretching/tearing of the tissue surrounding the suture) resulted in disruption of that microenvironment, perhaps a little serum or blood got to the suture, and this provided the right environment for the bacteria to resume growth.  Just conjecture.  --Scray (talk) 03:33, 25 January 2009 (UTC)
 * This paper reports multiple small-colony variant (SCV) staphylococcal infections, some indolent for decades. This paper reviews much of the literature on SCV organisms, noting that SCV strains have been described for a wide variety of Gram-positive and Gram-negative species.  Some, including SCV staph, can live dormant for long periods of time intracellularly.  --Scray (talk) 04:26, 25 January 2009 (UTC)

Equations of motion
I've seen the equations of motion for constant acceleration derived using both definite and indefinite integration. Which one is the better approach? —Preceding unsigned comment added by 65.92.228.224 (talk) 19:25, 24 January 2009 (UTC)


 * I think it's just personal preference. It just depends on whether you like to work with constants of integration or limits of integration, they end up being equivalent. --Tango (talk) 20:31, 24 January 2009 (UTC)

Kids holding books upside down
My mother just told me that when I was one or two years old, I held picture books upside down. She was naturally concerned about that, but the pediatrician told her that that was normal and I'd turn out all right, which I think I can confirm. At least I'm not holding the monitor upside down as I'm reading Wikipedia. ;-) But the explanation the doctor gave, or at least what my mother remembers, doesn't make sense to me: It had something to do with the image is projected upside down in the retina. This is true, of course, but it is equally true for the book as for anything else I see. Unless I had been first walking on my hands (which my mother said I didn't), I must have seen the world in the same way I saw any book. So I'm wondering if there is any explantion for this? Do other kids turn their books, too? (My mother didn't remember what my younger siblings did.) &mdash; Sebastian 22:43, 24 January 2009 (UTC)
 * It seems very possible that it's normal to hold a book upside down as a kid. Think about it: You're new to the world, and don't know what everything is supposed to look like. So you look at it one way and say to yourself, "Hey, that's a picture of something, but I don't know which way it goes, so I'll guess this way." Analogy to an older person: If you find a book of alien texts (which you have never seen before), and you try to read it, there's a good chance that you'd read it upside down, because you had no idea which way it should go. flaminglawyer 22:58, 24 January 2009 (UTC)
 * Yes, but that would result in you holding books upside down at most 50% of the time (probably less since you'll usually recognise something in the picture). Is there any explanation for holding them upside down almost all the time (which is what the OP seems to mean)? --Tango (talk) 23:05, 24 January 2009 (UTC)
 * Yes, that's what I meant. Of course, I can't be sure about the percentage; it is well possible that my mother selectively noticed the unusual cases. &mdash; Sebastian 00:21, 25 January 2009 (UTC)
 * The argument the doctor gave is hogwash though. Sure, the lens in your eye does result in the image on the retina coming out upside down and flipped left-to-right.  When the doctor was pinned to the wall by an anxious parent and needing to produce a passable answer RIGHT NOW, that's what he pulled out of his memory from Med School.  But as an explanation, it simply doesn't hold water.  You see all of the world that way up all the time - that's "normal".  That IS the right way up.  Holding the picture of the moo-cow in the book the opposite way around from moo-cows in the real world isn't explicable in that way.  I don't know what the explanation is...but that's CERTAINLY not it. SteveBaker (talk) 01:14, 25 January 2009 (UTC)


 * You are right, the image is indeed projected on the retina upside-down; and you are right again, this has nothing to do with how we perceive the "up" and "down" directions in the world. In human infants the connections in the visual cortex, as well as between the visual cortex and other areas of the brain, form largely under the influence of sensory stimuli. AFAIK, we are not born with visual concept of "up" and "down", even though our heads are quite different from our bottoms. The visual sense of "up" and "down", as well as "left" and "right", forms through visual stimulation, and it would have formed largely the same way if the image on the retina was not flipped. Humans learn very fast to use mirrors, so flipping the world around any axis is not a huge problem for our brain; not even for an adult brain. In fact, there are experiments in which adult humans are fitted with prism glasses that flip the visual world by 180 degrees; humans adapt to the flip and function more-or-less normally after a few hours or days. So no, you don't tend to hold the book upside-down because the image on the retina is flipped. Neither do you flip the trees and houses upside-down when you go for a walk (or at least so I hope). Two-year-olds do turn the books upside-down, but I always thought it is just out of curiosity; or more often they would just pretend to be reading, without actually realizing the book is upside-down. That is probably the explanation you are looking for. Now, if you have older siblings, it's a different story entirely. When an older kid learns to read, younger siblings tend to watch. The book can not be easily seen or read from behind the older kid's back; so the younger kids approach from the other side, and end up facing the book from the wrong direction. I heard stories about younger kids in large families who actually started reading upside-down, and had to relearn in school to read the "proper" way. That may also be the case. Hope this helps. --Dr Dima (talk) 02:14, 25 January 2009 (UTC)


 * I think you got it. Since I don't have older siblings, I go with the curiosity explanation. If I were a cat, I'd be dead now, despite the bonus lives. &mdash; Sebastian 02:46, 25 January 2009 (UTC)
 * I can still read anything upside down. It's a handy kind of obscure skill that I'm guessing many people have. Julia Rossi (talk) 05:58, 25 January 2009 (UTC)
 * Are you Gemma Williams?  Rockpock  e  t  07:00, 25 January 2009 (UTC)
 * No, the usual way is fine as well, no colour filters required. :) Julia Rossi (talk) 07:17, 25 January 2009 (UTC)
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 * ףסש ¡әәӋ 07:49, 25 January 2009 (UTC)
 * Reading upside down is a useful skill when reading a book to children. You can hold the book facing them so they can see the pictures and then lean over the top to read it. --Tango (talk) 17:13, 25 January 2009 (UTC)