Wikipedia:Reference desk/Archives/Science/2007 October 11

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October 11[edit]

Do you think that this is real?[edit]

I've been doing some reading about hybrid macaws today. In the course of which, I encountered this image:

Supposedly, it's a Hyacinth Macaw x Blue-and-yellow Macaw hybrid. I know that some of the hybrids do look a little strange and their appearances can very a lot depending on which species was the father/mother (and luck!) but this one just looks *wrong* (looks way too much like a photoshop cut+paste job to me). Would I be right in calling BS on this image?

OTOH, this image supposedly depicts the actual hybrid, which apparently really does exist. Cool huh? --Kurt Shaped Box 02:11, 11 October 2007 (UTC)

The lighting in the image is rather screwed up (probably due to using flash photography close up), but other than that, I can't see any obvious reasons for it to be fake - the image fits the shadow perfectly and there are no obvious discontinuities in the colouring or pattern. Laïka 11:55, 11 October 2007 (UTC)
The pic is too heavily artifacted for me to say for sure, but something about it just doesn't sit right with me. The head seems too large for the body, the colour shifts on the throat and breast seem too abrupt, and the shadow doesn't look right. Just below the shadows of the head, there's a shadow 'bump' that is of somewhat different quality (sharper, a little darker) and looks to me like it might have been added in. Any of those things could be accounted for by a simply poorly taken picture, but together they make it unconvincing. Again, overall poor quality makes it difficult to be sure. Matt Deres 00:07, 12 October 2007 (UTC)


At my school there are gulls that presumably feed on abandoned food after lunch. Normally there's few, if any, flying over the quad (where most of us eat), but every once in a while they all come out at once. Does anyone know why? Does any box shaped like someone (named Kurt) know why? — Daniel 03:17, 11 October 2007 (UTC)

My name is not Kurt and I'm not shaped like a box but I'd like to give a possible explaination. I heard some years ago that gulls (and vultures in Africa) operate a sort of territorial watching patrol system where they fly around in a certain area looking for food and keeping an eye on their neighbours. When they find food they drop out of the sky to scavenge it, meanwhile the neighbours of this gull, which have been watching him, see that he has dropped down to eat so they head for that spot. The neighbour's neighbours also see that their neighbours have zoomed down to sample something so they too head for the action. Very soon you've got more gulls than you were able to see beforehand. Forgive me for treading on what is cleary viewed as your territory KSB Richard Avery 07:36, 11 October 2007 (UTC)
Yup, you're pretty much spot on there, Richard. Gulls have very sharp eyesight and can presumably observe the behaviour of other gulls over fairly large distances. It's not always the case that they're all airborne at the same time either. Gulls like to sit on high rooftops and watch the other gulls - a lot.
I also wouldn't put it past the gulls to know roughly what time of the day that the kids eat their lunches too. When I was at school, I remember seeing gulls starting to congregate in large numbers on the rooftops of the school buildings overlooking the playground from about 11am onwards.
Speaking from my observations of Herring and Lesser/Great Black-backed Gulls, it's often the case that a single bird will spot food and immediately sound the famous 'MWAAK! MWAAK! MWAAKMWAAKMWAAKMWAAKMWAAK!' cry. Whether this is a joyous announcement of the presence of food to the flock or a warning to the other gulls to stay away from that individual's find is largely irrelevant (I suspect the latter) - the net result is the same, in that the other gulls hear it and scoot over to take a look. --Kurt Shaped Box 08:54, 11 October 2007 (UTC)
Dear god, we're imitating animals now? Ok, let me do a fruitfly then: bzzzzzzzzzz. :) DirkvdM 09:32, 11 October 2007 (UTC)
Hey - leave Kurt alone - we only have one gull expert and he's it! (Which is weird because we have about 1e6 relativity/quantum-theory experts!) If he says it goes "MWAAK!" - then as far as I'm concerned, that's the end of the debate! SteveBaker 16:06, 11 October 2007 (UTC)
That is spot on, and I think it's very ironic that we claim to know more about invisible particles than about macroscopic animals. I may wax philosophical, and charge our educational establishment with focusing on the wrong things; but of course, this could lead to a long and heated debate about the merits and demerits of theoretical sciences. Nimur 04:36, 12 October 2007 (UTC)
I wouldn't dare dispute Kurt's expertise on gulls. But living in a student house, I think I can consider myself an expert on fruitflies (aka beerflies in Dutch). :) DirkvdM 18:20, 11 October 2007 (UTC)
How are you on barflies? --Kurt Shaped Box 22:08, 12 October 2007 (UTC)
Beer bad. DirkvdM 09:47, 13 October 2007 (UTC)

Connecting two tanks automatic siphon?[edit]

I am wondering if this is physically possible: Imagine two water tanks. The tanks are connected together by a hose which connects to the bottom of both tanks. Assuming both tanks are at equal height, and the hose is below the water level of the tanks, it is obvious that water will flow from the tank with more water to the tank with lesser water until both have equal water height.

Now if the hose is primed with water, then the hose can actually run above the height of both tanks and the siphon action will cause water to flow from tank with higher water level, to tank with lower water level.

My question is when the water levels are equal and the siphon action stops, what difference in height between the two tank water levels will be required for the siphon to restart on its own. I know this would depend on the height of the hose above the water level of the higher tank at least...--Dacium 04:10, 11 October 2007 (UTC)

Assuming the hose didn't empty out and is still primed, the difference needed is either negligible or zero, I'm not sure which. — Daniel 04:16, 11 October 2007 (UTC)
I think you will find when the water levels are equal a vacuum will form in the higher part of the hose - it does not stay primed. The question is does the water level need to reach past the height were this vacuum is, or does it only need to reach a lower height to arrest the vacuum (as you said only a little difference)? Might have to try this practically!--Dacium 04:25, 11 October 2007 (UTC)
Why would you expect a vacuum to form? Air pressure will keep that from happening. (Well, unless the hose extends more than about 33 feet [10 meters] above the water, but in that case it wouldn't work as a siphon to begin with.)
In effect, the two tanks linked by a siphon form a single system. Any change in water level on one side will cause a flow through the hose between them to level it out.
--Anon, 04:33 UCT, October 11, 2007.
A museum, perhaps the Boston Museum of Science had an interesting demo once. There were two pairs of small plexiglass tanks and the two tanks in each pair were linked by a lever to a control knob. By turning the knob, you could raise one tank and lower the other. One pair of tanks contained water and was linked by a siphon. The other pair of tanks contained a Slinky looping over the tanks from one to the other. The cool part, of course, was that the two mechanisms behaved essentially identically. When you shifted the relative levels of the watery tanks, the water in the two tanks obviously flowed through the siphon until both tanks contained water at the same absolute level. And the Slinky tanks did the same thing, shifting Slinky coils until the "solid" portions of the Slinky stood at the same absolute level.
Atlant 12:31, 11 October 2007 (UTC)

charged particle and electromagnetic radiation .[edit]

when charged particles(electron and photon) are accelerated ; they emits electromagnetic wave(photon) . where the photon comes from ? is there any store-house of photon inside a charged particle ? what happen if a charged particle emits all of its stored photon ? what is the direction of emitted photon by a charged particle when it is accelerated ? —Preceding unsigned comment added by Shamiul (talkcontribs) 04:13, 11 October 2007 (UTC)

A charged particle does not emit photon when accelerated in general. It only emits photons when accelerated by receiving electro-magnetic energy (ie another photon).--Dacium 04:31, 11 October 2007 (UTC)
What about cyclotron radiation? --JWSchmidt 04:56, 11 October 2007 (UTC)
Actually all charged particles emit when accelerated. For all practical purposes this acceleration happens through EM interactions. In a cyclotron it is accelerated by magnetic force, for example. There is not really a photon in the electron, the photon is the form energy takes when it is released. It is the same when electrons move between energy levels in an atom. Photons are constantly destroyed and created (emitted and absorbed) there is no more a store when an electron is accelerated than there is in a lightbulb before it is turned on. See Bremsstrahlung. The direction of emitted radiation depends on the acceleration. In synchrotron radiation (circular acceleration) it is emitted in a cone in the direction of the particles travel, perpendicular to the acceleration. In general the two directions of the particle, before and after emission act like an antenna. Cyta 10:19, 11 October 2007 (UTC)
There is no 'storehouse' of photons - photons (like all matter) are just made from energy (E=Mc2, etc). When a photon is emitted, it's just a manifestation of energy being lost by the emitter. When you use electricity to power a lightbulb, the energy from the electricity gets pushed out as a bunch of photons (little bundles of energy) - which rush off at the speed of light until they hit something and are absorbed by it - transferring their energy into whatever they hit, and being 'destroyed' in the process (typically, the energy they give up just makes the thing they hit a tiny bit hotter). Photons are being created and destroyed all the time in vast numbers as they transfer energy in the form of light/radio/x-rays/etc. SteveBaker 15:57, 11 October 2007 (UTC)

Source of water to create homeopathic remedies[edit]

Where do homeopaths obtain water to use in the dilution and succussion process by which they create homeopathic remedies?

The rest of this is background as to why I ask this question. It arose from discussion about the homeopathy article.

The prior history of the water used in dilution seems to be relevant. "since water will has been in contact with millions of different substances through its history, critics (of homeopathy) point out that any glass of water is therefore an extreme dilution of almost any conceivable substance, and so by drinking water one would, according to homeopathic principles, receive treatment for every imaginable condition."

Thank you, Wanderer57 05:39, 11 October 2007 (UTC)

You've discovered one of the many reasons why homeopathy is considered a pseudoscience; its basic theory is irredeemably faulty.
Atlant 12:33, 11 October 2007 (UTC)
Don't they use distilled water? 14:39, 11 October 2007 (UTC)
Related Wikipedia article: Water memory (does not answer the question). There is some discussion in this article of possible sources of water used by Samuel Hahnemann and contaminants found in water used by some manufacturers of homeopathic remedies. --JWSchmidt 14:59, 11 October 2007 (UTC)
Since the medicinal effect (such as it is) can only possibly work by the placebo effect - it only matters that both patient and the doctor believe it will work. It may be that either the doctor or the patient would only believe it to work if distilled water were used (or water from the slopes of Mount Kilimanjaro or water blessed by the Pope during a month with an 'R' in it). Hence it may well be that distilled water is required...but not for any scientific reason. All of this careful dilution nonsense is just there to convince people along the chain that the stuff will work such that when they give it to the patient, they don't give off "this is complete bullshit" vibes that would throw off the placebo effect. There is absolutely no science behind it though...please don't ever think there might be! SteveBaker 15:49, 11 October 2007 (UTC)
I think the original question is about the "logic" from the homeopathic point of view for the previous "memory" of the water they use.
That's why I cited distilled water. They could say that the distillation process "destroys" the water memory. Maybe they really say that, I don't know.
So, this "amnesic" water will have only the memory of the substances it eventually had contact in the homeopathic process.
And will be more higienic, too. 15:58, 11 October 2007 (UTC)

How do “selfish gene” principles account for biased demographic effects?[edit]

A science writer declared that there are more boys than girls born (about 55/45 ratio) because Mother Nature compensates for all the boys who will die prematurely through violence and foolhardiness. This got me thinking as to how this would square with the principle of the “selfish gene”. If the process of evolution is driven purely by the fitness of an individual organism, then how do community-wide concerns about demographics end up influencing the biological processes of reproduction in individual females? Myles325a 06:03, 11 October 2007 (UTC)

Read over sex ratio first. 55/45? Nah. --Cody Pope 06:14, 11 October 2007 (UTC)
Keep in mind, even where there might be a skewed ratio, the selfish gene principals would only only be working on genes in the X and Y chromosomes. If it were the case that there was selection for more males than females in a population, it might be because sex-linked genes that led to a higher male population were advantageous, either linked to some other selective pressure, or merely because they were linked with the sex-determining chromosome and therefore expressed more frequently. -- JSBillings 10:33, 11 October 2007 (UTC)
For the mother to have a chance of passing on her genes, she needs to give birth to a child that maximises it's its chance of reproducing by not being one of an excessively large number of females or one of an excessively large number of males. So a species will tend to produce a male/female ratio that comes closest to the ideal radio in adulthood (taking into account whatever environmental effects may kill of males in early age for example). If there were a gene that caused production of 100 males for every female, then that gene would survive just fine for a few generations - but it wouldn't be able to spread across the whole of the community without self-destructing. So the only genes that can survive are the ones that produced a 50/50 split (or whatever ratio is best suited to the environment) - so there is evolutionary pressure to produce children in the right probabilities. SteveBaker 15:40, 11 October 2007 (UTC)
Corrected a quite painful spelling mistake. --Taraborn 14:07, 14 October 2007 (UTC)

Silver nitrate staining[edit]

Co2+ was precipitated with ammonium sulfide and visually enhanced with silver nitrate - I understand that cobalt sulfide is black; what reaction takes place with silver nitrate? What compound is formed? --Seans Potato Business 07:07, 11 October 2007 (UTC)

You mean, "why does silver nitrate turn black if left exposed"? I know it has to do with exposure to light, or more specifically sunlight if I recall correctly. Since silver nitrate breaks down in liquid form to form silver and nitrate ions, it would have to be a reaction between either of those, and compounds on your skin, or on surfaces. It would then logically have to be a reaction between the silver and something else, since I doubt the nitrate ions can readily find other positive ions so quickly. Luckily, everything has electrons, so those can reduce the Ag+ ions to become stable silver atoms, therefore the stain is pure silver.-- 07:40, 11 October 2007 (UTC)
The methode would be imilar to black and white photography, but on cobalt sulfide the layer of silver, which would form with silver nitrate and sun light, would turn into the black silver sulfide fast, because silver has a high afinety to form the sulfide. The result would be black colour like the silver itself so you get a black surface.--Stone 08:35, 11 October 2007 (UTC)
See also Silver stain. --JWSchmidt 14:47, 11 October 2007 (UTC)

Controlling earthquakes by setting them off[edit]

Inspired by the above question about nuking a volcano: Along fault lines, tension builds up and then when it is released, an earthquake takes place. Now if you know that there will be an earthquake, but not when, then wouldn't it be nice if you could release the energy and thus have more but smaller earthquakes at moments you yourself set, so people can prepare them selves for it? For example by setting off a big bomb at a crucial position (like in A View to a Kill). How much energy would that require? Would a nuclear bomb be needed or would it even be enough? And if the energy is released in many small doses, then might it be possible to build humongous shock-breakers that absorb the energy and store it in some fashion that it can be retrieved gradually and made to good use? For example compressed air or even (considering the huge amount of energy) compressed water? How much energy is released in an earthquake? The article doesn't seem to say. DirkvdM 08:35, 11 October 2007 (UTC)

There was an article along these lines in the New Scientist of 30th June 2001: [1]. I have to go to a lecture now, but I've found the article and should be able to give more information later (if you haven't got access to a library or the online NS yourself). Algebraist 10:56, 11 October 2007 (UTC)
There is a calculation of earthquake energies and a comparison against nuclear weapon energies here. Gandalf61 11:09, 11 October 2007 (UTC)
Note though that the energy released by an earthquake is much less localized than the energy released by a nuclear weapon. By analogy, the sun releases thousands of megatons worth of energy every second or so, but only a small fraction of that is focused on earth, and even of that energy it is diffused over a very wide area. The important fact about nuclear weapons—and making the megaton scale make sense on an inuitive level—is that all of the energy is released into a relatively small area and in an extremely rapid fashion. -- 12:57, 11 October 2007 (UTC)
The problem with controlling earthquakes is strongly related to understanding them in the first place. In order to know exactly where to place a nuclear explosive, how powerful it should be, and when to trigger it, we'd need to know a lot about the environment of the fault into which we'd be placing it. The trouble is that we mostly don't - we have very (very very) poor imaging for the subterranean environment: it's a complex 3D system of multiple faults, where each fault is a complex 3D structure made from solids with pockets of aggregates or sand, lots of (moving, sometimes) underground water, all floating on a fulminating chaotic soup of magma. If we could properly image fault systems over time we'd be able to build much better models of how earthquakes start and stop, and we'd be able to explain all their different characteristics. Right now geologist's understanding of earthquakes is akin to doctor's understanding of internal medicine in times when they weren't allowed to do dissections - they're just feeling the externalities and making (mostly statistical) guesses about what's going on in there. The missing information (what you'd need to understand, and finally predict, earthquakes) is just the same information you'd need before you could really meaningfully pursue the discipine of thermonuclear tectonic engineering. And given that all the danger, and much of the expense, of earthquakes arises from their unpredictability - if you could adequately predict them, you wouldn't need to manipulate them. -- PrettyDirtyThing 11:44, 11 October 2007 (UTC)

Concerning the tapping of the energy (for which a controlled release seems rather essential), how much energy is there in the first place? Gandalf's source says a really huge 9.5 magnitude earthquake releases about 1019 J. The world's energy consumption is about 5 x 1020 J per year. So to cover that we'd need to tap all (!) the energy of 50 such earthquakes each year, but Richter magnitude scale says these occur less than once a year. A 6 magnitude earthquake releases 6 x 1013 J, so we'd need to tap 10 million of those. Not only are there nowhere near that many (less than a thousand), but we'd need to build way too many of such powerplants for it to be realistic. And that's assuming 100% efficiency, while we'd be lucky to get 10% efficiency. So even if this were doable in some ingenious way, the yield would be such a small fraction of the world's energy demand that it's not worth considering. So the energy source is out the window, I suppose, or did I make a mistake somewhere? Hold on, to put this in perspective: the bottom table in the Richter article says a 12 magnitude earthquake (which would split the world in half - do I read that correctly?) would release the same amount of energy the Sun shines down on Earth every day. I'm clearly barking up the wrong tree here (barking mad pseudo-scientist I am).
As a life-saver, the controlled release might still be an option, though. DirkvdM 13:25, 11 October 2007 (UTC)

On the concept of avoiding major quakes by inducing smaller ones more often: in some cases it might be possible to do this without anything as dangerous as explosives, by simply lubricating the fault. See this PDF newsletter, starting at page 6. But all the problems that Pretty describes would still apply, and I guess this method would only work if the fault wasn't badly "locked", so I doubt we will ever see this method widely used. --Anonymous, 23:07 UTC, October 11, 2007.

Tapping the world's rotational energy[edit]

Here's the barking mad pseudo-scientist again. :) Suppose that somehow we could tap the rotational energy of the Earth, how long would that last and how dangerous would it be? Here's a calculation: Earth's mass is 6 x 1024 kg. Its circumference is 40,000 km. Let's for the sake of easy calculation say that that mass is concentrated on the surface of a cylinder with a circumference of 10,000 km. So the mass travels at a speed of 10,000 km per day. There are about 100,000 seconds in a day, so that's 100 m/s. E = mv2, so that would be 6 x 1024 kg x (100 m/s)2 = 6 x 1028 J. The world energy consumption is 5 x 1020 J, so at the present consumption rate that would last us 100 million years. Now I'm only a pseudo-scientist, so first question: is this a correct calculation / realistic approximation? Second question: what effects will slowing down Earth's rotation have? (And hasn't this been asked here before?) And the third and hardest question to answer: how might we do this? (Fourth bonus question: how mad am I, really?) DirkvdM 14:11, 11 October 2007 (UTC)

A stationary satellite in orbit with a (really, really) long pole dangling down to touch the earths surface with a dynamo at the end? Think outside the box 14:20, 11 October 2007 (UTC)
Essentially tidal power is exactly tapping the earth's rotational energy so the third part of the question ain't that tough. --BozMo talk 14:23, 11 October 2007 (UTC)
There's a better approximation in our article on rotational energy (one that treats the Earth as spherical rather than cylindrical); it gives a kinetic energy of 2.58×1029 J. (So you're off by less than an order of magnitude. That calculation assumes a uniform density to the planet; the kinetic energy will actually be a bit lower because Earth's core is denser than its crust, giving a lower moment of inertia.) As BozMo notes, tidal effects have been gradually slowing the Earth's rotation for billions of years: [2]. Every century, a day gets a little more than two milliseconds longer. It hasn't hurt us so far, but it does cause some intriguing problems for some astronomers.
Perhaps more worrying is what might happen if we were able to tap a significant amount of that energy rapidly and make use of it on Earth. The waste heat produced by whatever processes we put the energy in to could heat the Earth's climate significantly. (For comparison, the entire Earth's surface absorbs about 1024 joules of energy from the Sun over the course of a year.) TenOfAllTrades(talk) 15:17, 11 October 2007 (UTC)
  • I don't have a link, but I believe some space agency tested an energy production scheme a few years ago where you lower a line into the atmosphere from a satellite, and the resulting static (?) charge provided power. If we had a magical nanotech rope we could perhaps attach one end to the South Pole, and one end to the Moon, and use the yo-yo action to generate power, but that's harnessing the moon's eccentric orbit rather than the Earth's rotation. --Sean 16:06, 11 October 2007 (UTC)
This was the Tethered Satellite System, flown on the STS-75 Shuttle mission. Essentially, a satellite on a multi-kilometer wire was extended from the shuttle into the ionosphere, generating a voltage on the wire. In fact, enough voltage was generated that it arced from the wire to the mechanism paying out the wire (which was at ground potential relative to the wire), melting the wire and resulting in the loss of the satellite. Definitely proved the technology though. Arakunem 00:36, 12 October 2007 (UTC)
Of course, you can tap the rotational power. You just need something that is held still while the earth rotates under it. The bulge of the tides, kept fixes by Earth's gravitation is quite suitable. See tidal power for more. The link Sean was referring to is 17:28, 11 October 2007 (UTC)

After edit conflict:
First of all, a pat on my own back for getting so close with such a crude calculation. :) Concerning the tidal energy, 2 ms per day per century means 100,000 / 0.002 x 100 = 5 billion years until the Earth stands still (exactly when the Sun will consume the Earth - a coincidence, I assume). So that would be only 1/50 of the energy we demand now and that is assuming we can harvest all that energy. And even if we could do that, what effect would the ending of tides have on life on Earth? Doesn't sound like a good option. Concerning the heating up of Earth, I hadn't thought of that, good point. But what would the effect be compared to the energy already in the system, I mean percentage-wise, so to say? My gut feeling tells me that would be smaller than any effect the Sun could have, such as with the help of greenhouse gases. But if we would take enough energy out of the rotation to meet our present energy demand, then the speed of slowing would be more like 0.1 second per day, or about an hour per century. Still slow enough for life to adapt through evolution, I assume.
Then again, I'm a bit tired now, so any of the above may be erroneous and that is probably also the reason I don't get the three suggestions. Sean's yoyo effect sounds amusing, but assuming we could pull that off, that would mess up the Earth's axis tilt, and that sounds rather too dangerous. Think outside the box's magnet is probably meant to use the Earth's magnetic field, but I don't see how (definitely not my field). And I don't get Sean's static energy line. Is that maybe to do with the triboelectric effect? Please elaborate, if you remember any more details. DirkvdM 18:02, 11 October 2007 (UTC)

The Earth's rotation slowing won't be linear. In times near to the present, you can model it as a linear slowing, but over long periods of time, that won't be appropriate. The primary source of tides is the Moon, but the slowing of the Earth's rotation is lifting the Moon into a higher orbit - which weakens the tides and so the rate of slowing of the Earth's rotation will gradually decrease. Also, as the Earth's day length gets close to the Moon's orbital period, the raised tides will stay in a similar position on the Earth, so tidal friction will be much reduced. Eventually, the Earth and moon would be tidally locked to each other - so the rotation won't stop as you suggest. Richard B 12:23, 12 October 2007 (UTC)

Ah, followed 85's link and that indeed uses the Earth's magnetic field. But it only powers a space station, so it couldn't begin to make a dent in mankind's total energy demand. And the article points out an obvious flaw that you already hinted at - the space station isn't 'held still'. Anything that hangs in space and doesn't have a really huge mass will pick up Earth's rotation more than generate power. Or not? The article suggests something different. Really too tired now, I'll come back tomorrow. DirkvdM 18:08, 11 October 2007 (UTC)

I think the worries about waste heat from this scheme are not a pressing environmental concern. You basically can't do it -- at least, not on a scale that's going to extract any noticeable fraction of the Earth's rotational kinetic energy, prior to such time as we can build structures on the scale of the Earth.
The Earth has a certain angular momentum, which is conserved; you can't change it (you could conceivably throw some of it off into space, but then you'd be throwing a lot of the energy with it). To extract rotational KE from the system without changing the angular momentum, you have to change the moment of inertia. And we can't change it, very much, because we don't have anything big enough to change it much. If we ever do build things that big, I think current estimates of world energy consumption will no longer be terribly relevant. :-). --Trovatore 18:22, 11 October 2007 (UTC)
Angular momentum isn't a problem. By tapping the Earth's rotation via tidal power, we're transferring that momentum to the Moon: over the centuries, the Moon's orbit gets higher. --Carnildo 22:02, 11 October 2007 (UTC)
What I said was, we can't change it much. Only a minuscule fraction of the Earth's rotational KE is available for extraction in that way. (Well, I think it's minuscule; I haven't done the actual calculations. But note that the Moon's contribution to the moment of inertia goes up only as the square of its orbital radius, whereas the tidal effect diminishes with the cube of the radius.) --Trovatore 22:11, 11 October 2007 (UTC)
Ah, yes, the principle of an ice-skater extending his arms. It wouldn't have to be such a big structure, though, at least not the result - how to start construction is entirely different matter. I've once heard of a plan to make a space lift - AH, we have an article. Of course we have an article. :) Before reading it, this is what I was thinking. Once in place, the top of the cable is rotating faster and can thus 'suck up' something from Earth. This is a way to send stuff into space. But we can also tap the energy of whatever is flying off up the lift. The heavier the stuff we 'send up', the more energy we get out of it. So we'll end up doing a lot of that and eventually we'll have the material up there to build a second layer around Earth. And a third, etc. Could be useful for loads of things. But the article doesn't seem to mention this (haven't read it all). It actually assumes this will cost energy. I don't get that. The principle of what I just said is correct isn't it? I mean from a basic energy conservation pov. DirkvdM 08:00, 12 October 2007 (UTC)

Effects of concrete under high temperature[edit]

I was curious why some people blamed the collapse of the WTC for its steel construction. I checked the melting points of both iron and carbon, and they are very high. Of course they will undergo plastic deformation before melting completely, but I was wondering what are the effects of concrete under extreme temperature? What is the melting point of concrete? Will it undergo deformation before melting? At what temperature does this happen? Would the WTC have survived if it was constructed primarily out of concrete as opposed to steel? 14:07, 11 October 2007 (UTC)

I cannot provide a good source but as far as I know: generally in fierce fires concrete survives for much longer because the heat transfer/conduction into the concrete is much slower and it takes longer to heat up. After Piper Alpha some of the North Sea oilcos started coating the steel members on platforms with concrete because of this; a few centimeters gives an extra hour to the steel survival. At what temperature concrete eventually fails, and how long it takes to heat up depends on the type of concrete but also critically on how damp the concrete is. You can pour molten iron into a ceramic mould so you know survivability ie better anyway. --BozMo talk 14:21, 11 October 2007 (UTC)

The trouble is that concrete is a terrible construction material by itself - it has to be reinforced with steel rebar - as soon as the temperature gets up high enough for the rebar to give way - your concrete loses it's ability to resist twisting and tensile forces (it's really only any good under compression) - and the building collapses anyway. The extra thermal insulation the concrete would have provided might have prolonged the life of the buildings somewhat - but they'd have collapsed sooner or later anyway. But a bigger consideration is whether the impact of the aircraft itself would have caused more damage in a purely concrete construction. It's really hard to say - but it's certainly not obvious that concrete would have been better. I vaguely recall that there was a problem with the construction of the WTC anyway - wasn't it discovered that the steel beams had not been coated with the right fire-resistant stuff? SteveBaker 14:34, 11 October 2007 (UTC)

Yes, the spary-on fireproofing wasn't designed to resist the forces that resulted from airplane crashes, so it tended to blow off the structural steel. After that, it was just a question of how quickly the steel would become too plastic to carry the static loads.
Atlant 16:34, 11 October 2007 (UTC)
A bit off topic but informative: Collapse of the World Trade Center#The collapse mechanism. - hydnjo talk 15:33, 11 October 2007 (UTC)

Under very high temperatues, concrete tends to spall off. Atlant 16:36, 11 October 2007 (UTC)

A lighted torch should not be turned on concrete. Concrete always contains some moisture which may cause the concrete to explode. --Duk 15:19, 12 October 2007 (UTC)
  • I see two fallacies about steel here. One, you cannot figure out the melting point of a compound or alloy just by figuring out the melting point of its elements. For example, this kind of C8H16 melts at 14.59 °C, while this kind of C8H16 melts at -101.7 °C. Two, as the WTC reports showed, steel begins plastic deformation at something like half its melting point, and it doesn't take much deformation for the top floors to come crashing down. --M@rēino 20:27, 11 October 2007 (UTC)
That wasn't very helpful. We knew all that already. The question is, at what temperature does the steel used in the WTC undergo plastic deformation, and how does reinforced concrete compare to that. Malamockq 23:12, 11 October 2007 (UTC)
Hey, it might not be helpful to you, but it is helpful to, who had not been corrected by anyone else for the error about steel's melting point in the original question. --M@rēino 23:59, 11 October 2007 (UTC)
Popular Mechanics did a huge report debunking the most popular "9/11 myths." It has details about the collapse that might be helpful to you. You can find it here.
Mrdeath5493 05:52, 12 October 2007 (UTC)

black contact lenses[edit]

In science fiction tv shows and movies, when a character goes evil or changes (like on buffy with Oz turning into a warewolf or Dark Willow) or is possesed by an alien, they often have black eyes - I mean totally black. Is this black contact lenses or ink or what? And, as Oct 31st is coming soon, where can one buy the requirements to have their own eyes turned totally black? Picture of a cloud 14:11, 11 October 2007 (UTC)

Computer generated imagery perhaps? It would be a simpler solution, but I'm sure black contacts exist. They certainly can be made. —Preceding signed but undated comment was added at 14:20, 11 October 2007 (UTC)
They're "decorative" or "cosmetic" contact lenses. More information in the article. You can buy them by mail order from lots of places.--Shantavira|feed me 14:21, 11 October 2007 (UTC)

Crystallization and entropy[edit]

Quick question: does crystallization represent an increase or decrease in entropy? I think it's a decrease but not sure. Thanks! 15:11, 11 October 2007 (UTC)

Decrease, as crystals are more ordered than a random jumble of molecules. Someguy1221 15:40, 11 October 2007 (UTC)
Please see Entropy. The picture says a thousand words. --slakrtalk / 22:56, 11 October 2007 (UTC)

Okay, thanks! 18:18, 12 October 2007 (UTC)

Can one have echolalia with thoughts? Or is that called something else?[edit]

Just wondering, is it possible to have echolalia occur with simple thoughts, instead of language. I've heard that small children haven't yet developed the social or whatever other skill is needed to maintain thoughts that aren't out loud. (I thought that was inner voice but it doens't look like it. It might be, though.) So, would someone high on the autism spectrum possibly not repeat words so much as thoughts after a few seconds? Also, would it be considered echolalia when a person simply thinks something internally a number of times in a row after hearing it, instead of repeating it? In other words, possibly processing the information but able to do it internally? —Preceding unsigned comment added by (talk) 15:44, 11 October 2007 (UTC)

Interesting that it has a name. I always thought that was just something children did, and the supposed 'delayed echolalia' in that article (with no sources) just sounds like "quoting", which my parents used to get annoyed at us for doing... From the sounds of the article, it can be for the pleasure of the sounds rather than buying time/processing (although it mentions that too). I'm intrigued to see what an actual knowledgable person replies... 17:16, 11 October 2007 (UTC)

Caudate nucleus left/right asymmetry[edit]

As I understand, the caudate nucleus is usually asymmetrical. Does anybody know which side is bigger in normal subjects, left or right? Lova Falk 16:48, 11 October 2007 (UTC)

According to Encyclopaedia Brittanica, "in boys without ADHD, the right caudate nucleus was normally about 3 percent larger than the left caudate nucleus; this asymmetry was absent in boys with ADHD." [3] Rockpocket 17:57, 11 October 2007 (UTC)
Thank you! Your source says boys without ADHD: right>left, boys with ADHD: right=left. I just found another source [4] that says the opposite: normal children 72% left>right, ADHD children 63% right>left, due to smaller left side. Very contradictory... Lova Falk 14:11, 12 October 2007 (UTC)
Ahh. Welcome to the world of sampling bias! Personally, I would trust your source above mine. Rockpocket 17:23, 12 October 2007 (UTC)

Photoconductive cells[edit]

What is photoconductive cells? How photoconductive cells work? (also include detailed photoconductive cells technologies circuit diagrams characteristic curves photoconductive cells component diagrams technical shets) What are their applications and explanations. —Preceding unsigned comment added by (talk) 17:40, 11 October 2007 (UTC)

Photoresistor would be a good place to start your research, followed by typing Photoconductive cell into a Google search and reading to the top few hits. Rockpocket 17:46, 11 October 2007 (UTC)
Yes. Please dont pose your homework question like that. You must show that you are willing to do some of the work yourself by searching. We will gladly point you in the right direction but we wont hand the answer to you on a plate. —Preceding unsigned comment added by (talk) 00:22, 12 October 2007 (UTC)

Expansion of Universe and expansion of expansion measurement device[edit]

Dodgy title, eh?

Anyway, my Physics teacher bought up a subject when studying the expansion of the Universe that if the universe is expanding then surely it cannot be measured as the tools being used to measure the expansion are also expanding. I notice the Big Bang article has a small section on why this might not be true - to assume planets and other masses as seeds on an expanding loaf of bread, but he seems not convinced - For my own understanding and the wanting to trip up an Oxford MSc does anyone have a better answer to this problem?


-Benbread 18:01, 11 October 2007 (UTC)

I instantly (well, almost) got your title because I have thought of something similar. When you get sucked into a black hole, you get spaghettified (love that word). But so does everything directly around you - including the atoms? So to an outside observer (as if they could observe this) you might turn into spaghetti, but from your own vantagepoint it's business as usual. One snag, though, assuming you go in feet first, the laws of physics have altered in your feet more than in your head, and that must be very confusing to the blood. Or am I now falling into the trap myself? (btw, Red Dwarf experienced a similar phenomenon in one episode). DirkvdM 18:29, 11 October 2007 (UTC)
I think the theory is that the gaps between things are expanding - not matter itself - ((note if you take all matter to be expanding you can derive a gravitational like force between objects - the 'force' depends on size not mass.. this is probably irrelevant to what you are asking..)) —Preceding unsigned comment added by (talk) 20:17, 11 October 2007 (UTC)
NOTE if the space of the universe is expanding and the matter in the universe is expanding at the same rate - you wouldn't notice.. (except for possible 'gravitational' like effects mentioned above)
The current view is definately that things are moving apart - but the things stay the same size.. —Preceding unsigned comment added by (talk) 20:41, 11 October 2007 (UTC)
Accelerating universe lists some of the sources of data that can be used to not only measure expansion of the universe, but also change in the rate of expansion. Data for Ia supernovae have been interpreted as indicating a change in the rate of expansion over time (billions of years). Most physicists seem to think that this observation is not confounded by expansion itself since it relies on observations of light that seem to be well understood. I'd expect a physics teacher to be able to explain these kinds of data and their conventional interpretation. --JWSchmidt 21:58, 11 October 2007 (UTC)
Things stay the same size because the laws of physics haven't changed, and the forces keeping atoms together in molecules, planets in the solar system, and stars in the galaxy are much stronger than the repulsive force that's pushing the universe apart. Only over great distances, such as those between galaxies, is this expansion easily observable (presently). Someguy1221 23:22, 11 October 2007 (UTC)
Yes essentially if there was no gravity etc, everything would be stretched (like dots drawn on a balloon being inflated) but the other forces act to hold things together. So galaxies act more like things glued onto the inflating balloon. The best evidence for measuring the expansion of the universe is red shift data. This is essentially light waves being expanded with the universe compared to our telescopes which stay the same size. From this the speed of other galaxies compared to ours can be compared. We can see from this that a galaxies speed away from us is proportional to its distance from us (Hubble's law) which can be best explained by an expanding universe. Cyta 06:52, 12 October 2007 (UTC)
I remember reading something about the measurements of the speed of light. They were progressively changing (moving faster and faster I believe) until we locked them to the cesium atom timing. Some people still believe that the speed of light is changing as the universe expands, as a result time is slowing down (or speeding up?). One guy even claims this is the reason for the anomoloy of the velocities on the voyager space probes (he claims to caculate their speed exactally if he is allowed to vary the speed of light).... puesdoscience at its best i think, but who knows--Dacium 23:47, 16 October 2007 (UTC)

ISEL vs TUNEL[edit]

Is there are difference between In Situ End Labeling (ISEL) and Terminal deoxynucleotidyl Transferase Biotin-dUTP Nick End Labeling (TUNEL)? Are there enzymes other than TdT in use for ISEL? --Seans Potato Business 18:02, 11 October 2007 (UTC)

Here are some articles that describe the differences: [5] and [6]. --JWSchmidt 20:47, 11 October 2007 (UTC)


Is there any possible defence against Tasers that is safe both for me and the attacker ? —Preceding unsigned comment added by (talk) 18:37, 11 October 2007 (UTC)

  • Run away. --Sean 19:52, 11 October 2007 (UTC)
  • Thor Shield. You can build it right into body armor. They might even be able to make a less-conspicuous-looking Thor Shield T-shirt; I'm not sure. --M@rēino 20:32, 11 October 2007 (UTC)
get your body completely electroplated? —Preceding unsigned comment added by (talk) 00:19, 12 October 2007 (UTC)
Not if they were using it for self-defense. If you aren't hurt, you'll hurt them. — Daniel 01:16, 12 October 2007 (UTC)
Some have advocated loudly shouting "Don't tase me, bro!" repeatedly, but studies show this is ineffective. - Nunh-huh 02:42, 12 October 2007 (UTC)
Someone has now proposed/is now weaving metal fibers into a sweater or vest. These will conduct the majority of the Taser's electrical current rather than allowing it to flow through you.
Atlant 16:01, 12 October 2007 (UTC)
Is Kevlar effective against it? —Preceding unsigned comment added by (talk) 18:25, 14 October 2007 (UTC)
IMNSHO, probably not. The voltage used in Tasers is pretty high and would likely penetrate between the weave of the Kevlar fabric. I'd go for short-circuiting conduction rather than insulation here. Of course, if the Kevlar were woven tightly enough to prevent the barbed probes from getting "latched in"...
Atlant 16:15, 15 October 2007 (UTC)
I think thick leather would do. But I would rather see them banned - therefore I support human rights organisations... /SvNH 20:24, 14 October 2007 (UTC)
Against the "cattle prod" types typically sold to civilians, non-conductive clothing should suffice (a dry suit would be perfect, if a bit conspicuous). The dart type used by police MIGHT be stopped by a very thick layer of a non-conductive material, but I wouldn't want to rely on it. Modern dart tasers use an electrical arc rather than needles to deliver the charge, so simply preventing the darts from entering the skin is no good. 14:50, 17 October 2007 (UTC)

Are basic anhydrides basic according to Arrhenius's definition?[edit]

If a substance releases hydroxide ion OH− according to Arrhenius it is said to be basic, OK. However, let me consider Sodium oxide. When hydrated it gives sodium hydroxide, but is it really a base? I'm asking this question because I sought over the Internet and didn't found any examples of Arrhenius bases being basic anhydrides (metal-oxygen). Thanks —Preceding unsigned comment added by (talk) 19:14, 11 October 2007 (UTC)

According to " A substance is classified as a base if it produces hydroxide ions OH(-) in water" - so the answer is yes it is a base. 20:13, 11 October 2007 (UTC)


what are pervious rocks? —Preceding unsigned comment added by (talk) 20:19, 11 October 2007 (UTC)

sorry to be so boring, but did you actually type in the word 'pervious' in the search box. I just did and it gave me the right info. please try doing that next time! —Preceding unsigned comment added by (talk) 00:29, 12 October 2007 (UTC)
But Geogaphy does nothing useful in the search box! The pervious rocks allow water or other fluids to pass through. Graeme Bartlett 06:03, 12 October 2007 (UTC)
OK it was useful enough to cause me to fix some spelling errors! Graeme Bartlett 07:08, 12 October 2007 (UTC)