Wikipedia:Reference desk/Archives/Science/2008 April 10

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[edit] April 10

[edit] Huge sandbanks!!!

Hi. When I went to a nearby plaza recently, I saw some huge sandbanks, the remnants of giant snowbanks that have melted and left their sand, dust, and other material in a huge heap. I mean, even with a huge snowbank, you would have thought that it would have eventually melted and left behind maybe a few deposits of dust, right? Well, no. It appears that due to this winter's amazing snowfall amounts, the snow contained extra amounts of sand, and created a heap. Now, I know that what usually happens is, snow from the streets gets shovelled onto the lawns, the snow contains dirt, as the snow melts, the dirt is left behind, the melting water forms mini-rivers flowing over silt, and as the water from the dirt evaporates, it turns into sand, and spring wind blows the sand across the street, forming mini-sandstorms, and the cycle repeats again. However, at this plaza, from which probably about 20,000 square metres of snow was shovelled into one place, a huge snowbank, and later a resulting heap developed. Well, this heap is huge, and I think it must weigh 1000 tonnes. It's still wet, but I think as the water evaporates this heap will remain. First of all, where does all the dirt come from? Yes, I know that as the snow is shovelled, it must contain massive amounts of sand from eroded ashphalt, tar from the cars and roads, remnants of road salt, dirt carried from grass by water to the roads, sand carried from elsewhere from dust blown from previously melted snowbanks, silt from nearby places, dirt carried by walking people, grains of metal from the machinery of the snowploughs, dust from industry, manufacturing, and heavy machinery, atmospheric dust, as well as dust, pollen, and pollutants carried down by snowflakes, but why such a massive amount? Also, why does a snowier year tend to bring larger heaps, is it because more snow requires more shovelling and picking up the dust from the roads, or because of more dust in the snowflakes deposited? Also, what will happen to the heap? Will it stay until next winter and keep building up, will it be manually shovelled away by heavy machinery, or will it be blown away by the wind and eroded by water? Either way, wouldn't that create massive sandstorms in the area as it dries up and is carried to nearby areas? If a sudden windstorm blows on the dust, would it bring sand to areas kilometres away? If it rains (heavy rains are in the forecast within the next few days), will the rain cause it to stick to itself, or erode it away? Or will some people just use it as a bike ramp? Will it affect local wind patterns? Thanks. ~AH1(TCU) 00:46, 10 April 2008 (UTC)

Apologies for a banal answer to such a cosmic question, but are you sure it couldn't have been the remnants of the sand (sometimes mixed with salt) which is typically spread on snowy/icy roads to increase traction? The snowier the winter, the more of that they have to spread, and it all ends up somewhere. —Steve Summit (talk) 02:40, 10 April 2008 (UTC)
As to what will happen, I suggest phoning the organization that arranged for the snow to be piled up there and asking them. That would either be the plaza management if the snow was all from the parking lot itself, or else your local city hall (or county government or whatever) if they arranged for snow from elsewhere to be dumped in the parking lot. --Anonymous, 03:26 UTC, April 10, 2008.

[edit] decision sequences

Is there a publication which shows decision/results sequences in which subjects had a choice of doing one thing or another, for instance to stay inside and stay warm or go outside without adequate clothing and become cold, with each decision sequence ending in a result, for instance the number of persons that developed a cold as the result of following one sequence of another? 71.100.160.37 (talk) 02:33, 10 April 2008 (UTC)

Lots of publications (both fiction and non-fiction) feature decision/results sequences. Can you be more specific? Also, from our common cold article: "Although common colds are seasonal, with more occurring during winter, experiments so far have failed to produce evidence that short-term exposure to cold weather or direct chilling increases susceptibility to infection, implying that the seasonal variation is instead due to a change in behaviors such as increased time spent indoors at close proximity to others."--Shantavira|feed me 09:03, 10 April 2008 (UTC)
Colds are a good example but I am really looking for longer sequences with many more decision points such as someone ending up as a CEO versus a security guard position. BTW - One of the motives for having a statistically based sequence is that I completely disagree with the conclusion set forth above, which is based on experiments and not everyones actual experience. 71.100.160.37 (talk) 21:45, 10 April 2008 (UTC)

[edit] Echidna name?

Why are echidnas named after the monster of the same name? Is it because Echidna was half human/half snake and monotremes seem part mammal, part reptile? Vultur (talk) 03:08, 10 April 2008 (UTC)

They both have the same etymology (echidnas are not named after the mythological monster). Echidna the mammal comes from Latin echinus, from Greek εχίνος (ekhinos, meaning hedgehog), from εχίς (ekhis, meaning snake). As you can see in Echidna (mythology), they both come from the root εχίς. 70.162.25.53 (talk) 03:35, 10 April 2008 (UTC)
To confuse things a bit, the animal kingdom also has a genus named Echidna which fits the mythological description glancing eyes and huge snake, great and awful, with speckled skin, eating raw flesh beneath the secret parts of the holy earth (Don't know about the "fair cheeks"). According to the article on Homonym (biology) the name was first proposed for the aquatic genus and thus the eel has priority. The spiny genus was renamed Tachyglossus. ---Sluzzelin talk 11:30, 10 April 2008 (UTC)

[edit] murdoch

what's wrong with this guy's face excuse my bluntness. I'm not asking for medical advice on his behalf yada yada —Preceding unsigned comment added by 79.122.13.205 (talk) 13:04, 10 April 2008 (UTC)

When one gets older, the skin looses elasticity. Also, it looks like he's pushing his chin back into his throat, which causes your neck to look flattened. -- MacAddct  1984 (talk • contribs) 14:16, 10 April 2008 (UTC)
"When one gets older, the skin looses elasticity" sounds like half of a skin-care ad! Can you tell me what the other half would be, in an NPOV way? (ie what is the solution? I've always wondered what the science is behind it once you remove the marketing). Thank you! —Preceding unsigned comment added by 217.116.40.195 (talk) 15:49, 10 April 2008 (UTC)
Haha, well I don't think there is any 'solution' for it, it's just a fact of life; however, I'm sure many skin product companies assure their customers that their products will guarantee them wrinkle free skin. Maybe botox injections? -- MacAddct  1984 (talk • contribs) 16:01, 10 April 2008 (UTC)
Agreed, unless it's radical intervention, the rest is likely B$. Instead of my first guess which was sun damage], I'd say it's at least because he is 77, long days and nights and an unkind photographer. ; ) Julia Rossi (talk) 00:13, 11 April 2008 (UTC)
Nah, Murdoch's just half Shar Pei. :-) StuRat (talk) 14:28, 11 April 2008 (UTC)

[edit] why can't humans focus on two objects simultaneously like the chameleon? please also include the answer to my email - *redacted* thank you.

why can't humans focus on two objects simultaneously like the chameleon? —Preceding unsigned comment added by 203.193.135.51 (talk) 15:44, 10 April 2008 (UTC)

Take a look at Binocular_vision#Field_of_view_and_eye_movements. It seems chameleons are actually a very rare example. Human vision works by binocular vision, which requires both eyes to be looking at the same object to allow for depth perception. -- MacAddct  1984 (talk • contribs) 16:32, 10 April 2008 (UTC)
Daniel Dennett has speculated that human consciousness may be a virtualization of a parallel processing machine as a serial processing unit (computing is moving in the opposite direction). The stream of consciousness that results would be hampered if our vision were divided into two exclusive fields. Imagine Reason (talk) 18:37, 10 April 2008 (UTC)
I can; can't everyone? Edison (talk) 19:04, 10 April 2008 (UTC)
Apparently they've done the exact experiment on people with brain injuries whose two sides of the brain can't communicate. They are shown different pictures for each eye, and they end up making up stories that reconcile the differences without any conscious effort. Imagine Reason (talk) 19:33, 10 April 2008 (UTC)
Thanks to maybe misreading Edison, there's Strabismus, and I've got this pic stuck in my head for the day.[1]. Or did you mean you can walk and chew gum at the same time? Julia Rossi (talk) 00:32, 11 April 2008 (UTC)
Anyone who experiments with stereoscopic photography is likely to develop the ability to voluntarily adjust the convergence of the two eyes, as when combining two photos into a stereoscopic view. This amounts to voluntarily causing the eyes to be crosseyed or wall-eyed. Others can trick the eyes by selecting a distant object and a nearby finger: look at the distant object and you see the finger double. Look at the finger and you see the distant object double.See Ben Turpin for an example of a performer who could roll his eyes around independently. Microscopists may learn to look with one eye through the lens and with the other at apiece of paper where they are sketching what they see. A photographer/videographer may become able to look with one eye through the camera eyepiece and with the other simultaneously at the larger scene. In selective attention experiments, people have been shown able to divide the attentional field even within the view from one eye to more than one area in the visual field. Edison (talk) 14:40, 11 April 2008 (UTC)

[edit] semiconductor resistivity

What is FOUR POINT PROBE METHOD in the measurement of semiconductor resistivity? —Preceding unsigned comment added by Shraktu (talk • contribs) 17:08, 10 April 2008 (UTC) Shraktu (talk) 17:13, 10 April 2008 (UTC)

Main article: Four-terminal sensing
-- MacAddct  1984 (talk • contribs) 18:04, 10 April 2008 (UTC)
WHAAOE! —Steve Summit (talk) 00:59, 11 April 2008 (UTC)
Specifically, semiconductor test labs have a microscope-aided four-point probe for this task. This website from UC Berkeley provides theory and photos. Per WHAAOE, we should have an article with photos of this tool but I can't find any... here's an external site with photo-micrographs for a specific nano-wire application. Nimur (talk) 15:49, 11 April 2008 (UTC)

[edit] Computer Simulations of Black Holes

I've watched a few computer simulations of black holes and can't help but wonder if they're slightly misleading

As far as I know a black hole is supposed to have a gravitational pull so strong that not even light can escape so in theory if you were to actually be in the vicintiy of one you would never know it existed because all the light that is usually reflected off surfaces in order for you to see them would actually be 'sucked' into the black hole.

This makes me think that the only way you could actually tell visually that a black hole was 'nearby' would be that there would be an unusual absence of light in a particular direction.

I guess my question is are these simulations purely to allow people to learn about the path of anything approaching a black hole or is there some scientific truth to the multitude of colours displayed in these simulations? —Preceding unsigned comment added by 82.44.178.36 (talk) 20:39, 10 April 2008 (UTC)

Can't comment on the simulations you have been watching unless you give a link, but black holes are not exactly black. See our article on Hawking radiation for the reason. SpinningSpark 21:42, 10 April 2008 (UTC)

If I recall correctly, a black hole heats up nearby gases until they emit X-rays along the 'poles' of the singularity. Plus, if you see a space in the night sky where there's no light, that suggests the presence of a black hole. Correct me if I'm wrong, anyone, it's been a while since I took much interest in astronomy. Vranak (talk) 22:56, 10 April 2008 (UTC)
Black hole#Techniques for finding black holes seems pretty relevant. Someguy1221 (talk) 23:03, 10 April 2008 (UTC)
It's not that the black hole heats up gases; it's that if there is lots of matter around it will start to rub against other matter as it is pulled down into the black hole. Outside of the event horizon, what you see is the result of this friction. It's a lot of energy. See quasar. I think it's pretty neat—that something as apparently simple as friction is probably responsible for the most powerful energy sources in the universe. --Captain Ref Desk (talk) 23:20, 10 April 2008 (UTC)
Yes, that's a better description of what's happening than I gave; but the fact remains that if the black hole weren't there, the gases wouldn't be heating up. Ergo, the black hole heats up the gases. Indirectly (gravitationally). :) Vranak (talk) 15:50, 11 April 2008 (UTC)
I thought that the black spots in the night sky were caused by intergalactic dust.[won't put faith in a reading passage on an exam again.] I doubt black holes suck up all the light. Imagine Reason (talk) 23:45, 10 April 2008 (UTC)
(e/c) Outside the event horizon, the gravitational field around a black hole is just like that of any other large body of mass, like a planet or a star. Gravitational fields bend the path of light, so if there is a star on the other side of the hole to you, you will see a gravitational lensing effect. Confusing Manifestation(Say hi!) 23:04, 10 April 2008 (UTC)


it is only mass that is beyond the event horizon that is 'invisible' because the light it gives off gets sucked in. However before the event horizon matter can orbit the black hole just like a satellite orbitting a planet. And due to the strength of the block hole it would not be surprising to find large amounts of matter orbitting. I would guess the colours are meant to just be clouds of gas that are slowly orbiting and eventually going to fall through the event horizon.--Dacium (talk) 23:07, 10 April 2008 (UTC)

[edit] Clock of observer at center of the earth

Please follow my reasoning for a moment: Suppose that there was a hollow cave at the center of the earth that allowed an observer A to "float" freely. The observer would, then, not experience any acceleration due to gravity, since an equal force would act on him from all directions (shell theorem).

However, here's the question. Suppose there was another observer B positioned a considerable distance away from the earth. Relative to observer B, would he perceive A's clock to run slower than his? My intuition tells me A's clock would run slower, since even though A does not feel any force acting on him, he's still surrounded by nearby regions of intense gravity. Is my interpretation correct? Dmitry Brant (talk) 20:42, 10 April 2008 (UTC)

I don't see why. A's clock, unlike another that's floating in space orbiting Earth, isn't accelerating. If you consider the equivalence principle, there's no experiment, so long as A stays within the shell, for him or her to determine if the location is within the shell or very far away from Earth. Imagine Reason (talk) 20:58, 10 April 2008 (UTC)
The answer is yes due to gravitational time dilation. The path that you have to follow from A to B is important in this calculation. It goes through a gravitational field. Graeme Bartlett (talk) 21:27, 10 April 2008 (UTC)
Hmm. That's what I was thinking. Although the local field might have zero strength, the gravitational potential is definitely lower at A. It seems pretty obvious that a photon emitted from A will have lost energy and thus been redshifted if it reaches B. --Prestidigitator (talk) 22:28, 10 April 2008 (UTC)
Gravitational time dilation reaches its maximum at the surface of the planet (were gravity is the strongest) and falls away to zero at the center. At the centre of the earth there is no gravitational potential and time runs normally. For the guy way out in space there will be a very small gravitational potential, so his clock will run slower. If you assume he is so far away that the gravitational potential is also effectively zero, both clocks are the same and there is no dilation.--Dacium (talk) 23:00, 10 April 2008 (UTC)
You're speaking in terms of gravitational force, not gravitational potential. Gravitational potential strictly increases as one moves out from the center of the Earth. Someguy1221 (talk) 23:06, 10 April 2008 (UTC)

What would be handy here is a solution of the Einstein equations for a spherically symmetrical body. The Schwarzschild solution is only valid outside such a body. This paper describes the Tolman VII solution with a density profile of \frac{3}{8 \pi} \cdot (\frac{1}{K^2} - \frac{r^2}{\beta^2 K^2}) for some constants K and β. Substituting β with 1 or values slightly above 1 into the (moderately complex) equations in the paper clearly shows that a stationary clock is running slower at the center than at the surface. Is there something like a general theorem for General Relativity saying that for any spherically symmetrical mass distribution clocks are never running faster at lower values of the radial coordinate? Icek (talk) 01:12, 12 April 2008 (UTC)

[edit] How does this collet work?

Hi all,

I just got a second-hand Black & Decker rotary tool, like a Dremel, which is supposed to have a "universal collet system". I'm pretty sure it has all the parts. However, I'm confused about how the collet works. On the assumption that it's the small metal sleeve with a wide head, then it looks almost exactly like the one at the bottom of the image here. However, it does not have any slits in it. It's a tube and the sides are completely solid. How is it supposed to tighten with no slits in it? I've tried putting in the "nut" part of the tool and tightening the nut, and I can still easily slip the bits that came with the tool.

Am I missing something? What is the point of a collet without slits in it?

Thanks! — Sam 23:52, 10 April 2008 (UTC)

You need to knock it right up hard! See Machine_taper —Preceding unsigned comment added by 79.76.215.59 (talk) 00:31, 11 April 2008 (UTC)
I don't think so. This is a small hand-held tool the OP is talking about. Here's an alternate suggestion. On my Dremel, I can completely remove the collet-tightening nut to change collets. The collets come in a couple of different (inside) diameters. Can you remove this purported collet completely? Perhaps it's in backwards? I don't know about a "universal collet system", but I am skeptical of a collet that claimed to be able to hold more than a small range of diameters. Do you have any way to measure (calipers?) the inside diameter of the collet and the outside diameter of your toolbit? JohnAspinall (talk) 14:36, 11 April 2008 (UTC)
Yes, it's removable, and I think "universal" was referring to the whole system, not that individual collet. What I can't understand, however, is how a collet without slits in it is supposed to work, especially given that it's the one they shipped with (assuming it's the original). There';s no way I can tighten the nut hard enough to actually maker a change in the diameter of the thing. — Sam 16:45, 11 April 2008 (UTC) —Preceding unsigned comment added by 63.138.152.238 (talk)

It looks to me like a keyless chuck, (I think they're also called universal chucks). It's a bit difficult to guess how all the parts in your photo go together but i'm guessing 'universal' as with keyless chucks means you screw the parts together and all the parts are interchangable. Looking at your picture, have you tried inserting the bottom left bit (on your pic) into the bottom right bit, then screwing it onto the main dremel unit? If that does work it's just a case of using the bits on your middle right and top left screwed on top of that to fit the appropriate accessories. Probably! Hope that works. Cosmic joker (talk) 00:26, 12 April 2008 (UTC)