Wikipedia:Reference desk/Archives/Science/2011 April 16

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April 16

Question about gravity and absolute zero temperature?

As both [Einstein’s and Newtonian] models of gravity are independent of temperature therefore “Does gravity cease and clocks stop or slow down at Absolute zero temperature?” Also this might help: http://www.jupiterscientific.org/sciinfo… 68.147.41.231 (talk) 02:47, 16 April 2011 (UTC)Eclectic Eccentric Khattak#1

Seeing that we've gotten much closer to absolute zero than we have to the speed of light, with no strange things happening (EDIT: with gravity and time; some other strange things happen, for example see superconductor), I think it's safe to say "no". 72.128.95.0 (talk) 02:52, 16 April 2011 (UTC)

Your link is broken, but I'm guessing you might be indirectly referring to a Big Freeze, which is one form of a heat death of the universe scenario. In any form of a heat death of the universe scenario, there will no longer exist any kind of astronomical objects, or even any macroscopic objects, that would produce anything significantly different from a constant curvature of spacetime, so you could kind of in a limited sense say that gravity at that point has ceased. And in any heat death of the universe scenario, entropy comes arbitrarily close to its asymptotic limit (in particular, a temperature of absolute zero is approached in a Big Freeze scenario), so at that point there will be essentially no thermodynamic arrow of time, so you could kind of in a limited sense say that at that point "time has ceased". But the same laws of physics will still apply, it's just that there won't happen to be any astronomical objects around for gravity to be meaningful anymore except for describing the universe as a whole, and there will no longer be any physical processes going on such that "forward in time" looks noticeably different from "backwards in time" locally, so the second law of thermodynamics will lose its significance.

At any rate, it's not like a temperature of absolute zero causes gravity to become insignificant. If you managed to create a macroscopic object with a temperature of absolute zero, it would still produce a gravitational attraction, to use the Newtonian description. Basically, any nonzero component of the stress-energy tensor causes a curvature in spacetime, to use the general relativity description, and thermal energy is in general only a minor contributor to the stress-energy tensor, so zero thermal energy does not at all imply a zero curvature of spacetime.

And a constant temperature of absolute zero doesn't really cause time to cease, either, even if there is no longer a thermodynamic arrow of time, and even though a clock can't actually function at a temperature of absolute zero. At the very least, even in a Big Freeze scenario, there will still exist a cosmological arrow of time. Red Act (talk) 06:28, 16 April 2011 (UTC)

Here is the link "http://news.harvard.edu/gazette/1999/02.18/light.html" Sorry about the broken one. —Preceding unsigned comment added by 68.147.41.231 (talk) 04:19, 17 April 2011 (UTC)

Einsteinian gravity is not independent of temperature. Temperature is energy and therefore has mass. 83.134.138.215 (talk) 06:05, 17 April 2011 (UTC)

Oh, your question actually has nothing to do with the Big Freeze scenario; I guessed wrong. Your question is really about slow light travelling through a Bose-Einstein condensate. Slow light just involves a quantum mechanical interaction between the photons and the atoms in the medium they're travelling through, and doesn't involve time itself slowing down, or involve gravity in any unusual way. Spacetime within and around the medium through which the slow light travels is still just a normal, unremarkable inertial frame of reference as in special relativity. For example, the decay rate of any radioactive nuclei within the medium will be the same as it always is. It's just that the group velocity of light through the medium in the frequency range of interest is less than c. Red Act (talk) 07:14, 17 April 2011 (UTC)

I was just thinking about

1. Einstein’s theoretical light clock but at microscopic level [either stationary or travelling] through a state of matter of Bose-Einstein condensate [c reduced to 38 miles/hr]

2. Thermal energy or heat can’t be transferred further [can become no colder] at absolute zero temperature and thus circumlocutory it cease if not at instantaneous motion. Thus as per above comment E=mc^2; so does mass remains constant with changing in thermal energy to a minimal motion of matter through a Bose-Einstein condensate in which c to reduce to 38 miles/hr OR at absolute zero temperature? EEK#1 —Preceding unsigned comment added by 68.147.41.231 (talk) 03:49, 18 April 2011 (UTC)

c isn't reduced to 38 miles/hr. There is a fundamental physical relationship between space and time, such that 299,792,458 meters of space is equivalent, except for a change of sign in a component of the metric tensor, to 1 second of time. c is that ratio between equivalent amounts of space and time, and is always exactly 299,792,458 m/s. One of the most important and easy-to-understand ways that c shows up in nature is that it's the speed at which light travels in a vacuum, so c is commonly referred to as "the speed of light". But referring to c as just "the speed of light" comes with the implicit understanding that that name is only actually accurate macroscopically when discussing light that's traveling through a vacuum.

Although slow light is a dramatic example, you don't need an exotic phase of matter, or an extremely cold temperature, to illustrate that light travels at a speed lower than c when it's not in a vacuum. Light travels at a speed significantly lower than c when traveling through ordinary materials at ordinary temperatures. For example, at room temperature, visible light travels through water at about 0.75 c, through glass at about 0.66 c, and through air at about 0.9997 c. The inverse of those numbers is called the refractive index of the material the light is going through.

Light traveling slower than c in a medium does not affect the value of c within that medium, and hence does not affect the ways other than the macroscopic speed of light that c shows up in physics, such as in the equation E=mc².

You could do calculations involving light clocks in which there's something other than a vacuum between the mirrors, although the equations would get a little messier than in the vacuum case. It's only within a vacuum that how fast light travels is the same for all inertial frames of reference. If the light is traveling through some medium other than a vacuum, you'd have to take into account not only how the speed at which light travels varies with what the medium is, but how it also varies with how fast the medium is moving. But it'd be conceptually straight-forward; since the light is traveling at a speed less than c, you can treat the light basically the same as a normal massive object that's traveling at that lower-than-c speed. You'd just need to mess around with things like the velocity addition formula that don't come up when analyzing the simpler case of there being a vacuum between the mirrors. And in the end, you'd wind up concluding that how space and time in one inertial frame of reference relate to space and time in another inertial frame of reference doesn't depend on what medium light happens to be traveling though in that vicinity. Red Act (talk) 21:28, 18 April 2011 (UTC)

Help! I have some questions about Venus, Uranus and the moon

1. "Does the planet Uranus get hit by earthquakes? I read this off of a website: "In the course of a study of tidal effects on earthquakes2, the astronomical positions of the planets have also been taken into account and a remarkable correlation between the positions of Uranus and the moment of great earthquakes has been established for a certain period. Gutenberg and Richter's data of all earthquakes equal or greater than magnitude 7; have been used." Could someone explain this to me?

2. A long-standing mystery of Venus observations is the so-called 'ashen light', an apparent weak illumination of the dark side of the planet, seen when the planet is in the crescent phase. The first claimed observation of ashen light was made as long ago as 1643, but the existence of the illumination has never been reliably confirmed. Observers have speculated that it may result from electrical activity in the Venusian atmosphere, but it may be illusory, resulting from the physiological effect of observing a very bright crescent-shaped object." In astronomy, Venus has been called the twin sister of Earth. Both planets are about the same size. If another planet could support life, it was once thought to be Venus (of course, now it's realized that the heat and sulfur make that impossible). What association do the Moon and Venus have in common in astronomy?

3. Does the planet Venus get hit by meteor showers? I read this information of a website: Venus is a Crescent V when it is most visible twice a day and for a long period. Also during this time it becomes much easier to monitor the ever going meteor storms ranging around and into Venus. Venus is known well for being the planet most hit by Meteor Showers. Go outside before sunrise, around 5:30 a.m. is best, and look East. The brightest object in that direction is the planet Venus. It looks like a star going supernova. Above Venus lies Saturn, and below, near the horizon, is Jupiter. Every 10 minutes or so you'll see a meteor streak among these planets. The meteors are pieces of Halley's Comet.

"Every year around this time Earth glides through a cloud of dusty debris from Halley's Comet," explains Bill Cooke of the NASA Marshall Space Flight Center. "Bits of dust, most no larger than grains of sand, disintegrate in Earth's atmosphere and become shooting stars."

"It's not an intense shower," he says, "but it is a pretty one."

Astronomers call it the "Orionid meteor shower," because the meteors appear to stream out of a point (called "the radiant") in the constellation Orion. The radiant is near Orion's left shoulder. But don't stare at that spot, advises Cooke. Meteors near the radiant seem short and stubby, a result of foreshortening. Instead, look toward any dark region of the sky about 90 degrees away. The vicinity of Venus or Jupiter is good. You'll see just as many Orionids there, but they will seem longer and more dramatic." Please give me more information. Neptunekh2 (talk)

While we wait for experts to answer your first two questions (and give a better answer to the last), I can answer the bit about meteor showers on Venus. Yes, all the planets get hit by meteor showers, but we can't see these from Earth, or at least not with the naked eye. The shooting stars mentioned are meteors in Earth's atmosphere that just happen to be highly visible near to the horizon where Venus can often be seen most clearly. There is no connection between the planet Venus and the Orionids except this visibility factor. I would like to see some evidence for the correlation that you mention between the position of Uranus and the time of earthquakes. I can think of no logical reason for any causality here. Dbfirs 07:02, 16 April 2011 (UTC)

1) If they are talking about the position of Uranus causing quakes on Earth, that's absurd. At that distance there's no way it could have an effect. The Moon could contribute to quakes, due to it's proximity, and possibly the Sun, due to it's size. StuRat (talk) 08:48, 16 April 2011 (UTC)

2. The OP mentioned but didn't ask about the Ashen light of Venus. An early astronomer believed that it was from the fires from celebration of a new Venusian emperor. This purports to be a picture of it] but I can't make out the emperor nor even his clothes. Cuddlyable3 (talk) 11:57, 16 April 2011 (UTC)

To me the "ashen light" looks pretty clear there - I can quite easily see the dark side of Venus, if that's what it is, against the background. If it's due to electrical activity it must be something pretty homogeneous throughout the dark side. But Earth has all sorts of weird upper atmosphere activity that people are discovering around all the time - and Venus has more sun and more atmosphere. Wnt (talk) 01:44, 17 April 2011 (UTC)

The OP will be aware that Venus has no moon for moonshine on the surface, though one was announced in 1686 and the object 2002 VE68 is in a 1:1 orbital resonance with Venus, so is considered a quasi-moon, but only for the next 500 years or so. Dbfirs 12:04, 16 April 2011 (UTC)

As for question 3, all planets are hit by meteorites, but the OP seems to be confused as to how meteors are observed. They are strictly atmospheric phenonomea and the ones the OP has seen are certainly in the Earth's atmosphere. The apparent streaking among the planets is merely the coincidence of alignment. Astronaut (talk) 12:44, 16 April 2011 (UTC)

This is the letter you're quoting: R. Tomaschek. "Great Earthquakes and the Astronomical Positions of Uranus." Nature 184:177-178 (18 July 1959). (To see more than the abstract, one needs an appropriate subscription.) The author found a fairly strong correlation between the location (the right ascension, actually) of Uranus in the sky and the occurrence of earthquakes of magnitude 7.75 and greater, during the years 1904-1906. It is not explained why he chose that particular cutoff for earthquake strength. The author fails to discuss in any detail the rest of the earthquake record; the source he was working from contained data up to at least 1950, but all we hear is that the correlation after 1906 "becomes less significant". Hm.

Among other issues, his significance testing fails to account for the effect of multiple comparisons. If one takes a three-year window and slides it across five or six decades' worth of the earthquake record, one will find clusters of earthquakes that correlate with other totally irrelevant measures purely by coincidence. Besides cherry-picking the six or seven percent of the earthquake dataset which best fits his thesis, we also don't have any information about what other phenomena were examined. (Did he look at the right ascension of a bunch of other astronomical objects and find no correlations? Weather conditions: rainfall, snow, temperature? The stock market?) The larger the set of data he was trying to match the earthquakes against, the greater the likelihood that such a match will occur somewhere by coincidence. If we don't know how many such comparisons were made, then we can't appropriately account for this source of false significance. There is, as always, an appropriate xkcd: [1]. TenOfAllTrades(talk) 14:35, 16 April 2011 (UTC)

This response to some criticism suggests that it is the hour angle of Uranus, not the right ascension that correlated with earthquakes (with an orbital period of 84 years for Uranus, RA doesn't change very much over the course of two or three years). Which doesn't change the fact that this was bull from the start... --Wrongfilter (talk) 16:00, 16 April 2011 (UTC)

I'd say that the immediately preceding letter (a response by a legitimate mathematician to the original paper, and which Tomaschek poorly and incompletely attempts to rebut in the abstract you've linked) is probably the most illuminating comment: Burr, E.J. "Earthquakes and Uranus: Misuse of a Statistical Test of Significance" Nature 186:336-337 (23 April 1960). He makes essentially the same points that I do, but does so much more eloquently. (This isn't the first time I've been scooped at the Ref Desk, but it may be the first time I've been fifty years late.)

I will note that Tomaschek's rebuttal is silent on the most important question—that is, how many different sets of astronomical or other data did he examine in searching for correlations with earthquakes?

Tomaschek's rebuttal also asserts – in defence of his silly claim – that the apparent effect of Uranus is significant (though smaller) when the entire fifty-year earthquake record is considered. This is unsurprising, since we already know that a portion of that record contains a small region with a (likely spuriously) high correlation. Tomaschek errs in including the 1904-1906 data when he asserts that his correlation exists across all of the data; you can't reuse data when you're trying to confirm a correlation. If one only looks at the remaining data covering the years 1907-1952(?), then one finds 111 earthquakes, 24 of which occurred within one hour of Uranus being in the desired location. The expected value is 1/6 of 111, or 18.5, so there were slightly more quakes than the mean we would expect by chance. If we assume that earthquakes are instead randomly distributed in time (and follow a Poisson distribution), that expectation value carries with it a standard deviation of 4.3. Our measurement is a trifling 1.28 standard deviations from the mean, something that we would expect to occur by chance about 20% of the time.

A more subtle point is that the earthquake events listed aren't actually all independent. In Tomascheck's Table I, there are several quakes that appear to be clustered in time and space and which I would expect are quakes followed by aftershocks. Between June 25 and June 27, 1904, there are three earthquakes at (nearly) the same location in Russia; the initial temblor and first aftershock falling outside Tomaschek's Uranus rule, and only the second aftershock fitting within. Similarly, there are a pair of earthquakes in or near Mongolia in July of 1905; the first quake fails the Uranus test but the aftershock squeaks in. Should initial quakes and aftershocks be treated the same way? It's just sloppy analysis and wishful thinking. TenOfAllTrades(talk) 19:05, 16 April 2011 (UTC)

Old Balls

This came up in a question on the Miscellaneous Desk. Someone found what is probably an American Civil War cannon ball which has a diameter of 3 inches and is covered in rust. Using a table found on Field artillery in the American Civil War, it ought to weigh 9lb 8oz. My thought was that oxidation would make it a bit lighter now; was I right or just talking balls? Alansplodge (talk) 08:18, 16 April 2011 (UTC)

The rust ought to make it heavier, not lighter. Rust is iron + oxygen + water, so it's heavier than just iron. None of the original iron atoms have escaped unless the rust has flaked off. On the other hand, if by 'lighter' you meant less dense, then you would be right - rust is probably less dense than iron. --Heron (talk) 10:23, 16 April 2011 (UTC)

A common high-school or freshman-college chemistry experiment is to take a piece of metal of known mass and oxidize it. From the mass increase, students then determine how much oxygen was added, and from that, various details about the metal and its oxididation properties. DMacks (talk) 14:58, 16 April 2011 (UTC)

The rust might make it heavier if it were all there, but for an iron ball that has been sitting around in the rain since the Civil War, a good bit of rust is likely to have flaked off and washed away. Looie496 (talk) 16:22, 16 April 2011 (UTC)

I suppose that was my chain of thought, but there wasn't much scientific enquiry behind it. Thank you for your answers all. Alansplodge (talk) 16:59, 16 April 2011 (UTC)

Translational acceleration

I am looking for a definition of translational acceleration, preferably one that is easy to understand. Lova Falk talk 09:09, 16 April 2011 (UTC)

It means acceleration (getting faster) in a straight line, as opposed to rotational acceleration, which means acceleration in a circle. See Translation (physics). --Heron (talk) 10:26, 16 April 2011 (UTC)

I think the exact meaning may depend on context. In Newton–Euler equations, "translational acceleration" is used to mean the instantaneous acceleration of the centre of mass of a body. In rotation around a fixed axis it is used to mean the tangential component of the acceleration of a general element of a body. Gandalf61 (talk) 10:42, 16 April 2011 (UTC)

Thank you! Something that confuses me is that the Thesaurus in the free dictionnary says that translational is: "of or relating to uniform movement without rotation". As far as I understand, uniform movement is the same as constant movement, and in that case there cannot be any acceleration. Lova Falk talk 11:11, 16 April 2011 (UTC)

A thesaurus is a reference work that lists words grouped together according to similarity of meaning (containing synonyms and sometimes antonyms), in contrast to a dictionary, which contains definitions and pronunciations. Never rely on a thesaurus for definitions! Cuddlyable3 (talk) 11:43, 16 April 2011 (UTC)

Always nice to learn more than I asked for. Thank you! But to return to my original question, is translational acceleration basically the same thing as linear acceleration? Lova Falk talk 14:02, 16 April 2011 (UTC)

Gandalf61's answer was better than mine, so the answer to your question is no. Linear acceleration is a special case of translational acceleration in which the object follows a straight line while its speed varies. If the path is not a straight line then the acceleration is still translational but not linear. --Heron (talk) 17:15, 16 April 2011 (UTC)

Strange archive search message

Whenever I search the reference desk archives I am told You may create the page "Your search terms prefix:Wikipedia:Reference desk/Archives", but consider checking the search results below to see whether it is already covered. Surely I may not do this; maybe someone could tweak the appropriate template.--Shantavira|^{feed me} 09:42, 16 April 2011 (UTC)

You used the Search box at top right of the page, which is for searching ARTICLES. When you want to want to search the ref. desk archives, use the Search archives button (in the header on this page). Cuddlyable3 (talk) 11:37, 16 April 2011 (UTC)

But I did use the search archives box. Try it. (I meant to put this question on the Help page but never mind.)--Shantavira|^{feed me} 12:43, 16 April 2011 (UTC)

I will echo this. This is the current response from the "search archives" box at the top of the page. I'm not familiar with the reference desk, but Cuddlyable3's answer suggests that what we're seeing is not what's intended. As Shantavira points out, we should not be offered the ability to create a page called Fred prefix:Wikipedia:Reference desk/Archives (which is what is presented in response to a search for 'Fred'). -- EdJogg (talk) 14:52, 16 April 2011 (UTC)

Yes, this seems to be a bug in the archive search engine. Fortunately it still gives sensible search results. I struck out my incorrect assumption above. Cuddlyable3 (talk) 22:47, 16 April 2011 (UTC)

An archive search box is just a convenient way to produce a search with a prefix operator you could have entered manually in the normal search box. bugzilla:21102 is: "don't propose to create the page when using prefix:, intitle: or +incategory:". PrimeHunter (talk) 01:15, 17 April 2011 (UTC)

Sensations and the vocabulary to describe them

In elementary school, children are taught that humans have five senses (sight, hearing, taste, smell, and touch.) These senses convert physical stimuli into signals that go through multiple layers of processing before they eventually reach consciousness. There are other signals (feelings? sensations?) processed by the brain that are not linked to simulation of the five traditional senses, at least not clearly so. Some examples:

• hunger
• nausea
• vertigo
• the feedback that you get about the tension in a muscle
• the feeling that you experience when you are in a free fall
• the feeling that you experience when you exercise at a level that your body cannot comfortable handle
• the feeling of disgust when you think of something unpleasant

These examples are not all of the same nature, especially the last one. However, at some level they are "signals" that rise to the level of consciousness.

Is there is systematic categorization of these "signals"? If so, where can I find a categorized list of terms for referring to the different signals/feelings/sensations? —Preceding unsigned comment added by 173.49.79.68 (talk) 12:36, 16 April 2011 (UTC)

Not very systematic, but our sense article lists quite a few more than the "traditional" senses, plus a few more in the see-also list. Part of the problem is that many of these senses do not have specific names.--Shantavira|^{feed me} 14:15, 16 April 2011 (UTC)

Proprioception! Lova Falk talk 14:17, 16 April 2011 (UTC)

Most of these can be caused by vestibular system, so it could be related to sense of balance, disgust however is an emotion - you perceive something, largely with the five traditional senses, and only then realize that it is disgusting ~~Xil (talk) 17:46, 17 April 2011 (UTC)

I think qualia is the word we're looking for to describe these sensations. 50.92.126.63 (talk) 22:25, 17 April 2011 (UTC)

I was just browsing list of common misconceptions and found five senses also listed as such - according to the list depending on how it is defined humans have 9 to 20 senses, including (in adition to 5 commonly known) balance, acceleration, pain, body and limb position, hunger, thirst, pressure, temperature, itching, full stomach, need to urinate or defacate and carbon dioxide level in blood ~~Xil (talk) 00:36, 18 April 2011 (UTC)

We just had a question where it came up that oxygen is also sensed. And does "taste" or "smell" cover the 'taste' sensation that seems to come from the artery at the back of the throat, the awful smell when a few microliters of DMSO gets on the skin or the bitter 'taste' when Neosporin is put on an open wound? What do you define it as when a particularly intense shade of red seems to flicker constantly, or a pattern of blue stripes creates spurious yellow diagonals, or any other sort of optical illusion one hasn't heard of? What name would I use to call the unpleasant sensation, neither itching nor nausea nor pain, of handling an erection before puberty, or using a finger to apply suction to the navel before about 25, or getting a hair or dust up into the urethra even long into adulthood? And what would I call the dizziness produced by the emotion produced by the sight of terribly fake things (like seeing a small motorized water fountain with a hyper-colored plastic shade sold as a "tranquility fountain" in a mall, to drown out the noise that too many people have resigned to), or the entrancing, encroaching feeling of the laughter of flowers, when vegetables stand by mutely in witness to horrors? The array of nerve receptors is likely even wider than we imagine, the transitions probably continuous, their targets spread throughout many different parts of the brain. The world is full of mysterious feelings and sensations that at least English has never bothered to put a name to, and I have no idea how much of them is shared by all, or some, or few, or none. Wnt (talk) 07:09, 18 April 2011 (UTC)

What do glucocorticoids do?

My 9-year old son has an Intelliquest Quiz Book "Gross and Yucky Human Body". The Intelliquest system uses an electronic wand to check the answers of a series of multiple choice questions. For the question What do glucocorticoids do? (no 156 in the book) the correct answer is (apparently):

D) Prevent you doing your homework

My son wanted to know why this was the correct answer (the others being: Prevent bleeding, Prevent infection, Prevent swelling.)

We did have a look at the glucocorticoid article, but not being degree level chemists we could not work out an answer for ourselves. Hence we come here to consult the collective wisdom of the reference desk! -- EdJogg (talk) 14:45, 16 April 2011 (UTC)

Judging by this line in the article, "glucocorticoid turns immune activity (inflammation) down", I'd say "prevent swelling" is a better answer. SemanticMantis (talk) 15:13, 16 April 2011 (UTC)

Our article on glucocorticoids is pretty sucky when it comes to conveying useful information. I think the information the question was trying to evoke is that glucocorticoids, especially cortisol, are activated by stress, especially chronic stress, and if elevated for a long time can have a variety of negative effects on brain function. Looie496 (talk) 16:18, 16 April 2011 (UTC)

I think glucocorticoids also have a vasoconstrictive effect that may counter bleeding, but their use on a bleeding animal is apparently debatable.^[2] But I think any book entitled "Gross and Yucky Human Body" aimed at kids should not be asking such a question unless it gives the answer somewhere inside! Wnt (talk) 20:18, 16 April 2011 (UTC)

To answer Wnt, it is a rather curious way of learning. The book contains a series of multiple choice questions, and the wand (which is pre-programmed with the book number, and hence the answers) keeps track of how many the child gets right. I think the idea is that through repeated attempts to get all the questions right, the facts are memorised. One reason for giving all the book details was on the off-chance someone knew if the answers were on-line!

I would guess that Looie496's answer is the most likely (assuming that the wand has been programmed correctly!) but it is a surprising question to find in a kid's book. Thank you all for your help. -- EdJogg (talk) 22:13, 16 April 2011 (UTC)

A dumb kid would get the answer faster, but even a genius should eventually figure out the right place to tell the author to stick that wand... =) Wnt (talk) 01:47, 17 April 2011 (UTC)

Infinite re-reflection

I remember a ,probably, childhood experience about mirrors. If you stand between two mirrors and position yourself so that you can see your rear reflection reflected in the mirror in front of you, and you look closer you will see that the same reflection back and forth, smaller each time is repeated apparently ad infinitum. Not something I ever worried about very much, but I did cogitate a bit about the reflections continuing down to the micro limits of light particles/waves, and wondered if this had anything to do with infinity. Recently I saw something on a N.Geo. documentary about 'fractal geometry'. This triggered the mirror memory, and I've tried several guesses at artical headings but can't find anything that mentions the mirror thing or anything about a connection with infiniy or fractal geometry. Am I seeing something that's not there? Layman. No advanced math please.Phalcor (talk) 18:51, 16 April 2011 (UTC)

Some thoughts:

1) The reflection isn't really infinite, since mirrors don't reflect 100% of the light that hits them, so get darker each time, eventually fading to black. I wonder what the maximum number of reflections ever recorded is ?

2) I see what you mean about the fractal nature of the reflections, each being the same as the original but slightly smaller (depending on the curvature of the mirror, of course).

3) Light has a dual wave/particle nature. It's an interesting thought problem to consider when the reflection stops, under both the wave and particle models. In the case of waves, there's no reason they can't continue to reduce in magnitude infinitely, although at some point they become indistinguishable from "noise". (Think of ocean waves below a certain size being impossible to tell apart from the random splashes and ripples on the surface.) But, as for the particle nature of light, each photon either is or isn't reflected, so there should be a precise time when the last photon fails to reflect, but instead turns into heat, I suppose. StuRat (talk) 19:01, 16 April 2011 (UTC)

Essentially I think you are talking about recursion. I remember Douglas Hofstadter's book Gödel, Escher, Bach has some pictures showing and discussing the effect you describe (except using cameras and monitors) to illustrate the various optical feedback strangeness that can occur. The "In science" section of the optical feedback article is likely to be of interest to you. Sean.hoyland - talk 19:45, 16 April 2011 (UTC)

I'm not sure if an infinite regress of mirrors is a fractal - I think a fractal branches at each increasing level of detail (like tree branches or snowflakes, but infinite). Anyone want to venture an opinion on the Hausdorff dimension of the image? Wnt (talk) 20:09, 16 April 2011 (UTC)

Relevant is our short Perfect mirror article. Comet Tuttle (talk) 20:21, 16 April 2011 (UTC)

Here [3] an article that shows (with photographs) how you can generate a fractal (similar to the lakes of wada) via the reflections produced by four colored, reflecting spheres (e.g. Xmas ornamnents). Enjoy! SemanticMantis (talk) 18:05, 20 April 2011 (UTC)

Brain scan lie detection

I was recently fascinated to read this paper about modern lie detection (available here temporarily) from the U.K. Is there a review of this technology that talks about accuracy rates for incarcerated convicts, captured belligerents, and whether it is possible to discern subjects' answers to questions when they refuse to speak? How much more accurate are these techniques compared to a typical polygraph? 99.39.5.103 (talk) 19:09, 16 April 2011 (UTC)

I suspect it would have the same shortcomings as regular lie detectors:

1) People are different, and can react differently when lying or telling the truth. Hopefully each person reacts consistently when either lying or telling the truth, but there's no guarantee.

2) Probably wouldn't work on people who don't know what the truth is, like those with mental diseases.

3) Still sounds like it has the same problem as the current test, that the results are highly subjective, and different experts may read the results differently (possibly depending on who is paying them).

4) Not very good info for a jury to base decisions on. That is, even if a certain portion of the brain "glowing" absolutely meant they were lying, this isn't obvious to the jury, they just have to take the word of an "expert". For juries that distrust so-called experts, this doesn't work.

It might have the advantage of stopping one method of fooling the test, by causing pain (say by stepping on a tack), to produce stress on truthful answers. Presumably the pain area of the brain would be distinct. However, a brain scan that involves swallowing a radioactive tracer sounds like it might violate the "first, do no harm" law. Doing that to diagnose a medical problem is one thing, but doing that for no medical benefit whatsoever sounds like iffy medical ethics. StuRat (talk) 19:26, 16 April 2011 (UTC)

I have to wonder if you read the paper. It was about (2) entirely. 99.39.5.103 (talk) 22:09, 16 April 2011 (UTC)

The use of fMRI for lie detection is still in early stages of being developed, and the technique has not been validated for use in a court of law, at least in the US. (Some strange things have happened in India, as I recall.) Our article on lie detection gives a brief description of it, with references. In my view this is not likely to become a widely used technique even if it can be validated, because it is very expensive, and because the procedure is pretty unpleasant, especially for people who are claustrophobic. (You're crammed into a narrow tube surrounded on every side by a huge mass of metal, and while the scan is running the machine makes a hell of a racket.) Looie496 (talk) 19:55, 16 April 2011 (UTC)

Thanks! I was amazed to read in the article that there are already two commercial vendors and a Mythbusters segment where 1 out of 3 were able to fool the test. 99.39.5.103 (talk) 22:09, 16 April 2011 (UTC)

(ec) I would be wary of assuming all such techniques will go away. See P300 or "brain fingerprinting", which sounds intimidating - but which is aimed specifically at recognition. I can think of very sinister uses.

The data here sounds significant-ish, but it's not entirely clear to me what data that Student's t-test was done on, and p<0.05 studies come up fairly often (someone posted a cartoon about that recently...) That 74% +- 17.5% consistency (50% baseline) sounds like something to watch.

I find myself suspicious of an ulterior motive with this study. It says the sample group included one murderer and one rapist, and I can't get over the feeling that somebody was interested in some single particular truth in the study. I also wonder if their conclusion - that schizophrenics make an active effort to lie, involving "executive decision" might potentially be used to make an argument that they know what they're doing is wrong, and hence are not legally insane. I'm likely wrong about where or what, but I smell something funny here. Wnt (talk) 19:59, 16 April 2011 (UTC)

Nationwide ground stop effect

After the September 11 attacks, a nationwide ground stop was ordered at all US airports, which lasted for several days. Our article about this is at Security Control of Air Traffic and Air Navigation Aids. At one point, I read that after a couple of days of no commercial (nor GA) air traffic, there had been a small average temperature increase that was attributed to the nationwide ground stop.

Has anyone compiled other studies or analyses, of atmospheric effects or any other effects, of the nationwide ground stop? Comet Tuttle (talk) 19:32, 16 April 2011 (UTC)

There is often a temptation to wrongly infer causation from correlation. Edison (talk) 19:39, 16 April 2011 (UTC)

Sounds like the sample size might be too small to determine if there is even a real correlation. StuRat (talk) 19:43, 16 April 2011 (UTC)

Christ, you guys, I wasn't born yesterday. I'm under the impression the claimants of the above weren't, either, though I could be wrong and it could be some news reporters speculating or something. But I seem to remember that atmospheric scientists were said to be excited about getting data from this event that would be unlikely to ever recur, and a definite temperature increase attributable to the stop was said to be one of the findings. Comet Tuttle (talk) 20:19, 16 April 2011 (UTC)

I heard that they were interested in how jet exhaust contributed to cloud formation, and a few days without jets gave them a nice "control", which they lacked before. But extrapolating from that to it having an effect on the global temperature is new to me. StuRat (talk) 02:50, 17 April 2011 (UTC)

You are looking for this or this. There may be other papers too, but that was what I found first. People have definitely looked at this issue. Dragons flight (talk) 21:05, 16 April 2011 (UTC)

I'd go with the 2nd paper, which says that it's difficult to draw conclusions over such a short time period, since local weather patterns have far more effect. StuRat (talk) 03:11, 17 April 2011 (UTC)

Antarctic ice sheet

Hi. What are the values of pressure, temperature and ice density over the rock of Antarctica beneath the ice, assuming 2.2 km of ice depth and bedrock elevation of 150 metres above sea level? Thanks. ~AH1 ^(discuss!) 20:41, 16 April 2011 (UTC)

Roughly 0.92 g / cm³ and 20 MPa (200 atm). Temperature will depend strongly on local conditions (e.g. surface temperature, geothermal flux, ice convection), but somewhere between -30 C and 0 C typically. Dragons flight (talk) 22:29, 16 April 2011 (UTC)

Citation needed! If you used hand-calculations to obtain those numbers, the least you can do is explain what type of calculation you used, so we can validate the assumptions you're making! Ice dynamics in Antarctica have been very heavily studied; and, as with all things scientific, the more you study it, the more you see it deviate from the ideal! Here's a few books: The West Antarctic Ice Sheet: Behavior and Environment; and more general books, Deformation of glacial materials and The Physics of Glaciers, that should help get you pointed in the right direction.

As an example of how complicated it can be, here's a NASA brief on active lakes underneath the ice - liquid water can form, flow, and re-freeze under there! Nimur (talk) 15:43, 17 April 2011 (UTC)

3 science questions

What is the bond that holds salt molecules together with water molecules and what is the most efficient way to break these bonds leaving the water by itself? Would bombarding the nucleus with alpha particles move the element up the table of elements and would high voltage electricity have a effect on this? Could waste materials be heated up to a high enough temperature leaving all the elements of the waste by themselves then could these be filtered by their density? —Preceding unsigned comment added by 82.38.96.241 (talk) 21:39, 16 April 2011 (UTC)

I'm thinking that moving up the table would involve cold fusion. No?190.56.105.59 (talk) 23:51, 16 April 2011 (UTC)

I believe that sodium chloride has an ionic bond, but I guess that's not what you were asking. As for how to separate water and salt, heat the water until it evaporates, using the Sun, leaving behind the salt. Of course, if it's the water you're after, then other desalinization methods work better. StuRat (talk) 02:40, 17 April 2011 (UTC)

For the first question, see solvation. In the case of polar solvents (like water) dissolving charged particles (like salt), the ion-water bonding is ofter thought about in terms of dipole moments and electrostatic interactions (like the above mentioned ionic bond, but much weaker). More detailed treatment would also consider the covalent nature of the solvent-ion bond.

For your second question: One typical way to remove charged particles is through ion exchange. Distillation is another, which requires not just breaking the water-salt bonds, but also the water-water bonds. Finally, in reverse osmosis, a pressure gradient across a semipermeable membrane is used to remove dissolved substances. Distillation (particularly Multi-stage flash distillation) is currently the most used method in the desalination of water, with reverse osmosis the second preferred method. Both methods require large amounts of energy to purify water: distillation takes about 23-27 kWh/m³ of water [4], while reverse osmosis systems are probably only somewhat more energy efficient. Buddy431 (talk) 03:56, 17 April 2011 (UTC)

If this article on reverse osmosis is to believed (and the calculations at least look plausible) the theoretical maximum efficiency in getting pure water from sea water through reverse osmosis is less than 1 kWh/m³, considerably more efficient than the 23-27 used in flash distillation. However, real reverse osmosis desalinization plants apparently operate at close to 2-4 kWh/m³ [5] [6]. These values are still much more energy efficient than the distillation plants, though I think distillation plants are easier to scale up to large sizes, and are liked due to their "simplicity, robustness and inherent reliability" (over reverse osmosis plants) [7]. The total volume of desalinated water from distillation is greater than from reverse osmosis, even though there are more reverse osmosis plants, if our article is to be believed. Buddy431 (talk) 04:15, 17 April 2011 (UTC)

If you have more than one completely unrelated questions like that you should open more than one inquires and title them something more relevant than "science question" which is as vague as it gets. Dauto (talk) 13:26, 17 April 2011 (UTC)

Thanks every one and will do dauto —Preceding unsigned comment added by 82.38.96.241 (talk) 20:12, 17 April 2011 (UTC)

Freedom of navigation in the Earth's outer core

Is there any way to predict whether the Earth's outer core is free of strands of solid material? In particular, the Earth's inner core may rotate at 1 revolution per 400 years. Is there any chance that some "mountain" poking out from the inner core might ram into some crystalline strand poking in from the mantle and suddenly the whole inner core is stopped or pushed off center? If so, would this have noticeable effects for those on the surface? Wnt (talk) 22:58, 16 April 2011 (UTC)

Not sure about the Earth's core, but there is some evidence that giant impacts may have perturbed the Earth's interior, causing giant mantle plume eruptions on the opposite side of the impact point, see e.g. here.

Hmmm, the formation of antipodal craters involves the coalescence of waves travelling along the surface of the Earth which comes together. But do these also affect the deep mantle? Wnt (talk) 01:04, 17 April 2011 (UTC)

The deep mantle would have to be affected, there is no other way for the antipodal volcanic effects to be so-precisely "antipodal", millions of years after the impact. The full article can be downloaded free of charge here It is made clear that the mechanism for antipodal volcanism is not known. Perhaps an oscillation mode where the core ocillates relative to the mantle can be excited by very large impacts that puncture the Earth's crust and give rise to large impact basins. You can then imagine that the oscillation preserves the information about the precise direction from where the impact was coming. Count Iblis (talk) 14:59, 17 April 2011 (UTC)

Also if this actually occurs then it could cause not only plume erruptions but also magnetic reversals. Phalcor (talk) 23:34, 16 April 2011 (UTC)

Any body remember any periodicity info on magnetic reversals.Phalcor (talk) 23:42, 16 April 2011 (UTC)

Our article magnetic reversal talks about the frequency and timing of them. DMacks (talk) 23:48, 16 April 2011 (UTC)

I think there is persuasive modeling of magnetic reversals with a simple liquid outer core model - though the disruption I'm asking about, if plausible, could cause all sorts of weird phenomena. Wnt (talk) 01:01, 17 April 2011 (UTC)

Sounds like you're concerned about navigation. Are you planning to take a magma proof submarine down there? A little early I think caus. nobody knows yet.190.148.132.157 (talk) 03:49, 17 April 2011 (UTC)

Sorry, I meant that just in the sense of the core rotating without bumping into things. I mean, it's not like land vs. air - it's solid iron in liquid iron. I wonder how regular it really is. And likewise, so far as I know, the mantle could have some kind of projecting webwork of odd high melting point minerals filling all the space in the outer core wherever the core hasn't been sweeping them away. Of course, probably not - I just don't know if it's known. Wnt (talk) 04:42, 17 April 2011 (UTC)

I see what you're getting at. I did not intend to be flippant. Well, not very anyway. Does'nt seem to be much info coming about that. In Hawaii some magma samples were very high in He3 which is thought to come from very close to the iron core. Let's face it. It's right under our feet but absolutely inaccessible. Much more inaccessible than space.Phalcor (talk) 05:42, 17 April 2011 (UTC)

Don't forget the reason why there are no mountains higher then nine kilometers on the surface of Earth: such a mountain would just collapse under its own weight. Something similar must be true to the inner core: it has to be spherical with only very small bumps because gravity forces it to that shape. (Okay, not really sphere but somewhat squashed because of the rotation of earth, but that's irrelevant now.) – b_jonas 10:16, 17 April 2011 (UTC)

On such time and length scales, if two "rigid structures" collide, they will elastically deform and/or melt/liquify. Energy might be lost to viscous damping, but it's unlikely it could cause any sort of total halting of the bulk motion of the inner structure of the Earth. Even though we call some parts "liquid" and some parts "solid", everything down there is convecting if you consider a long enough time-scale. A quick web-search turned up Turbulence and Small Scale Dynamics in the Core and Inner Core Dynamics; as you can see, a dynamic model (read: fluid model) exists even for the so-called "solid" inner core. Nimur (talk) 15:51, 17 April 2011 (UTC)

I should thank you for those references, which are interesting. One of them come close to saying that the outer core is homogeneous and well-mixed, though not literally swearing to it, and the various plumes and currents they described didn't seem very solid. They put an upper limit to the viscosity of the core, though I wasn't exactly sure what it meant. For now I'm going to assume that the scenario I pictured, which even I thought was likely outlandish, is truly not a factor. Even someday in the future, when the Earth's core nears freezing solid, they made the good point that gravitational coupling could stop it from spinning, so no collision need actually happen. Wnt (talk) 19:49, 19 April 2011 (UTC)