Wikipedia:Reference desk/Archives/Science/2013 February 18

= February 18 =

Why didn't Earth's gravity capture the asteroid 2012 DA14?
What I'm really asking is in what approach, distance, and speed should the asteroid that passed a little bit over 17,000 miles above the earth have approached earth in order for the earth's gravity to have captured it in such a way that the asteroid would have become earth's second moon? I ask because many of the planets' moons in the solar system, like the moons of Mars and many of the small moons of the Gas Giants, are actually asteroids that were captured by the planets' gravity in the distant past. 71.98.163.183 (talk) 00:06, 18 February 2013 (UTC)


 * I don't think it's as simple as coming in on the right velocity vector. Some complex gravitational interaction involving the Moon and Earth would need to slow it down, otherwise the speed it gains "falling" towards the Earth would be enough to make it too fast to achieve orbit.  Another option is that it could skim Earth's atmosphere, and this could slow it just enough.  And captured moons seem to be relatively rare, with most moons forming in place.  Our Moon might be stranger, still, resulting from ejected material from Earth after an early giant impact on Earth.  StuRat (talk) 00:24, 18 February 2013 (UTC)


 * Mars and Jupiter have greater densities of asteroids near them then out here, in the relative outskirts of asteroid-ville, and the giant planets have a Huge! capture radius and numerous moons (Ganymede is as big and massive as Mercury) to make the necessary slowdowns. Sagittarian Milky Way (talk) 01:49, 18 February 2013 (UTC)


 * The earth didn't capture the asteroid, but it had enough impact to change its orbital period around the sun. And keep in mind that for every Deimos or Phobos there could be thousands of asteroids that came near Mars and were not captured into an orbit. ←Baseball Bugs What's up, Doc? carrots→ 11:39, 18 February 2013 (UTC)


 * And it is not surprising that it was not captured into an orbit. If you just consider the two body problem of earth + asteroid, you get Kepler orbits, i.e. if the asteroid is not in an (elliptic) orbit at the start, its path will merely get disturbed by the earth, but it will not end up in an orbit around earth. For this to happen you'd need additional forces, e.g. from the moon, sun, other planets, Asteroid impact avoidance or perhaps crashing into satellites. bamse (talk) 13:08, 18 February 2013 (UTC)


 * So, in the case of Comet Shoemaker–Levy 9, did it kind of had all the right forces when it was captured by Jupiter in the 1960s and kept orbiting around the planet until one day the comet got so close to Jupiter and its Roche limit that the comet collided with the planet in July 1994? Would Comet Shoemaker–Levy 9 have been considered to be a moon, or at least almost a moon, of Jupiter for about 30 years? It seems that Comet Shoemaker–Levy 9 was the latest known case of a large celestial body (Jupiter) capturing a smaller celestial body. 71.98.163.183 (talk) 15:35, 18 February 2013 (UTC)
 * I guess you can kind of look at that comet as a model of how a planet can capture something and turn it into a satellite, but I think that the problem was that it took 2 years to revolve around Jupiter. 131.247.244.23 (talk) 15:25, 19 February 2013 (UTC)




 * Several of the responses above are correct, but perhaps aren't making the point quite clearly enough. The default, when a body approaches a much larger body, is a hyperbolic trajectory, as illustrated in the figure -- the smaller body exits moving in a different direction, but just as fast as it came in.  Very special and very unusual things have to happen in order for capture to occur. Looie496 (talk) 17:51, 18 February 2013 (UTC)
 * The OP asked why the earth didn't capture it, so maybe flip that around: What would have to happen in order for an approaching asteroid to be captured by earth's gravity and become a second "moon"? ←Baseball Bugs What's up, Doc? carrots→ 20:54, 18 February 2013 (UTC)
 * But wouldn't the earth's gravitational pull be stronger than that of the sun's gravitational pull where asteroid 2012 DA14 was at its closest point to earth? Or was the gravitational pull too weak for the asteroid to be captured by earth for orbit, and why? And what was the case with comet Shoemaker–Levy orbiting Jupiter for about 20 or 30 years? P.S: There were 2 edit conflicts in a row. 71.98.163.183 (talk) 23:29, 19 February 2013 (UTC)

"Cannon fire" following meteorite impacts
Looking through reports of the 2013 Russian meteor event (we really need a better title), I've come across reports of sounds said to resemble "artillery bombardment" following the explosion. This seems to match up with similar reports from the Tunguska event, including:"After 1.5–2 minutes ... six more thumps were heard, like cannon firing, but individual, loud and accompanied by tremors."and"After that such noise came, as if rocks were falling or cannons were firing, the earth shook, and when I was on the ground, I pressed my head down, fearing rocks would smash it."The first thought that comes to my mind is that the sounds could have been caused by larger fragments of the meteorite impacting the ground, but this seems unlikely since no fragments of the Tunguska object have ever been found, and so far no large pieces of KEF-2013 have been found either (at least as far as I know). Does anyone have a good guess as to what might cause this effect? Evanh2008 (talk&#124;contribs) 01:15, 18 February 2013 (UTC)


 * The process of it breaking up can be quite explosive. StuRat (talk) 01:18, 18 February 2013 (UTC)


 * Yes, but this is noise following the main explosion, by at least several minutes. Evanh2008 (talk&#124;contribs) 01:25, 18 February 2013 (UTC)


 * Watch the video of the shuttle breaking up. It happened in several stages. StuRat (talk) 01:47, 18 February 2013 (UTC)
 * The flashes from exploding meteors always travel at the speed of light while the radiated heat from the explosions travel at similar speeds. The reason why the noise is heard several minutes later is due to the fact that noise travels at the speed of sound. Therefore a noticeable delay occurs before the sound reaches the observer. 1.178.33.170 (talk) 01:54, 18 February 2013 (UTC)
 * I think you're still failing to understand that the sound I am describing is not the primary explosion of the object. I am well aware of the difference between the speed of light and the speed of sound. These are noises following the fireball and the associated sound. They may be reflections of that sound, but it is not simply a matter of delay. Evanh2008 (talk&#124;contribs) 02:03, 18 February 2013 (UTC)


 * The 'primary explosion' of the object was never accompanied by a fireball. In some videos you can clearly make out two meteors traveling side by side, one slightly overtaking the other long before the fireball erupted. Even the smoke trail itself shows two very distinct paths that lay side by side in almost perfect symmetry. This indicates that the meteor impacted the upper atmosphere first and exploded (broke apart) before it started heating up during re-entry. Therefore it seems likely that this initial 'boom' we hear was never actually accompanied by an explosion. There are also two distinct groups of sonic booms that seem to correspond closely to the initial eruption of the fireball followed by a final, very loud sonic boom that seems to correspond to the second, smaller eruption as the meteor breaks apart into several smaller pieces. The hundreds of smaller pops and crackles could very well have been caused by the hundreds (some say 'thousands') of tiny pieces still traveling along at supersonic speeds peppering the ground. Much of this lingering noise would have been accompanied by acoustic reflections bouncing off the Ural Mountains directly underneath the path of the meteor. The mountains themselves would have acted as a natural sounding board. 1.178.161.116 (talk) 05:26, 19 February 2013 (UTC)


 * I heard what you are talking about on the video. One animation that I saw had it breaking into one big piece followed by five smaller ones, but I don't know if that is accurate.  The smaller booms could have been from the smaller pieces.  Just my guess.  Bubba73 You talkin' to me? 02:13, 18 February 2013 (UTC)


 * It could have been explosions of bits separated by the main explosion, but more likely the subsequent bangs were sound reflections. I have a book describing a test nuclear explosion in the South Australian desert.  At distances of 200 - 300 km or or so, three bangs were heard - the primary shockwave travelling outward at ground level first, then a boom that was due to reflection of sound from the upper atmosphere (ie longer path), then a rumble from reflections off prominent ground features.  A high altitude explosion could be expected to produce a degree of reverberation from reflection from the ground and high altitude velocity of propagation changes.  Air temperature, and thus velocity of sound, reduces with increasing altitude until a certain height, whereupon temperature increases again.  Wickwack 124.182.151.79 (talk) 01:56, 18 February 2013 (UTC)
 * Ah, that's very interesting. I find the sound reflection idea much more plausible than my idea of fragment impacts. Thanks! Evanh2008 (talk&#124;contribs) 02:03, 18 February 2013 (UTC)
 * And the 30,000,000 tons TNT of the Tunguska event is alot of boom, even spread out. That is the size of 15,000 atomic bombs or millions of Oklahoma Cities. Sagittarian Milky Way (talk) 02:01, 18 February 2013 (UTC)

Every time I have tried to type here got an edit conflict :) so here is a short Q: Why do all the meteor videos start with a bang? ie there is no lead up sound, no build up to the bang, just silence then boom, then diminishing fireworks for a minute or so. Possibly because it broke apart almost immediately - two trails appear almost right from the start. Interested in any thoughts. Zeb253 (talk) 02:04, 18 February 2013 (UTC)
 * Because the film editor thought it had more impoect if it started with a good bang? Wickwack120.145.150.250 (talk) 02:14, 18 February 2013 (UTC)
 * Here is one video of the sound I am talking about - 30 seconds of near silence followed by the big bang and fireworks - no lead up to the big bang. http://www.youtube.com/watch?feature=player_embedded&v=MKx97csfPy0# - possibly part is the mics adjusting the sound level but still it does start suddenly but not end suddenly. Zeb253 (talk) 02:20, 18 February 2013 (UTC)

Evanh2008 - There is certainly a big bang at the start of the audio - my guess for the canon fire and fireworks after the bang is a combo - the meteor broke up in explosive stages (perhaps one big one and many more subsequent ones as the pieces got smaller and smaller), and of course it happened at such a speed that sonicboom/shockwave add to the noise, plus there will be some echo too. Zeb253 (talk) 02:10, 18 February 2013 (UTC)


 * Our Russian meteor article tells us that it was "[m]oving at a speed of 15 km/s to 18 km/s (34,000 mph to 40,000 mph)". If this speed is correct, it is about Mach 44 (assuming speed of sound at sea-level and 20&deg;C and thus 761 mph). Our sonic boom article explains that "[s]everal smaller shock waves can, and usually do, form at other points on the [ aircraft meteor]". Hence the multiple sounds heard. In addition, that article also says "[as] those who have witnessed landings of space shuttles have heard, when the aircraft is nearby the sonic boom is a sharper 'bang' or 'crack'. The sound is much like the 'aerial bombs' used at firework displays" -- Senra (talk) 14:10, 18 February 2013 (UTC)


 * Don't forget, it was traveling at about Mach 45, whereas the sounds that it was making traveled at Mach 1.  So, all of the noise that it made along it's path would take a while to "catch up" and reach observers who were near the main explosion later in the path. North8000 (talk) 17:45, 18 February 2013 (UTC)
 * According to NASA:Russia Meteor Not Linked to Asteroid Flyby 02.15.13: "The meteor entered the atmosphere at about 40,000 mph (18 kilometers per second)", "its estimated mass [is] 10,000 tons", "... the event, from atmospheric entry to the meteor's airborne disintegration took 32.5 seconds" and "Based on the duration of the event, it was a very shallow entry". I cannot calculate its terminal velocity but it would be considerably less that Mach 44 --  Senra (talk) 21:08, 18 February 2013 (UTC)


 * I wouldn't want to original-research this one without having at least five tapes with GPS coordinates for the location each one was taken and accompanying video so that they can be precisely timed based on the point where the trail turns from very wide to quite narrow. But maybe somebody will post that. :) Wnt (talk) 20:01, 18 February 2013 (UTC)

Historical accounts of past meteor events report the same phenomenon. A good example is the meteorite fall that occurred near Hatford, then in Berkshire (now in Oxfordshire), in England on 9 April 1628. Accounts of that are given here and here. The first link is a contemporary account as later published in Chambers's Papers for the People (1850) - not sure when it was first published. The second link (a pdf from an Oxfordshire museum) gives what it says are the only two known contemporary accounts of the Hatford meteorite fall. What I'm quoting here is from the museum's quote from that anonymous 1628 account (the wording in the Chambers's Papers for the People version is different and includes other details): "'In an instant was heard first a hideous rumbling in the air, and presently after followed a strange and fearful peal of thunder; it maintained the fashion of a fought battle. It began thus: first, for an onset, went off one great cannon as it were of thunder alone, like a warning-piece to the rest that were to follow. Then, a little while after, was heard a second; until the number of 20 were discharged, or thereabout. In some little distance of time after this, was audibly heard the sound of a drum, beating a retreat. Amongst all these angry peals shot off from heaven, at the end of the report of every crack, a hissing noise made way through the air, not unlike the flying of bullets from the mouth of great ordnance, and by judgement were thunderbolts; for one of them was seen by many people to fall at a place called Bawlkin Green...' - anonymous leaflet on the Hatford meteorite fall of 1628" That all sounds very much like what was observed and heard for this meteorite fall in 2013. Carcharoth (talk) 20:26, 18 February 2013 (UTC)


 * Very interesting indeed, thank you. I note that on this video you can hear the multiple explosions persisting for at least 90 seconds after the initial detonation. The physics of the sound are probably going to keep some researcher busy for a long time... Prioryman (talk) 20:46, 18 February 2013 (UTC)

tempature of LED
On the wikipedia page for Light-emitting diode (LED) it states as one of the advantages, "Cool light: In contrast to most light sources, LEDs radiate very little heat in the form of IR that can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED." My question is what does very little heat mean? How hot does the inside get? — Preceding unsigned comment added by 69.115.221.5 (talk) 03:27, 18 February 2013 (UTC)


 * While the total heat generated is low, each LED is tiny, so the heat inside it might be more than you'd think. StuRat (talk) 05:19, 18 February 2013 (UTC)


 * An incandescent globe operates essentailly as a black body radiator. The filament is heated by the electric current to a white heat temperature - 3800 deg C or higher.  In consequence, most of the energy is emitted as infra red i.e., heat, and only a small fraction of the energy is emitted as visible light.  In contrast, LED's operate completely differently, emitting visible light photons as electron- hole pairs collapse.  Heat in a LED is only generatted by the current floowing through the electrical resistance and is not an essential part of the light emitting process.  Hence the fraction of total energy emitted as light in a LED is very high compared to incandescents.
 * LEDs must be operated at an internal temperature less than the limit imposed by the chemically doped crystal structure - usually taken as 200 deg C. However, the reliability of LEDs, as with any semiconductor device roughly doubles for each 10 C drop in temperature.  For this reason, LEDS should generally be opereted at an internal temperature of around 55 C or less.
 * Keit 120.145.52.14 (talk) —Preceding undated comment added 11:25, 18 February 2013 (UTC)
 * While I don't disagree with the majority of Keit's post, I'd note that given the demands of lighting systems, particularly replacement bulbs for existing fixture, many LED manufacturers seem to be coming round to a higher limit usually 85 degrees C maximum for the internal junction temperature under normal operating conditions. Cree bins or rates most of their LEDs at this temperature, as does Philips/Lumileds (which they call real world operating conditions) and actually I believe most reliable manufacturers now do so, rather then the ridiculous 25 degrees C they used to use. You can also see this Osram document which also recommends a similar thing  and interesting enough the Indian Railways seems to have specific requirements at 85 degrees C . This does have negative effects on lumen maintenance and lifespan in general, as well as in performance but as I said, thermal management is not always easy, particularly when used in replacement style bulb. (And of course once it's binned at that temperature, if it meets your requirements the fact it could be better if cooler is not necessarily a big concern. Of course since the colour tends to change and the changing performance means the brightness changes, you do want to have a good idea of what the junction temperature is likely to be.) The absolute maximum BTW tends to be in the 120 - 150 degrees C range. You may also be interested in Thermal management of high-power LEDs. Nil Einne (talk) 12:28, 18 February 2013 (UTC)

Water heater producing hydrogen
From the instruction manual of my washing machine: WARNING: Under certain conditions, hydrogen gas may be produced in a hot water system that has not been used for 2 weeks or more. HYDROGEN GAS IS EXPLOSIVE. If the hot water system has not been used for such a period, before using the washer, turn on all hot water faucets and let the water flow from each for several minutes. This will release any accumulated hydrogen gas. Hydrogen gas is flammable; do not smoke or use an open flame during this time. How would an unused "hot water system" produce hydrogen gas, and is this a real hazard, or is it something that happened to somebody once so the lawyers now insist on a warning? --Carnildo (talk) 04:23, 18 February 2013 (UTC)


 * When two different metals are connected by a conductor, an electrochemical reaction occurs which will corrode one of the metals. In the case of a steel water heater tank and copper pipes, it is the steel that corrodes. To slow this process down, water heaters have a replaceable sacrificial anode rod that corrodes instead of the steel. High end water heaters often have two rods.


 * The most common rod material is magnesium. Magnesium sacrificial anodes have the advantages of not dropping debris into the tank and not expanding when they corrode. However, they can release hydrogen gas as they corrode. How much depends a lot of the chemistry of your water. Of course the warnings are for the worst case. When the water contains sulfur, bacteria can grow that produces hydrogen sulfide gas (that's the one with the "rotten egg" odor).


 * Aluminum anodes also prevent corrosion and do not produce hydrogen gas, but they expand as they corrode, often becoming larger than the opening they were inserted through. Also, debris from the aluminum anode drops into the bottom of the tank and can clog the hot water system. Also, decide for yourself whether you buy the stories about aluminum in the diet and Alzheimer's.


 * Zinc rods are really aluminum rods with a section of zinc The zinc lessens the odor of sulfur within the water. Otherwise they act kike any other aluminum rod.


 * If you have a bacteria problem, the first step is to set the temperature on your water heater to high for three days. Be careful not to scald yourself when using the faucet or shower. Another effective way to fight the bacteria is to oxygenate the water with hydrogen peroxide. Anaerobic bacteria thrive where there is little or no oxygen, and the hydrogen peroxide creates an oxygenated environment. One pint of hydrogen peroxide per forty gallons of tank capacity should be enough. Keep open flames away just in case there is some oxygen buildup. Run all the hot water taps until they run hot and then turn them off Let the water stand for two to four hours then run each hot water tap for about five minutes to flush the peroxide. Sometimes you have to repeat this several times until you kill all the bacteria. --Guy Macon (talk) 05:22, 18 February 2013 (UTC)


 * (ec) Galvanic action between different metals can set up a slight electric field, which might decompose water, by electrolysis, into hydrogen and oxygen. I would expect a microscopic amount, but, if it accumulates, it might add up to something substantial. StuRat (talk) 05:25, 18 February 2013 (UTC)


 * It's not electrolysis, which turns water into hydrogen and oxygen. It is anaerobic galvanic corrosion, which turns water and magnesium into hydrogen and magnesium oxide. This process is used in industry to create bulk hydrogen without the high electricity costs of electrolysis. Basically, you put magnesium in a 100% steam atmosphere and set it on fire as the steam flows past. As it burns it creates magnesium oxide and hydrogen gas. (Steam works better than water, because it avoids magnesium hydroxide forming). --Guy Macon (talk) 07:19, 18 February 2013 (UTC)


 * Why not use same metal in the whole hot water system? Electron9 (talk) 08:51, 18 February 2013 (UTC)


 * Most water heaters are steel to keep the price down, but there are copper water heaters that would match copper pipes. The problem is that a household water system needs to connect with the rest of the world, and re-plumbing your entire city with copper is impractical, so you would still have galvanic corrosion where your copper pipes connect to the water main. The answer to that would be a sacrificial anode somewhere in the system.


 * Plastic pipes are an interesting variant; you would still get an electric current between, say, a copper faucet and a steel water tank connected by a long plastic pipe, but the long column of water would have significant resistance, thus lowering the current and the corrosion. --Guy Macon (talk) 14:15, 18 February 2013 (UTC)

The girl with an IQ of 161
I've never understood what an IQ actually means. Using this girl as an example, is she able to read this entire reference desk page and understand everything everyone is discussing? I'm assuming she still has to "learn", but maybe her capacity for retaining and recalling information is advantageously high? Why is a high IQ usually equated with science and math, anyway? Can someone have a (technically) high IQ, but have no interest or even comprehension of higher and complex sciences and mathematics on paper, but be brilliant at something else? Mozart probably had a high IQ, I'm guessing? He could absorb music and spit it back out, but if he were alive today, would he have also been designing rockets on the side? Do polymaths have high IQs? What about people who were extremely good at one thing and one thing only, like someone who has an uncanny ability to learn and speak languages fluently, but more or less isn't good at anything else? – Kerαu noςco pia ◁ gala xies 08:34, 18 February 2013 (UTC)
 * Oh, and how much of IQ do you think is affected by impatience? – Kerαu noςco pia ◁ gala xies 08:34, 18 February 2013 (UTC)


 * See g factor (psychometrics) for the hypothesized quantity that IQ is supposed to measure. The extent to which such a thing exists is hotly debated (you'll probably get a lot of responses that say there's no such thing, because, apparently, that's what the "good" people are supposed to think ).  Even if there is such a quantity, of course, IQ, like any test, is an imperfect measure of it. --Trovatore (talk) 08:38, 18 February 2013 (UTC)


 * I happen to believe IQ tests are laughable metrics, but it has less to do with any moral judgement on equality and more to do with coming from a background in the cognitive sciences and knowing the difficulties involved in quantifying mental phenomena (though in the case of IQ tests I daresay anyone with common sense ought to be perplexed out how subjective weighting of different mental tasks is supposed to yield an absolute) and in knowing the well-documented origin of such tests in pseudo-science and general quackery (usually of the highly racist variety). This is one subject I typically can't help weighing in on -- the disconnect between the actual empirical study of the mind by those in a field of relevance and the perception amongst people who are not (even those who are undoubtedly are extremely gifted) can be pretty profound and fascinating -- but my time is limited just now, so it will have to wait until tomorrow.  In the meantime, to anyone interested in the subject, an excellent primer is Stephen Jay Gould's The Mismeasure of Man, which looks in detail at the methodology of the early IQ tests in the context of the broader trend towards ridiculous psychometrics of the time, including craniology and so-forth.  Of course, IQ tests have come a long way in trying to eliminate biasing elements with regard to social factors, but they haven't made much progress in terms of improving upon the general subjectivity in their basic assumptions and priorities between mental tasks, which is why they are of little interest to anyone doing genuinely empirical work in exploring the brain's (or the mind's) capabilities. Snow (talk) 10:17, 18 February 2013 (UTC)
 * Gould is exactly one of the ones I had in mind with my "good people" comment. I got a very strong impression, when reading Mismeasure, that while he certainly made valid criticisms of the scientific objectivity of the ones he criticized, his own was equally suspect. --Trovatore (talk) 10:21, 18 February 2013 (UTC)
 * It's an interesting book for him; most of his other supposedly broad-audience works I've found to be pretty dry. He obviously felt impassioned about this subject.  But for all of that, I don't know that I ever felt his objectivity was flawed in that particular work.  He certainly busts some balls, but it's hard to find fault with his arguments, as well documented as he makes them. Snow (talk) 10:26, 18 February 2013 (UTC)
 * To me it comes across as a straight polemic. I cannot claim to have studied the topic in any depth, but I find it very difficult to trust the objectivity of a worker who writes polemics on the subject he studies.  His argument that g is nothing but principal component analysis is particularly facile in my view. --Trovatore (talk) 10:30, 18 February 2013 (UTC)
 * Hold that thought, back to discuss this in greater detail tomorrow. (All in the interest of more fully answering the OP's inquiry, of course! :) Snow (talk) 10:36, 18 February 2013 (UTC)


 * (Trovatore, you just committed the unspeakable crime of having an unmatched tag.) The g factor, which IQ tests are designed to measure, is the "average" of a wide variety of abilities that make up intelligence. As an analogy, suppose a company wanted to evaluate how good a potential employee is.  It might consider a wide variety of factors that affect "goodness".  How much experience does he have?  How patient is he? How logical is he? Does he have good people skills? Motivation? Passion? Flexibility with work hours? All of these factors are part of his goodness, but someone with very high goodness doesn't necessarily score highly in every aspect, and someone with very low goodness could score highly on a few aspects.  The same is true with IQ tests.  Getting a high score is far easier if you're good at everything, but you could be absolutely hopeless at a few things and still score highly if you're a genius at everything else.  --140.180.243.51 (talk) 09:19, 18 February 2013 (UTC)


 * IQ is the attribute measured by IQ tests. HiLo48 (talk) 10:23, 18 February 2013 (UTC)


 * In much the same way ghosts are the things pursued by ghost hunters. ;) Snow (talk) 10:28, 18 February 2013 (UTC)


 * Yes, an excellent analogy. HiLo48 (talk) 10:35, 18 February 2013 (UTC)
 * Except, IQ strongly correlates to performance on a wide range of other skill-tests, from rifle marksmanship to average salary over the person's entire career. In this August 2010 discussion, I attempted to refute the point that IQ is meaningless.  I will concede that IQ is not the only factor in human cognitive performance, but it is a reliable and repeatably-measurable metric.  The accuracy and precision and repeatability of modern tests are all surprisingly high.  And when judging people in the statistical aggregate, which is often necessary for large organizations, it can efficiently summarize other performance metrics.  If misused or misinterpreted, the data-point can lead to very invalid conclusions.  But it's not fair to say that IQ is pseudoscience.  On the whole, it is a measurable, quantifiable, repeatable metric.  Any conclusions drawn from this metric are falsifiable and can be substantiated by experiment.  Nimur (talk) 13:25, 18 February 2013 (UTC)
 * Nimur, do you have citation for the IQ-marksmanship correlation? I'd like to read more about it. It's not too surprising, since IQ is correlated with height.Dncsky (talk) 14:39, 18 February 2013 (UTC)
 * Yes, in the discussion I linked above, or directly linked here: Whole and Part Methods in Learning a Perceptual Motor Skill (1954). The study compared some three hundred infantrymen at Fort Knox; trained some in marksmanship using Method A and others using Method B; and analyzed the results.  A secondary analysis of the same data treated the the training method a a null factor, and instead compared the marksmanship results against each man's previously-measured intellectual aptitude.   And finally, they analyze whether higher-intelligence riflemen learn better using Method A or B.  There is no reason to slander the intellectual aptitude of "army guys."  Completion of an ASVAB (part of which is essentially an IQ test) is mandatory for most recruits to most branches of the armed services in the United States; and the intellectual entrance requirements for many career military tracks are as stringent as those for some universities.  At the very least, the armed forces have standardized empirical data, which can be used to objectively analyze their recruits: many universities do not even have such data, or rely on a third-party corporation to provide it.  Nimur (talk) 15:01, 18 February 2013 (UTC)
 * What a jolly good read, thanks. I have retracted the joke; it was made in poor taste and inappropriate for the RD. A certain Australian must be rubbing off on me. For the record, I will soon be one of the "army guys" and the joke was intended as self-deprecating humor.Dncsky (talk) 15:10, 18 February 2013 (UTC)
 * Yeah, but consider an anologous Athletic Quotient, which is measured by a test which inncludes things like a 100 yard dash, a bench press, ability to catch a fastball, triathlon, etc. I think the average person would see that there is some validity in such an overall measurement, but would realize that it's really looking at a wide variety of individual skills, which are only slightly related; if you have some sort of neuromuscular disorder, for instance, you'd do badly overall, but marathon running and weight lifting skills probably aren't that tightly correlated. Same for IQ; if you brain is defective, you do badly overall, but there's no real evidence that there is a specific function used in visualizing what the object in a picture would look like if rotated and also used in reading comprehension. Gzuckier (talk) 14:57, 18 February 2013 (UTC)
 * As I mentioned above, IQ is measurable and tests can be designed that stand up to scientific scrutiny. That does not equate to "any conclusion about IQ is immediately true."  There are many hypotheses about IQ that can be tested and proven false.  If your hypothesis is that IQ correlates (or does not correlate) to performance on spatial reasoning tasks, then I am not sure about the answer; but I am sure there are experiments that have tested that hypothesis and published their findings.  Nimur (talk) 15:01, 18 February 2013 (UTC)


 * The issue with IQ tests is that they are statistically valid without necessarily being individually deterministic. That is, IQ tests are valid when averaged across wide swaths of population: for enough people, they seem to be roughly useful predictors of certain aptitudes and skills they may have.  If you average out IQ results over many millions of people, then the outliers get averaged out.  That is, there are going to be some people for whom the IQ test is woefully wrong in terms of acting as a predictor of anything.  These people are common enough that, anecdotally, enough people will know someone like this, to make it seem like the IQ test is worthless.  And for that person, it is.  However, when that person's results are averaged into "all of humanity", it doesn't make much of a difference at all.  So, are IQ tests useful?  Yes, except when they aren't.  -- Jayron  32  16:24, 18 February 2013 (UTC)
 * As a random comment, not an answer, I would suggest that the layman's terms of "dull" and "bright" are about as useful. IQ measures something, but just what is hard to say.  If you do a chemical reaction and the mixture turns some color, sometimes you can surmise something, but it doesn't really tell you much for sure, and that is about what IQ tells you.  It is a single scalar measurement of a very complex matrix of many kinds of data. Wnt (talk) 19:57, 18 February 2013 (UTC)
 * Wnt just neatly and concisely said what may very well have taken me (is about to take me?) several paragraphs to voice.  It's a bit of a case of not seeing the forest (the concepts in this case) for the tress (the raw mathematical data and various trends found therein).  Nimur makes some decent arguments above as to why the field cannot be said to be completely without structure, but statistical self-consistency is not the only measure by which the extraordinary claims of this field must be analyzed.   That people or sample populations can repeat similar testing scores under similar conditions is not in itself surprising.  What said results actually say about the nature of intelligence and where people rank in some abstract spectrum is another matter entirely.  Though the specific claims that are made may vary some between tests, they mostly share in common an assumption that "this is a reasonable measure of some form of general and/or practical intellect." -- an assumption that is just not established by any line of reasoning that doesn't rely heavily on subjective assesments.  Oh, look at that, managed to keep it to one. :) Snow (talk) 00:42, 19 February 2013 (UTC)


 * In 1967 I applied for a job with the Australian Government Public Service. At that time, entry was by sitting a 3-part exam.  When I sat it, there were about 1500 applicants for 120 jobs.  The exam results were ranked 1 thru to 1500, 1 = highest score, 1500 = lowest score, and they sent job offer letters to the folk with rankings 1 thru 140, knowing from experience that about 20 would not take up the job offer.  No interviews were conducted.  The three parts to the exams were a) Mathematics, pitched at about 1st year high school level, b) General knowledge: multi-choice - typical questions were things like From what direction does the sun rise in the morning? East, West, North, South;  Is the differential in a motor vehicle connected to - engine, radiator, gearbox, prop-shaft; c) A raven's style IQ test.  I should think that every one would have done well in Part A and B, so the IQ test was probably the decider.  I was ranked 37, so I got a job.  They sent a list of all 140 successful applicants, with their ranking, to each succesful applicant.  I still have my copy, and I now have known most of the succesful applicants for 46 years.  Some turned out a dead loss.  Some are now top leaders in major businesses.  So how does the ranking correlate with the career success?  Answer - no correlation AT ALL!  And nothing I have experienced in my career ever since has indicated to me that IQ and aptitude testing has any value at all. Ratbone 124.178.182.45 (talk) 00:21, 19 February 2013 (UTC)
 * I don't know how you compared the success of those selected against that of the ones not selected. For quite a few jobs doing an interview after selecting candidates that way actually reduces the number of good outcomes compared to not doing any interviews so I wouldn't dismiss it so quickly. There's no 100% successful way to select people to employ - and in fact even selecting only the most suitable is often a wrong thing to do as one wants a mix with some people willing to do the nitpicky stuff and others to have grand ideas and others who just hold a team together properly. Dmcq (talk) 13:06, 19 February 2013 (UTC)
 * If you read carefully what I said, you'll see that where I said there was no correlation between ranking and carrer success, I was referring to the subset of candidates who received job offers - these are the people I got to know as work colleagues and friends. Having said that, I do know three people, who were schoolmates, who were not given job offers (and were ranked further down on the list), and all three have had careers that indicate they should have being given an offer.  One was ranked at about 1000 as I recall.  One is a senior police detective, one is head of a plumbing firm emplying hundeds of people, and one became a grocery store manager.
 * In my careeer, which includes leaving govt service after 10 years and becoming a professional engineer and later becoming a manager, I have plenty of experience of working with people selected by interview conducted by others, people selected by interview conducted by myself, and plenty of experience where selection was based on interview backed up by IQ/aptitude testing. Interviewing is certainly not perfect, and a lot of managers/selectors don't know how to run interviews.  Never-the-less, interviewing is far far better than any other method.   When I interview candidates for jobs, I try and create a relaxed atmosphere and put the guy/girl at ease - pretty much have a friendly conversation, getting them to tell me what they've done, problems they've solved, things they are proud of. People they admire and why.  I'll ask them why they applied for the job. At some stage, I'll switch and put them under pressure, hit them with questions to see how they think.  Then I'll back off.  After the formal interview, I'll walk them around the plant, unless they are hopeless.  If they are good, I'll find some excuse to talk to one of the technicians, and get the candidate involved in the conversation, which may range from where is a good place to buy pizza to whatever the tech is working on.  I seldom make a mistake on who to hire.  I worked for one company that had a policy of backing up interviews with IQ/aptitude testing done by an external consultant.  The consultant provided a detailed report based on raven testing and another IQ test that I found out was supposed to screen high school students for higher study.  These detailed reports never got it right, and fortunately we ignored them. Ratbone 124.182.51.4 (talk) 01:29, 20 February 2013 (UTC)


 * As to IQ it is designed as a measure of mostly problem solving ability and as people said above doesn't measure a number of important factors like being able to get on with others. Intelligence tends to be a bit more integrated than athletic ability, but even so whilst it is true that a top class runner won't shine against a top class weightlifter but compared to your average man on the street they probably still would wipe the floor with them at most things athletic. IQ can be thought of as an equivalent of measuring general fitness by seeing how people get on with a range of athletic tasks. Dmcq (talk) 13:06, 19 February 2013 (UTC)
 * Not really. It's more like trying to measure athletic fitness by measuring things like how much weight you can lift by bending your big toe, or how much air you can pump by fluttering your eyelashes.  There are IQ tests and there are IQ tests, Stanford Binet and Raven's Progressive Matrices being probably the most well known and most used.  SI has perhaps some merit, not much.  Raven's just tests your ability to pick visual patterns and has no bearing on anything in the real world at all, not withstanding what some psychologists would have you believe.  And it is amazing what rot some psychologists read into Raven scores.  Ratbone 58.170.151.23 (talk) 12:19, 21 February 2013 (UTC)

Disease transmission from sharing a cup?
What is the likelihood of contracting a disease, illness or infection from the sip of a shared cup at a social gathering in church? 140.254.226.238 (talk) 15:20, 18 February 2013 (UTC)


 * Yes, I recognize that some churches individualize cups. 140.254.226.238 (talk) 15:24, 18 February 2013 (UTC)


 * I'm curious about this too, always seemed instinctively to be a really bad idea - perhaps some sort of alcohol content helps kill the germs, hopefully someone can find us a study --nonsense ferret  17:01, 18 February 2013 (UTC)


 * I don't think the alcohol in church wine is sufficient for sterilization, you'd need something more like vodka. Also, as I note below, the rim may carry the microorganisms, and not have any wine on it. StuRat (talk) 17:09, 18 February 2013 (UTC)


 * Do you think churches should use something like vodka or individualize cups instead? 140.254.226.238 (talk) 17:12, 18 February 2013 (UTC)


 * Individual cups, yes. Straight vodka, no.  That would burn people's throats. (Enough alcohol to kill bacteria cells also kills the cells lining your throat.) StuRat (talk) 17:19, 18 February 2013 (UTC)


 * Well, if you can distill wine and count it as valid for sacramental use when it condenses, perhaps one could merely boil the alcohol and the congregation could take inspiration from the ... :) Wnt (talk) 19:50, 18 February 2013 (UTC)


 * Well, quite a few diseases can be transmitted by saliva, especially if the recipient has a cold sore or chapped lips, allowing the bacteria to enter thru a crack in the skin. The  previous person to use the cup might have had similar lip problems, and have bled on the cup rim.  Exposure to air quickly kills many microorganisms, but the time between sips may not be long enough.  Inside the mouth and throat, there may be other breaks in the skin.  Once the sip makes it to the stomach, disease transmission is unlikely, as stomach acid is sufficient to kill the small bacteria load. StuRat (talk) 17:04, 18 February 2013 (UTC)


 * This is actually much closer to "medical advice" than many of the redacted topics - however, public health seems to be something traditionally left to guesswork (and the will of God). I would think of meningitis (e.g. Neisseria meningitidis, Streptococcus pneumoniae) and (if cleanliness lags behind godliness) hepatitis A transmission, among other things. Wnt (talk) 19:48, 18 February 2013 (UTC)


 * If anyone is interested in references that address this question (I see Wnt has supplied some nice wikilinks for diseases during my EC), please see the study entitled "Survival of bacteria on the silver communion cup", here . The article is on Jstor, and should be freely available to the public. I found this reference by searching google scholar for /disease transmit cup/, and this came up as the third hit. Granted, the work is old, and there are probably much more recent studies. These can be found by perusing the list of works that cite this one, etc. SemanticMantis (talk) 20:12, 18 February 2013 (UTC)


 * Should be, alas is not. But you're right I should have looked more carefully.  Even so... I imagine every species of bacteria is potentially its own experiment.  Anyway, respected sites  support cup sharing for meningitis. Wnt (talk) 22:17, 18 February 2013 (UTC)
 * Ok, my link assumes institutional access, which we obviously don't all have. However, I believe anyone can get to it if they register (for free) at Jstor's "register and read" program, here . From there, the title quoted above should get you to the paper. (and the "no refs" comment was not directed at anyone individually, and you at least did give some relevant wikilinks). SemanticMantis (talk) 22:26, 18 February 2013 (UTC)
 * And to save time for the only mildly curious, the study concludes "Evidence is presented which indicates that bacteria swabbed on the polished surface of the silver chalice die off rapidly. Experiments on the transmission of test organisms from one person to another by common use of the chalice showed that approximately 0.001 % of the organisms are transferred even under the most favorable conditions; when conditions approximated those of actual use, no transmission could be detected. Only small numbers of bacteria from the normal mouth could be recovered from the chalice immediately after its use by 4 persons. It is concluded that in practice the silver communion cup is not an important vector of infectious disease" -- But I'm not sure how this conclusion would hold up under modern scrutiny. SemanticMantis (talk) 21:44, 18 February 2013 (UTC)


 * Yea, the main thing missing from that statement is how long they waited afterwards to do the test. Since there may only be seconds between uses of the cup, the test would need to be done within seconds to be valid. StuRat (talk) 21:51, 18 February 2013 (UTC)
 * They figured out that 5 minutes was too long to wait. So they had person A drink, then immediately person B, then took samples within a minute. (I'm not quoting directly because of formatting issues, the quote above took me several annoying minutes. If you don't trust my paraphrasing, please register and read the paper ;) SemanticMantis (talk) 22:31, 18 February 2013 (UTC)
 * I also should note that their conclusions are based on the treatment wherein the cup was wiped with a cloth between drinkers (which I think is common practice in USA christian churches that have a communal communion cup) -- so that this claim of "not a big risk" is specific to communion cups which are wiped, not drink sharing in general. As Wnt points out above, several diseases are known to propagate via casual drink sharing. SemanticMantis (talk) 23:47, 18 February 2013 (UTC)
 * Who drinks wine at church? It's blood. --Onorem♠Dil 12:29, 21 February 2013 (UTC)

Water jet that pops up after somethings drops in water
When something is dropped in water, a thin jet rises up (like this). What is the technical name for it? I cannot recall. It was something like williamson jet. Does anyone here know the right name? —  Hamza  [ talk ]  15:32, 18 February 2013 (UTC)
 * According to Wikipedia's page on Drop (liquid), it looks like it's called a "backjet". 140.254.226.238 (talk) 15:37, 18 February 2013 (UTC)


 * Thankus Maximus. —  Hamza  [ talk ]  16:08, 18 February 2013 (UTC)

Sulfuric acid and nitric acid for cleaning glassware
In Introduction to Organic Laboratory Techniques two solvents are suggested for cleaning glassware:

1. Acetone

2. A mixture of concentrated sulfuric acid and concentrated nitric acid

Isn't #2 kinda dangerous since it could lead to nitration and thus explosive compounds like RDX and TNT?

Googling "cleaning glassware sulfuric acid nitric acid" gets me plenty of other textbooks suggesting the exact same formula, albeit as an alternative for the standard chromic acid. Dncsky (talk) 16:50, 18 February 2013 (UTC)


 * Geez, if you need to use dangerous chemicals like that the clean your glassware, maybe you'd best just replace it. I sure wouldn't want to inhale those fumes, either. StuRat (talk) 16:57, 18 February 2013 (UTC)


 * The glassware under question typically isn't a set of Ikea bowls. Depending on the reaction vessel under consideration, you're talking $50 or more each - sometimes much more if it's a specialized piece of apparatus. "Toss and replace" is impractical if there is a simple and inexpensive way of cleaning them. Sulfuric & nitric acid aren't all that hazardous if handled properly, and "wouldn't want to inhale those fumes" isn't an issue if you have a exhausting fume cabinet, as would most who are doing such procedures. - Regarding nitration, yes, that can lead to explosive compounds, but generally you would remove most of the organic material by soap&water/solvent washes prior to the acid bath. The acid bath is primarily to get rid of residual contamination. Only a small amount of nitrated compounds will be made, they'll be dissolved in an excess of solution, and there's enough excess nitric acid that they'll likely be completely oxidized. Care must be exercised, though. Consult with an experienced organic chemist (most love to talk shop, especially if you're paying for the beer) before attempting. - That said, using concentrated sulfuric and nitric acid isn't all that common in my (limited) experience. Perhaps if you have some persistent contamination and nothing else works. Most organic labware gets cleaned with soap&water and an acetone rinse. If that doesn't work, a soak overnight in dilute acid is usually done. Dilute sodium hydroxide baths are sometimes also used, although that can etch glass and ruin ground glass joints, so base baths are typically used sparingly. As Pavia says, the sulfuric/nitric mix would only be for "troublesome stains and residues that adhere to the glass despite your best efforts". -- 67.40.213.4 (talk) 17:56, 18 February 2013 (UTC)


 * OK, but basic test tubes and flasks can be replaced cheaply. StuRat (talk) 18:02, 18 February 2013 (UTC)


 * Test tubes are nearly always designed to be disposable.. especially the smaller ones that you fit into fraction collectors. Flasks on the other hand can be quite a bit more expensive, especially precisely calibrated volumetric flasks.  Back to the original question, as 67 pointed out you'd really only be using this sort of acid rinse in a functional hood so the fumes aren't much of an issue. Not to mention, we do keep both dilute sulfuric acid and dilute nitric acid in the teaching labs here, it can't be too hazardous if they let freshman/undergrads use it in genchem.  For really persistent contaminants you can use concentrated nitric acid; I've certainly cleaned cuvettes that way on occasion.  It's certainly more cost effective than replacing a quartz cuvette! And yes, base baths will etch glass rather quickly, so as already stated above you wouldn't want to use them on anything with joints or fittings.  (+)H3N-Protein\Chemist-CO2(-) 19:12, 18 February 2013 (UTC)


 * Cleaning classware with nitric is on par with lighting a barbecue with liquid oxygen . Spectacular but dangerous. Don't try it at home folks – in your neighbors garage yes – it invokes a lot of 'wows' etc (followed by the sound of sirens from the emergency services ) but not in your own house please.--Aspro (talk) 22:55, 18 February 2013 (UTC)


 * Note, "concentrated" is of course a relative term. Nowhere did I say that I was cleaning cuvettes with fuming nitric acid. I think it's a little silly to make such a big deal about nitric acid without even factoring in concentration.  Yes, you probably don't want to pour even moderately concentrated nitric on exposed skin or bad things would happen.. but I doubt many solvents would pass the "I can pour it on my arm without adverse affects" test, so that's an odd safety requirement.  (+)H3N-Protein\Chemist-CO2(-) 02:11, 19 February 2013 (UTC)
 * I'd compare it with things normally used to clean glassware, like detergent and water (distilled water in this case). In that context, these cleaning methods seem quite extreme.  I suppose they have their place, on expensive items which can't be cleaned in any other way, but their use should certainly be limited.  StuRat (talk) 02:17, 19 February 2013 (UTC)
 * Not limited. No reason it should be either. We used to keep a garbage pail of dilute acid for the first day's soak and a garbage pail of sodium hydroxide for the second day's soak then a day in the oven to dry. For every single piece of glassware. Chemists work with far more dangerous chemicals all the time. Which is why we have lab coats - and gloves, safety glasses, fume hoods, eye washes, emergency showers, acid spill kits, mercury vacuums, gieger counters, lead aprons, inert atmospheres, etc. Rmhermen (talk) 15:52, 19 February 2013 (UTC)
 * Small clarification, my statement "we do keep both dilute sulfuric acid and dilute nitric acid in the teaching labs here" in no way implied that it's used for cleaning glassware or that we let the undergrads just randomly play with strong acid. (+)H3N-Protein\Chemist-CO2(-) 02:19, 19 February 2013 (UTC)
 * But there are some professionals who specifically specify fuming nitric acid though, hence my question. In any case, thanks for all the responses, guys. It's been a real eye opener. Dncsky (talk) 04:44, 19 February 2013 (UTC)


 * If 'you' know what effect that RFNA has on baked-on encrusted carbonated glassware then fine. I'm just saying that for those folks that can obtain  access to these chemicals  – they should not try it at home – especially, if they live next door to me and if they don't know the A to W to archive  the right dilution.  De-rusting the  gas  tank on your classic car  with nitric.... forget it … a little knowledge can be a dangerous thing. “Hi Aspro, can you be right neighbourly and lend me some of you battery top up acid? My son's  following this recipe where he boils up some cotton wool in nitric and now he need a little sulphuric... I'll tell you this, that 'ol tree stump in the back yard will be out the ground tomorrow – just you wait and see.” So just saying.--Aspro (talk) 15:31, 19 February 2013 (UTC)
 * The original question was about suggestions offered by an "Organic Laboratory Techniques" handbook for cleaning specialized scientific glassware in a laboratory setting. I don't think anyone was suggesting that this was general wisdom for cleaning everyday household items. (+)H3N-Protein\Chemist-CO2(-) 14:36, 20 February 2013 (UTC)

Arthur C Clarke Space Elevator
Hi, I read the quote from Arthur C Clarke again today about the space elevator being built "50 years after everyone stops laughing". What did he mean by this? Did he just mean 50 years after everyone starts taking the idea seriously, or is there something more I'm missing? I'm sure its really obvious but my brain is slow today... 80.254.147.164 (talk) 16:55, 18 February 2013 (UTC)


 * I think your 1st instinct was right. That is, it will take some 50 years of development to make it work, and that can't start until the idea is taken seriously. StuRat (talk) 16:58, 18 February 2013 (UTC)


 * NASA started spending money on space elevators in 1995. So we can be fairly sure that at least those people had stopped laughing.  So we're about 40% of the way into Arthur Clarke's 50 years.  That would put the space elevator completion date around 2045.  I don't think any of the developers believe that's possible.  However, Clarke left himself a loophole - he said "50 years after everyone stops laughing" - I'm pretty sure there will still be laughing right up the the first day it actually works!


 * Sadly though - it's looking like he may not be correct. I think enough people have taken the idea seriously - and between the tremendous difficulties with making a sufficiently light/strong tether - and the much more problematic issues of stability - he may have been wrong about whether it'll ever be built.  However, there are lots of people working on various aspects of the problem - and a good amount of research money is going into solving these problems - so nothing is certain.


 * SteveBaker (talk) 20:32, 18 February 2013 (UTC)


 * Well, even if the version he had in mind, only supported at the top and bottom, doesn't work, you could always have one which is powered and has upward propulsion at various points along it. StuRat (talk) 21:57, 18 February 2013 (UTC)
 * No-one questions that (given sufficiently strong materials) it would work once erected: the difficulties lie in the erection and the dangers of failures during that process. {The poster formerly known as 87.81.230.195} 84.21.143.150 (talk) 13:57, 19 February 2013 (UTC)
 * No, no, no. That is absolutely not the case!  There are severe concerns about the stability of the cable due to things like Coriolis effect forces on the elevators.  These things start the cable swaying - and damping out that motion is difficult with the vast majority of the system being above the atmosphere and extremely flexible.  As StuRat points out, you might be able to put rocket motors at intervals along the cable to counter this motion - but then the system needs power and fuel and all sorts of other things that make the whole concept much more difficult.  There have been suggestions that you could cancel out these forces by running the elevator very slowly (like 2 months to get from bottom to top) - or by having multiple elevators climbing the cable at the same time (which makes the strength requirements of the cable vastly bigger - and means that you can't bring the elevators back down again once they're up there).
 * Then there are issues of how to handle wear on the cable and small breakages due to flaws in the materials. Those are exceedingly hard to repair on-the-fly.
 * With our present level of knowledge, the space elevator is far from a "do-able" problem - even if the cable strength issues can be resolved using something exotic like carbon nanotubes.
 * SteveBaker (talk) 14:29, 19 February 2013 (UTC)
 * For a single robot crawler, a single cable should suffice, but as loads increase, its really just a matter of designing a larger structure to handle these. --Modocc (talk) 17:06, 19 February 2013 (UTC)
 * But that's just not true. The technology doesn't scale at all well. SteveBaker (talk) 20:50, 19 February 2013 (UTC)
 * The current technology doesn't scale, for the material needed is not even here yet. Strength-weight ratios might improve dramatically though with better models and fiber design. Once we have made one cable that works well, more will be made and used in new designs that do scale. -Modocc (talk) 00:05, 20 February 2013 (UTC)
 * I would hate to be on such an elevator when an asteroid or meteor came along and severed it. ←Baseball Bugs What's up, Doc? carrots→ 14:36, 19 February 2013 (UTC)
 * If that catastrophe were to happen, I'd make sure one could fly away! :) I imagine too that a large spiraling suspension bridge type construction would work, because each counter-weighted (the weights are in orbit, which can be bundled with a large structure, like with marionette control bars) cable would need to support only a very small portion of the road (or tracks). The structure can have lanes for both directions of travel and the suspension cables would be isolated from the vehicles. The vehicles which are capable of traversing this bridge could also be wheeled like when one climbs up and down a mountain with an auto. With a large enough structure, the incline can be gradual, but with a shorter near vertical incline, a coilgun type system should get you to orbit and back. At some point, if we ever are able to manufacture cables that are strong enough, it could become commonplace to be taking out one's favorite retrofitted computer driven auto to an upscale establishment. Modocc (talk) 16:12, 19 February 2013 (UTC)
 * That at least isn't a problem! These machines are unlikely to ever carry people.  The elevator would travel through the atmosphere quite slowly and that would expose people to huge amounts of dangerous radiation as it passes through the Van Allen belt.  Astronauts only survive this right now because the spacecraft are moving really fast and don't hang around in the dangerous areas for very long.  The power requirements for the elevator and the nature of a friction grip on a valuable, highly stressed cable means that the elevator will rise very slowly.  They'd spend hours to days of travel through Van-Allen belts - and that would fry the astronaut without many tons of lead shielding - which kills the performance of the machine and makes for impossibly strong cables.  The space elevator would have to be a cargo-carrier only.  But the probability of an meteor/asteroid/comet hitting the cable is utterly negligable - there are much larger concerns here.  For example, an earthquake or tsunami (for example) would induce dangerous waves along the cable.  Lightning strikes on a wet cable in the rain would be a serious risk also - so it's likely that the base station will have to be mounted on a ship and moved around to avoid storms...which brings a whole other cluster of issues. SteveBaker (talk) 20:50, 19 February 2013 (UTC)
 * "...friction grip on a valuable, highly stressed cable..."???? You apparently didn't comprehend my design method. The bridge spans put a static downward load on the many cables which are numerous (essentially as many as are needed). The vehicles have no contact with the cables and therefore no friction. There are reaction forces, but their effects will depend on the bridge's resistance to these and other environmental loads due to its inertial mass which, as a mass, will be considerable. The bridge is suspended and anchors need not be inelastic, thus probably earthquake resistant. Storms might be a factor... again it depends on the mass and plenty of bridges weather storms (and floods), but the whole thing doesn't have to (although it probably will) extend all the way to the ground and/or it could have a few light-weight tethers to the ground to give it lateral stability. Depending on the orbit one wants to achieve it could take hours perhaps, and far less if a rail system is used, thus shielding (we will need some substantial shielding anyway, something the astronauts could not afford much of due to mass restrictions) may or may not be much of a factor. -Modocc (talk) 21:46, 19 February 2013 (UTC)
 * Respondents here have clearly read the PR blurb and missed the math. Read this:  http://www.sciencedirect.com/science/article/pii/S009457650800338X - it's not saying that the elevator is impossible, but very likely so inefficient that it won't ever be economically viable.
 * SteveBaker (talk) 20:50, 19 February 2013 (UTC)
 * Very first sentence: "The space elevator offers an alternate and efficient method for space travel." Its unstated conclusions are behind a paywall, but why bother? -Modocc (talk) 21:46, 19 February 2013 (UTC)


 * I can access the whole article, and its conclusion is almost the exact opposite of what you claim. Quotes:
 * "The effect of climber transit on the base of the space elevator is negligible..."
 * "However, due to the relatively small mass of the climber and the fact that the ribbon itself is not actually deployed or retrieved, the decay or growth of the oscillatory terms is small. This result distinguishes the dynamics of the space elevator from those of a typical tethered satellite system, where the growth of tether oscillations during constant rate retrieval can be very large."
 * "Since both of these values have upper bounds, the ribbon libration they cause is bounded as well; it will be of the order of milliradians at most."
 * "Still, the ribbon will experience undamped oscillations about its vertical equilibrium position. The period of such oscillations would be about five to six days. Even though the amplitude of these oscillations is small, it is desirable to minimize, and if possible, eliminate it."
 * "Three simple climbing procedures that aim to minimize or eliminate residual oscillation upon climber arrival have been presented [...] The second climbing procedure aims to eliminate ribbon oscillations. It is applicable for the case where a single climber is sent from one point on the ribbon to another at a constant speed. If the climber is sent at the moment when the ribbon reaches its maximum eastward propagation with a specific climber mass and velocity pairing, the ribbon will reach equilibrium at the moment when the climber arrives at its destination; the oscillation can be eliminated by using the Coriolis force on the climber." --140.180.243.51 (talk) 00:03, 20 February 2013 (UTC)

Completely fictional, but Kim Stanley Robinson described the fate of a space elevator that was severed at anchor point (in space) in Red Mars as mentioned above by Baseball Bugs. I've never liked the idea in the first place, but Robinson's description of the thing crashing down was enough to convince me that—feasibility and other subjects touched upon above aside—this is one of the most frightening ideas we've ever come up with as a race. Which means it'll probably happen several centuries from now, at some point in the far future when celebrities own their own moons. – Kerαu noςco pia ◁ gala xies 22:45, 19 February 2013 (UTC)
 * Personally, I found the concept in SimCity 2000 of a microwave energy satellite going off-track far more disturbing. Entertaining in a game, though. &mdash;  The Hand That Feeds You :Bite
 * As you said, that description is entirely fictional. Since any plausible material for a space elevator will be lightweight, the cable's terminal velocity will be very low.  It might (or might not) be enough to kill a person with a direct hit, but there's no risk of damage to buildings or trees.  --140.180.243.51 (talk) 00:03, 20 February 2013 (UTC)
 * If we did manage to make a space elevator we'd make it big not small, something capable of lifting hundreds if not thousands or tens of thousands of tons at a time and the cable might be many yards across. If it was snapped it certainly would cause damage. Probably you'd want shelters at the bottom just in case and have it some distance from anything else. Dmcq (talk) 17:31, 20 February 2013 (UTC)
 * Current designs call for a ribbon rather than a cable. In the event that it breaks at the upper end, most of the ribbon will burn up on re-entry, and the bits that don't will have the terminal velocity of a sheet of paper. --Carnildo (talk) 23:22, 21 February 2013 (UTC)
 * Tons of metal falling down will case damage. Dmcq (talk) 11:55, 22 February 2013 (UTC)

Salticid eyes
Hello,

what colors do the retina layers of salticids recognize? Is it blue, red or green or how is it constructed?

Greetings HeliosX (talk) 19:35, 18 February 2013 (UTC)


 * From the Wikipedia article on Jumping spider, it says that those types of spiders have tetrachromatic vision and are sensitive to UV light. Their retina presumably can detect up to four colors. See Tetrachromacy. 140.254.121.36 (talk) 19:52, 18 February 2013 (UTC)


 * Also enjoy this nice book article "How jumping spiders see the world", here on google books . Of note is that they have a "staircase" retina, which they move to focus, rather than their lenses! SemanticMantis (talk) 20:17, 18 February 2013 (UTC)


 * They also have one of the most robust visual cognition systems of all invertebrates; with certain species like the domestic jumping spider in particular, you can note that they will track the movement of a large entity like a human (and even individual body parts thereof) with much finer attention than the overwhelming number of critters their size. To the OP, this article describes the specific frequencies of light which the photoreceptors of the retinas are sensitive to, with regard to two specific species.  Snow (talk) 07:34, 19 February 2013 (UTC)

Identify guinea fowl-like bird with white cheeks and orange wattles
My nephews and I went out to feed the geese in the US North East this weekend in freezing weather. We found instead what I think of a standard guinea fowl with coal-colored feathers with small white spots. But they had bright white cheeks, yellow beaks, and a small bright orange wattle along what would be the jawline on a human on each side. Can anyone suggest what species these might be or what variety of what species? Google image searches get me things that look like the same genus but not identical. Thanks. μηδείς (talk) 20:37, 18 February 2013 (UTC)


 * Could it be Helmeted Guineafowl? 140.254.121.36 (talk) 20:55, 18 February 2013 (UTC)


 * (EC) There is a a lot of variety in guinea fowl. Some of them have white cheeks, yellow beaks, and orange wattles. I think there must be some plasticity in the color of the cheeks and wattles, even within species (think overall health and diet). Anyway, check out the helmeted guineafowl. Our article shows one with bluish cheeks, but this zoo page shows theirs with white cheeks. Is that close at all? SemanticMantis (talk) 20:58, 18 February 2013 (UTC)
 * Related: does anyone have refs of feral GF flocks in the NE USA? I had not heard of that before... SemanticMantis (talk) 21:00, 18 February 2013 (UTC)


 * Common Birds of the New England Area — Preceding unsigned comment added by 140.254.121.36 (talk) 21:09, 18 February 2013 (UTC)

First, I am absolutely certain they are Guinea Fowl, which aren't native to the US. Second, while they do look like helmeted guinea fowl from the neck down, they have bright yellow beaks, no wattles at the nares or above the "jawline", and bright orange ones at that, and bright white cheeks--they could be Crayola Crayon standard colors or the focal colors of Berlin and Kay. Third, they have absolutely no helmet or crest whatsoever--even though in body shape and the color and pattern of the plumage they are identical to standard helmeted guineas, and their beaks seemed a bit less robust. There were six individuals, and they were identical, no apparent sexual or age variation, so I am wondering if perhaps they were hens or juveniles or juvenile hens. Their white heads were closer to the Pearl variety, but still no crests and their wattles were smaller than and less forward pointing, basically only the size of the white are of the wattles on this Pearl specimen, as if the white part were orange and the red part snipped off. Pearl. μηδείς (talk) 01:57, 19 February 2013 (UTC)

Standard components in Edison, Tesla, and Marconi's day
Where'd they go to get, say, a resistor of X ohms, a capacitor of Y microfarads, or an inductor of Z henries when implementing one of their designs? Not Radio Shack or Mouser, I presume :) 67.163.109.173 (talk) 22:48, 18 February 2013 (UTC)


 * Edison had his own staff, and I imagine the others did, too, or made the items themselves. StuRat (talk) 23:01, 18 February 2013 (UTC)


 * I wold like to know (for knowledge's sake, not that I would expect to be able to do it in my garage) if records/documentation for their "recipes" and construction methods/procedures to construct components with specified component values have been preserved. 67.163.109.173 (talk) 23:06, 18 February 2013 (UTC)


 * Michael Faraday's Experimental Researches... contains many experimental descriptions that are very cook-book-recipe-like. By the era of Edison and Marconi, electrical components were already commercially available.  For example, Federal Telegraph Company is famous in these parts; they used to sell great big ferrite cores and integrated radio systems, dating back to the very early era of radio.  Telegraph and telephone companies, like Western Union and Bell and AT&T (the original corporation) were already building large scale telecommunications systems, and they manufactured support electronics on industrial scales.  Nimur (talk) 23:48, 18 February 2013 (UTC)


 * Thanks. Related to first precision components, I always wondered; the first calibration measurement devices to be able to measure to a given precision, how were they calibrated, given that prior to them, one could not know that their components were accurate to that given precision. For example, the first capacitors asserted to be accurate to within 10 microfarads, the tester used on them would need to be able to measure within 10 microfarads to know what value they were. So the device to test the first capacitor with a value within 10 microfarads of its nominal value needs to be able to do whatever it does to indicate "yes" within 10 microfarads of that value. Doesn't that device need such level precision capacitors in it? But prior to this moment, no such precision capacitor has been made (knowably with any degree of confidence). This questioning could go for resistors or inductors, I suppose, not just capacitors, which I used as an example. 67.163.109.173 (talk) 00:29, 19 February 2013 (UTC)


 * Well, for simpler measurements, like length, they just carved a line in stone somewhere, declared that to be a foot, and measured everything else against that. I suppose a similar method could have been used with more complex measurement devices. StuRat (talk) 01:50, 19 February 2013 (UTC)


 * I am an electrical engineer by occupation, and electronic engineer by qualifications and hobby, and have been involved in a bit of chemistry. The great thing about electrics and electronics is that it is quite simple and easy, if a pretty bit tedious, to home-make basic components because they are calculable, and you can build up from there.  For instance, given the physical dimensions, the capacitance of an air-spaced capacitor can be calculated with a high degree of precision (> 4 place accuracy is not at all difficult with a modicum of enginuity).  The inductance of a toroidally wound air core inductance can also be calculated from its physical dimensions with a high degree of precision.  I wish chemistry was like this!
 * In a home electronics lab, all you really need is an accurate DC voltmeter, and accurate frequency meter (both easy to get without much expense: 5-digit digital voltmeter ~ $200, 6-digit frequency meter about the same), and an accurate capacitor (home-made), and you can construct apparatus to relate all measurements back to these, for calibration purposes. 100 years ago, labs used the Weston Cell as an accurate voltage for calibration purposes.  Oscillators were compared with others by counting beats and ultimately referred back to a mechanical clock, whose accuracy was assured by comparison with observations at astronomical observatories.
 * Ratbone 60.230.235.171 (talk) 02:39, 19 February 2013 (UTC)


 * Standards might be determined in world-class physics labs, such as the work of Maxwell and Thompson in the 1860's with standard cells and carefully managed galvanometers, or Lord Rayleigh and (his faithful companion Mrs. Sidgwick) to define a column of mercury of some length of some cross section at some temperature as having a specified resistance. See . They followed on the heels of Davy, Faraday and others. The telegraph industry made use of standards and measuring instruments by the 1850's, to achieve commercial success. A useful and practical lab standard resistance could be made with a certain length of resistance wire, such that it has a desired resistance with a stated precision at a stated temperature, and correction factors could be applied for other temperatures.  Lab supply houses could make and sell to experimenters such standards. Bridge circuits could be used with standard voltage cells, standard resistances, inductances or capacitances to measure or adjust other secondary standards. An early 20th century or late 19th century electrical lab had big, expensive and delicate instruments for precision measurements, museum pieces today, just to achieve similar precision to a digital meter today costing a few hundred dollars.Edison by the 1870's could buy or have whatever measurement devices he needed. Tesla in the 1890's could just write Westinghouse and have motors or generators built for his lab, or could have his workmen build new devices.  Experimenters did not always need state of the art accuracy in measurements, compared to the physics labs developing standards with many digits of accuracy.  Edison (talk) 16:58, 19 February 2013 (UTC)

freezing point of water
Would a solution with 250 ml of rock salt and 25 ml water 125 ml ice have a much lower freezing point than a solution with 175 ml of rock salt, and 25 ml water 200 ml ice? At the beginning of my experiment, the 400 ml beaker was half full with ice, and the rest was rock salt (with a tiny bit of rock salt). Alot of rock salt was added to the beaker, over the course of the experiment. — Preceding unsigned comment added by 99.146.124.35 (talk) 23:29, 18 February 2013 (UTC)


 * Does our freezing-point depression article help? Mikenorton (talk) 23:37, 18 February 2013 (UTC)

If the extra rock salt dramatically lowers the freezing point of the ice in the 400 ml beaker, how would that impact the freezing point depression constant of the fluid inside the vial that is positioned inside the 400 ml beaker? All of the freezing point depression constant Kf values of the water that was mixed with citric acid inside the vial were much lower than what they should be (0.5 and 0.65 is what they came out to be. — Preceding unsigned comment added by 99.146.124.35 (talk) 23:41, 18 February 2013 (UTC)


 * Citric acid ? As for the effect of more rock salt, there's only so much salt that can be dissolved in the (liquid) water.  Beyond that, more won't change it's properties, it will just sit on the bottom.  However, rock salt has lots of other junk in it, some of which might be dissolved by water and affect it's properties.  In a chemistry experiment, I expect you'd want to use pure table salt (non-iodized sodium chloride). StuRat (talk) 02:35, 19 February 2013 (UTC)


 * If you want to REALLY lower the freezing point of water, you can make a saturated solution of calcium chloride and see how cold you can get it without freezing it. :-)  24.23.196.85 (talk) 05:01, 19 February 2013 (UTC)
 * Rock salt will lower the freezing point of water by maximum −21.12 °C for saturated solution with 23.31 wt% of NaCl. See also . Ruslik_ Zero 07:54, 19 February 2013 (UTC)