Wikipedia:Reference desk/Archives/Science/2012 November 9

= November 9 =

Antimatter meteor/comet
What would happen if an antimatter meteor or comet hit the Earth? --168.7.238.36 (talk) 06:22, 9 November 2012 (UTC)


 * A large one would pretty much destroy the planet, while a small one might just destroy all life on Earth. However, if it hit at a shallow angle, the initial explosions when it first contacted the atmosphere might blast it back into space before it did much damage.  Fortunately, there don't actually seem to be any.  Within our solar system, I suspect that even the sparse gas here would cause sufficient explosions on it's surface to break it up.  StuRat (talk) 07:27, 9 November 2012 (UTC)


 * Incidentally, are you aware that one theory is that the Tunguska event was a tiny antimatter meteor which struck Russia ? StuRat (talk) 17:42, 9 November 2012 (UTC)


 * See antimatter comet. StuRat (talk) 17:44, 9 November 2012 (UTC)


 * You should read the short story "Flatlander" in the collection Neutron Star by Larry Niven. We have an article on it, but the article is basically one big spolier, so just get the book Neutron Star at the library, or buy it used for a penny at Amazon. μηδείς (talk) 15:49, 10 November 2012 (UTC)


 * I once (1993? 1984?) heard Robert L. Forward say that if you put an anti-iron cannonball on the ground it would probably sizzle, not explode. Whether this implies anything for an anti-body crashing at escape velocity, I don't know. —Tamfang (talk) 16:21, 10 November 2012 (UTC)


 * It would surely do a lot of radiation damage even if it sizzled / bounced off the atmosphere. The solar winds would make the object highly visible, too, so you could probably defeat it by crashing a kill vehicle into it at the right angle (OR). The annihilation would do good enough to make any explosive warhead utterly redundant. - ¡Ouch! (hurt me / more pain) 17:31, 10 November 2012 (UTC)

The minimum diameter for an incoming antimater asteroid travelling at 30 km/s to reach the surface is about 50 km. If you approximate the problem as an ideal photon rocket, then the change in velocity w.r.t. to the initial rest frame of the asteroid is:

$$v = c\frac{m_{1}^{2} - m_{2}^{2}}{m_{1}^{2} + m_{2}^{2}}$$

where m1 is the initial asteroid mass and m2 the final asteroid mass. Then we put m2 = m1 - delta m and expand to lowest order in delta m. We then put delta m = pi r^2 rho_at L where rho_at is the density of the atmosphere and L the effective lengtht it travels through the atmosphere, as this is the amount of mass it scoops up in the atmopshere and this will be annihilated. The initial mass is 4/3 pi r^3 rho_as, where rho_as is the density of the asteroid. You then find:

$$r = \frac{3\rho_{\text{at}}L c}{4\rho_{\text{as}}v}$$

which gives the minimum radius of the asteroid needed to reach the surface. For an incoming asteroid at speed 30 km/s the change in velocity is v = (30 + 11.2) km/s. In reality the radius it can be a bit less than this because the ideal photon rocket model overestimates the propulsion from annihilation. Count Iblis (talk) 18:08, 10 November 2012 (UTC)


 * Whether it reaches the surface seems rather besides the point. The energy released by the annihilation of a 49 km diameter antimatter asteroid will vaporize the Earth's crust, in any case. StuRat (talk) 23:14, 10 November 2012 (UTC)


 * As for a really tiny antimatter meteor, that might fry anyone in a direct line-on-sight, and the blast wave might kill people hundreds or thousands of miles beyond that. People 90 degrees from it (or a bit past 90) might have the best chance at survival, since the blast would be most diluted, there.  At the opposite side of the Earth, the blast wave again builds up to a deadly level. StuRat (talk) 23:21, 10 November 2012 (UTC)

Lead poisoning
I have an extension PCB(a pci card of some sort) that is not compliant with ROHS That is built with lead-solder, DOES handling it pose a threat to health or something of that nature, like getting poisoned if you are to have contact with the solder joints on it? Should you always wash your hands thoroughly after handling it? — Preceding unsigned comment added by 77.35.32.100 (talk) 09:38, 9 November 2012 (UTC)


 * There is nothing to be concerned about. No need to wash hands beyond the usual requirements. Electric and electronic devices have been made with tin-lead solder for around 100 years without any problems whatsoever from handling.  If there was any problem, it would have occured with the factory people doing the soldering - and for the first 70 years or so it was mostly done by hand.  No problems have been reported.  Same with electronics hobbyist and radio hams - who have always, and still do, assembled their devices with tin-lead solder.  Problems are unknown.  I have an electronics background myself.  Employers generally banned the consumption of food at the workbench, and recommended washing hands before eating.  But this was more about keeping the workplace clean and avoiding the transmision of microbes (thereby keeping teh workers healthy and not off sick) than about lead.  Later I worked in a diesel engine facility, and they had the exact same rules, and zero use of lead.
 * In circuit boards of at least reasonable quality, after soldering, the solder side of the board is coated with a type of varnish. This varnish is very thin and generally is not visible.
 * You need to understand the reason for the use of lead free solder in recent years. It has been determined that very small amounts of lead affect intelligence, and most people have medically significant lead in their bodies.  First, various countries banned lead in gasoline.  That had little effect, except in the USA, where the number of gasoline engined cars and trucks per unit area is much greater than in other countries (and even in the USA, significant lead levels in humans still occur).  So the Europeans decided to ban the use of lead altogether (an exemption has been made for lead-acid batteries).  However, lead is still ubiqitous in the environment.  One of the major causes of lead in our bodies is the use of lead-based paint in houses.  In most advanced countries it was banned decades ago, but every house gets sanded back and repainted once in a while, releasing the lead as dust into the air.  There are lots of other reasons for lead in the environment that far swamp out any nanoscopic source in electronic cards.
 * Keit 120.145.147.142 (talk) 10:35, 9 November 2012 (UTC)


 * I should add that one problem with lead in landfills, or any stable element, is that it doesn't decay. So, while something like feces ceases to be a problem after it's been buried a few years, this isn't the same with lead.  It's permanent.  As new layers of soil are deposited on top, the lead will eventually be buried so deeply that we will be unlikely to have much contact with it, but this will take thousands of years. StuRat (talk) 17:24, 9 November 2012 (UTC)


 * The lead in the solder would be released back into the environment if it is discarded into the earth and the lead gradually corrodes over the years. As others have mentioned there is no  hazard by handling it.  But don't eat it or grind it to dust. Graeme Bartlett (talk) 11:01, 9 November 2012 (UTC)
 * Yeah, like everybody says, the major health risks of lead-based solder are water pipes, where it's been banned for a long time, and for the solderers who are inhaling the vapors over a lifetime. Although, I would definiely recommend against eating a lot of it. Gzuckier (talk) 19:11, 9 November 2012 (UTC)

Ive heard that by getting lead solder inhaled or in your blood your gonna get deaf partly blind and and have no sexdrive plus seizures and have problems with fertility im gonna def check all of my home appliances on whether they meet up with the rohs standards and requirements — Preceding unsigned comment added by 77.35.33.248 (talk) 10:29, 10 November 2012 (UTC)


 * Yes, so don't grind lead up to a powder and snort it. Otherwise, you should be fine. StuRat (talk) 07:08, 12 November 2012 (UTC)

What was the purpose of the Galileo thermoscope
I try to understand what were the purposes of Galileo galilei when he made the thermoscope? It seems that the him purpose was not been to make a scale of the heat, but he meant to see or prove something... Do you know what was it? מוטיבציה (talk) 14:00, 9 November 2012 (UTC)


 * Why do you discount the idea of making a scale of the heat? This was a non-trivial scientific and instrument-based question in the 17th century, and there was considerable debate at the time what temperature really was (e.g. was it a substance or was it something else? We now know that it is identical with movement on a very small scale, but they didn't know that then). --Mr.98 (talk) 14:29, 9 November 2012 (UTC)


 * First off, let's point out to those that aren't aware that Galileo's thermoscope was completely different from a Galileo thermometer, which wasn't even invented by Galileo. With that out of the way, even if you don't have an absolute scale on the device, you can have a rough relative scale - if I use this hot object, the column is visibly greater than if I use this cold object. By playing around with it, you can find out interesting things, like a piece of metal which feels much colder than a piece of wool actually doesn't register any different with the thermoscope. (It's different because how warm/cold something feels is due to the rate of heat transfer, rather than absolute temperature - I don't know if Galileo did such an experiment, but it something he could have done.) quotes a contemporaneous source saying he used it to "examining the degrees of heat and cold" - which at a time when the concept of heat was still fuzzy is no small thing.  says that experimenting with his thermoscope lead Galileo to formulate a hypothesis about the nature of heat. That's a lot of what the early scientists did. There was very little existing knowledge, so they played around with toys and other scientific instruments, coming up with theories of why things behave as they do. It was less hypothesis testing and more "hmm.... that's funny". It's probably worth pointing out that prior to constructing a thermoscope, it was probably a question if one *could* construct an absolute scale for measuring temperature. Note that once it was constructed,  and the previous references indicate that researchers very quickly applied various reference points to the thermoscope, which with intermediate graduations became a thermometer. -- 205.175.124.30 (talk) 03:32, 10 November 2012 (UTC)
 * Thank you for this information. I asked this question because I read once, that Galileo wanted to prove something about the atmosphera... I don't know what it is. (maybe that the crowding in the buttel\vessel depends in changes of atmosphera). I believe that Galileo approach to this experiment in order to looking for something. Is there any Information about in him writing or him pupils writing? (except of 33 origin) מוטיבציה (talk) 16:20, 10 November 2012 (UTC)

Converting candela per square metre to "photon density"
Greetings!

Given the luminance (in candela per square metre) and wavelength of a light source, I'd like to obtain its "photon density" (a measure that's new to me but used to describe stimuli in some vision research papers; its units are given as photons/cm2/s). One paper specifies that a red light source has a wavelength of 635 nm, a luminance of 34 cd/m2, and a photon density of 1×1014 photons/cm2/s (I include these values as they can - presumably - be used to check that a computation using the first two values correctly results in the third).

The candela is defined as a measure of "luminous flux per unit solid angle"; if it could be converted to an energy measure, the conversion should simply be a matter of computing the energy per photon of the light source using $$E=h\nu=h\frac{c}{\lambda}$$, dividing the overall energy by the per-photon energy, and then accounting for the change from m2 to cm2. Based on the fact that 1 candela = 1 lumen⋅steradian and 1 lumen = 1/683 Watts, 1 candela should be 1/683 Watt⋅steradian. There are 4π steradians in a sphere, so for a light source emitting light equally in all directions 1 candela should be equivalent to 0.0184 Watts.

Coming back to the values given in the aforementioned paper, 34 cd/m2 becomes 0.63 W/m2 and the energy per photon for a 635 nm wavelength light source is 3.13×10-19 W⋅s (1 Joule = 1 Watt⋅second). Dividing the first value by the second (to obtain the number of photons) results in 2×1018 photons/m2/s ("photons" is a unitless quantity here in the same way that "cycles" is for frequency (cycles/s), so the units are technically s/m2); finally, dividing by 1002 to convert from s/m2 to s/cm2 results in 2×1014 photons/cm2/s.

As this result doesn't match the value given in the paper, something is evidently amiss. The only thing I can see that might be incorrect is the assumption that the source emits light equally in all directions, but I would appreciate any suggestions or corrections. Hiram J. Hackenbacker (talk) 18:05, 9 November 2012 (UTC)


 * If I'm reading it right, the basic problem I see is that the candela is a measure of light emitted from a source, while the the photon density is a measure of the light received at the target. So, in astronomy terms, one is absolute magnitude and the other is apparent magnitude.  The apparent magnitude varies not only with the absolute magnitude, but also the intervening distance and particles.  So, there is no direct conversion between the two, and a conversion can only be done when you know what lies in between. StuRat (talk) 19:10, 9 November 2012 (UTC)


 * The candela is indeed a measure of the intensity of the source (and not directly comparable to luminance measures), but the stimulus in the paper is characterized by its luminance given as candela per square metre, which should be comparable to other per-unit-area measures (the paper used a Ganzfeld-type stimulus, which is basically a surface illuminated such that it has uniform luminance, which is why the surface is characterized by a luminance measure rather than one of luminous intensity). I converted candela to Watts via an assumption about the effective point source illuminating the Ganzfeld surface (which may or may not be valid) but this was just for the purpose of converting the given candela per square metre value to one expressed in more useful units, so the luminance of the surface is still a per-unit-area measure after the conversion and should remain comparable (and convertible, hopefully) to photon density. Hiram J. Hackenbacker (talk) 19:34, 9 November 2012 (UTC)


 * You may enjoy reading Labsphere's introductory guide to integrating photometry. This guide is not overtly commercialized: it actually provides a lot of solid physics and mathematics that is useful for people who are interested in quantitative measurement of light.  A large quantity of educational literature is available on the company's website in the science section.
 * Particularly note their list of various units (page 2); it provides a handy breakdown between flux, flux-per-area, flux-per-solid-angle, and flux-per-area-solid-angle. Each of these quantities represents a different useful parameter in photometry and radiometry.  The candela is a measure of photometric photons per solid angle - approximating a standardized human perception of light - so you can't directly convert it to total number of photons.  This critical distinction is elaborated in the guide, and is covered in our article on spectrophotometry.  Nimur (talk) 20:00, 9 November 2012 (UTC)


 * Thanks for the suggestion, Nimur; I'll take a look at the resources you've linked to. I'm beginning to suspect that, rather than performing a conversion of some sort to obtain the photon density, the various papers I've seen that use it actually directly measured it with an appropriate instrument (it'd be nice if that was mentioned in the papers, though). Hiram J. Hackenbacker (talk) 14:37, 11 November 2012 (UTC)
 * Many instrument vendors use the CIE standard to convert between radiometric and spectroradiometric photons; but others use (name-brand equipment-maker)-proprietary-standard-number-(whatever), which you have to obtain by calling up their sales team and hoping you can make sense of whatever half-truths they tell you about the equipment. (This tends to be impossible, especially if you're a low-volume customer).  In theory, you can use these data to convert to absolute photon-count.  In practice... you can't.  For that reason, it's much more useful in practice to know how the measurements were acquired - what instrument make and model, and how the lab bench was setup - instead of trusting the vendor to follow any "standard" that has "colorspace" or "perceptual curve" in its name.  Always compare measurements under conditions that are as identical as practical.  Nimur (talk) 19:50, 11 November 2012 (UTC)

Deer
Why do mother deer run off their young bucks?--Wrk678 (talk) 23:12, 9 November 2012 (UTC)


 * To prevent inbreeding is the evolutionary answer. Psychologically it will most likely have to do with smell, which is one reason humans don't often commit incest--relatives don't smell good.  Curious whether there's a ref for this. μηδείς (talk) 23:17, 9 November 2012 (UTC)


 * Human relatives don't smell good? Clarityfiend (talk) 23:28, 9 November 2012 (UTC)


 * You never heard siblings tell each other "you stink"? —Tamfang (talk) 16:15, 10 November 2012 (UTC)


 * But if our natural smell is covered by artificial scents, wouldn't that mean that we start to get sexually interested in our relatives? Comploose (talk) 17:44, 10 November 2012 (UTC)


 * See Body odor and subconscious human sexual attraction, where it says "The increased attraction between people of dissimilar MHCs is also hypothesized to prevent incest and its possibility of producing birth defects." And it has TWO cites.  -- Jayron  32  23:46, 9 November 2012 (UTC)


 * I'd say it a bit differently: The mother deer runs him off so he will go spread his genes, which are half hers, as widely as possible, thus ensuring that they are passed down. StuRat (talk) 02:03, 10 November 2012 (UTC)
 * Recent but little-known paleontogy studies show that ancient deer discovered DNA eons before Watson and Crick came along. ←Baseball Bugs What's up, Doc? carrots→ 18:46, 10 November 2012 (UTC)
 * "Has the deer a little doe?" "Yeah, two bucks." -The Three Stooges. Gzuckier (talk) 04:35, 13 November 2012 (UTC)


 * And I get threatened with eternal damnation for awarding people RD-best stars? What a fucking crock. μηδείς (talk) 23:11, 10 November 2012 (UTC)
 * Not eternal, just purgatory. I found Stu's comment "The mother deer runs him off so he will go spread his genes..." to be pretty funny, as if a deer would know anything about spreading genes. That might be the practical effect, but that doesn't mean the mother reasoned that out. ←Baseball Bugs What's up, Doc? carrots→ 08:12, 11 November 2012 (UTC)
 * Please see Strawman. You did not upset people for congratulating them for good answers.  We should do that.  You upset people when you placed stars after their answers with no context which implied some sort of official standing in such an award.  You should tell people that they give good answers.  You shouldn't try to establish a system of semi-official looking awards that implies that it is endorsed by the ref desk as a whole.  Your actions in awarding those stars should unambiguously represent you singularly, and not the entire reference desk.  Also, when people object to something you do, please don't invent something they didn't object to, as though your invented reason were the actual reason they were upset.  Instead, pay attention to words people use, and the exact reasons why they object to your actions.  -- Jayron  32  23:17, 10 November 2012 (UTC)
 * I would also point out that despite suggestions to the contrary when μηδείς was blocked, when the issue of the stars first came up there was clear consensus it was inappropriate to add them without signing, even if we accept (I'm not saying I do) there wasn't consensus against adding the stars. Just to be sure I rechecked the original discussion Wikipedia talk:Reference desk/Archive 93 and I can't see anyone suggesting adding them without signatures was okay. In fact nearly everyone who didn't just say don't do it specificially mentioned the need for signatures. Most also mentioned the need to do it on a new line and otherwise make it clear it wasn't part of the original response beyond just adding signatures. The flaw with the hidden 'signature' method was raised by at least 2 people (me and 203), with no response that I saw, so it was resonable to assume it was accepted. In any case, anyone who mentioned the need for signatures can be taken to be suggesting the need for something compliant with Signatures unless they said otherwise. And a hidden 'signature' clearly does not comply. Note that while neither WP:TPO or WP:SIG seems to have been specifically linked to in the original discussion on the stars, it's resonable to assume a regular is familiar with these guidelines. And in any case, the basic requirements were discussed as relevent to the specific example. Nil Einne (talk) 13:35, 11 November 2012 (UTC)

Inbreeding is pretty common in nature so I dont think thats the reason--Wrk678 (talk) 01:45, 11 November 2012 (UTC)


 * Actually no. Outcrossing is the norm in the wild, given that inbreeding is almost always genetically undesirable and deleterious to the population (see inbreeding depression). Almost all species have mechanisms that minimize the possibility of inbreeding as much as possible, while at the same time maximizing the possibility of outbreeding. Some examples:
 * The formation of harems, with a dominant male maintaining control of a number of females. Male offspring in such groups are forced to leave as soon as they attain sexual maturity. Lions exhibit this. Deer exhibit this briefly during the mating season.
 * The formation of sex-specific social groupings and sex-biased dispersal. Elephants, for example, have highly social females that form matriarchal herds. Young male elephants will stay with the female herds but will depart at maturity to join bachelor herds of young male elephants. Fully mature bull elephants are usually loners. Cats, as well, have social females, but itinerant males. In some species of ants, alate queens take flight first, followed by the drones. Deer exhibit this throughout much of the year.
 * Communal breeding, usually involving mass migration to mating grounds during the mating season. This ensures that normally scattered populations will mingle with each other and exchange genetic information. Examples include penguins, flamingos, and whales.
 * Sequential hermaphroditism, where all individuals are a certain sex at a certain age, but change into another sex once they reach a certain age. This is common among fish like groupers and other serranids. It ensures that all offspring hatching from the same clutch of eggs (and thus be of the same age) will be of one sex at the same time, making it impossible for them to mate, though in certain special circumstances, one of them may assume the opposite sex if absent.
 * Dichogamy in plants, where flowers produce pollen when the stigmas are nonreceptive or vice versa, therefore avoiding self-pollination. This is sometimes timed (synchronous dichogamy), e.g. in avocados, when the flowers first open they are functionally female. They then close. On the following day, they again reopen, but this time as functionally male flowers. This is coupled with the fact that avocados exhibit two morphs, and in a larger system known as heterodichogamy, one avocado morph open their flowers in the morning, while another open theirs in the evening. The result is an overlap in the flowering times, and at any given time an avocado tree possessing flowers that are functionally one sex will always have other trees that have flowers of another for cross-pollination.
 * Negative assortative mating. A form of nonrandom mating where individuals will mate more frequently or mate exclusively with individuals of a different phenotype/genotype. In simpler words: opposites attract.
 * Extreme sexual dimorphism. E.g. in scale insects, velvet ants, and cockroaches, males are winged, while females are not.
 * Highly motile juvenile stages, like planktonic larvae in marine animals. This ensures that offspring will be scattered far and wide before they even reach sexual maturity, making it unlikely that they will mate with a sibling or a close kin.
 * Kin recognition. Aside from identifying recipients of nepotistic behavior, kinship recognition is also used to avoid mating between individuals recognized as close kin. What μηδείς pointed out earlier is an example of this. The use of pheromones is a common method of kinship recognition and inbreeding avoidance. This includes humans, reinforced by physical recognition and social mechanisms like incest taboos.
 * Etc. There are others of course. Some of them so elaborate, it's a wonder they even find mates at all. --  O BSIDIAN  †  S OUL  09:07, 11 November 2012 (UTC)