Wikipedia:Reference desk/Archives/Science/2015 July 26

= July 26 =

How long does it take for a new habitat to change an organism's DNA?
I'm not sure if my question makes sense. I was wondering if, theoretically speaking; my ancestors and I reside in Norway for the last 300 years or so (light-skinned and blue -eyed) and travelled to a warmer climate like Dominican Republic or Africa, how long and what mechanisms by which a person's DNA is changed to look like the current inhabitants? — Preceding unsigned comment added by 67.142.96.127 (talk • contribs)
 * How much sex are you having with the natives? -- Jayron 32 01:57, 26 July 2015 (UTC)
 * The DNA itself doesn't change. What happens is that certain organisms reproduce and others don't, and the recombined DNA of those that reproduce is passed on.  Imagine if there's a comet that strikes the earth, and only aquatic creatures and those that live buried in underground dens survive, while those that live above ground are roasted to death. (KT event).  Then all the elephants will go extinct in a few days, if not hours.  That is a drastic "change" in the organism's DNA.  For human evolution, disease (think smallpox in the Americas) is the biggest game changer.  Other traits like hair color can change very slowly within a population.  What matters is the selection pressure. μηδείς (talk) 02:07, 26 July 2015 (UTC)


 * Assuming no interbreeding with the locals (so, assuming a large number of Norwegians accompany you) and ignoring tanning, the skin changes would be the result of Natural selection. One of your descendents could be born with a genetic mutation (not like sci-fi super powers, but a benign mistake in combining the parents' DNA) that causes them to produce more melanin than their ancestors (or their immediate ancestors at any rate).  That descendent would be less likely to develop skin cancer and have more resistance to sunburning, so s/he would lead a healthier life and so be more likely to reproduce.  A mutation could also occur to make them produce less melanin, which would make them more prone to sunburning and skin cancer, and so make it harder for them to survive long enough to reproduce as much as the darker-skinned descendent.  The darker skinned descendent would have their own children, and some of their descendents could have a similar mutation, increasing melanin and so their chances of survivability.  It is theoretically possible (though practically impossible) that the change could happen after a single generation (i.e. you have a kid who blends right in with the Khoisan).  What would be more likely is that it'd take many generations.  I'd guess that 1,000 years would still be theoretically possibly but practically impossible, 10,000 years as possible but remarkably fast, and that it could definitely happen in about 100,000 years.  Of course, this is based on reversing how long it took African colonists in Europe to lose their melanin (giving them more vitamin D), a number that might have been thrown off by interbreeding with neanderthals.  Ian.thomson (talk) 02:16, 26 July 2015 (UTC)


 * I don't think it took nearly 100,000 years for white skin to develop when Homo Sapiens Sapiens moved to Europe. According to Europe, modern humans appeared there some 40-43 thousand years ago.  Not sure if we know how long it took for skin color to lighten after that.  Does the earliest European modern human artwork from which color could be determined show black skin ?  If so, when did it change ? StuRat (talk) 04:19, 26 July 2015 (UTC)
 * Why use artwork when you can use genetics? See Human_skin_color, Light_skin. It's also not just time, one hypothesis is that there was relatively little selection towards lighter skin in Europe until about 6-10k years ago, roughly when certain behavioral and agricultural changes occurred. It seems from the research linked there that there were few if any light-skinned Europeans before ~20k years ago. SemanticMantis (talk) 14:19, 26 July 2015 (UTC)

Scar and hair biology
If someone has a small scar on the chin that makes a beard clearing... Can he transplant hair on the scar? If there are some Dermatologists that claim that they can transplant hair on this scar with about 60% success, are they talking the truth from a biological perspective? Can hair really grow on scars?... Maybe they mean that only some areas in the scar are living skin? What is your final conclusion on this? Thank you!!! Ben-Yeudith (talk) 04:42, 26 July 2015 (UTC)

If you are talking about hair transplant in scar area then answer is "yes". but one should have enough blood supply at that particular area.. t u r b o  05:20, 26 July 2015 (UTC)


 * Turbo, Could you please elaborate even more? Can you give more details of what of the sub-questions the "yes" applies? Thank you. Ben-Yeudith (talk) 13:31, 26 July 2015 (UTC)

Can we have speed greater than c ?
In Michelson-Morley experiment, we saw that the speed of light in space is equal in all inertial frames and in relativity, light speed is defined to be the maximum. Is it really impossible to get speed slightly greater but equivalent to light speed such that if we calculate that speed as light speed, we won't be wrong?Sayan19ghosh99 (talk) 07:52, 26 July 2015 (UTC)
 * Are you asking how sure we are that nothing travels slightly faster than c? Experimentally I think all we can do is measure approximate masses. The mass of the photon is known to be very close to zero, but I suppose that leaves open the possibility that it's a tiny imaginary number (which would make it a tachyon). Theoretically, in quantum field theory, tachyonic particles make the vacuum unstable, which is a very strong reason to think they don't exist. (See Tachyonic field.) -- BenRG (talk) 08:57, 26 July 2015 (UTC)


 * My (non-expert) understanding is that the true c, from the viewpoint of relativity, is the velocity that can be attained only by truly massless particles. If photons had a tiny bit of (non-imaginary) mass, they would necessarily have to travel a bit slower than the relativistic c. However, this would also mean that cphoton, unlike the true c, would no longer be a constant in all frames, which would be quite easy to detect: experiments that measure this are thus experiments which can put the masslessness of the photon to the test.


 * There are also more sensitive tests available: for more on these, see John Baez' page here, and Photon -- The Anome (talk) 13:38, 26 July 2015 (UTC)


 * I might be wrong, but I think the OP is saying something like "What if we accelerate something to light speed, and keep on accelerating it ... even if it is still calculated as moving at the speed of light, couldn't it be going faster?" And the answer there is that it takes an infinite amount of energy to accelerate any object with mass to lightspeed, and it would have infinite relativistic mass; if you could apply two infinities of energy it would have two infinities of relativistic mass, but that doesn't really mean anything, and it doesn't ever happen, because if it happened even once we'd have all been sucked away by the infinite gravity of the object, no matter where it is in the observable universe.


 * That said, photons experience a phenomenon that is sort of like deceleration, in the red shift that occurs as space expands. After all, a galaxy that is coming at us at a slower speed might seem to decelerate and eventually turn back and move away from us as billions of years pass.  The photons can't go slower than light speed, but they do lose energy.  You might compare this to the difference in overall energy in them going from 0 * infinity to 0 * infinity / 2 (or whatever the redshift is at their source), but that of course isn't really proper mathematics. Wnt (talk) 14:59, 26 July 2015 (UTC)
 * Just to be clear, you CAN have 'speed' grater than C, quite easily depending how you define "speed". What you can't have is information faster than C. Our article Faster-than-light which no one has linked yet, is quite good! Vespine (talk) 22:40, 26 July 2015 (UTC)
 * A shadow can travel faster than C. But of course a shadow is not a thing. Void burn (talk) 23:36, 27 July 2015 (UTC)
 * Shadows are indeed mentioned in the article Vespine linked. Along with many other classic examples. Someguy1221 (talk) 00:01, 28 July 2015 (UTC)

Why in Bohr's stationary orbits an electron completes integer wavelengths?
Why mvr=nh/2π and 2πr=nλ ?Sayan19ghosh99 (talk) 07:53, 26 July 2015 (UTC)


 * In the Bohr model it was just assumed to be that way. The only justification for it was that it led to a correct formula for the spectral lines of hydrogen.
 * In real quantum mechanics with the Schrödinger equation, it's for more or less the same reason that musical instruments have discrete harmonics. -- BenRG (talk) 08:36, 26 July 2015 (UTC)


 * To continue a bit with the above, look at the Schrödinger equation article itself, specifically the first time-dependent formula: iħ multiplied by the partial derivative over time of the wave function is equal to the Hamiltonian of the wave function. Solutions to that equation always (???) seem to take the form eiħ [phase], so the wave function ends up as a three-dimensional loop in space, generally with a positive lobe and a negative lobe, as shown.  I'll admit that one of the little mysteries I missed is where the negative end of an 1s orbital is, but I think it must be tucked in there somewhere...  anyway, the point is that the wave function has to go "all the way round" in a way that ends up quantizing its angular momentum in units of ħ. Wnt (talk) 10:55, 26 July 2015 (UTC)

How much can ice reduce the temperature in a room.
If you put two liters of ice in 20 cm3 room, how much degrees will the temperature drop, when the ice melts?--Scicurious (talk) 17:03, 26 July 2015 (UTC)


 * 20 cm3 is a very small room. You couldn't even fit 2 L of ice in there.
 * Assuming you meant 20 m3, you can work it out easily. Density of ice is about 0.92 kg/L, latent heat of fusion is about 335 J/kg, density of air under typical conditions is 1.2 kg m-3, specific heat of air is 1004 J/(kg K). The rest is just arithmetic.
 * If the ice is colder than 0 C you need to take account of the heat absorbed by warming up the ice to the freezing point. Short Brigade Harvester Boris (talk) 17:17, 26 July 2015 (UTC)
 * However, the adjustment for the ice being below 0 C is not large, because the latent heat of fusion is large compared to the heat capacity of ice. Robert McClenon (talk) 17:52, 26 July 2015 (UTC)
 * Generally true, which gives me the opportunity to correct my units: latent heat of fusion is about 335 J/g, not 335 J/kg. Short Brigade Harvester Boris (talk) 21:53, 26 July 2015 (UTC)

How did we get images of asteroid 2011 UW 158?
A week or two ago it came screaming by the Earth and the radio telescope in Puerto Rico took some pictures. Cool, radio pictures of screaming asteroids, but how was it done? One report said they used a 20 tw signal. How do you generate 20 tw of anything, much less a microwave? So they generate this signal, and point it at the asteroid, and the signal bounces back, but how do you make an image out of returns? Are they scanning the asteroid, pixel by pixel? Or are they somehow able to focus the return signal and get an image all at once, like a camera? 50.43.33.62 (talk) 19:14, 26 July 2015 (UTC)
 * Generating 20TW of (peak) power is actually quite easy: you really only need a very short pulse. When this pulse is scattered its temporal profile and spectrum change. From these you can reconstruct the shape of the surface. Ruslik_ Zero 20:07, 26 July 2015 (UTC)


 * The technique of synthetic aperture radar is used to image objects. Twenty Terra Watts is not really a lot of energy, since a watt is defined by joules per second . I.E. If one was to release (transmit) one joule in a nano-second that would would equate to a 100,000,000 watts.. should the transmit power last for a full second.--Aspro (talk) 20:48, 26 July 2015 (UTC)

Easy? Sure it is, but with what? (Watt?) I can imagine all kinds of equipment that could be used for this, like capacitors and batteries and transformers, but what are they actually using? Tin cans? Diesel generators?

I can see how the temporal profile can tell you that some parts are farther away, and spectrum change can tell you if something is moving closer or away, but how do you turn that kind of info into an image? 50.43.33.62 (talk) 02:22, 27 July 2015 (UTC)
 * If you assume that the object is rigidly rotating, the timing and Doppler data alone should give some inferences about shape, even without any fancier tricks such as are presumably described in the article that I haven't looked at yet. —Tamfang (talk) 06:25, 30 July 2015 (UTC)


 * What report quoted twenty terawatts? Are you certain they didn't mean effective radiated power?  Power is a complicated parameter for RADAR!  Start by reading RADAR signal characteristics.
 * https://en.wikipedia.org/wiki/Arecibo_Observatory#General_information50.43.33.62 (talk) 06:17, 29 July 2015 (UTC)
 * "The Radio Telescope" mentioned in the original question probably refers to the Arecibo Observatory. Arecibo usually operates passively, but for some experiments it can transmit as a true RADAR, as well: read about how the transmitter works; when it does so, it  typically uses about 150 kilowatts of average power - that is how much power the electrical generator is providing.  (That website is a bit old, citing an operations manual from year 2001, and there have been many facility upgrades at Arecibo since then - so use all of these numbers with caution)!  The transmitter is often used to send a pulsed RADAR signal; so at any given fraction of a second, either zero watts or 2.5 megawatts are being transmitted.
 * That peak pulse number provides true instantaneous power. To compute peak ERP, you multiply the power with the antenna gain to derive a (fictitious) equivalent as if the energy was transmitted in all directions.  This allows us to use simplfications in the RADAR equation.  In actual fact, the energy is directed by the RADAR antenna: it is not a spherically-isotropic source.  Radio astronomers like ERP because it's the amount of power that an equivalent spherical cow would emit.
 * In specific, you can read about the high priority NEO survey conducted by Arecibo to image 2011 UW 158: Current NEO surveys from the Solar System Sudies group at Arecibo Observatory. Their nominal operational power was scheduled to operate at  900 kilowatts for the survey.
 * For big antenna facilities like Arecibo, antenna gain is enormous - so you can get very strange values when you look at "effective" radiated power. The physical quantity of energy per unit time, on average, is still only the amount supplied by the electricity source, and is not "terawatts."
 * Nimur (talk) 14:29, 27 July 2015 (UTC)
 * Excellent reply from Nimur, as always. To answer the OP's specific question, the transmitter device is a klystron. Tevildo (talk) 22:12, 27 July 2015 (UTC)


 * One way to boost the power in a pulse, but keep the same energy, is to generate a sweeping frequency. Then the signal is put through dispersion so that the earlier frequency comes out at the same time as the latter frequency, and all the power piles up into a very short pulse. Graeme Bartlett (talk) 22:57, 27 July 2015 (UTC)