Wikipedia:Reference desk/Archives/Science/2013 February 26

= February 26 =

4K resolution tv
Does an 84 inch 4K tv have a similar ppi to a 32 inch Full HD tv? If not, are the differences insignificant due to limitations with the human eye? Clover345 (talk) 00:58, 26 February 2013 (UTC)


 * I'm not quite sure what a "4K tv" is, but I imagine you mean it interpolates between the pixels in a 1080p signal, and adds new pixels, accordingly. While this could make it look less blocky, it will make it more fuzzy.  The only way to make an 84 inch screen look good with a 1080p signal (or, God-forbid, a lower res signal), is to sit a long way away from it.  (Incidentally, the Computer Desk might be better for this type of Q.) StuRat (talk) 01:13, 26 February 2013 (UTC)


 * Stu, you might at least look it up in Wikipedia before replying: 4K resolution. It refers to TV (and other devices) designed to display ultra high definition television signals.  This technology is just emerging and most people won't yet have access to UHD sources.  Dragons flight (talk) 01:23, 26 February 2013 (UTC)


 * I didn't find an article under 4K resolution television, which is what I looked for. Looking at that article, they apparently have switched from labeling resolutions by vertical lines to horizontal, no doubt to trick people into thinking they are getting more than they are.  How annoying.  I'm aware that such a signal isn't likely to be available anytime soon, hence my comments on interpolation to upconvert a lower res signal.  StuRat (talk) 01:29, 26 February 2013 (UTC)


 * UPDATE: Someone has now added a redirect so that link now works. StuRat (talk) 16:22, 26 February 2013 (UTC)


 * To answer the original question, a 4K UHD TV will have similar PPI to HD TV that is 1/2 its size. So a hypothetical 84 inch 4K TV will have similar pixel density to a 42 inch 1080p HD TV.  Dragons flight (talk) 01:26, 26 February 2013 (UTC)


 * Regarding whether your eye can resolve such detail, you might read the technical section in Retina display; or the more generic angular resolution article. Nimur (talk) 02:22, 26 February 2013 (UTC)


 * 1920 minutes of arc is only 32 degrees, and some can see up to 20/10 (twice as sharp) so not insignificant. With the 84" 4K set you can start seeing pixels at $$84/((sqrt(16^2+9^2))/16*3840*2)*tan(90-1/120)$$ ° inches away — 89.5% of the screen's width, even up to 1.79x the screen's width if you have Chuck Yeagar's eyes. And I want a monitor that remains pixeless as I walk 4 times closer than the couch like it's the future, the 8K TV is for me. Sagittarian Milky Way (talk) 03:29, 26 February 2013 (UTC)


 * A 4K tv has 4,000 pixels per scanline. A regular "1080p" tv has 1,080 pixels per scanline.  So an 84" 4K TV has the same per-pixel angular resolution as a 21" 1080p TV when viewed at the same distance.
 * HOWEVER: There are no good sources of 4K content. The highest resolution you get from a bluray disk is 1920 pixels (and that's either interlaced or at a reduced frame rate - so the "quality" is arguably no better than 1080p) - but I doubt that many disks are mastered at that resolution.  Broadcast & cable TV tends to top out at 1080p.
 * What this means is that your 4K tv has to stretch the lower resolution image to fit the screen - and almost certainly, that will result in an image that is no sharper than a 1080p tv of the same size viewed at the same distance.
 * Right now, the only use for a 4K TV might be as a computer display...some graphics cards can generate signals at that resolution. Probably there are video games out there that would benefit from being displayed on a 4k TV versus a 1920 pixel computer screen...but even that is highly problematic because the amount of calculation needed to compute a video image that large tends to degrade the frame rate - making for a worse experience overall.
 * I could imagine someone who edits digital movies from finding a benefit in this technology - but otherwise 4K TV is a solution looking for a problem.
 * SteveBaker (talk) 14:46, 26 February 2013 (UTC)


 * Steve, apparently you didn't read the article or my comment on it above. From the lede: "The term 4K refers to the horizontal resolution of these formats, which are all on the order of 4,000 pixels".  My comment: "Looking at that article, they apparently have switched from labeling resolutions by vertical lines to horizontal, no doubt to trick people into thinking they are getting more than they are.  How annoying.". StuRat (talk) 16:25, 26 February 2013 (UTC)


 * There's content and GPU problems with 4K screens. That's why it's future tech instead of current tech. I'm using a 23.625" 1080p TV as a monitor and I wish it was 4K so I could stop seeing pixels. That is one reason to use 4K now, if you can afford it: my pixels look honking huge, and I don't want to go back to a small monitor for smaller pixels again. My crappy 0.6 gigapixel per second graphics card feeds 2 million pixels at 60Hz with no problem, except for intensive graphics like Google Earth with lots of icons to draw. Crysis 3 at 4K would test even very good modern cards, but you could always turn the resolution down if need be. Sagittarian Milky Way (talk) 18:57, 26 February 2013 (UTC)


 * I don't think TVs make very good computer monitors, since they are designed for more distant viewers. While they are fine if you're sitting across the room, they're not if your face is right in front of them.  I have an only slightly better res 1600×1200 monitor, and it's 24 inches diagonally, and I sit close to it, too, but I don't see individual pixels.  I believe this is because the gaps between the pixels are intentionally kept smaller on computer monitors. StuRat (talk) 22:50, 26 February 2013 (UTC)
 * That was true in the past, when computer monitors and TV's were CRT based, and TV was analog TV. One of my computers is an old PC (bought in 1995).  I pusrchased with it back then an NEC Multisync 1600 x 1200 monitor - a top model in its' day.  It had a dot pitch of 0.25 mm and a video bandwidth of >100 MHz.  This is way way outclassing a CRT based analog TV of same diagonal size (21 inch) the old top of the line Philips I have has a dot pitch of 3.3 mm and an RGB video bandwidth of only 5.5 MHz.  But the NEC monitor cost a lot more than the TV too - about $2000 vs ~$600, equivalent to about $8000 today.  Recently the NEC monitor stopped working.  I replaced it with a 19" diagonal LED digital TV Teac model LEDV19U83 Full-HD.  It has a "VGA" input and so can be used as a computer monitor.  It cost $139.99, cheaper than the cheapest 1600 x 1200 monitor the shop had, and does 1600 x 1200 @70 Hz.  I can't tell you what the dot pitch and video bandwidth is, as it is not specified in the manual.  But it clearly outperforms the old NEC monitor.  Why is it so?  Because it was difficult and costly to make high resolution CRT's, and high power electronics was required to drive them.  But LED screen manufacturing cost is mainly a function of the screen area, and not the resolution.  The driving electronics are low power and simplified. Ratbone 121.221.5.16 (talk) 02:32, 27 February 2013 (UTC)

In what respect two protons (or any two fermion) are not identical?
No two electrons of the same atom are identical. If we choose two electrons of two different atoms having same orbit, same subshell, same spin, same quantum numbers, would they be identical? I know two protons (or any two fermions) are not identical. In what respect two protons (or any two fermion) are not identical? Is it mass, charge, shape, size, or anything else? 106.218.233.58 (talk) 03:00, 26 February 2013 (UTC)
 * I'm certainly no expert but according to Identical particles, electrons ARE in fact identical, as are the other fermions and bosons. Vespine (talk) 03:17, 26 February 2013 (UTC)
 * Then, what about Pauli Exclusion Principle 106.218.233.58 (talk) 03:23, 26 February 2013 (UTC)
 * As you say, they're part of orbitals of (and have quantum numbers with respect to) different atoms (coordinate systems or frames of reference for the wavefunctions). Whatever equation you use to describe 1s, for example, is based on an origin at the center of the nucleus. If you expand your perspective to include two different atoms, there's an offset for the actual position of that center, which is different for different atoms in your world. DMacks (talk) 03:53, 26 February 2013 (UTC)


 * Protons/electrons are not identical. The concept you're referring to is distinguishability (no article?), which has a very specific meaning.  Each electron is described by a wavefunction, and any system of two electrons can be described with a system wavefunction that's a combination of the two electrons' wavefunctions.  Electrons are indistinguishable because if switch their wavefunctions, the system wavefunction remains identical.*  This property is called exchange symmetry.  Nobody, not even God, can tell if the two electrons were exchanged or not; an exchanged system behaves exactly the same as an unexchanged system.  Distinguishability doesn't hold for classical particles, because if electrons were classical, you could always mark one of them with chalk, or paint one of them red, and you'll always be able to distinguish them.
 * * Actually, this is not true. Since what matters for any measurement is the square of the wavefunction, the wavefunction itself can either stay the same or switch sign.  Bosons do the former; fermions do the latter.  --140.180.254.250 (talk) 07:42, 26 February 2013 (UTC)
 * Not sure if it's what 250 was looking for above regarding distinguishability, but we do have an article on indiscernibility. It's framed more philosophically, going back to Leibniz, but I think it is relevant for the current question as well. If someone has more confidence on the matter, perhaps they could be bold and add a redirect. SemanticMantis (talk) 16:03, 26 February 2013 (UTC)
 * The article I linked quite plainly says: indistinguishable or indiscernible particles, are particles that cannot be distinguished from one another, even in principle. Am I misunderstanding or is the article wrong? Vespine (talk) 21:49, 26 February 2013 (UTC)
 * I think this is just a nomenclature issue. I think the party line is that protons are not identical (i.e. there is more than one proton), but that they are indistinguishable (or perhaps indiscernible). I only linked the indiscernibility article for background context, not to say anything written here is in error. Basically, in physics and philosophy, identity and discernability are separate issues. As discussed in the linked article, identity of indiscernibles is philosophically (or perhaps physically) debatable, while indsiscernibility of identicals is a (relatively) settled issue. SemanticMantis (talk) 14:47, 27 February 2013 (UTC)


 * Identical particles have the same rest mass, electric charge, and spin angular momentum, but can be distinguished by position, linear momentum, orbital angular momentum, and direction of the spin angular momentum. Quantum particle indistinguishability is stronger than merely having some properties in common, though—it also affects statistics.


 * Indistinguishability is another example of a property that's often called "purely quantum" but actually survives in the classical wave limit. If you put one joule of horizontally polarized light (governed by Maxwell's equations) into a mirrored box whose walls preserve the polarization, and later add another joule with the same polarization, and then extract one joule, you can't say which of the original joules it was, even in principle. On the other hand, if the first joule was horizontally polarized and the second was vertically polarized, you can say which of the original joules you extracted. That's the difference between being indistinguishable (Bose-Einstein statistics) and merely having some identical properties ("classical statistics"). I'm less clear on how you can get Fermi-Dirac statistics classically, but there appear to be papers on the arXiv showing how to do it, e.g., physics/0601014. -- BenRG (talk) 17:35, 28 February 2013 (UTC)

T-90 tank Shtora system
What is the reason that Shtora was able to disrupt 6 kornet missiles from a total of 10 http://fofanov.armor.kiev.ua/Tanks/TRIALS/19991020.html 87.236.232.97 (talk) 08:10, 26 February 2013 (UTC)
 * I have to admit that I'm confused as to the exact thrust of your question. Are you asking why Shtora is at all effective against 9M133 Kornet missiles?  Because Shtora is an electro-optical countermeasure and the Kornet is laser-guided.  Are you asking why it's not 100% effective?  Broadly, because countermeasures rarely are.  Specifics probably depend on the nature of the test.  Our article suggests that while it has a 360° field of use, it may not cover all 360° with equal effectiveness simultaneously.  Additionally, there is a time delay to deploy its aerosol screen, and said screen also dissipates in a fairly short amount of time.  It's also possible that the system wasn't being run in fully automatic mode, which introduces the possibility of operator error.  Finally, you might be asking why specifically 6 of 10 missiles were disrupted, in which case little can be said except that that's how probability worked out in that particular test on that particular day. &mdash; Lomn 14:39, 26 February 2013 (UTC)

How much sensation does a tortoise/turtle have in its shell?
I've heard of occasional occurrences of people graffiting the shells of living large tortoises at wildlife parks - sometimes by carving into the shell with a knife (and also the repair attempts afterwards, using polyfilla type stuff and silicone sealant). I gather that it's more common for people to write stuff on the shell with marker pen or tippex or something, but anyway...

To what extent would the tortoise feel someone cutting their name into its shell? Or are the whole outer layers of shell pretty much dead tissue and sensation-free? --Kurt Shaped Box (talk) 08:46, 26 February 2013 (UTC)


 * Turtle shell does not seem to address this question. I googled [can turtles feel their shells] and well as [can turtles feel pain on their shells], and a variety of contradictory answers turned up. ←Baseball Bugs What's up, Doc? carrots→ 10:09, 26 February 2013 (UTC)


 * I searched google scholar for /turtle shell innervation/ and found this paper, titled "Carapace and plastron sensitivity to touch and vibration in the tortoise." :It seems to address your question rather rigorously and scientifically, but you might need to get to a library to read it. The abstract says "Neural impulses in response to tactile stimulation of the shell were recorded from afferent nerve fibres in tortoises (T. graeca and T. hermanni). It was found that there is a mechanoreceptive innervation in the superficial layers of the shell which is sensitive to transient stimuli, particularly to vibration at frequencies up to 100 Hz. Receptive fields pertaining to single and small groups of individual afferent fibres were mapped: the fields were sharply circumscribed and distributed in relation to the scutes of the shell. The tactile innervation that was found would be consistent with a capacity for recognition and accurate localization of innocuous stimuli and may play a central role in courtship and mating behaviour."


 * So, it seems that at least one tortoise can feel at least some touch on its shell. To get into more detail, I'd look to see what papers have cited that one. SemanticMantis (talk) 14:19, 26 February 2013 (UTC)
 * But vibration is a funny thing. If my hand is on the table and I can feel you bang on the table 4 feet away, just because I can detect superficial vibration on the table top of on its legs inches or feet away doesn't mean that the vibration receptors (perhaps Pacinian corpuscles, as in mammals) exist in the shell.  My point is this -- the shell is firmly attached to the living tissues of the tortoise and so the organim can feel when you tap on the shell, for instance, but detecting pain would require free nerve endings or other end bulbs that I'm not qualified to respond about -- but perhaps this distinction ought to be made rather than confusing the two very different things of  "can a tortoise feel its shell being manipulated" vs. "is carving a living tortoise's shell a painful stimulus to the tortoise."  DRosenbach  ( Talk 18:12, 26 February 2013 (UTC)
 * I had similar thoughts, but this line " It was found that there is a mechanoreceptive innervation in the superficial layers of the shell which is sensitive to transient stimuli"- seems to indicate that there are nerve endings in the shell. That being said, the scutes are mostly (entirely?) keratin, which has no innervation. So, as to the two questions you pose, my guess (based on the linked readings) is that a turtle could sense being carved on, but would not feel pain unless the scutes were penetrated. SemanticMantis (talk) 20:10, 26 February 2013 (UTC)
 * Big Tortoises (Aldabras,etc) that are too heavy to lift can be "steered" quite readily by slapping on the side scutes to steer and on the rear scutes to get them to move forward. Zookeepers commonly use this to move them from one exhibit to another, or bring them inside at night. They will usually only move about 2 metres, then you have to let them rest for a few minutes before they will respond to more signals. 124.191.176.235 (talk) 05:44, 27 February 2013 (UTC)
 * Not that I doubt you, but you know this how? μηδείς (talk) 02:39, 28 February 2013 (UTC)
 * From a long and varied career as a zookeeper. I know there's a massive body of knowledge in past and current zookeepers, much of which is unrecorded and will die with them. 124.191.176.235 (talk) 07:06, 28 February 2013 (UTC)

I read the paper Semantic linked to. Quite on topic is the following: The level of sensory endings within the shell was determined by testing the effect of progressive abrasion of the outer horny scute. Complete penetration of the scute caused abolition of the locally elicited response (Fig. 6). Partial abrasion, however, led to an increase in response (Fig. 7). These results support the view that the receptive surface under examination lies near the underside of the scute. An accompanying diagram of the experiment helps elucidate their conclusions - basically, they assert that sensory nerve endings may be found a mere 0.5mm beneath the surface of the shell. They show earlier in the paper that stimuli as gentle as a brush stroking over the shell can be felt by the tortoise. They also show that not all areas of the shell are equally sensitive. Someguy1221 (talk) 03:05, 28 February 2013 (UTC)

About the gravity field of black hole
Some time we see news about black holes in center of galaxies, but nothing is there in references about exact properties of black holes gravity field. When the black hole in center of galaxy creates strong gravity field how is space time shape or figure of such field? Does it never done calculating or drawing such field. Is the potential diagram 1/r? --Akbarmohammadzade (talk) 09:19, 26 February 2013 (UTC)


 * A black hole is nothing special on the outside other than being a big lump of mass in a very small area. Its gravitational potential indeed drops as 1/r, as with any planet or star. Near the surface of a black hole, however, you start seeing the neat tricks of gravity when you can orbit a supermassive body at extremely tight radius: relativistic effects like frame dragging, tidal forces that lead to spaghettification, etc. SamuelRiv (talk) 15:28, 26 February 2013 (UTC)

thank you
 * With shortages have been monitored in theories and existing explanations it can not be considered that any central black hole be able to tide  whole galaxy members !!!A.Mohammadzade  — Preceding unsigned comment added by 2.187.66.54 (talk) 21:04, 26 February 2013 (UTC)

endosymbiotic theory of prokaryotic evolution
how can natural selection explain why, after the anaerobic primitive cell ingested aerobic bacterium to become its mitochondrion been able to divide so that after mitosis, both daughter cells have mitochondrions? it is like me ingesting a worm but not digezsting and when i divide to reproduce, the worm also replicate so that i and my sons could have worms inside our bodies.TTLOAFH (talk) 10:26, 26 February 2013 (UTC)
 * Cells can have loads of mitochrondria inside them though many only have one, they reproduce inside the cell. What would have evolved is a way of properly regulating the number rather than having them reproducing unchecked inside the cell. Dmcq (talk) 13:25, 26 February 2013 (UTC)


 * In a recent BBC documentary it was mentioned that sexual reproduction might have evolved 1.5 billion years ago to deal with this problem. Count Iblis (talk) 14:18, 26 February 2013 (UTC)


 * Unless I am missing something, that doesn't make sense. Mitochondria have their own DNA, so they are unaffected by sexual recombination.  The fact that mitochondria reproduce asexually even in cells that reproduce sexually is what has made mtDNA a useful tool for tracking ancestry (on the maternal side). Looie496 (talk) 16:13, 26 February 2013 (UTC)
 * Or even simpler to counter Count Iblis's comment, sexual reproduction does not affect mitochondrial reproduction. Instead of a cell doubling and the daughter cells doubling and so on and so forth to create new life forms, sexual reproduction merely allows for the separation (haploid state) and combining of alleles (diploid state) to further mingling within what amounts to a gene pool -- but the first new diploid cell then undergoes mitosis to form its own diploid, multicellular organism and the mitochondria in each of these cells must split.  DRosenbach  ( Talk 18:04, 26 February 2013 (UTC)


 * See about ten minutes into this programme. Count Iblis (talk) 18:34, 26 February 2013 (UTC)
 * Seems extremely unlikely to me, there are large numbers of such cells that never bother with sex and there are other much better reasons for sex. If you listen to that - where did this neighbouring cell with mitochrondria come from if not from straighforward splitting instead of sex? Splitting happens far more often. Dmcq (talk) 18:11, 27 February 2013 (UTC)

Reversing light-emitting diode
Can a LED work in reverse order, transforming light into electricity instead of electricity into light? OsmanRF34 (talk) 14:14, 26 February 2013 (UTC)
 * That would be a solar cell, which is not a form of diode. So no, you can't have an LED that does this (because it wouldn't be light-emitting), nor can you have another form of diode that does this (they don't do that), but you can have a semiconductor-based circuit that does this. &mdash; Lomn 14:27, 26 February 2013 (UTC)
 * Not correct. LEDs and photovoltaic cells are all semiconductor PN diodes.  Shining light on any semiconductor diode will generate electrical power.  However, the efficiency of LEDs in converting light into electrical power is abyssmal, and of little if any practical use.  Googling "LEDs as photodiodes" will give you a multitude of web pages describing experimental results.  Ratbone 120.145.40.231 (talk) 14:37, 26 February 2013 (UTC)
 * Ratbone is correct, as a bit more thought and searching would have told me. In fact, here's a recent article discussing a new high-efficiency LED-type solar cell design. &mdash; Lomn 14:49, 26 February 2013 (UTC)
 * A reversed LED might make a poor power generator, but it works well enough to make quite a decent light detector. I saw a great project once that used alternating rows of LEDs configured to emit and detect light respectively. The sensitivity was set so the detectors wouldn't trip until an object, such as your hand, came close enough to reflect a significant portion on the light emitting LEDs, making a pretty decent close proximity, 'touch less' switch. I searched for a couple of minutes but I can't find it. Vespine (talk) 23:28, 26 February 2013 (UTC)

Why "Hornblower's Sign?"
A test for injury to certain shoulder muscles is called "Hornblower's sign." Is it named after some Doctor Hornblower, or is the particular arm position involved thought to resemble that used in "blowing a horn?" Edison (talk) 15:53, 26 February 2013 (UTC)
 * I couldn't find a specific statement about this, but in the literature it is commonly used as "hornblower's sign" or "hornblower sign", and combined with the fact that this (inability to rotate the shoulder joint) is a problem that would naturally occur in somebody holding a horn for hours a day, the answer seems reasonably clear. Looie496 (talk) 16:07, 26 February 2013 (UTC)


 * A thorough search of Google has failed to turn up any famous medical people called Hornblower, although Hornblower is an English surname. However, I found Orthopedic Physical Assessment, by David J. Magee (p.313), which for some reason also gives the name of the test in French, "Signe de Clairon" which translates as "Bugle Sign". This seems to confirm your second suggestion. Alansplodge (talk) 16:15, 26 February 2013 (UTC)

One photon at a time
Is it possible to connect a light emitting diode to a battery in such a way that light is only emitted one photon at a time? Either one photon for each push on a switch or a continuing series of photons with significant time gaps between.

Thanks, Wanderer57 (talk) 16:27, 26 February 2013 (UTC)


 * Yes - see this news report from 2001. But it does not sound easy - for example, the prototype device had to be cooled to 5K. Gandalf61 (talk) 16:43, 26 February 2013 (UTC)


 * You might also find shot noise enlightening. You can use any method to reduce the intensity of photons down to one photon per unit time, where one unit of time is a completely arbitrary duration.  If you observe for n units of time, you expect to see n photons; but in practice, you actually observe n photons on average.  The error is due to shot noise.  If you expect to emit (or observe/absorb) exactly one photon, you will actually sometimes see zero photons, and sometimes one photon, and sometimes some other number of photons.  The better that you can control your experiment, the less shot-noise will affect your measurement.  This is a good thought-experiment that can help develop intuition about the statistical interpretation of quantized observations.  It's also very easy to set up as a real experiment, in a simple lab, using very common equipment - like a digital camera and a dark shoe-box.  Cooling the device will help eliminate thermal noise.  Thermal noise is a different physical effect that superimposes on the shot noise, contributing to the experimentally-measured value.  What do you know, there's an MIT OpenCourse lab detailing this exact experiment! Nimur (talk) 17:12, 26 February 2013 (UTC)


 * It is relatively easy to get a low number of photons per time by e.g. start with any light source and put enough absorbing material in front of it. Getting exactly one photon on the push of a button (not just one on average) is more difficult. In quantum mechanics this is called a Fock state with one photon and there is some information on how to create these in Fock state. Ulflund (talk) 17:33, 26 February 2013 (UTC)
 * I don't see how the "quantum mechanical case" is any different from the "classical" case, except that one uses the somewhat obtuse bra-ket notation. Shot-noise describes quantized events, whether they are microscopic or macroscopic in origin; the arrival of a single photon is still a single quantized event, whether the origin of that photon is described using a bra-ket equation or by putting several ND filters in front of a light-bulb.  The photon doesn't know the difference.  If I put a thousand ND filters in front of an incandescent bulb, and used a button to turn on the bulb, I might see one photon at my detector.  Haven't I constructed a "macroscopic quantum-mechanical system" that can emit one photon at the push of a button?  It's the same system, whether you use difficult mathematics to describe it or not.  Nimur (talk) 17:58, 26 February 2013 (UTC)
 * Well, as you said, your approach is not very reliable; that section of the article appears to be about reliably getting one photon and not 0 or 2. -- BenRG (talk) 02:50, 27 February 2013 (UTC)
 * Is there reason to believe the signal-to-noise ratio would be any different at all? The OpenCourseWare paper I linked above derives the signal-to-noise ratio of shot noise, from first principles; if we are concerned with only one photon, then we are definitionally in the limiting-case of a Poisson distribution, and no matter what system you construct to determine when the emission happens, - quantum-mechanical or otherwise - the signal-to-noise-ratio is always the same.  "According to Poisson statistics, the variance of n is the mean... ."  Or, to put it another way, when your expected value is one photon, your variance is also one photon.  Nimur (talk) 04:07, 27 February 2013 (UTC)


 * You seem to misunderstand, the research grade single photon sources are specially designed so that releasing two photons is simply not an option (typically for a specific wavelength and interval). For shot noise, there is a Poisson distribution such that the probability of seeing n photons p(n) is always greater than zero for all n.  One can tune such a system so that the probability of getting two photons within a given time interval is arbitrarily low (at the expense of reducing the one photon rate), however the one-photon research sources are different.  They are physical systems such that the probability of getting n photons is exactly zero for all n greater than 1.  For example, you might be watching a quantum dot in its lowest excited state, and it can only collapse once (and emit one photon) without the researcher intervening to re-excite it.  Dragons flight (talk) 04:36, 27 February 2013 (UTC)

Log laws
Log laws tell me that the integral of dx/x is lnX + C so why is the integral of adT/T equal to aln(T2/T1)? Clover345 (talk) 16:48, 26 February 2013 (UTC)


 * You're looking at the difference between taking the antiderivative (the indefinite integral) and evaluating a definite integral. (Bear in mind that one property of logarithms is that log(A)-log(B) = log(A/B). ) TenOfAllTrades(talk) 17:07, 26 February 2013 (UTC)


 * Shouldn't this be on the Math Desk ? StuRat (talk) 17:08, 26 February 2013 (UTC)


 * I know the integral of dT is T2-T1 but won't that mean the integral of adT/T is a(T2-T1)lnT? Clover345 (talk) 17:11, 26 February 2013 (UTC)
 * (EC)You are confusing indefinite integrals with definite integrals. The first expression you gave is for an indefinite integral, and the C is the constant of integration. In the second expression, you are effectively integrating a dT/T from the limits of integration (T1,T2). Integrals don't notice scalar multiples, so you pull that outside. When integrating dT/T, over (T1,T2) you first get Log(T2)-Log(T1). Then, the Log identities (Logarithm) tell us that the difference of two Logs is the Log of the quotient. Remembering we took the scalar out, that gives us \int_T1_T2 (adT/T)=a Log(T2/T1). Make sense? (I am at work and busy/lazy. If anyone cares to pretty print my math, feel free :) SemanticMantis (talk) 17:13, 26 February 2013 (UTC)
 * Further detail, you are making a very common mistake of trying to treat the factors of the integrand separately. To handle (non-scalar) factors correctly, you'd need integration by parts. But this problem doesn't need that, it is just a simple application of the fact that \int dx/x = log x + c, and fundamental theorem of calculus (version applied to definite integrals). SemanticMantis (talk) 17:18, 26 February 2013 (UTC)

The indefinite integral of adT/T is aln T + C. The definite integral of adT/T, evaluated from T1 to T2, is found by first plugging T2 into the indefinite integral, to get aln T2 + C, and then subtracting from that what you get when you plug T1 into the indefinite integral, which is aln T1 + C. Performing the subtraction gives a ln T2 - a ln T1 = a(ln T2-ln T1) = a ln(T2/T1). Duoduoduo (talk) 18:18, 26 February 2013 (UTC)

Angiosperms
The article on peppers doesn't indicate that this species (and its many member cultivars) are angiosperms, even though the various peppers are flowering seed-producing plants, which according to how I understand it, should rank them as angiosperm spermatophyes. Nonetheless, the pepper article calls them eudicot instead of angiosperm, and the eudicot article refers to the term 'eudicot' as a clade along with 'angiosperm', which seems to mean that one clade is not contained within the other -- so are eudicots (and therefore peppers) angiosperms?  DRosenbach  ( Talk 17:52, 26 February 2013 (UTC)
 * Eudicots are a subset of angiosperms. Basically angiosperms divide into monocots, dicots, and a few weird things such as the magnolia.  Monocots can be thought of as grasses and things that are similar to grasses.  Dicots are the other major group, with over 100,000 species, and the great majority of existing dicots are eudicots (which basically means "true dicots"). Looie496 (talk) 18:43, 26 February 2013 (UTC)
 * To further explain, a clade is simply a group containing all the descendants of a given ancestor species, and nothing that is not descended from that single ancestor species. It is entirely possible for one clade to be contained in another clade. Looie496 (talk) 18:46, 26 February 2013 (UTC)
 * Eudicots are a subbracnh of angiospems, the largest, and containing what we might consider typical flowering plants with two seed-leaves, flowers usually with five, or sometimes four petals, and leaves with branching veins. Think rose, oak, poinsettia, sunflower, chili peppers, and even cactuses.  Monocots are a large branch of angiosperms distinguished by one seed leaf, leaves with parallel veins, and three-part flowers.  Think grasses, orchids, [lilies, onions, bananas, palm trees.  There are various other more primitive angiosperms, including the magnoliales, that generally resemble dicots but are more primitive, and inlcudes magnolias, laurales, and black pepper.  This very simplified diagram of all the angiosperms is a little easier to understand than the cramped diagrams we have at our own article.μηδείς (talk) 18:58, 26 February 2013 (UTC)

when a capacitor discharges, is it all converted to heat + light? what percent light? if light can't escape, is it all converted to heat?
I don't understand something. could a single charged capacitor in theory heat a room just as well as the equivalent amount of charge going into any other radiator/heater occupying the same space, in terms of heat generated? I realize that the discharge could produce light - but if you enclose the whole thing in something (like tiles) then doesn't the light also have to convert to heat as it dissipates from bouncing around?

What I mean to say is this: look at this answer

http://answers.yahoo.com/question/index?qid=20110416105219AALeYzb

THe guy (first response) does a calculation which shows that a "large" 100000µF 100 volt capacitor in the end stores "500 joules" as compared with 5400 joules in an AAA battery.

However if you short the AAA battery it still won't instantly discharge 5400 joules as heat. Whereas a capacitor has very low inherent resistance and I imagine causes a big spark and instant discharge.

So my question is this: if you tried to use a "500 joule" power source (of any kind) to heat, for example an AAA battery that is 9/10th drained :), then is it guaranteed to do just as much heat as instantly discharging a "500 joule" capacitor inside a container where light has to bounce around?

Or is there some kind of extra inefficiency (with respect to trying to turn charge into heat, which is our goal) due to the instant discharge? I can think of one off-hand: the noise if it makes a spark noise is mechanical energy some of which would leave before it turned into heat.

is that hte only loss? otherwise is a capacitor an "optimal heater" for that many joules (as compared with a power source discharging that many joules over some time into some heating-device) when instantly discharge? How much are the losses from the nosie? THanks. 178.48.114.143 (talk) 19:00, 26 February 2013 (UTC)


 * Any sound and light produced from the spark will be negligible compared to the heat, and most of this will end up as heat anyway. The capacitor resistance will not be quite zero, but, even if the capacitor takes half a second to discharge, 500 joules in 0.5 seconds is just the equivalent of a 1Kw heater switched on for half a second -- not very much heat to heat a room, but possibly sufficient to damage the capacitor or its leads.    D b f i r s   20:19, 26 February 2013 (UTC)


 * It seems from your question that you are thinking of discharging the capacitor or battery by short-circuiting it. If you short a battery, the energy is almost entirely dissipated in its' internal resistance (assuming it isn't damaged, which for many batterries it will be).  In doing so, almost all of the energy is converted directly into heat, raisning the temperature of the battery.  However, with capacitors, compared with batteries of similar size in terms of energy storage, the internal resistance is a lot less.  This means that the inductance of of the "short circuit" must be taken into acount.  It means that shorting a capacitor results in emission of significant amounts of electromagnetic energy - in laymans' terms, short-circuiting a capacitor results in practice in a brief emission of a radio wave.
 * If the desire is to confirm that energy is energy and can regardles of source be converted into heat, a better way is to discharge either the capacitor or the battery into a resistance, sized in both cases so that the discharge time is long enough to avoid inductive effects so that conversion to radio wave energy is negligible. Then, in both cases, virtually all the stored energy will go into heating the resistor and then be transfered to the surrounding room.  In this case, a capacitor large enough to store the same as a battery will heat the room to the same degree.  In discharging into a resistance, there is no need to cause sparks and light or sound.  Keit 58.170.129.186 (talk) 01:30, 27 February 2013 (UTC)


 * As regards the light produced by a discharging capacitor: any light which is trapped (for example, by putting the capacitor in a box) will be converted into heat (in our example, it's absorbed by the walls of the box which heats them up). As for what percentage of energy becomes light, this depends on how the discharge occurs. Light will be created if the discharge causes a spark though this is technically because the spark heats the air it passes through and anything sufficiently hot will radiate light. Even if there is no spark, small amounts of light will be emitted because the capacitor itself will be hot. The power emitted as electromagnetic radiation depends on the temperature of the capacitor (and the spark if present) and can be calculated from the Stefan-Boltzmann law. Some of this radiation will be light but much of it may be elsewhere in the electro-magnetic spectrum (e.g. infra-red). How much of the radiation is actually visible light can be calculated from Planck's law. Again I stress: the discharging of the capacitor produces heat and then light is emitted because it is in the nature of hot things to emit light in order to achieve Thermal equilibrium with their surroundings. Eaglehaslanded (talk) 03:10, 27 February 2013 (UTC)
 * All good theory. But, if you actually DO the calculations, you'll find that in just about every possible real life non-sparking case, and ignoring deliberate light creating strageies like discharging into a light globe, LED, etc, the amount of light so emitted is a negligible fraction of the system energy.  It's a bit like saying light is emitted from the hot water when you make a cup of tea.  Keit 121.221.5.16 (talk) 03:39, 27 February 2013 (UTC)

What kind of fish is this
The BBC news story Giant 6ft long ling fish caught off Shetland doesn't say what kind of fish it is. Can anyone identify it? -- Q Chris (talk) 19:14, 26 February 2013 (UTC)
 * Yes she does, she identifies it as a ling (although not the specific species) in the first sentence. μηδείς (talk) 19:21, 26 February 2013 (UTC)
 * interesting phenomena - I, too, missread and fused ling long' to long. Just like 'Paris in the the spring', I guess. Zarnivop (talk) 03:23, 27 February 2013 (UTC)
 * I'm sure the 'ling' in the strapline was not there when it was originally posted (BBC English is riddled with errors) because I read it deliberately, several times, wondering why they used just the word 'fish' without any sort of identification. 6ft long isn't big for many fish, but of course very big for a shore caught ling. Richard Avery (talk) 08:07, 27 February 2013 (UTC)
 * I can confirm it was not. -- Q Chris (talk) 12:40, 27 February 2013 (UTC)
 * You are correct, Ave, it wasn't, or I would not have bothered even to listen. Shows how much you can trust the BBC are doing the honorable thing. μηδείς (talk) 02:37, 28 February 2013 (UTC)

T-90 Shtora System
It seems that you understood my question more than me, but what I meant from the question is why the Kornet was expected to be not harmed by Shtora as stated in the conclusion of that page "Report of Shtora-1 EOCMDAS trials is confusing. Being laser-guided, ATGM Kornet should not suffer any interference from Shtora as it only affects IR SACLOS ATGMs" 149.200.151.156 (talk) 20:54, 26 February 2013 (UTC)
 * You should edit the earlier question to maintain context rather than posting new questions. Our Shtora article indicates that it is effective against lasers, at least in some wavelengths, so perhaps your report is in error.  Alternately, your report suggests that flawed test procedures could be the reason.  Finally, I note that I find any technical military details highly suspect when provided by a site labeled "Sauron's Creations" -- that's not exactly Jane's.  For instance, elsewhere on his site (on the Shtora page he specifically says it's effective against laser-guided missiles, in agreement with our article. &mdash; Lomn 00:23, 27 February 2013 (UTC)

Anisotropic aquifers
What are some factors that make àn aquifer ísotropic?


 * Have you intentionally misspelled isotropy? You will need to specify what you're looking for in a little more detail.  When I think about isotropy and reservoirs, I think of two general categories: porosity/permeability, which can be anistropic, and affect fluid flow; and stress/strain tensors, which can be anisotropic, and affect propagation of sound waves during a seismic survey.  There are dozens of other characteristics that could be anisotropic.  I'm surprised that we have no article on reservoir mechanics, but we have rock mechanics, Reservoir modeling, reservoir simulation, and so forth.  And, whenever I find something missing on Wikipedia related to water wells, I go straight to the Schlumberger reservoir and geology encyclopedia website: here's a query for isotropy, which prompted me to recall that electromagnetic surveys are also used in the study of water and hydrocarbon reservoirs, and electromagnetic properties are frequently anisotropic.  Nimur (talk) 23:12, 26 February 2013 (UTC)

Meat tastes
Why do cheaper meats tend to have less of a taste compared to more expensive high quality meats? Some of the low cost meats, especially minced, can have very little taste. Clover345 (talk) 23:02, 26 February 2013 (UTC)


 * Virtually all of the taste in meat is in the fat, and cheaper cuts have less marbling. You can make burger out of lean pork and beef fat or lean beef and and lamb fat and it tastes like the meat the fat came from. --Guy Macon (talk) 23:06, 26 February 2013 (UTC)


 * At the cheapest end, we have pink slime, where the citric acid or ammonia fumes they are exposed to in order to kill the bacteria may also denigrate the flavor. StuRat (talk) 23:11, 26 February 2013 (UTC)


 * People prefer meat with more taste and pay more for this. Cheaper meats are the same phenomenon in the opposite direction. OsmanRF34 (talk) 23:59, 26 February 2013 (UTC)


 * Yeah - exactly, it's a market-demand matter. If cheaper meat tasted better than the expensive stuff, more people would buy cheaper meat and fewer would buy the expensive stuff.  Laws of supply and demand would then cause the cheaper cuts to get more expensive and the expensive stuff would get cheaper.  So no matter which parts of the animal are better tasting, those are always going to be the most costly.  It's economics rather than biochemistry that's at work here!  SteveBaker (talk) 14:47, 27 February 2013 (UTC)


 * Agreed, although "tasting better" is subjective. In the case of fish, for example, most people like them to taste "less fishy".  Lobsters used to be cheap, until they became fashionable to eat.  Similarly, chicken wings were cheap, until preparation methods were developed allowing people to get a maximum amount of salt and fat with each, by frying them and serving them with ranch dressing, etc. StuRat (talk) 16:28, 27 February 2013 (UTC)


 * Or, in the case of red meat, some people actually prefer the tougher, leaner (and therefore milder-tasting) cuts. 24.23.196.85 (talk) 00:42, 28 February 2013 (UTC)


 * I think this is only true in limited cases. For example, the most traditionally expensive beef cuts are the very tender cuts with 'subtle' (i.e., little) flavour, whereas the cheaper cuts were the tougher cuts (usually stewed or similar, to render them tender) which were much more flavourful. Lobster is much less strongly flavoured than the cheaper crab.
 * However, fresher, less processed meat, from animals that have exercised and eaten a more interesting diet, and which has not subsequently been injected with water, tends to have more flavour. And that does cost more. 86.129.248.199 (talk) 21:33, 27 February 2013 (UTC)
 * The breed of the animal might be a factor as well. In the UK Aberdeen Angus beef and rare-breed pork are regarded as being particularly high quality. Itsmejudith (talk) 10:55, 2 March 2013 (UTC)

Homosexuality in identical twins
If one twin is gay, what is the likelihood of an identical twin being similarly gay versus that of a non-identical twin? — Preceding unsigned comment added by 82.132.236.194 (talk) 23:35, 26 February 2013 (UTC)
 * See Biology_and_sexual_orientation. OsmanRF34 (talk) 23:55, 26 February 2013 (UTC)
 * Also see Homosexuality. The bottom line is, nobody really knows at this time -- so please remember to check back in ten to twenty years. 24.23.196.85 (talk) 23:59, 26 February 2013 (UTC)