Wikipedia:Reference desk/Archives/Science/2016 March 24

= March 24 =

Is gray matter of CNS - just collection of the bodies?
I'm reading now in a textbook that "Cell bodies are located in the gray matter of the CNS, and their collections are called ganglia in the PNS and nuclei in the CNS." Does it say that the gray matter of CNS is just collection of the bodies and the white matter is the collection of the Dendrites? If it is, what is the reason for the grey color of the bodies? is it because of that the bodies have nuclei?93.126.95.68 (talk) 00:06, 24 March 2016 (UTC)
 * The difference in color between gray matter and white matter is largely that the latter contains myelin whereas the former does not. -- Jayron 32 00:13, 24 March 2016 (UTC)


 * A big reason is neuromelanin - see . Indeed, there is a substantia nigra in the brain ... yes, we have all been slandered with a racial epithet. :)  Also a locus coeruleus.  The source I cite suggests all these wonderful colors are about protecting the brain from free radicals and metals.  Quite possibly they're right.  But melanin's highly complex structure reminds me of an earlier era when strands of sugars bound to four nitrogenous bases in random order were assumed to have some modest structural role ... I wouldn't rule out the possibility of a big surprise here. Wnt (talk) 00:18, 24 March 2016 (UTC)
 * Thank you for your comments. What about the text that I brought? What does it mean to say? 93.126.95.68 (talk) 00:38, 24 March 2016 (UTC)
 * It says basically that Grey matter is where the cell bodies of the neurons are located. Concentrations of gray matter are called ganglia in the peripheral nervous system and nuclei in the central nervous system.  If you read those articles, you'll learn more about those terms.  -- Jayron 32 01:30, 24 March 2016 (UTC)
 * The answers about are pretty much correct. However, I must quibble with the idea that "cell bodies are in gray matter, dendrites in white". This is only the case for neuronal cell bodies, and even then not exclusively so. There are plenty of oligodendrocytic, astrocytic, microglial and endothelial cell bodies in the white matter. Also, whilst neurolaminin is indeed important in giving the substantia nigra its colour, it is not responsible for the making the grey matter grey (it's more a dirty yellow sort of colour in fresh brain anyway, the white matter is indeed fairly white in my experience). Fgf10 (talk) 08:18, 24 March 2016 (UTC)

Is it known which substance (ingredient) in cow milk causes to sleepiness?
93.126.95.68 (talk) 01:35, 24 March 2016 (UTC)


 * This source claims that any protein-rich food should induce drowsiness, so milk can do so, but not exclusively milk. -- Jayron 32 01:45, 24 March 2016 (UTC)


 * Is that also true for turkey? Or is it just because we eat too much of it at one sitting? ←Baseball Bugs What's up, Doc? carrots→ 01:47, 24 March 2016 (UTC)


 * Yes, overeating definitely causes sleepiness, as does the excess protein and tryptophan. StuRat (talk) 01:50, 24 March 2016 (UTC)


 * Mother Nature's way of directing your body's focus toward digestion? ←Baseball Bugs What's up, Doc? carrots→ 01:55, 24 March 2016 (UTC)


 * Basically yes. While tryptophan can cause sleepiness, and tryptophan (as an amino acid) is present in the proteins of milk and turkey (and lots of other foods), the sleep-inducing effects of tryptophan cannot be induced by turkey or milk alone.  I believe the source I cited notes that one would need to eat a stomach rupturing 40 pounds of turkey meat to get enough tryptophan to induce drowsiness.  Protein-rich foods are themselves enough to induce drowsiness, according to same.  -- Jayron 32 02:02, 24 March 2016 (UTC)


 * Yep, like a snake who swallowed a pig. StuRat (talk) 02:02, 24 March 2016 (UTC)


 * Isn't it supposed to be warm milk ? That suggests it's a psychological thing, since warm milk is associated with being a baby, much like the white noise of the womb,  and the fetal position, things also sometimes used to make you "sleep like a baby". StuRat (talk) 01:54, 24 March 2016 (UTC)


 * Google "warm milk and sleep" and many claim it's a myth. However, it also says milk contains tryptophan, as with turkey. ←Baseball Bugs What's up, Doc? carrots→ 01:57, 24 March 2016 (UTC)


 * See source above. While enough tryptophan can, of its own accord, induce sleep, you cannot get enough of it through either milk or turkey, or really any foodstuff, to do so.  -- Jayron 32 02:04, 24 March 2016 (UTC)


 * Besides the too small amount of tryptophan to cause drowsiness, I wonder if the problem is also that it does not reach the brain when taken orally. Llaanngg (talk) 18:58, 24 March 2016 (UTC)


 * Presumably because different things work for different people. As far as finding one universal sleep aid, well, propofol might work, but Micheal Jackson found that's not a wise choice.  StuRat (talk) 02:02, 24 March 2016 (UTC)


 * Milk is also rather high in sugar content. Hyperglycemia.  I used to drink several gallons a week.  I gave it up when I was diagnosed as diabetic.  Nowadays when I do have a sip it tastes as sweet as fruit juice. μηδείς (talk) 02:32, 24 March 2016 (UTC)


 * There is an article analyzing the link between food and drowsiness. Llaanngg (talk) 19:00, 24 March 2016 (UTC)

The article on "specific impulse" seems to mix up force and mass (physics)
In the article on Specific impulse. It specifies that "If mass (kilogram or slug) is used as the unit of propellant, then specific impulse has units of velocity." this statement may be correct. But! "If weight (newton or pound) is used instead, then specific impulse has units of time (seconds).". Weight is supposed to have unit in mass not force? I suspect there's some serious mixup here. Anyone care to scrutinize this from a physics standpoint ? Ferrofield (talk) 04:01, 24 March 2016 (UTC)
 * It's correct. Thrust is force (mass * length/time^2)).  If mass is used, the rate of consumption is mass/time.  Divide and you get (length/time) which is velocity.  Weight, though, is also force (a kilogram is mass, a pound is force - knowing weight is not mass is critical here).  (force/(force/time)) = time.  It's a measure of efficiency.  You are either measuring how quickly the rocket is getting lighter for a given thrust to weight ratio or you are measuring how far the rocket has moved for a given amount of time.  --DHeyward (talk) 07:10, 24 March 2016 (UTC)
 * Shouldn't it say "If force [N] is used instead, then the specific impulse has units of time (seconds)." ? Ferrofield (talk) 15:25, 24 March 2016 (UTC)
 * In physics, weight is a force. Weight can be measured in pounds, which are a unit of force.
 * In many other contexts, we know that the weight is measured on Earth, so we can easily translate between weight and mass: these two parameters are related by the g (little g) standard gravitational constant.
 * When people are sloppy, they interchange mass and weight freely. But in our article on specific impulse, we are being precise: weight is a measurement of force.  "Pounds" are a unit of force, not a unit of mass.
 * Some people choose to clarify this distinction by separately defining two different units: Pound (force) and Pound (mass) - but they're really just formalizing a sloppy conversion. To make matters even worse (!) - even if the author intends to distinguish between "pounds force" and "pounds mass", and diligently works to ensure correctness, in common speech many English speakers elide the qualifier and simply say "pound", yielding a linguistic collision.
 * In our Wikipedia article on specific impulse, and most of our other physics articles, we totally avoid this confusion by using pounds to refer only to force. If you want to use an imperial unit of mass, use the slug (mass).  Alternately, you can use SI units.
 * Nimur (talk) 16:32, 24 March 2016 (UTC)


 * You could use "force" but it would be more confusing, I think. People understand intuitively "Thrust to weight ratio."  That's a dimensionless term since thrust is Newtons and weight is Newtons (or "pounds").  The "impulse" is thrust to weight ratio with the added "and thrust to weight ratio improves because the rocket is expelling part of its weight in Newtons/sec)." The dimensions work out but what's missing and might be confusing is it's an integral approaching an ideal Dirac delta function.  Ideally, a rocket would have infinite thrust and expel all it's fuel in 0 seconds so it doesn't have to lift the fuel.  That 0/0 equation has an integral solution.  The difference in using mass (slug/kilogram} or weight (pound/Newton) lets the engineer calculate lift efficiency.  Using mass, a moon rocket can be compared to an earth rocket as it will be the same efficiency, but using weight shows the difference in the the amount of burn necessary.  An identical rocket motor expels mass at the same rate on the moon as it does on earth and has the same thrust but not the same weight.  The rocket weighs less on the moon so the thrust to weight ratio is better - that means less burn time and less fuel (i.e. "seconds" of burn).  When calculating efficiency and capability, both metrics are useful/needed but you can't lose sight that it's an integral and units aren't independent of the fuel, mass of the rocket or thrust.   --DHeyward (talk) 16:56, 24 March 2016 (UTC)
 * Tiny nitpick: an identical rocket on the moon has about one atmosphere less back-pressure on the exhaust system - which has a nontrivial effect on the exhaust mass rate and the flow expansion characteristics. To really get into the details, we'd have to do some "rocket science" - but anyone who looks at, say, the iconic bell nozzle of the Apollo Service Module Service Propulsion System - and compares the bell nozzle shape to, say, the F-1 engine, you can almost intuitively feel that one of those engines was designed to exhaust into vacuum, and one was designed to eject exhaust at sea level.  For any particular ambient backpressure, there is a particular nozzle expansion shape that will encourage laminar flow for optimal energy and momentum extraction, even at the same Isp.  Nimur (talk) 17:44, 24 March 2016 (UTC)


 * Must admit that on first reading, the article did not make complete sense to me either. So,what may help too, is a sentence or two explaining that the velocity of the exhaust is dependant on the total molecular weight (molecular mass) of the combustion products. Which is why Hydrogen & Oxygen is favoured for the higher speeds required for obit insertion and of interplanetary probes. In practical engines, the unit of mass ejected has little to do with the resulting velocity. One could construct a whooping big engine that ejects a ton of lead oxide per second but  stays firmly on the launch pad or a smaller  engine with the same starting wet weight  that ejects  a ton of super heated H2O per second  and brakes the sound barrier within seconds of being launched. Therfore,  this  article would benefit I think, from an explanation that all fuels are not equal and that very much affects SPI per unit mass. As a thought experiment: imagine one is sitting just outside the airlock on the ISS with two balloons. One is full of hydrogen and the other (smaller) balloon is filled with the same mass of gas but of CO2. There is no air resistance to worry about. Let go of both necks at the same time. Which balloon will zoom away fastest? The math is the same and  the starting mass is the same but it is the velocity of the exhaust  that makes the difference to the SPI. This molecular mass bit,  the article  fails to include explicitly for clarity. Other than that,  I think that the article is pretty comprehensive and well structured.--Aspro (talk) 20:31, 24 March 2016 (UTC)

Wouldn't "If force (newton or pound-force) is used instead, then the specific impulse has units of time (seconds)." be more correct? Currently it seems to use weight as a substitute for force and a lot of loosely specified imperial units. Ferrofield (talk) 02:21, 26 March 2016 (UTC)

A reflection on black hole event horizon (physics)
A black hole event horizon is supposed to be the final end of interaction of anything falling into it with the outside world, besides hawking radiation and preservation of information. Now suppose the gravity exerted by the black hole is measured externally and an object less massive than the black hole falls into it. Will the gravity of the black hole plus an object outside of it become less in magnitude when the external objects falls into it? Such that the event horizon has the ability to chop gravitational interaction? Ferrofield (talk) 05:06, 24 March 2016 (UTC)
 * No, the mass of whatever falls into a black hole adds to the mass of the black hole. -- BenRG (talk) 09:13, 24 March 2016 (UTC)
 * Indeed, the mass (and therefore the gravitational force), the electrical charge and the spin of the incoming object all add to whatever the black hole had before it 'consumed' it. Conservation laws do apply to black holes. SteveBaker (talk) 13:02, 24 March 2016 (UTC)
 * So light which is an electromagnetic phenomena with the propagation of 3*10^8 m/s can't break free. But gravity with the same wave nature and propagation of 3*10^8 m/s goes straight through. Kind of like the black hole where selective about which wave types it permits to escape. It's like it's permeable for some type of waves and not others. Perhaps there are other wave types yet undiscovered that would enable one to see what's inside? (if 2800 light years won't degrade the resolution too much) Ferrofield (talk) 15:17, 24 March 2016 (UTC)
 * Um, stop right there. Your statement " gravity with the same wave nature and propagation" isn't necessarily an established way to think of gravity.  Gravity has not been properly explained by the standard model and thus analogies between gravity and other standard model forces (like electromagnetism) and their gauge bosons like photons or light are not valid.  You simply can't treat gravity like light and ask what a black hole does to gravity.  While the standard model did not exist at the time of Einstein, he DID have an intuitive sense that gravity as a force did not work like other forces, which is why he treated gravity as a pseudoforce and developed general relativity to explain the phenomenon of gravity without having to resort to treating gravity like light or other phenomena, which causes all sorts of problems.  -- Jayron 32 15:58, 24 March 2016 (UTC)
 * Also best keep apart gravity itself and that ripple called "gravity wave" (caused by some interaction of bodies of huge mass) which is more precise a ripple in time or time-space, not a ripple in gravity!! --Kharon (talk) 17:03, 24 March 2016 (UTC)


 * The equivalence principle implies that you can treat gravity in more or less the same way as electromagnetism for this purpose (at least for a small object falling into a large hole). The Standard Model's gauge forces are actually similar to gravity; mathematically they are like general relativity applied to compact extra dimensions. And electric and gravitational charge are both preserved, so there is no difference that needs explaining here. -- BenRG (talk) 18:26, 24 March 2016 (UTC)


 * As SteveBaker said, electric charge (which is the electromagnetic equivalent of mass) is also preserved. It doesn't disappear. There is no difference between gravity and electromagnetism/light in this regard. There are various ways of thinking about this. One way is that the field is an entity unto itself which can't disappear for geometric reasons, much like you can't untie a knot in a rope if you don't have access to the ends. Another way is that you never actually see the object cross the event horizon; it's always just outside, where it can still "emit" the field for you to detect. -- BenRG (talk) 18:26, 24 March 2016 (UTC)


 * Yeah, the question of how the gravity escapes the black hole is a long-standing one. One thing I wonder -- if you look at something falling into the event horizon, it is said that it is always hypothetically visible since there is always the chance of some absurdly red-shifted photon finally outrunning the infalling space (???) of the event horizon and getting out.  But you can't see it because the photon emission is minimal.  However, from our frame of reference the mass of an infalling object, travelling at almost the speed of light, should grow without limit, right?  And that tremendous and super-fast-moving mass ought to emit some kind of gravitomagnetic radiation, shouldn't it?  Which leaves me thinking someone who knows a hell of a lot more physics than I do could work out some kind of limit calculation for how much the gravitomagnetic emission vs. the redshifting comes out to be.  If you can redshift gravity that is.  Am I just gibbering? :) Wnt (talk) 23:39, 24 March 2016 (UTC)
 * If the sun would disappear in an instant. How long would it take for us to know? I suspect the propagation delay is 3x10^8 m/s. So whatever it is, it has some properties that can be explored. Say if one can design a device that can sense really small gravity changes and have high angular resolution. And have this sent to orbit close by a black hole. Perhaps one would discover something about what happens on the inside. Ferrofield (talk) 21:43, 25 March 2016 (UTC)
 * That's pretty much what the LIGO and Virgo interferometers do. They recently detected the collision of two black holes with enough angular precision to say where in the sky it happened. SteveBaker (talk) 02:15, 28 March 2016 (UTC)

Egg white whipping with contaminants
Can egg white be whipped into a foam if it already contains sugar and cocoa powder or will these prevent that? Seans Potato Business 07:59, 24 March 2016 (UTC)
 * Contamination from copper seems to help with whipping eggs, and the foaming is a result of denaturing and air insertion. I can't imagine sugar and cocoa powder preventing denaturing of the egg whites.  It might affect the form of the foam, but that's just a guess.  Ian.thomson (talk) 08:21, 24 March 2016 (UTC)
 * Those things make whipping more difficult if they are added at the very start, but I don't think they will prevent it. Note though that even a little bit of fat can make it impossible to whip egg whites. Looie496 (talk) 14:30, 24 March 2016 (UTC)
 * This suggests to me that one needs to look into if certain substances will bind to or interact with the substances in the egg white in a way that prevents denaturing or stable bubble walls. Ferrofield (talk) 15:21, 24 March 2016 (UTC)
 * Sugar improves the stability of whipped egg white - Meringue actually discusses this issue in some detail although the section lacks sources. Roger (Dodger67) (talk) 16:15, 24 March 2016 (UTC)

Entropy. Statistical mechanics. Information ~ Energy
37.53.37.94 (talk) 20:07, 24 March 2016 (UTC)


 * The author doesn't believe this: he defines it. He has explicitly stated that his hypothetical system has N independent variables that may each take only one of two possible states.  This is definitional or axiomatic - he constructed the problem so that he could work out the math that follows from it.
 * This formulation is nearly identical to what shows up in my textbook on thermodynamics, Stowe's Introduction to Thermodynamics and Statistical Mechanics. It's a pretty standard formulation, because it's a simple way to work the math.
 * I've never heard of this particular author - so I'm not going to vouch for the entirety of everything else he's written: in particular, his unique terminology "exergy" is not commonly used in other books on statistical physics. Reading through some more of his material, I'll say this: his book is clearly the work of a non-native English speaker, and it doesn't look like he had a very good editor.   I would not use his book as your primary reference for anything, let alone for information on standard formulations of thermodynamics - even if some parts of his book are correct, there are plenty of much better resources.  I can personally vouch for several other great books on statistical mechanics if you're interested; and we have a list of references in our article on thermodynamics.
 * Nimur (talk) 20:10, 24 March 2016 (UTC)
 * Exergy wasn't taught in the physics classes I remember either, but it has been around a while and I have encountered it in the wild before. I'm too lazy to try and verify if he is using the terminology in the same way that others have used it.  Dragons flight (talk) 19:50, 25 March 2016 (UTC)


 * But with this formulation he calculates entropy wrong way. He must take $$j=1,2,...,m$$. If particle states are 0 and 1, $$j=1,2$$.37.53.37.94 (talk) 20:53, 24 March 2016 (UTC)


 * in particular, his unique terminology "exergy" is not commonly used in other books on statistical physics -- This is book not on statistical physics. This book about exergy. And this word is not unique [see http://www.collinsdictionary.com/dictionary/english/exergy ]. Quality of the book isn't the subject. I've leafed through your book and it does not contain info I'm looking for. In your book you  write $$\Omega = \omega ^N $$ too. So your book also does not agree with wiki article Boltzmann's entropy formula. E.g. let system have 5 non-distinguishable atoms of helium. Every particle can be in state 0 or 1. Two atoms are in state 0, Three atoms are in state 1. How will you calculate number of system states $$W$$, which is used in formula $$S=k\log W$$? And why? 37.53.37.94 (talk) 04:47, 25 March 2016 (UTC)

37.53.37.94 (talk) 21:12, 24 March 2016 (UTC)
 * Here's some questions for you to consider: Is there more information in one-minute of white noise or one minute of the Crab canon? Or what about a one-minute of me speaking this comment? Shannon would say white noise has far more information, but that doesn't necessarily match up with our intuitive notion of information. As for generation: can a farm consisting of sheep, pigs, and cattle generate information? Note that one of the most popular Diversity_indexes used is in fact just Shannon's entropy, more on that here . So if we calculate the Shannon diversity of a given paddock, it will change on a day-to-day basis based on the which animals are there, and we could perhaps claim that the animals are creating information, sort of Maxwell's livestock as an analog to Maxwell's demon. My point is that it's pretty easy to compute some H and say "there's this much entropy/information in this system", but the interpretation of that claim is subtle and often depends on many other implicit assumptions and models. SemanticMantis (talk) 14:26, 25 March 2016 (UTC)