Wikipedia:Reference desk/Archives/Science/2007 December 14

= December 14 =

Stressful situations
Not sure if this goes in the science section but I guess it could be part of psychology, and behavior. What are good ways or I guess some could say, good exercises for the brain before an anticipated stressful (on the mind) event. Example, a test, mentally laborious work, etc. I think I could go for some new tips. My methods are just trying to relax, or getting some light meditation on anything that isn't too heavily related. I try to ease my mind from the harsh reality of solving problems. --Agester (talk) 01:14, 14 December 2007 (UTC)
 * There are some interesting results from experiments with laboratory animals that suggest physical exercise can be "good exercises for the brain". Starting point in the literature: Neurobiology of Exercise. see also --JWSchmidt (talk) 05:33, 14 December 2007 (UTC)

Disease
I'm looking for the disease that causes victims to fall into an apathetic state. They will fall into a deep slumber, they can be roused to perform tasks, but if allowed to rest they will go back to sleep and eventually die. Some survived for months like this before succumbing. An outbreak that afflicted millions occurred at the same time as the Spanish flu of 1918, it is apparently dormant or gone now. Thanks. 75.175.30.112 (talk) 01:50, 14 December 2007 (UTC)
 * Read about it in Encephalitis lethargica. Graeme Bartlett (talk) 02:22, 14 December 2007 (UTC)

Numerous Questions About Space
First off, I realize that the information on this subject is limited, but I would greatly appreciate any information. Anyway, to the questions:

Is our solar system unique in that it has 8 planets? Is this considered a high number for a planetary system?

Do most planetary systems have asteroid belts? Do some have have multiple belts?

Sometimes, in video games, you will see that a planetary system has a asteroid "field" as opposed to a "belt." Basically, there is an elliptically shaped area of asteroids in the upper left corner of the map. Is this really possible to have this in a planetary system?

Do all planetray systems have an equivalent of the Kuiper belt?

In sci-fi, spaceships often make a "jump" to some sort of FTL speed. However, in reality, wouldn't they have to be outside of the asteroid and Kuiper belts so as to avoid collisions?

Lastly, is it possible to build a space station in a random spot in the solar system (ie. it wouldn't be orbiting a planet)? Or would it just get pulled into the Sun? What about if it was placed in an asteroid belt? 24.125.31.205 (talk) —Preceding comment was added at 03:01, 14 December 2007 (UTC)
 * I apologize I can't answer any of those, but to maybe point you in the right direction, wikipedia does have an article on planetary systems (other than ours) as well as links to the various known systems. Also, there is a section Kuiper_belt that states "As of 2006, nine stars other than the Sun are known to be circled by Kuiper belt-like structures. They appear to fall into two categories: wide belts, with radii of over 50 AU, and narrow belts..." Interesting questions though! -- MacAddct &#xF8FF; 1984 (talk &#149; contribs) 03:16, 14 December 2007 (UTC)
 * For your last question, Lagrange points can be a good place to put things. Algebraist 03:39, 14 December 2007 (UTC)


 * For question three, no. A random blob-like arrangement of asteroids orbiting a sun would pull itself together under its own gravity, or would have to form with sufficient velocity that it would spread out, thus ruining the appearance the video game designers sought. For question five, it depends on what FTL involves.  Since there is no known way of traveling faster than light, you can make up whatever rules you want. For question six, Newton's law of universal gravitation necessitates that any body orbiting the sun will have the same orbital speed for a given distance out, regardless of its mass. So a (relatively) light space station would be able to stay in orbit just fine without a planet nearby, although it may still require corrections due to gravity from other objects.  Someguy1221 (talk) 03:44, 14 December 2007 (UTC)


 * The fact is, we still don't have a lot of data on other solar systems than our own. We have found planets orbiting stars, and the number is growing all the time. But these all have to date been massive gas giant planets - not little dinky ones like our own. So, although we do know about other solar systems out there, we simply cannot tell right now how many planets are in any of them. So there's no way to know for sure whether our system is "typical" or not. Saukkomies 04:42, 14 December 2007 (UTC)

Let's take these one at a time:
 * Is our solar system unique in that it has 8 planets? Is this considered a high number for a planetary system?
 * Our ability to find planets orbiting other stars is not good enough to find small planets - and when there are many planets, the effects of their gravity on the parent star becomes too complicated to decypher - so it becomes hard to count the planets. We know that a large proportion of the stars we have looked at have at least one planet - and we know that some of them have more - but we don't know enough to answer your question yet.
 * (Just clarifying, here) Let's think about how we find those planets orbiting other stars. Basically, there are several ways to find one.  Simplest is to track a star well enough to realize that it's not moving in a straight line.  If it's track looks like a wave, then that must be caused by the mass of something orbiting it.  Clearly, such an orbital mass has to be big enough to move a star, so that's a pretty big planet, like Jupiter (or larger).  Next, we can observe a start long enough with a spectroscope to be pretty sure what it's spectrum is.  Then, if we see a dip in either total output or most frequencies, we can assume that something got between us and the star.  If it happens periodically, like the star is "normal" most of the time but every 90 days it goes down to another constant level for 30 minutes, we're probably seeing the effects of a planet moving in orbit, causing a partial eclipse.  Again, the planet would have to be pretty honkin' big for us to see this.
 * There are other ways, but mostly they all boil down to "the star is so far away we can only detect huge planets." If we assume that other systems are laid out like ours, and we see something that looks like Jupiter, then it's likely that they also have something like earth/venus/mercury/mars, and also saturn/neptune, but these are assumptions.  We can't see the little ones. -SandyJax (talk) 19:49, 14 December 2007 (UTC)
 * That's a good explanation. Little planets only put very small ripples onto the motion of the star - so they are harder to detect than the bigger ripples caused by huge planets.  But also, planets further away from the star cause smaller ripples than those close-by so the outermost planets may cause ripples that are too small to measure.  Worse still, if there were as many planets - all large enough and close enough to detect in this way, the star would be pulled back and forth by all of them and it's motion would be quite haphazard.  If the interactions are small and you have this much complexity, it can be tough to sort out what planets or what masses are at what distances and what orbital periods are involved.   Since it may take many years for a planet to make a complete orbit of the star, you may have to watch the motion of the star for a very long time in order to see enough of the motion to make accurate measurements of all of it's planets. SteveBaker (talk) 13:45, 16 December 2007 (UTC)


 * Do most planetary systems have asteroid belts? Do some have have multiple belts?
 * Again, we don't know - asteroid belts would be even harder to find than planets.
 * Sometimes, in video games, you will see that a planetary system has a asteroid "field" as opposed to a "belt." Basically, there is an elliptically shaped area of asteroids in the upper left corner of the map. Is this really possible to have this in a planetary system?
 * If that 'patch' was orbiting the star as a group - then, yes, that could happen if (say) two larger bodies collided while in a similar orbit and produced a bunch of fragments that continued to orbit the star. However, over time, the asteroids mutual gravitation would tend to pull them back together again - so this probably couldn't last for tens of thousands of years without something more complicated going on.
 * Do all planetray systems have an equivalent of the Kuiper belt?
 * Probably - but again, such things are too hard to measure with the instruments we have. We know SOME of them do.
 * In sci-fi, spaceships often make a "jump" to some sort of FTL speed. However, in reality, wouldn't they have to be outside of the asteroid and Kuiper belts so as to avoid collisions?
 * There is no such thing as a "jump" and faster-than-light travel is impossible - so there is no "in reality". Some scifi series (eg StarTrek and StarWars) have no problem with you just zipping off to lightspeed once you are in orbit.  Others (eg the Isaac Asimov 'Foundation' series) specifically state that you have to fly a long way away from any gravitational sources before you can jump.  However, it's all pure fiction...completely and utterly bogus.
 * Apart from the fiction of the question, though, the answer is "no" in terms of the asteroid and Kuiper belts. The densities of asteroids, comets, and other hazardous rocks is far too low to make a significant hazard to spacecraft.  Consider that any NASA probes going to Jupiter and beyond just sail blithely through the asteroid belt, and that the only collision was purposeful.  If, however, you were a fanatic about minimizing risk (and this is not an unreasonable assumption), there's another trivial solution: leave the plane of the ecliptic.  &mdash; Lomn 13:54, 14 December 2007 (UTC)
 * In all of the SciFi I've seen, worm-hole travel is used for FTL travel from one location to another. As such, you do not travel through any obstacles between the origin and destination point.  So, asteroids, planets, stars, and all other spacecraft are easily avoided.  You only have to worry about someone else using the same wormhole or having something sitting right at the destination point. --  k a i n a w &trade; 15:39, 14 December 2007 (UTC)
 * You need to read more SciFi! There are many more mechanisms than wormholes.  I recall that the book (and less great movie) "Contact" used wormholes.  In StarWars, very little is said about how it works (a wise move!).  The implication in StarTrek is that space is warped (hence "warp drive") so you are somehow scrunching space up in a region immediately around the ship so you can be moving slower than light - yet still getting from A to B at a speed that keeps the plot humming along.  This is claimed to be why they don't use their drives near planets (which don't much like being scrunched up) - and in one episode of The Next Generation, it is revealed that space is somehow weakened by the repeated traversal of fast ships because of all of this scrunching and unscrunching of space.  So wormholes are definitely not involved in StarTrek (although they are occasionally used as plot devices).  In other books (I'm thinking especially of the god-awful "Lensman" series by E.E.'Doc'Smith), they simply assert that Einstein was wrong...which is unlikely - but no worse than 'bunching up space' I suppose.  In a few books, the mechanism is like teleportation - you go from A to B by "folding" the universe so that the points A and B are actually touching (like folding a piece of paper so that two dots - one in each corner - are touching.  In others it is claimed that the spacecraft stays still and the universe is moved around it (how does this help?), in even more creative series ("The Hitchhikers guide to the Galaxy") FTL space travel in "The Heart of Gold" works because of quantum improbability - your spacecraft (because of Schrodinger's equation) has a finite probability of being anywhere in the universe - so getting to where you want instantaneously isn't impossible - it's merely highly improbable.  You just have to get REALLY lucky - for which they have an infinite improbability generator.  It appear that this "saves all of that tedious mucking about in hyperspace".  Or in a later radio show in the series a craft exists that takes advantage of the fact that restaurant bills never add up to the same total that each person is supposed to pay.  This mathematical property is unique to restaurant bills and can be exploited for hyperspatial travel - so your spacecraft takes on the appearance of a rather cosey little Bistro - and when the check arrives - and doesn't add up - KERPOW! You get where you need to be.  Asimov proposes this extra 'hyperspace' - perhaps extra dimensions.  The 'jump' takes zero time to occur - but the horribly complex math required to do it limits the distance and frequency at which you can jump.  It contains the handy plot device that forces you to take days to weeks to travel conventionally into and out of gravity wells which complicate the horrific math to an unbearable degree.  I could go on - there are hundreds of ways this has been written about.  SteveBaker (talk) 16:03, 14 December 2007 (UTC)
 * Edit Conflict: I was about to say the same thing, Kainaw. While hitting a small bit of debris at light speed might do a significant amount of damage, a lot of shows get around many FTL travel-problems by "bending space time", allowing one to fold space and make your current position and destination right next to each other. -- MacAddct &#xF8FF; 1984 (talk &#149; contribs) 15:44, 14 December 2007 (UTC)


 * But because it is fictional, fictional laws may apply. In Asimov's Foundation universe, the calculations involved in making a FTL jump become impossibly complex when there is any kind of gravity field nearby.  So they have to fly at tedious speeds for days or even weeks to get to a point where a jump is possible.  Perhaps in those circumstances, even the large distances between asteroids and comets would be too much to make this possible...maybe leaving the plane of the ecliptic turns a 2D calculation into a 3D calculation - thus making it even harder to figure out.  It's useless to speculate on the constraints that an entirely fictional device might impose. SteveBaker (talk) 15:41, 14 December 2007 (UTC)


 * Lastly, is it possible to build a space station in a random spot in the solar system (ie. it wouldn't be orbiting a planet)? Or would it just get pulled into the Sun? What about if it was placed in an asteroid belt?
 * You could theoretically build it in an orbit around the Sun - moving (as a planet does) in a large ellipse around the Sun. But if you were trying to be "stationary" with respect to the Sun, it's gravity would pull your space station to a messy end.  It could certainly be build in the asteroid belt too.  Asteroids in the asteroid belt are VERY far apart - if you stood on one asteroid, it's very doubtful you'd be able to see any other asteroids around you - even with a good pair of binoculars.  So - yeah - you could build a station out there.  In general though - you'd want to build where you have plenty of materials for the construction.  If you wanted one out in deep space - vastly distant from any moons or planets or whatever - then you'd probably want to build it in orbit around the earth and then use rocket motors to move it to wherever you wanted it to be.  Building in the asteroid belt would be a good idea - there are plenty of nickel/iron asteroids that could be melted with a solar powered furnace and refined to make the metal parts of your space station.  You could alternatively simply hollow out a regular rocky asteroid to make your space station.
 * SteveBaker (talk) 04:59, 14 December 2007 (UTC)
 * There are some asteroid fields, formed mainly when asteroids enter the Lagrangian point of another planet - the combination of the gravity of the sun and the planet combine to produce a small patch of stable asteroids; the Trojan asteroids near Jupiter are a good example. Laïka  13:39, 14 December 2007 (UTC)


 * Yes - but the Lagrangian points of the planets aren't stationary - they still orbit the sun. Also, the Lagrange points are exactly that - points - so any object that is not precisely at the lagrange point is going to have some sort of forces causing it to be unstable over the very long term. SteveBaker (talk) 15:41, 14 December 2007 (UTC)
 * The L4 and L5 Lagrangian points (where Jupiter's Trojans are located) are stable regions, provided the mass of the object at the point is insignificant relative to the masses of the two main bodies. The other Lagrangian points are unstable as you've noted, though orbits about them exist that are more stable -- though probably not stable enough. &mdash; Lomn 16:23, 14 December 2007 (UTC)
 * Addendum: perhaps even the L1-L3 orbits are sufficient -- SOHO has been "at" the Earth-Sun L1 point for over 10 years now, still on its original fuel supply for orbital adjustments, and ACE has been there nearly as long. A space station could presumably be refueled often enough to use this orbit indefinitely. &mdash; Lomn 16:28, 14 December 2007 (UTC)

More fuel efficient vs. less air polution (cars)
I know this is some what of an opiniative question but what is more important (for cars) using less gas or giving off less pollutants? Personally I think its more important to give off less pollutants.thanks--Sivad4991 (talk) 03:14, 14 December 2007 (UTC)


 * Using less gas makes it cheaper to use the car, thus making it more common. In addition, the money saved on gas can be used to buy carbon offsets or, for that matter, it can go to any charity. Of course, the answer depends on how much less gas and how much less pollutants. You wouldn't want to buy a car that uses a million times as much gas and gives off 1% less pollutants, would you? — Daniel 03:58, 14 December 2007 (UTC)

I guess what im asking is should we be trying to find way to decreise our fuel usage of trying to find ways so that our cars give off less pollutants. --Sivad4991 (talk) 04:06, 14 December 2007 (UTC)


 * The two problems are related. When you burn gasoline perfectly correctly, you end up with carbon dioxide and water. Carbon dioxide is a pollutant - so burning more gasoline produces more of the stuff - burning less produces less.   However, it's very tough to do a perfect job of burning the gasoline - and some of it gets combined with oxygen and nitrogen from the air and some of the carbon dioxide to make carbon MONOXIDE (which is pretty poisonous and an even worse greenhouse gas than carbon dioxide) - and also nitric oxide - which is another  rather nasty, poisonous pollutant.  Worse still, nitric oxide reacts further to form nitric acid - which washed out of the air when it rains in the form of "Acid Rain"...which is yet another major problem!  The catalytic convertor in your car removes some of the nitric oxide in the exhaust gasses, turning it back onto plain oxygen and nitrogen - but it doesn't do a perfect job.


 * So - the less fuel you burn - the less pollutants you emit - as simple as that.


 * SteveBaker (talk) 04:34, 14 December 2007 (UTC)


 * Note that diesel fuel is more polluting than unleaded gasoline, but it gets more miles per gallon in almost any equivalent engine. That's why you see a lot of hybrid designs with diesel engines - manufacturers are trying to push that number higher. SamuelRiv (talk) 05:15, 14 December 2007 (UTC)

Plants and carbon
I know that plants take in carbon dioxide and let out oxygen and that they need carbon dioxide to live. But is carbon good for plants just like carbon dioxide is? And if so could it be used in fertalizers? Corect me if im rong but when carbon dioxide is split it becomes sepret carbon and dioxide atoms. Is dioxide a gas and if so is it harmfull to the enviornment. If it dosnt become a dioxide atom then what dost it become and is what it becomes harmfull to the enviornment? thanks --Sivad4991 (talk) 03:27, 14 December 2007 (UTC)
 * "Dioxide" is just good old oxygen or O2. Photosynthesis goes like this:
 * 6 CO2(gas) + 12 H2O(liquid) + photons → C6H12O6(aqueous) + 6 O2(gas) + 6 H2O(liquid)
 * So plants aren't left over with just pure carbon. Edit: And the C6H12O6 molecule is a sugar molecule that plants uses/stores for energy. --MacAddct &#xF8FF; 1984 (talk &#149; contribs) 03:45, 14 December 2007 (UTC)

All of this chemistry happens in the leaves - CO2 comes in through tiny holes in the leaf and water drawn up through the roots meets it there. Sunlight provides the energy to perform the chemistry and Oxygen and Sugar comes out. The oxygen is released from the leaves into the air and the sugar is further transformed into cellulose which is how the plant makes up it's structure. Wood and leaves are mostly cellulose. Plants can't use raw carbon - their biology simply isn't designed for that because there isn't much plain carbon sitting around where plants can take advantage of it. Carbon doesn't dissolve in water - so there would be no easy way for the plants roots to pull the carbon up into the stems and leaves where it would be needed. SteveBaker (talk) 04:24, 14 December 2007 (UTC)

A solution to car pollution (hey that rhymes)
OK for my last question, does any one know of a website where I can offer my ideas and solutions I have for car pollution? thanks for every ones help on my continuoS QUESTIONS! --Sivad4991 (talk) 03:54, 14 December 2007 (UTC)


 * Since you had no understanding of how cars generate pollution (two questions ago) and didn't know really basic chemistry (that the "dioxide" in "carbon dioxide" is really just oxygen)...(one question ago) - I think it would be EXCEEDINGLY unlikely that anything you have to say on the matter is going to be of much interest to anyone who matters. You need to learn a LOT more before you stand any chance of offering ideas and solutions that are even remotely likely to work and not to have been thought of many times in the past.  If you have an interest in helping to solve this major worldwide problem, I strongly suggest you take some chemistry and automotive engineering courses - learn what we already know about this before you try to form your own ideas.   If you'd like to explain what you have in mind right here, I'm sure we can tell you whether it's already been thought of - or whether it stands a chance of working. SteveBaker (talk) 04:42, 14 December 2007 (UTC)


 * How about a little WP:CIVILity here? And who knows? Maybe he or she has an idea unfettered by conventional thinking. Clarityfiend (talk) 09:36, 14 December 2007 (UTC)


 * You may be interested to read up on the One True Solution.--Shantavira|feed me 12:18, 14 December 2007 (UTC)


 * I don't see that Steve Baker was at all rude in his comments - he was just being realistic. It's quite common for armchair engineers (with little or no actual engineering knowledge) to think their crazy ideas are somehow better than what the experts come up with.  Steve's suggestion was quite good- if you're serious about making contributions to the field, you should first learn what the people before you have already figured out.  And, from there, perhaps you can make some real improvements.  But approaching the problem without the necessary education is just a waste of time.  Friday (talk) 19:30, 14 December 2007 (UTC)

i under stand what you guys are say and i would like to find out what people have alredy thought of but you dont necessarly need to know all about chemistry or automotives to come up with ideas. Maybe all we need to do is change one part on a car. And i did know how cars create exaust.--Sivad4991 (talk) 21:08, 14 December 2007 (UTC)


 * OK, define what you mean by pollutants? CO2? Hydrocarbons? FYI the catalytic converter had already reduced a lot of pollution in cars (CO2 emission isn't really counted as pollution). --antilivedT 21:49, 14 December 2007 (UTC)


 * There's a famous quotation about the difficulty of someone from outside of a given field making significant contributions to it. Does anyone know the one I'm talking about?  It has of course happened before, like Luis Alvarez and paleontology, and Albert Einstein and eye surgery.  --Sean 00:50, 16 December 2007 (UTC)
 * I know we're straying way off topic here, but what's the story about Einstein and eye surgery? I see you linked to laser eye surgery. Did Einstein make any statements about laser eye surgery that were dismissed by the medical community? --NorwegianBluetalk 14:54, 16 December 2007 (UTC)


 * The 'out of field' Einstein story I like the most is the one about how he liked to stroll around the grounds at Princeton and took to chatting with one of the gardeners there. As they pass one particular row of plants, Einstein remarks that they are growing much faster than the others and asks the gardener what kind of plants they are.  It seems they are haricots.  The gardener was later heard to remark that "Einstein doesn't know beans." SteveBaker (talk) 13:35, 16 December 2007 (UTC)

Carbon at extremely low temperature
Have the properties of carbon, (particularly isotopically pure carbon) been investigated near absolute zero?Thanks, Rich Peterson130.86.14.90 (talk) 06:49, 14 December 2007 (UTC)


 * From perusing the literature, carbon resistors still function at 1K, but aren't used below this. So presumably it's not terribly affected at the few kelvin area, but I can't find a reason for its lack of use at lower temperatures.  This suggests to me that either something funky happens to carbon below 1K, or it's just a technical limitation.  Someguy1221 (talk) 08:10, 14 December 2007 (UTC)
 * One thing I'm wondering about is if at low temperatures, pure diamond or graphite might have interesting electical or heat-conduction properties. Also, what about phonons near 0 K?130.86.14.90 (talk) 01:59, 15 December 2007 (UTC)

Volume of universe compressed
If the entire mass of the universe was compressed to the density of a neutron star, how much volume would it take? MilesAgain (talk) 07:43, 14 December 2007 (UTC)


 * The mass of the observable universe is estimated at 3x1052kg, and the average density of a neutron star is about 3x1017kg/m3. So you're looking at a good 1035 cubic meters, which would produce a neutron star 400 million kilometers across (pretending it maintains this same density).  This assumes all matter in the universe has the same compresibility as nucleons, which is certainly not true for all mass in the form of neutrinos, EM/gravity waves, or dark matter. But I hope this helps! (and I hope I did my math right) Someguy1221 (talk) 08:07, 14 December 2007 (UTC)
 * Also, you can't give a figure for the mass of the entire universe, since there is no agreement on the shape of the universe (or evidence to suggest one). For certain possible shapes, the universe could well have infinite extent and infinite mass, and there is no law of physics that would inherently prohibit such universe. Someguy1221 (talk) 08:12, 14 December 2007 (UTC)


 * Of course, the mass of such a neutron star would certainly be above the Tolman-Oppenheimer-Volkoff limit, so it would collapse into a black hole. —Keenan Pepper 16:56, 14 December 2007 (UTC)


 * How large would the event horizon be? --Carnildo (talk) 22:55, 14 December 2007 (UTC)


 * Calculate it yourself. $$r_s = \frac{2GM}{c^2}$$ (Schwarzschild radius). —Keenan Pepper 00:56, 16 December 2007 (UTC)
 * Plugging in Someguy's mass of the universe into KP's equation gives around 4 * 1025 meters, as compared to around 3000 meters for a black hole with the mass of the Sun (though I'm not sure they can be that small; I know they can't start out that small).  --Sean 01:10, 16 December 2007 (UTC)


 * I think the most interesting thing about that calculation is that the black hole is larger than the equivalent mass neutron star (for which I ignored the possibility of black hole formation). A nice example of how the average density of a black hole surprisingly decreases with mass.  Someguy1221 (talk) 04:55, 16 December 2007 (UTC)


 * But that's hardly a fair comparison. You're recording the diameter of the matter in the neutron star and the diameter of the event horizon for the black hole.  To be fair, you have to compare the diameter of the matter in the black hole - and that's zero (well, depending on the position of the observer because the speed at which the material collapses into that singularity becomes relativistic in the final stages).  SteveBaker (talk) 13:28, 16 December 2007 (UTC)


 * Your comment reminded me of the counterintuitive fact that planets above a certain size (e.g., Jupiter), get smaller when you add more stuff to them. --Sean 14:31, 16 December 2007 (UTC)


 * Interesting. That's the same order of magnitude as the size of the observable universe. --Carnildo (talk) 00:42, 18 December 2007 (UTC)

polar capacitor
is there any symbol of polar capacitor is available in PSPICE schematics version 9.1(student version).193.251.135.125 (talk) 07:43, 14 December 2007 (UTC)
 * Please, do not crosspost. See WP:RD/C. ›mysid (☎∆) 12:49, 14 December 2007 (UTC)

How close to a black hole before you are killed?
Lets say you are an astronaut in a space suit. How close can you get to a black hole of stellar mass, lets say 1 or 2 stellar masses, before you are killed? What is the mechanism that kills you? 64.236.121.129 (talk) 15:39, 14 December 2007 (UTC)


 * The mechanism is extreme tidal forces, otherwise known as spaghettification. Gandalf61 (talk) 15:44, 14 December 2007 (UTC)
 * How close would you have to be though. 64.236.121.129 (talk) 15:50, 14 December 2007 (UTC)
 * Estimate the force needed to tear apart a human, and do the math. Gandalf gave you the necessary pointer to tidal forces.  -- Coneslayer (talk) 16:06, 14 December 2007 (UTC)
 * I wouldn't be asking if I already knew how to do that. Haha. Mr. Obvious comes knocking. 64.236.121.129 (talk) 18:16, 14 December 2007 (UTC)
 * If you really want to be able to figure things out, instead of depending on the Reference Desk for the rest of your life, I would encourage you to get started on basic math and physics. Work your way through algebra, trigonometry, and calculus.  Get a good understanding of units of measure.  Learn the basic physics.  So far, you've been skipping over the basics (Ohm's Law, Newton's law of universal gravitation, gravitational end electric potentials) straight to the complicated phenomena (lightning, black holes).  Even if it doesn't seem sexy, you'll be far better off in the long run if you master the basics first.  -- Coneslayer (talk) 18:36, 14 December 2007 (UTC)
 * Naa. 64.236.121.129 (talk) 20:10, 14 December 2007 (UTC)
 * Isn't the part of the point of this desk that people who are untrained in some area but want to know something about it can ask people who are trained to figure it out for them? I mean, maybe 64.236.121.129 is a horticulturalist...if I had a question on how to grow some plant I wouldn't expect them to say "Go study biology & botany and then learn horticulture and the figure it out yourself." 202.37.62.105 (talk) 04:42, 19 December 2007 (UTC)
 * See also: event horizon -- MacAddct &#xF8FF; 1984 (talk &#149; contribs) 16:10, 14 December 2007 (UTC)


 * (ec)
 * This depends heavily on your height, your position relative to the hole, and the mass of the black hole. Using simple Newton's physics (and assuming I didn't screw up my algebra), you end up with: $$K = {2h G m_1 m_2 ( r + 1 ) } / { r^2 ( r + h )^2 }$$ where K is the killing force, G is the gravitational constant, m1 is your mass (negligible and can be ignored), m2 is the black hole's mass, r is the distance between the black hole and the closest part of your body, and h is the distance between the closest part of your body to the black hole and the furthest part of your body from the black hole. Plug in the values you are interested in and see what the difference in force between your head and toes will be. Then, decide if that is enough to kill you. -- k a i n a w &trade; 16:12, 14 December 2007 (UTC)


 * The only objective way I've heard of this calculation being done is to calculate where the tidal force of the blackhole exceeds that on the surface of the Earth, as this is a very certain lower bound on what will kill you. And I don't think you should be calling a factor negligable when you're multiplying something by it. Someguy1221 (talk) 19:23, 14 December 2007 (UTC)


 * How long from who's point of view? Your speed as you approach the event horizon will approach the speed of light - so your experience and mine (I'm the one in the nice, comfy space ship a long way off - OK?) will be very different.
 * Also, isn't it the case that you get cooked by the gamma radiation long before spaghettification sets in?
 * SteveBaker (talk) 17:20, 14 December 2007 (UTC)


 * Maybe, if we are assuming there is an accretion disk - but then we could assume a sufficiently shielded space suit too. Gandalf61 (talk) 17:48, 14 December 2007 (UTC)


 * I think gamma radiation or heat would kill before tidal forces would. 64.236.121.129 (talk) 18:15, 14 December 2007 (UTC)


 * The larger a black hole is, the weaker its tidal forces are at the event horizon. I'm no GR expert, but if I recall the numbers correctly, it could be hours for a several million solar mass black hole, and days, weeks or even much longer when you get up to the billions of solar masses black holes (time to lethal tidal forces after passing the event horizon). But what Steve said about point of view is very important, but for different reasons.  From the faller's perspective, eon's worth of starlight enters the blackhole behind him in mere minutes, tremendously blueshifted. So yes, for a very large black hole, you'll be fried by blueshifted starlight long before the tidal forces get you.  Someguy1221 (talk) 19:19, 14 December 2007 (UTC)


 * I think blue-shifted infalling radiation is only a problem if our astronaut is (somehow) hovering just above the event horizon. However, I was assuming our astronaut is in free-fall. Gandalf61 (talk) 19:51, 14 December 2007 (UTC)


 * ...the light's going to come in behind you no matter how you're falling...Remember that as far as the falling observer is concerned, nothing special happens as he passes the event horizon; everything changes smoothly. He can still see the infalling starlight right up until being squished, getting ever more blueshifted. Someguy1221 (talk) 19:56, 14 December 2007 (UTC)


 * Surely if the astronaut is in free-fall then infalling starlight coming from behind him should be red-shifted, not blue-shifted. Do you have a source for you blue-shift version ? Gandalf61 (talk) 20:15, 14 December 2007 (UTC)


 * Yup. (The author is an astrophysicist)  Someguy1221 (talk) 20:26, 14 December 2007 (UTC)


 * Well, my sources say different - you don't see "eon's worth of starlight", and infalling starlight from behind you is redshifted, not blueshifted:
 * "Your trip from the event horizon to the singularity is so short that most of the light from faraway distances doesn't have time to reach you so that you can see it."
 * "Once inside the horizon, you are doomed to hit the singularity in a finite time, and you witness only a finite (in practice rather short) time pass in the outside Universe. In order to watch the history of the Universe unfold, you would have to remain outside the horizon, the Schwarzschild surface."
 * "...there's no way that light from future events far away can get to me. Faraway events in the arbitrarily distant future never end up on my "past light-cone," the surface made of light rays that get to me at a given time."
 * "...blueshifts appear in directions transverse to observer motion, while redshifts are always seen in directions towards or away from the black-hole center" Gandalf61 (talk) 21:33, 14 December 2007 (UTC)


 * Define short, or just wrong. For truly massive black holes, the distance between you and the black hole can be quite extreme. The black hole at the center of our own galaxy (which is not even that big) has a Schwarzshield radius of about half a light minute.
 * "Also time outside would appear to be running much faster, so we would be able to see the evolution of the universe "flash" before our eyes"
 * "light from the universe "falls" into the black hole it gains energy. This means that its wavelength gets shorter. Blue light has a shorter wavelength than red light - so very simply things would appear to get bluer" Blueshifting in incoming light has nothing to do with relative velocities, it's entirely thanks to gravitational blueshift
 * Regardless of how much time worth of light actually reaches you, you're going to get a very deep tan. Someguy1221 (talk) 01:59, 15 December 2007 (UTC)
 * Someguy asked if I could comment on this issue, as I have some experience in the area. The question is rather interesting, but it might take me a while to come up with a complete answer, as most of my references are elsewhere and I'm rather busy this weekend. My guesses are that you probably wouldn't see the evolution of the universe flash before your eyes, and that any possible gravitational blueshifting argument would also have to take into account possible redshifting from the velocities involved. However, the matter is rather complex, and the answers are rather unclear; despite the frequent discussions of the environment inside event horizons, it's not clear to me that a person, or indeed any sentient creature or even any computational device, could survive at all in such a situation, even ignoring tidal forces and radiation. I'm of the opinion that most musings on the subject are probably incorrect, especially when written for a popular audience. --Philosophus T 08:34, 15 December 2007 (UTC)


 *  Your appeal to gravitational blueshift is incorrect, as the classic analysis of gravitation redshift/blueshift assumes that the source and receiver are stationary with respect to one another. See, for example, Einstein's On the Influence of Gravitation on the Propogation of Light, which says "Let the two material systems S1 and S2, provided with instruments of measurement, be situated on the z-axis of K at the distance h from each other ..." - note that h remains constant throughout the analysis. This analysis would apply to an astronaut who was hovering at a constant distance from the singularity (they would, of course, have to be outside of the event horizon) - I agree that infalling starlight would appear blueshifted to such an observer. However, our astronaut is not stationary with respect to the distant stars - he is accelerating away from them, and towards the singularity. Therefore he observes infalling starlight to be redshifted, not blueshifted. Gandalf61 (talk) 17:56, 15 December 2007 (UTC)


 * You can't just discount the effect entirely; you now have two competing influences. I suppose then it depends on just how fast can the black hole accelerate you away from the stars, what velocity you approach the event horizon at initially, whether you're fighting or helping the black hole on the way down, and whatever other wierd GR effects would be happening. Someguy1221 (talk) 04:53, 16 December 2007 (UTC)
 * You know, I think my main difficulty here is that the only GR I know I learned from a somewhat crazy physicist who had a habbit of proposing the construction of space stations inside black holes that would fight gravity to record data as long as possible...Someguy1221 (talk) 04:59, 16 December 2007 (UTC)
 * What exactly would be the point of that? If the space station is inside the black hole then none of the information it might record would ever reach the rest of the universe!  Sure, it finally answers our debate about redshift/blue shift - but the radio transmission (being "light") can't ever escape beyond the event horizon...by definition...so we'd never know what the answer was! SteveBaker (talk) 13:24, 16 December 2007 (UTC)


 * Remember you'd probably die long before the force is strong enough to tear you apart. For example, it's possible the force may stop the blood from reaching your brain depending on your position meaning you'll blackout and eventually die (as can occur with pilots). g-force may be helpful Nil Einne (talk) 16:40, 16 December 2007 (UTC)

Water + Hot oil
Why does hot oil react the way that it does to water? When I put wet strips of potato into a deep fat fryer, is there a relation to the reaction that occurs when I throw water over a chip pan fire? --Seans Potato Business 16:19, 14 December 2007 (UTC)


 * When the oil or pan is hot enough, water will very quickly go from liquid to steam. When submerged in oil, it accelerates upward out of the oil - often causing the oil to splatter.  When it hits a dry pan, it will bounce as the part that touches the pan will accelerate upward, pushing against the liquid.  Both are similar - the water is quickly becoming steam.  By the way - that is how I was taught to test the heat for cooking.  If the water doesn't jump off the pan or jump out of the oil, it isn't hot enough. --  k a i n a w &trade; 16:23, 14 December 2007 (UTC)
 * I was once told by my wise mother that fat content had something to do with it, as I noticed one time frying onions (I believe) didn't make the oil 'spit'. -- MacAddct &#xF8FF; 1984 (talk &#149; contribs) 16:49, 14 December 2007 (UTC)


 * I was told the same thing when I learned to cook. The thicker the oil, the more it splatters.  It makes sense.  As the water shoots out, it will carry more oil with it if the oil is thick. --  k a i n a w &trade; 16:52, 14 December 2007 (UTC)
 * That's presumably correlated with an oil's smoke point, right? -- MacAddct &#xF8FF; 1984 (talk &#149; contribs) 16:59, 14 December 2007 (UTC)


 * I don't know, but I doubt it. I cook with multiple oils (all refined).  I noticed that all but one have a smoke point of 450F.  However, they splatter much differently. --  k a i n a w &trade; 17:05, 14 December 2007 (UTC)


 * I heard that that's not the right method of testing if the oil is hot enough is wrong, because the hot oil could burn your skin. A better method is dropping small crumbs of the breading if you're frying a breaded meat/fish/vegetable, or small crumbs of the breading of something else.  If the oil is cold, it just sinks to the bottom.  If it's hot, small bubbles will push it around for a while.  &#x2013; b_jonas 16:15, 15 December 2007 (UTC)


 * I agree dropping water into hot oil is a bit dangerous should be avoided. Your suggestion is a good idea and I believe a cube of bread is also a useful test. How long it takes to brown will vary depending on the the temperature and some recipes/whatever will give this, there's also this general guide which has a fairly small temperature range . Of course, a thermometer suitable for the purpose is probably your best bet for consistent, simple and reliable deep frying Nil Einne (talk) 16:34, 16 December 2007 (UTC)
 * Aside - this is why you should NEVER use water to try and put out a burning liquid (such as oil) unless you're a trained firefighter and know exactly what you're doing. The water will superheat into steam in a fraction of a second and explode flaming oil everywhere - see . Exxolon (talk) 20:37, 16 December 2007 (UTC)


 * I feel that it is important to clarify what I meant by testing oil with water. I dip the tip of my finger in water and fling a single drop of water into the oil.  I do not take a cup of water and dump it in.  A single drop of water pops into steam when it hits the surface of the oil.  When testing a hot (dry) pan, it bounces.  I hope this explanation keeps someone from trying to pour water into hot oil. --  k a i n a w &trade; 00:06, 17 December 2007 (UTC)

Dams
OK so at a normal dam there is water on one side and air on the other. At the bottom of the dam force of the water pressure pushes horizontally is against the dam and here it is at its greatest. So the dam must be thick at the bottom but can be less thick at the top. However, behind the dam there are sometimes mini-dam-things, which can raise the level of the dam further. These don't have to be thick at the bottom because the water pressure from one side cancels out the pressure from the other. But what are these mini-dam-things called? —Preceding unsigned comment added by Swithlander (talk • contribs) 18:06, 14 December 2007 (UTC)


 * Are you referring to a floodgate? I've always called them flashboards, but apparently that is not the correct term. --  k a i n a w &trade; 18:33, 14 December 2007 (UTC)


 * Flashboards and Stoplogs are mentioned in the article - could use a picture though. The main reason gravity dams are wide at the bottom isn't because the water pressure is greatest at the bottom, it's to give a wide foot space and enough mass to keep the dam from being rolled downstream by the water pressure. --Duk 18:50, 14 December 2007 (UTC)


 * I don't quite understand what you mean by "raise the level of the dam further" if they have water on both sides. Just to break the log jam, I'll offer cofferdam. Flashboards, meanwhile, at least in New England, are usually big sheets of plywood-like material held vertically at the top dam by metal rods or pipes. The strength of the rod/pipe is engineered so in a (flash) flood, they will bend, allowing the flashboards to wash away and releasing the first four or so feet of water formerly held back by the dam, lowering the risk of a more-serious flood should the dam breach later (and removing from the dam the load of those extra feet of held-back water ).


 * Atlant (talk) 22:56, 14 December 2007 (UTC)


 * I don't understand the question either -- does "behind the dam" mean upriver or down? I'd think it means upriver, but if the downriver side is meant, perhaps we're talking about buttresses, as in the photo of The Dalles Dam for example? The buttresses are there, I'm guessing, to allow for a higher and less thick dam -- that is, the water pressure behind the dam is offset by the strength of the buttresses? I have no idea if this is actually the purpose of the buttresses, just another wild guess about what the OP meant; ie, These don't have to be thick at the bottom -- you mean "these kind of dams"? Pfly (talk) 00:27, 16 December 2007 (UTC)

Rising CO2 content in atmosphere: effect on human breathing?
The Keeling Curve shows rapidly increasing carbon dioxide levels in the atmosphere since 1958:

''It shows a steady increase in mean atmospheric CO2 concentration from about 315 parts per million by volume (ppmv) in 1958 to over 380 ppmv by the year 2006. This increase in atmospheric CO2 is considered to be largely due to the combustion of fossil fuels, and has been accelerating in recent years.''

We know that increasing CO2 levels are affecting Earth's temperatures.

What I want to know is at what point it will affect human breathing. We have gone from 315 parts per million by volume to 380 ppmv in the last 50 years. Since the trend is accelerating, we may at some point in the future have double the amount of CO2 in the atmosphere as we had in 1958.

I'm sure we won't fall over and die suddenly, since the change spans decades or centuries, but won't there be noticeable effects on the efficiency of breathing at some point? What is that point?

Jawed (talk) 19:42, 14 December 2007 (UTC)


 * I don't know a specific answer, but one factor to consider: What is the normal local variation in CO2 levels already?  In other words.. If you, your spouse, and your dog all sleep in the same room, with poor circulation, do you effectively have a higher CO2 level at night?  On the other hand, what if you happen to sleep in a room full of plants?   Maybe we already deal with local variations that are bigger than the global ones.  If they're not a problem, then maybe it's not a significant factor.  Friday (talk) 19:46, 14 December 2007 (UTC)


 * (edit conflict) I could not quickly find the numbers in Wikipedia, but my thinking is that (a) the CO2 concentration of exhaled breath is huge compared to ~400 ppm, and (b) the exchange efficiency of a breath is relatively poor (that is, a good fraction of the air in your lungs is retained each time you breathe). If both of these claims are true, then we should be pretty insensitive to atmospheric CO2, because the CO2 in our lungs is dominated by the effects of our own respiration, not the atmosphere. -- Coneslayer (talk) 19:48, 14 December 2007 (UTC)


 * The federal government considers concentrations greater than 5000ppm to be unhealthy to adults, although estimates on what constitutes a danger to one's health are as low as 1000ppm . The LD50 of carbon dioxide is 100,000ppm, although prolonged exposure at lower concentrations would also be fatal.  Someguy1221 (talk) 19:53, 14 December 2007 (UTC)
 * I recall reading an old book by Asimov and someone else (nonfiction) about the different kinds of planets we might find and be able to inhabit. I'm not sure how definitive it is, but they put human limits of CO2 at 7 torr... Man, I should've taken better notes. Æµ§œš¹  [aɪm ˈfɻɛ̃ⁿdˡi]  21:45, 14 December 2007 (UTC)


 * Here is the handy scale of CO2 concentrations in parts per million (ppm) distilled from our articles on Carbon dioxide, Carbon dioxide in the Earth's atmosphere and other similar places:
 * 180 ppm to 280 ppm - historical CO2 levels have oscillated back and forth between these limits from over half a million years ago until about 1940.
 * 280 ppm to 380 ppm - rise in CO2 levels from 1940 until today in "clear air" locations.
 * 600 ppm - typical present day value in big cities, etc.
 * 1,000 ppm - causes discomfort in more than 20% of people.
 * 2,000 ppm - majority will feel a significant discomfort, many will develop nausea and headaches.
 * 5,000 ppm - maximum safe level for healthy adults over prolonged (8 hour) exposure. Children, elderly & sick people can tolerate 'significantly less'.
 * 30,000 ppm - safe for brief exposure.
 * 40,000 ppm - immediately dangerous to life and health.
 * 45,000 ppm - exhaled breath
 * 50,000 ppm - Dangerous when inhaled. Exposure for more than half an hour causes acute hypercapnia
 * 70,000 ppm to 100,000 ppm - unconsciousness in only a few minutes.
 * 100,000 ppm - Car exhaust (up to 150,000 in California due to laws placing limits on carbon monoxide emissions).
 * Not a pretty picture - but that's the facts. SteveBaker (talk) 21:55, 14 December 2007 (UTC)


 * So...you're saying that someone who stands too close to my face while talking could be considered an "immediate [danger] to [my] life and health"? o_O Someguy1221 (talk) 21:59, 14 December 2007 (UTC)


 * If you were both in a very small, totally enclosed space so that your exhaled breath could not be replenished with fresh air...then yes. But your breath is warm - and warm air rises over cold air - so your breath doesn't leave your nose and just kinda sit there in a big invisible ball ready to be breathed back in again.  It floats upwards and outwards and disperses out into the room (watch what happens to the smoke when a smoker exhales - that's a pretty good indication of what's happening to the CO2 in his/her breath).  Also, read carefully - 50,000 ppm isn't harmful until you've been breathing it for half an hour - you've got a few minutes even at 100,000 ppm.  But if you put a plastic bag over your head, you'll die from rebreathing your own exhalations quite quickly (Hint: Don't try this at home folks!).  Of course we're only talking about CO2 levels here.  If you don't have adequate amounts of fresh air, you'll also be depleting the oxygen content of the air and running out of oxygen is also fatal.  The numbers above are for when there is adequate oxygen but the CO2 levels are becoming poisonous.  Recall the events of Apollo 13 (see the movie!) - they had plenty of oxygen for the trip - but the CO2 "scrubbers" were not removing the CO2 from the air - and they are all nervously watching the CO2 meter creep upwards as they construct the famous 'mailbox' gizmo to fit a square box into a round hole and get the CO2 scrubbers working again.  That was a case of three guys in a confined space breathing each others exhaled breath. SteveBaker (talk) 13:12, 16 December 2007 (UTC)


 * Standards seem to have changed since I looked at them: the 8-hour limit I remember was 10,000 parts per million -- higher than any substance. --Carnildo (talk) 23:05, 14 December 2007 (UTC)


 * We are now at a CO2 level of almost 400 ppm. According to this page on CO2 Health Effects and toxicity levels:
 * http://www.inspect-ny.com/hazmat/CO2gashaz.htm
 * Carbon dioxide levels above 1500 to 2000 ppm are likely to be reached only in unusual circumstances (being enclosed in an airtight closet for a long time)
 * I went to Keeling's website at ucsd.edu:
 * http://scrippsco2.ucsd.edu/home/ and got their data in Excel format:
 * http://scrippsco2.ucsd.edu/data/in_situ_co2/monthly_mlo.csv
 * If you solve for an exponential equation fit, you get:
 * co2_ppm = 0.1062 * e^(0.0041 * year)
 * Plugging in 1000 ppm, you get year = 2232. So in the year 2232 everyone on earth will start to feel as if they've been locked into an airtight closet when they breathe?
 * Jawed (talk) 06:45, 15 December 2007 (UTC)


 * Yep - that sounds about right. We aren't really likely to have CO2 toxicity problems because if we don't do something to fix global warming in a lot less than 225 years then climate-related problems will get us long before CO2 toxicity does.  However, that may well not be true for other plants and animals around the world.  The numbers above are for humans...maybe they also apply fairly well to other animals with lungs (maybe) - but we would also need to worry about what they are for pollinating insects and other vital bits of our ecology. SteveBaker (talk) 13:12, 16 December 2007 (UTC)

Absolute Zero Possible?
According to the Wiki article on Absolute zero, the coldest place that has been discovered to date is in the Boomerang Nebula, which the article states is –272.15 degrees Celsius, or about -458 degrees Fahrenheit, or 1 degree Kelvin. Here's an article published in the Sydney Morning Herald that discusses this discovery. The article mentions that this place in the Boomerang Nebula is actually colder than the background cosmic microwave radiation, due to the special influence of the nebula's central star that is acting like a giant refrigerator. It does this by shooting out super cold gas.

So here's my question: if there were more of these type of refrigerator stars in the universe, is it possible that scientists may some day discover one that is able to achieve absolute zero? What I'm asking is - is it actually possible to reach absolute zero, or is this an unattainable goal due to some reason I can't think of right now? -- Saukkomies 20:11, 14 December 2007 (UTC)


 * Quantum Mechanics says no. Zero point energy? Wily D 20:16, 14 December 2007 (UTC)


 * No, it doesn't: It only says that there is energy in the system if the temperature is absolute zero. Icek (talk) 02:59, 15 December 2007 (UTC)


 * See the Third law of thermodynamics. 207.148.157.228 (talk) —Preceding comment was added at 20:49, 14 December 2007 (UTC)


 * Definitely not. Reaching absolute zero is a bit like reaching the speed of light - you can get close - but you can't quite actually get there.  The 3rd law of thermodynamics is a law. SteveBaker (talk) 21:26, 14 December 2007 (UTC)
 * Isn't that a faulty anology since there actually is something that can travel at the speed of light - light itself?82.182.116.8 (talk) 21:36, 14 December 2007 (UTC)
 * Change "there" to "past" ;-) Someguy1221 (talk) 21:41, 14 December 2007 (UTC)


 * Individual particles can obtain a state of minimum energy. This is in fact the normal condition of most particles in a Bose-Einstein condensate.  However, the third law of thermodynamics says that no macroscopic process can ever remove all of the thermal energy from an ensemble of particles.  In other words, any macroscopic collection of particles will always have an average temperature above absolute zero.  Dragons flight (talk) 21:48, 14 December 2007 (UTC)

These are great answers! Thanks. So, do you think it would be a good idea to add something to the Wiki article on Absolute zero that mentions this? I think one of you guys who just replied to my question would be perfect a choice for that (hint hint). -- Saukkomies 16:59, 14 December 2007 (UTC)


 * It looks to me like the basic parts of it are all stated pretty clearly in the intro of that article, and even the details are there too. The sections on reaching towards absolute zero even mention the Boomerang Nebula. Of course, rewording to make the important parts even more clear especially for non-phyisicist readers would always be welcome! DMacks (talk) 22:28, 14 December 2007 (UTC)

Of course, if you don't mind going below 0K, you can also talk about negative temperatures. Well actually, they're infinitely hotter than 0K, but a lower number. Scientists are weird:) DMacks (talk) 22:24, 14 December 2007 (UTC)


 * Well you find a better way to designate a temperature that happens to be hotter than infinity :-p Someguy1221 (talk) 01:23, 15 December 2007 (UTC)


 * Okay, I read most of that article on negative temperature, and now my head hurts. It's well written, but I do think it's a bit foggy for even a somewhat clever non-physicist like myself to fully grasp. It presupposes a certain level of mathematical ability or familiarity with physics on the part of the reader. Of course perhaps I am trying to shortcut the process of mastering these deeper level physics concepts by not reading a physics textbook or taking a physics class, and instead am trying to get it all through the "Cliffs Notes" method of reading Wikipedia articles. Heh. But I do want to understand Negative Temperature. It's completely fascinating to me right now. So is there some book out there written for non-physicists like me that would do a decent job of explaining this stuff? Just to give you an idea of my level of comprehension, I have read Hawking's "A Brief History of Time", and was able to understand most of it. But that wiki article about negative temperatures eludes my grasp. Perhaps because it is sort of isolated on its own - there's not a lot of supportive background other than linking to other Wiki articles and trying to do so in a comprehensible way. Sometimes wikipedia is awesome, but perhaps it has a drawback in instances like this... --Saukkomies 17:40, 14 December 2007 (UTC)


 * I've never thought of negative temperatures as anything with profound meaning. It's really just a quirk of defining temperature as $$ \frac{1}{T} = \frac{dS}{dE}$$, where E is energy and S is entropy, if one is given a strict and absolute measure of entropy. For some systems, temperature is simply allowed to have a negative value, and this transition takes place as the graph of dS/dE crosses the energy axis. Someguy1221 (talk) 01:23, 15 December 2007 (UTC)

So actually the claims about absolute zero being "impossible" are not entirely correct, or at least not entirely precise (which is sort of appropriate, because the whole concept of "temperature" is not entirely precise in the first place).

A macroscopic object consists of finitely many atoms, each of which has a discrete spectrum of energy states, and then there are also states describing the interactions among the atoms, which form a discrete spectrum provided the whole system is bound.

So if just by chance the whole thing happens to radiate all its thermal energy away at once and reach its global ground state, do we call that "absolute zero" for the system? Well, I don't know what else "absolute zero" could mean for this system. And it is certainly possible (if extremely improbable -- cf infinite monkey theorem) for this to happen. (By the way, responding to WilyD -- no, the fact that some zero-point vibrational energy remains does not mean the system is not at absolute zero. The zero-point energy is precisely the energy that does remain at absolute zero.)

Now there are a couple of difficult subtleties with this scenario. One is that we could never know that the system was in its global ground state. Another is that temperature is defined as though the spectrum were continuous (it's defined in terms of the derivative of the internal energy with respect to the log of the number of states having that energy, but when we're talking about a discrete spectrum it's not so clear just how you take a derivative). --Trovatore (talk) 23:18, 14 December 2007 (UTC)

Absolute Zero Question, Part B
Okay, after reading the links that were included in the above answers, I became stumped on another related question. Let's say that the theory that states that the Ultimate fate of the universe is a Big Freeze, as opposed to the other possibilities such as the Big Rip, the Big Bounce, the Big Crunch, etc. I noticed in the Wiki article about the Big Freeze that it says that ultimately such an end to the universe would result in everything reaching Absolute Zero. So, assuming this is correct, then I am now puzzled. I was totally grokking what SteveBaker said about comparing Absolute Zero with the Speed of Light. It makes sense to me to think of it as an unreachable condition. But then I read this business about the Big Freeze producing Absolute Zero, and now I'm once again banging my head against the wall trying to get a handle on this. What exactly will happen to "stuff" in the universe if the Big Freeze takes place? Will it all turn into some kind of subatomic quanta? Will everything become nothing? My mind is reeling from trying to figure this one out... Please help! -- Saukkomies 17:26, 14 December 2007 (UTC)


 * The absolute zero point is only approached asymptotically, so only at infinite time do you get to absolute zero. Of course at any nominated time the temperature is not at absolute zero, so really it is never reached, just approached.  The same situation again. Graeme Bartlett (talk) 23:48, 14 December 2007 (UTC)


 * Okay, I think I've caught the scent of your trail, Graeme. It sounds like there are different kinds of time, is that correct? I mentioned I'd read "Brief History", but it was several years ago since I last peaked into it, and well, that book sort of takes a lot of re-reads to get to the point where I could recall most of it... So - we have different kinds of time, and one of these kinds of time is infinite. Does that mean that infinite time is connected to the Big Rip or Big Freeze theories? Because if I recall, Hawking discusses how if there was a Big Crunch that time would start to go backwards once the universe started to collapse upon itself. I say I remember this, but I don't understand it. I just take his word on it. Am I somewhere in the neighborhood here, or way out on a limb? -- Saukkomies 19:55, 14 December 2007 (UTC)
 * The going to absolute zero theory heat death only applies if the universe expands for ever. If time stops, the universe contracts, or time goes into reverse, the temperature will not go to zero.  If there is a big rip a different thing happens because the macroscopic world is taken apart leaving only widely separated particles.  I think that the concept of temperature will not apply in that case. The concept is like in maths $$1 / t $$ where you can get this as small as you like but cannot actually get it to zero, the concept of a limit. Graeme Bartlett (talk) 20:56, 15 December 2007 (UTC)
 * What it actually means is that for any given temperature above absolute zero, no matter how close, after a certain point in time the temperature stays lower than that. It has absolutely nothing to do with anything infinite. By the way, how can time stop or go in reverse? If t is time, dt/dt will always be one. (In plain english, time always goes forwards at a rate of one second per second. On second in the future is always exactly one second in the future. No more, no less.) It would have to be zero for time to stop, and negative for time to go backwards. It can end, but I don't see how it could stop. —Preceding unsigned comment added by DanielLC (talk • contribs) 17:30, 16 December 2007 (UTC)

Why do Cells divide?
I heard that cells divide to grow, reproduce, renew, and to repair, but what does each of these stages mean? —Preceding unsigned comment added by 68.228.14.91 (talk) 21:29, 14 December 2007 (UTC)
 * See cell division (or mitosis for how human cells usually divide). But very simply put, for a multicellular organism (like you), cells need to divide to build up your body. The reason you can't just have a handful of ever expanding cells form a multicellular organism is caused by surface area to volume ratio.  Basically, cells need to be tiny to work they way they're supposed to, so they need to divide as you grow. For the same reason, cells need to divide to replace any damaged or dead cells in your body, as they can't just expand to fill in the space and function of the dead cell. As for reproduction, it's very simple.  If cells never divided, reproduction simply wouldn't happen! You can't make a new organism without splitting it off from something that was already alive. If a single celled organism never divided, there could never be more of it, and when it dies that would be the end of it.  For multicellular organisms, if we just lopped off some existing cells to form a new person, we'd eventually run out of cells. Someguy1221 (talk) 21:40, 14 December 2007 (UTC)
 * That helps(a bit)... 68.228.14.91 (talk) 21:48, 14 December 2007 (UTC)
 * There are links between cell division and DNA repair. --JWSchmidt (talk) 01:48, 15 December 2007 (UTC)

Speed of light
When the light ray moves, what exactly makes it move at the speed it moves? In other words, when the ray travels (for ex. in vacuum), it has to move at a fixed constant speed, how exactly is it regulated to move exactly at this speed? or, in more other words, how does light know that it has to move at a particular 299,792,458 m/s in vaccum? I understand that this perhaps is because of some kind of weird forces or something of this sort acting on photons which always is equal for all photons no matter how much their energy is, but what is it exactly? Is there something strange happening in the sub-photonic level? - DSachan (talk) 22:58, 14 December 2007 (UTC)
 * Electromagnetic theory explains this. Basically is due to the wave and field properties of light.  The forces are not weird, and are amoungst the best known, they are magnetism and electrostatics. The actual value is down to the permittivity and permeability of a vacuum. Using differential equations it is possible to solve the speed that a change in the fields move at, and it is the speed of light. Graeme Bartlett (talk) 23:57, 14 December 2007 (UTC)
 * I know where it is derived from but I want more physical picture of the process and if possible intuitive also (which I think is very difficult here). In perfect vaccum (though this is not possible), there is no matter. So, how do I imagine, what is going on with the quantities like magnetic permeability etc. which appear in the wave equation. Also, if I take the particle theory and not consider the wave theory of light, how would I go about dealing with the speed? Photons are also fundamental particles of nature, so what is going on between photons when they move? - DSachan (talk) 13:15, 15 December 2007 (UTC)
 * Here's a way I have of grasping this idea, which might be completely wrong, and if so I would appreciate being corrected on it. At any rate, as something travels at very fast speeds it begins to get compressed away from the direction of its travel - it becomes foreshortened. However, this is not apparent from the perspective of the object traveling fast. So, for example, if an astronaut was on a spaceship that was traveling at half the speed of light, he, his spaceship, and everything in it would be foreshortened. This would be apparent to an observer standing on a stationary point looking at him and his spaceship through a telescope, but the astronaut himself would not be able to tell that he and his ship were being compressed. Now, as matter approaches the speed of light, it becomes increasingly compressed. Once matter finally crosses the threshold and actually achieves the speed of light, it becomes so compressed that it ceases to be matter anymore and turns into something else: electromagnetic energy, or light. In other words, it has become infinitely compressed. Once it has reached the state of being infinitely compressed, it is impossible for it to get more compressed than it is, for one thing it is no longer even in a state in which compression even can happen. So that is why it can not go faster than the speed of light, because there's no way for it to compress any further. So, does this idea have any basis in reality, or am I way off target? -- Saukkomies 20:05, 14 December 2007 (UTC)
 * Well, not exactly. First of all, nothing with mass can reach the speed of light.  To do so would require an infinite amount of energy, because not only does length decrease as you approach the speed of light, but mass increases.  Also, the major factor in relativity is not spatial compression, but time dilation.  As you approach the speed of light, time slows down, so light traveling away from your spaceship seems to be moving faster than it does to someone watching you fly past.  That way, from both perspectives, the light moving ahead of your spaceship appears to be moving at the speed of light. (see a bit more on it here)
 * Second of all, the speed of light actually does vary depending on the medium it's traveling through. Light travels through air, water, and glass more slowly than it does through a vacuum.  It's even been slowed down to as little as 6 miles per second.  So, while the speed of light in a vacuum is a constant, the speed that light can travel is not.  As for why the speed of light is that particular number, well, that's one of the great mysteries of science. --  Hi  Ev  01:53, 15 December 2007 (UTC)

I prefer the "modified" relativistic explanation: If you assume the laws of physics are the same in every inertial reference frame, then it is necessarily the case (through some annoying math) that one and only one invariant speed exists (the Mermin article in the reference gives a good derivation, if you have access to it). It can further be shown (through more annoying math present in Mermin's article) that any particle with zero rest mass must travel at the invariant speed, whatever it may be. And then quantum physics can show that photons have zero rest mass, or something. So, this explanation doesn't say why the invariant speed is whatever it is, but it's a very sound proof that light must travel at it. Someguy1221 (talk) 01:39, 15 December 2007 (UTC)
 * I never thought of that. In mathematics, there are only two fundamental numbers: the additive inverse (zero) and the multiplicative inverse (one.) All other numbers derive from these. In physics, there are only two fundamental speeds with respect to any reference frame: zero, and one times the speed of light. -Arch dude (talk) 02:53, 15 December 2007 (UTC)
 * You mean identity, not inverse. And there's nothing very fundamental about zero speed in physics, since it's frame-dependent. Algebraist 04:04, 15 December 2007 (UTC)


 * It's slightly unfortunate that the cosmic speed limit wound up being called "The Speed of Light" - because it makes light sound rather more special than perhaps it ought to be. Photons happen to travel at the cosmic speed limit - other particles happen not to.  This has some rather important consequences.  Special relativity says that your "rest mass" is multiplied by the "Lorentz factor" $$ { 1 \over \sqrt{1 - v^2/c^2} }, $$ to get your relativistic mass. (where v is the speed that you're moving and c is the cosmic speed limit).  So, if your rest mass is some non-zero number - then you can't be moving at 'c' because v2 would be equal to c2 - and you end up with dividing 1 by the square root of 1-1 - which means dividing one by zero...which means you'd have an infinite mass (or an undefined mass if you are a mathematician), not good!  But the reverse problem exists for light.  Photons have a small, measurable 'relativistic' mass at the speed of light.  So what is their mass if you slowed them down?   Well, to calculate rest mass from relativistic mass, you'd have to divide by the Lorentz factor...but that's infinity...and dividing by infinity is another one of those things that mathematicians get all sulky about.  So we can't calculate the rest mass of a photon...it doesn't really have a rest mass...and if we can't calculate it's rest mass, we can't use the Lorentz factor to calculate its mass at any speed less than c either!  So in a sense, the mathematics is what forces light to travel at the cosmic speed limit.  If it somehow managed to go at any other speed, the mathematics that underpins our universe would 'blow up' and produce all sorts of annoying infinities!  This argument is somewhat a "tail wagging the dog" thing...but which came first?  The behavior of light whose consequences are the Lorentz transform and special relativity?   Or perhaps (and I prefer this), relativity is "how the universe works" and light merely travels at that speed because it must. SteveBaker (talk) 16:43, 15 December 2007 (UTC)