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  #1  
Old 06-27-2007, 07:09 PM
Liberal Liberal is offline
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Are time and gravity related through entropy?

Frankly, I'm not even sure if I'm asking the question correctly. So, I'll tell you what tripped it, and maybe it could be framed better.

Consider pouring water out of a glass. If we stipulate that time is unidirectional, then the water isn't going to go back into the glass (at least not via the same timeline). But by the same token, it would be a violation of the laws of gravity for the water to rise and go back into the glass. Is entroy a principle that is applicable to both impossibilities (or near impossibilities)?

Same same for whatever quantum gravity might be. If hot water is mixed with cold, then the warm water will not separate back out into hot and cold. Both for reasons of time and of entropy. Quantum gravity then would be some relation between time and entropy with respect to the particles' states of temperature just as ordinary gravity is with respect to a thing's state of proximity.

I hope someone can help me make sense of this. Maybe the question is completely ridiculous, and if I understood the principles better, I would know it.
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  #2  
Old 06-27-2007, 08:26 PM
Malodorous Malodorous is offline
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Quote:
Originally Posted by Liberal
Consider pouring water out of a glass. If we stipulate that time is unidirectional, then the water isn't going to go back into the glass (at least not via the same timeline). But by the same token, it would be a violation of the laws of gravity for the water to rise and go back into the glass. Is entroy a principle that is applicable to both impossibilities (or near impossibilities)?
I don't think so, if we simplify your thought experiment a little and dropped a simple particle instead of a glass of water, then the process would have zero gain in entropy. In theory we could take all the kinetic energy that the particle gained in falling and use it to power a (ideal) machine, which would then have exactly enough energy to return it to its orignal height. In thermodynamic terms then, the process would be completly reversable.

Of course a real machine would have moving parts and such, that would loose some energy due to frictional heating and the like, and thus not be able to lift our particle up to its orginal height. But so far as I can reason, that wouldn't have anything to do with gravity. The same would happen if the particle was originally set moving by any other forces.

Wiser heads will have to tackle the quantum gravity bit. Mixing cold water with hot will certainly be non-reversable and increase entropy, but I don't see how that is due to gravity. Indeed, they'll come to a thermal equilibrium even if they're far away from any large sources of gravity.
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Old 06-27-2007, 08:30 PM
scm1001 scm1001 is offline
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I am no expert but I think there are several concepts here.

Entropy is a order disorder transition. Gravity (in simple terms) can be thought of in terms of fields, in the same way as electrostatics and magnetism etc. I think I understand entropy well because it is simply a mathematical concept without any physical baggage - I am not sure I "understand" fields quite in the same way.

There are two tendencies in nature (i) increase entropy and (ii) lower energy. Time is said to progress when either or both happen. (a physics/philosophical question - does time exist outside of these tendencies?)

I suspect on one level there is little to connect entropy and gravity. There may be a much deeper connection, but the way you are describing it the only connection is that both appear to go in one direction under time
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Old 06-28-2007, 07:49 AM
Der Trihs Der Trihs is offline
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I'd think that an object obeying gravity ( by "obey", I mean not being held up or moved in opposition to it by some other force ) is an example of entropy; the object is seeking a lower energy state. I don't think that would be special to gravity, however, and would apply to any force.
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Old 06-28-2007, 08:02 AM
ralph124c ralph124c is offline
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entropy increase relates to the non-reversability of time. in this respect, time is unlike the three dimensions of space-you can reverse any of the spatial dimensions, but you cannot reversew time-it 'flows" in only one direction.
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Old 06-28-2007, 12:24 PM
Liberal Liberal is offline
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Originally Posted by ralph124c
entropy increase relates to the non-reversability of time. in this respect, time is unlike the three dimensions of space-you can reverse any of the spatial dimensions, but you cannot reversew time-it 'flows" in only one direction.
Does gravity do the same? "Flow" in only one direction; i.e., can one invoke an antigravity?
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Old 06-28-2007, 02:08 PM
Napier Napier is offline
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A physicist offering two perspectives:

Time appears to flow like a river, or like some "stuff", because we evolved brains to manipulate it. That is, the brain's only purpose is to create causes whose effects we then get to enjoy. If you couldn't cause effects, you couldn't use a brain. But physically time is significant because it is the agency that orders causes before effects. I don't know that you can make any bigger claim for it than that. It isn't made of anything, if you like.

Entropy, in the thermodynamic sense in which the term was coined, is the integral of the inverse temperature of a body, with respect to heat. If you put two bodies at different temperatures in contact with one another and let them equilibrate, their total entropy will be lower at first and become higher.

The laws of gravity and temperature are macro scale descriptions of behaviors. Liquids don't flow backwards up into their containers, and bodies that have uniform temperature don't spontaneously grow gradients, because it's very unlikely, though on the basis of quantum mechanics both these things would have to happen occasionally. VERY occasionally. But if the universe really is infinite in size (as cosmologists seem convinced), then both these things have happened an infinite number of times. In fact, they both happened together to French-speaking intellegent purple 9-toed turtles pouring drinks called FizzyPop an infinite number of times.
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Old 06-28-2007, 03:48 PM
Liberal Liberal is offline
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Why has it never happened to me?
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  #9  
Old 06-28-2007, 04:16 PM
Phlosphr Phlosphr is offline
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Originally Posted by Liberal
Why has it never happened to me?
Perhaps it has and it was imperceptible to you at the time.
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  #10  
Old 06-28-2007, 04:55 PM
Chronos Chronos is online now
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Gravity is just as time-reversible as any other force. Throw a ball up into the air and catch it again, and film the process: The ball starts off moving up quickly. As time goes on, it slows down, until eventually, it stops at the apex of its trajectory, and starts moving down. At first, it's moving down slowly, but it speeds up as it falls, until it hits your hand again quickly.

Now reverse the film. The exact same thing happens. The ball starts off moving up quickly, slows down, stops, moves down slowly, and speeds back up.

It's only when you get a whole bunch of things interacting that time-reversal fails, and that's due to entropy taking over. But that's true regardless of whether gravity is involved. A whole bunch of balls in deep space joined together by springs (no gravity involved) will settle into an entropic state, and a single planet orbiting around a star (nothing but gravity) will stay in a state of low, almost constant entropy.
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  #11  
Old 06-28-2007, 08:05 PM
Stranger On A Train Stranger On A Train is offline
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Although Napier has offered a definition of entropy, I think the concept needs to be illustrated more explicitly. The energy we call "heat" is simple randomized kinetic energy; that is, it is a bulk measure of the kinetic energy of all the little particles that are bouncing around in a fluid or vibrating in a solid continuum. Unlike mechanical kinetic energy--in which we can identify a body with a certain mass moving at such-and-such a speed--a body with heat energy may not be going anywhere at all. We mesure the heat in a body in terms of its temperature (relative to an "absolute zero" state of minimum atomic motion) and density, and calculate the movement of heat-energy in a system via its difference in temperature from one location, portion, or cycle, to another via the laws of thermodynamics. These are the basic rules of thermodynamic behavior, analgous to Newton's laws in mechanics, Maxwell's equations in electrodynamics, and so forth.

The zeroth law--so named because it was so obvious that no one really thought it needed to be stated until it was realized that you have to make things painfully clear to undergraduate students in the hope that they'll learn anything--is familiar to algebra students as the associative principle; if two states are in equilibrium, a third state in equilibrium to one is in equilibrium to the other. Duh. The second law merely states that energy can't be created or destroyed, i.e. the principle of conservation of energy that is a known factoid even to people who paid no attention in basic eighth level science. Pretty boring stuff, really, and not even worth cracking open a book to read about.

However, the second and third laws get far more interesting, in that they really get into the guts of thermodynamics, which is all about entropy. What is entropy? Entroopy is a measure of the disorder of a system, or alternatively, how homogenous it is. We'd prefer systems in which energy is all tucked away into one high temperature corner (thermodynamicists call this a "reservoir") and then free to flow--by a process we direct--to a low temperature corner, doing for us valuable work in the process, like making an engine go. And rules #2 and #3 get into how this business works.

The second can be stated in a large number of different ways, but in essence, it says that no real world process is genuinely irreversable; you always "lose" energy. Of course, the smartacres in the upper rows will now crow about this being a contradiction to the previous rule, but in fact the supposed loss that occurs is in the form of energy no longer available for use; it's still in the system, but it is now just random energy, incapable of being put to a direction. We quantify by the means of entropy, about which I'll say more in a second. Rule three is even more interesting; it says that you can't even stop the total level of system entropy from increasing; even if you manage to separate out the hot particles from the cool particles in one region--Maxwell's famous daemon--you'll get a corresponding increase in the disorder of the system somewhere else, such at overall disorder is always greater than what you started out with.

Eventually, you end up with this system which is which is cooler than your high temperature corner was and hotter than your low temperature corner was, and all the same temperature throughout, thereby giving you no flow from which to get any work done. Imagine if all those monkeys in the room with the typewriters kept punching the "F" key over and over; you're never going to get the collected works of Shakespeare, or even a few lines from Hamlet, no matter how long you wait. This is what is meant by thermodynamics use of the term heat-death. It's more significant than merely the inability to make your system do work, incidentially; it also means that ultimately the system can't store information.

Now I've been talking in terms of closed systems, and in engineerirg idealized heat engines and steam cycles are used, like assuming that the horse is a sphere to make the math easier, for the sake of being able to get something done. In the real world, however, most systems are nonequilibrium thermodynamic (NET) systems, and although the also follow these rules implicity, the systems are so complex that you can't just draw a box and call them adiabatic and in "pseudo-equilibrium". Nonetheless, entropy occurs in those systems, too. Oh, you can make your house cooler by turning on the air conditioner, but only by making the outside hotter, and using a lot of energy in the process to reverse the normal flow of heat. You're just lucky that there's enough mass outside to accept this additional heat-energy, plus what you create in generating electricity for the compressor in the air conditioner.

So that's entropy in a nutshell. What about gravity? As Chronos notes, gravity, like all fundamental forces, is conservative; that is to say, in absence of any interactions with a system involving heat, you'll gain the same amount of energy going down as you did climbing up. Our Lady of Perpetual Gravity knows nothing of the crass prolitariat of Thermodynamics, working as they do at an intermediate stage of complex interactions between atoms and molecules (intermolecular forces) and pissing away their time bouncing from one covalent bond to another. Gravity has nothing to do with this; she maintains her stoic, unbending, highly directionalized and ordered grip between bodies of distinguished mass. The reason water doesn't naturally pour up into a glass has nothing to do with thermodynamics (and indeed if that coarse rabblerouser Entropy had anything to say about it, water would pour every which way without regard to the attractive grace of Lady Gravity) but rather energy; when you pour water out of a glass, you're giving away the potential energy required to lift the liquid up to a certain level, and giving it away to the stream of liquid flowing downhill in the form of highly directed kinetic energy.

However, review of the third law of thermodynamics tells us that no real world process, even one involving a pure fundamental force, is every without entropy on the macro scale, and this is true with pouring water as well, for the water we pour on the ground doesn't say in a neat coil to be elevated back to the glass with the same amount of energy it gained from being dropped, but rather splashes all about in a big mess that will take more energy to collect it than it is worth. Similarly, while we can carefully direct a stream of water back up to the glass, doing so requires the generation of more energy, which inevitably loses some of the applied work to random heat. In the end, the Morelocks must eat in order to keep the machinery working, and their diet is whatever is wasted in the process of doing work in the real world.

Gravity appears to have only one pole--an attractive one--although there's no clear reason why this should be so. There are various theories that posit a number of different ways in which gravity could have positive (repulsive) components, but there is no accepted notion of "anti-gravity" and no practical demonstration of any repulsive gravity effect that has stood up to investigation.

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Old 06-28-2007, 08:27 PM
Stranger On A Train Stranger On A Train is offline
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Urk...that should be "There are various theories that posit a number of different ways in which gravity could have negative (repulsive) components,".

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  #13  
Old 06-29-2007, 06:49 AM
Liberal Liberal is offline
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Thank you, Stranger! Great explanations. Do all forces have poles (or a pole)?
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Old 06-29-2007, 09:26 AM
Napier Napier is offline
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Another insight about thermal energy being able to do work, and entropy: You can think of mechanical work as a force moving something a distance. If you stand still and slide a box across the floor, you're outputting a positive amount of work. But if you and the box are both already shifting around randomly, and equally vigorously, when you apply a force to the box for a little while the box may just as well do the work to you - that is, the two of you may shift closer together, and the amount of work is negative. Similarly, relative mechanical motion between atoms in something very hot and something very cold predictably has the hot thing's atoms doing work to the cold thing's atoms, but not so if they are at the same temperature.
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Old 06-29-2007, 01:38 PM
Stranger On A Train Stranger On A Train is offline
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Another error: "The second law merely states that energy can't be created or destroyed," should read, "The first law merely states..." I'll never succeed as a copyeditor.

Quote:
Originally Posted by Liberal
Do all forces have poles (or a pole)?
A pole is just the focus of a field. It can be thought of as a singularity, where the field strength is infinite, and from which the force declines by some relation, which for electrodynamcs and gravity is 1/r2. Electrical charges have two types of poles--positive and negative--and two opposing poles create an eletric dipole moment, which can be thought of as more or less an analogue to a moment of inertia (its resistance to change in rotation) in a massy body. When you stick charges together, you amplify their effect, though like electrical charges repell, requiring some other attractive force to keep them together without totally cancelling out their effect; hence, atomic structure where positive neuclons (protons) draw in negative electrons which "orbit" about the nucleus in a shell. The protons are held in the nucleus as the result of strong interactions between fundamental particles that make up the nucleons, resulting in the "nuclear force" or residual strong force between protons and electrically neutral neutrons. This is also fantastically complicated and the guts of it are described, as much as we can describe it, by quantum chromodynamics (QCD).

Magnets also have poles, of course, and a dipole moment but the poles are never found individually in nature for reasons that are rather complicated to explain but persuade us that magnetic monopoles don't exist in nature, thus there is no such thing in practice as a magnetic charge. (Magnetic fields are quantified in strength by their magnetic field strength, H, and magnetic flux density, B.) Gravity, in contrast, has only "positive" poles as far as we can tell. Why (or whether) this is so is one of the fundamental and unanswered questions in gravitation.

All of these fundamental forces are actually abstractions of fundamental particle interactions on the quantum level; for instance, the electromagnet force is the result of interactions between electrically charged particles via photons, as described (among other things) in quantum electrodynamics (QED). (The magnetic component of the field is due to movements of these charged particles.) Strong and weak interactions, which create the "internuclear" forces are due to exchanges of gluons and the fundamental massy bosons which transfer and mediate color charge interactions between quarks and leptons, respectively. The gravitational force is thought (by students and researchers in quantum gravity) to be the result of interactions between massy particles of gravitons. What are gravitons? Nobody really knows, and aside from a few basic properties they have to have (be massless and chargeless, spin 2, interact with all other particles) we can't say anything about them, we certainly can't detect them individually, and indeed it may be impossible to do so. As a result of all of this, it's generally easier for any practical purpose to treat forces as fields and the sources of interaction as poles.

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  #16  
Old 06-29-2007, 04:38 PM
Liberal Liberal is offline
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Could time be described mathematically as an entropic field?
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Old 06-29-2007, 09:37 PM
Stranger On A Train Stranger On A Train is offline
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Quote:
Originally Posted by Liberal
Could time be described mathematically as an entropic field?
I don't really know what you mean by an "entropic field". However, actions in which entropy occurs (any thermodynamic process, or more generally, any process involving macro-scale mechanics of any sort) tend toward greater entropy as time increases, giving us a sense of direction of time and cause & effect in any real process. If you see broken coffee cups reassembling themselves, or smoke uncombusting back into coal, you can be assured that you are moving backward in time. Either that, or somebody threaded the wrong end of the reel as a gag.

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