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Thread: Entropy and thermodynamics

  1. #1 Entropy and thermodynamics 
    Forum Freshman Incision's Avatar
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    Why is there an inevitable increase in entropy when the laws of physics are invariant under time reversal? The time reversal symmetry of physical laws appears to contradict the second law of thermodynamics


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    Time reversal invariance will apply to simple systems only. As soon as you're dealing with statistical systems of large numbers of particles time invariance disappears.

    A collision between two billiard balls ( or two atoms of a gas ) will look the same, and all physical laws are obeyed, wether going foreward or backwards in time ( take a video and play it foreward or backwards ).

    A cup falling off the table and shattering into 100s of tiny pieces, is not the same, and will not obey the same physical laws, if those same hundreds of tiny pieces have to have the exact position, energy and velocity to come back together and re-form the cup in the time reversed video.


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    I hope this isn't far off-topic, but what implications does entropy have for the formation of complex structure (i.e. life) on this planet? Is this one of those low-probability instances where the entropy of a system seems to decrease? Could we say that life on this planet is a constant struggle against entropy (which we will eventually lose)?
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    Quote Originally Posted by Incision View Post
    Could we say that life on this planet is a constant struggle against entropy (which we will eventually lose)?
    That's a famous saying, isn't it? As it is nature's tendency for entropy to increase, it is life's tendency to struggle against its increase or something like that?
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    I do not remember exactly, but it seems I saw previously such question from Singapore somewhere. And this is quite correct and good question.

    So, the answer is quite direct: the opinion that "the laws of physics are invariant under time reversal" is only a history-making delusion. Really the forward and reversed processes in quantum physics have unequal differential cross-sections (in contrast to the integral cross-sections). There are, obviously, the so-called hidden parameters, which Einstein expected to find.

    So, any contradiction with the second law of thermodynamics is really absent. But I do not know how long else this Copenhagen's sleep will last. For additional information see e-print arXiv:0706.2488v5.
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    Time reversal is not very likely in "random" processes - such as quantum level events. At each hierarchical level of organization of physical event we have such processes (chaos, chance, quantum) which prevent back-prediction at that level ( you cannot specify with arbitrary precision the immediate past any more than the immediate future, at any given level). The paradoxical aspect of time irreversibility with reference to the Second Law is created when "cause/effect" is misunderstood as fundamental structure rather than human framing - as somehow more basic or fundamental or "objective" than the other principles of explanation appropriate at a given level.
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    In the same line of thought, I've read that Entropy is itself purely subjective. It is the amount of information we don't know about a system at any given time. It cannot increase unless the volume of the system increases and, the only way for that to happen is by a subjective smoothing of the fine information.

    It is odd to struggle against something we have created. Entropy is not inherent to the universe. Since matter and light etc. are not conscious, they cannot measure themselves nor can they make predictions as to what they're going to do; therefore, if there is nothing to make objects subjects than there is no entropy.

    Edit: Not that you're wrong Incision. All I'm saying is the struggle against entropy only exist within the human desire to create and act within the universe (on many different levels).
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    Quote Originally Posted by Incision View Post
    what implications does entropy have for the formation of complex structure (i.e. life) on this planet? Is this one of those low-probability instances where the entropy of a system seems to decrease?
    No. If you eat food and use oxygen then the entropy of the system that contains you is increasing.

    Could we say that life on this planet is a constant struggle against entropy (which we will eventually lose)?
    You could say that in a bad poem I suppose.
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    Quote Originally Posted by Bunbury View Post
    Quote Originally Posted by Incision View Post
    what implications does entropy have for the formation of complex structure (i.e. life) on this planet? Is this one of those low-probability instances where the entropy of a system seems to decrease?
    No. If you eat food and use oxygen then the entropy of the system that contains you is increasing.

    Could we say that life on this planet is a constant struggle against entropy (which we will eventually lose)?
    You could say that in a bad poem I suppose.
    Lol, kind of what I was thinking about the poem. Pynchon covered entropy.

    Is the entropy really increasing or is the volume merely being changed and becoming more fine.
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    Pynchon covered entropy.
    Yes, but what was all that about the octopus?

    Is the entropy really increasing or is the volume merely being changed and becoming more fine.
    The Second Law applies to isolated systems. If you isolate a human body it dies and entropy increases.
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    Quote Originally Posted by Bunbury View Post
    Yes, but what was all that about the octopus?
    Ha! I have not read Gravity's Rainbow..yet. Kudos to you if you did. That's quite a job.

    The Second Law applies to isolated systems. If you isolate a human body it dies and entropy increases.
    I just erased two paragraphs. We agree that an increase in entropy is synonymous with information becoming more fine and also, that entropy is subjective? If so, my last post was wrong because (as stated) a subjective increase in entropy is analogous with information becoming more fine. My bad.

    Are there any completely isolated systems? And, to clarify, you're saying a human body is not an isolated system.

    The real issue I'm having is with human confines.
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    Of course a human body is not an isolated system. It takes chemical energy (food) from the surroundings, uses some of it for motion, thinking, everything else, and poops, pees, breathes or sweats out the leftovers. How can it be an isolated system?

    You can apply the second law to non-isolated systems - we do it all the time - but you have to account for all the ins and outs. I don't really know what you mean about information becoming more fine. As an engineer I am used to talking about energy, not information. We let the project managers mess up the information side of things.

    Yes, I read Gravity's Rainbow, which was quite a struggle, and could not now tell you what it was about except in general terms. Amazing thing is that Pynchon also wrote a wonderful book called Mason and Dixon, about the mapping of the southern boundary of Pennsylvania and down the Delmarva Peninsula with a strange loop around a church somewhere in Delaware (IIRC) - the Mason-Dixon Line of course - and it's nothing remotely like GR. I recommend it to anyone interested in US history, but it doesn't deal with entropy.
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    I see what you mean. I really need to buff up on the actual definitions to physics' lexicon. I'm more of a literature/psychology/philosophy guy but, I find systems analysis to be relevant to all those subjects. I miscomprehended your original post.

    Maybe you can explain how they can say an open system has "supplies of energy that cannot be depleted" (It is a wiki quote). That statement would not be accurate if the universe is a closed system. My issue with accurately comprehending physics right now (besides the lack of all proper training) is whether or not to take everything out to the nth power or to be practical.

    Have you read Pynchon's short story Entropy? It deals more with the information theory approach but is a relatively quick read.
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    an open system has "supplies of energy that cannot be depleted"
    I find that to be an odd definition. You can hypothesize such a system, where the energy supply, say a hot reservoir, is maintained indefinitely at a constant temperature, but if that reservoir is maintained by, say, fossil fuel combustion, then the fuel will eventually run out and the reservoir go cold. So "cannot be depleted" presumably really means " can be maintained at constant conditions for a long time", or long enough to accomplish the task as hand. For instance a power generating station might be designed to operate for 25 years before it needs to be demolished and replaced. The supply of coal will last much longer than 25 years, so for all practical purposes the plant can be designed as if the energy supply cannot be depleted. But in an absolute sense it is not true.

    Perhaps someone else can offer a better explanation.
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    Maybe they literally mean the sum energy in the system always stays the same. BUT, that would obviously not apply to practical systems such as those you posted.
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    In the same line of thought, I've read that Entropy is itself purely subjective.
    Not at all. For any given defined physical situation it's a hard number, a defined quantity, the same for all observers.

    If you take a thermodynamics class, the professor will expect you to calculate the change in entropy from such events as dropping a hot brick into a swimming pool. The number will be the same for everyone doing the calculation - there will be exactly one correct answer.
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    I'm gathering my information from this paper and statistical mechanics lectures by Leonard Susskind (though I do not think he says the word 'subjective'). I'm not ready to believe they're wrong.

    http://necsi.edu/projects/baranger/cce.pdf

    Final Two Paragraphs:

    "The conclusion is that our dimensionless entropy, which measures our lack of knowledge,
    is a purely subjective quantity. It has nothing to do with the fundamental laws of
    particles and their interactions. It has to do with the fact that chaos messes up things; that
    situations that were initially simple and easy to know in detail, will become eventually so
    complicated, thanks to chaos, that we are forced to give up trying to know them.

    One final question: if entropy is really totally subjective, why does it appear so objective
    to so many people?
    Ask any physical chemist. You will be told that entropy is a
    fundamental, permanent property of matter in bulk. You can measure it, you can calculate
    it accurately, you can look it up in tables, etc. And the answer to that new paradox is:
    large numbers. It is well-known that very large numbers have a way of making probabilities
    turn into absolute certainty. But it would take another paper to do justice to that subject."

    So I don't think you're wrong either but this paper (for non-mathematicians) seems reliable.
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    One final question: if entropy is really totally subjective, why does it appear so objective
    to so many people?
    Ask any physical chemist. You will be told that entropy is a
    fundamental, permanent property of matter in bulk. You can measure it, you can calculate
    it accurately, you can look it up in tables, etc. And the answer to that new paradox is:
    large numbers. It is well-known that very large numbers have a way of making probabilities
    turn into absolute certainty.
    There is no paradox, simply a modern scientific description of the physical world. The author seems disturbed that physical reality as science describes it is based ultimately on probability calculations and the involvement of large numbers. But all physical "certainties"are similarly based. If that disqualifies something from being "objective", then nothing in the carefully described physical world is objective.

    Are you prepared to declare temperature, pressure, density, chemical reaction rates and products, color, nuclear fission, and the like, "purely subjective"?
    Last edited by iceaura; August 21st, 2011 at 04:16 PM.
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    I'm not trying to be purposely argumentative. The author is a professor at MIT- I don't think he is disturbed by anything he is writing about. He is merely bridging the gap for lay enthusiasts (like myself). I agree with your point. It is counter-productive to get that technical over terminology. When he specifically said that entropy was subjective, he was saying so because it is the amount of information we don't know about any system. Without humans, there is nothing to describe the entropy of a system. If we knew everything about a system there would not be any entropy. That is seemingly the point he is making. It is as if entropy is a principle that is applied to physical systems but, exists without them (in computer science and other theoretical systems).

    Your last sentence is not a panacea to the argument. It is obvious that all those things are subject to human measurements. As Nietzsche would say, science "creates values for a value creating power."
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    Also, I want to know (for my own knowledge) what is specifically wrong with that paper. I don't want to go about thinking it is reliable. .
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    The idea that entropy is subjective seems to me to be a true but useless concept that arose when our understanding of entropy shifted from being a purely engineering term that allowed steam engines to be designed, to being a term to describe the motion of particles in a macroscopic way. Entropy is nowadays described as “disorder” but I doubt if steam engine designers thought of it that way. It’s the concept of disorder that leads to entropy being thought of as subjective, because disorder is meaningless unless referred to some human datum.

    If you have ten pennies and throw them on a table some will land heads up and others will land tails up. If you place them carefully so that all are heads up you would say they are more ordered than the randomly thrown ones. Nature however couldn’t care less which way up the coins are, and in terms of physics there is no difference between the two cases. The concept of disorder is a purely human one, but is not essential to the understanding and use of entropy.
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    I wouldn't class entropy as "subjective" myself. I think "emergent" is a better word for it. One can take a cue from heat, or more properly temperature, like in an ideal gas. When you look at any one atom, it has no property of temperature. But a collection of atoms has a temperature, which is a measure of average motion. A red-hot iron bar isn't quite the same as an ideal gas, because the motion is more vibrational rather than linear. But what is the same is that when it has a high temperature, it isn't subjective. It will burn you. I prefer to think of entropy in terms of thermodynamics rather than statistics, and then I like to take it a bit further and think in terms of energy alone. In that context entropy it's a measure of the energy that is not available to do work. It's a measure of sameness.
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    Quote Originally Posted by Farsight View Post
    it's (entropy) a measure of the energy that is not available to do work. It's a measure of sameness.
    Not that simple. If you take saturated water at 200C and its saturation pressure and add heat at constant pressure you convert the water to steam at the same temperature and pressure. That steam can now do work for you, yet you have increased its entropy. The missing piece is that in order to raise the entropy of the water so it can do useful work you had to burn some fuel, and the net result is a greater increase in entropy for the system as a whole. So while you are correct in saying the increase in entropy means there is less energy available in the whole system to do work, if you just look at the steam the opposite is true.
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    Don't make things complicated, consider just closed systems. In that case Farsight is right, although I have a problem with his assertion that a single atom has no property of temperature. It definitely does !

    In a closed system, once temperature equilibrium is estabilished, such that there are no temperature differences which can be exploited to do work, then entropy is maximized.
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    In a closed system entropy can decrease by exchange of heat with the surroundings. I think you may have meant to say an isolated system.
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    I wonder if it even is something thermodynamic for within the engine as system. The W or dW in the thermic equation is not the mechanic work the water in the turbine does to the turbine but a change of internal energy within a thermic cycle that goes round. It can,t do work for the engine and all run stationary. The water in the kettle doesn,t do that work as costing energy (it,s not fuell).


    Most of the thermic work done by water is warming and evaporising "coolingwater" opposite side of the thermic cycle inside to where the turbine is. That evaporation is never acknowledged as part of the work. It,s only regarded as coolingwater loss as if steamengines get a random total shower. That,s not the case. The cooling is done where the temperature is relatively low to keep the cycle going. Biggest part of the entropy is the difference between the thermic energy to the coolingwater (including later on where the water is dumped with often a constant fog hanging over a lake) and the effective cooling. Is that entropie ? Maybe but it is visible thermic energy just a bit hard to calculate on a constant fog hanging over a lake where the coolingwater is dumped.
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    Quote Originally Posted by Incision View Post
    Why is there an inevitable increase in entropy when the laws of physics are invariant under time reversal? The time reversal symmetry of physical laws appears to contradict the second law of thermodynamics
    The simple answer is that even if the laws of physics are invariant under time reversal, it doesn't mean that this possibility could ever occur. In fact. irreversibility implies that it this is impossible. As Planck stated it, " there are some things in nature that are irreversible".
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    The thing about Time and Entropy, is information is the only part of reality that makes time a relevant concept. Entropy is a measure of how much useful energy is available, but you can also think of it as the amount of order in the system, or better yet, as the degree to which the system's order makes sense from the perspective of the order that exists in our larger system. Maybe another system based on another order would see a different amount of entropy?

    Our larger system has a certain defined arrow of time, but what if some other systems might have that arrow pointing in another direction?



    Quote Originally Posted by Incision View Post
    I hope this isn't far off-topic, but what implications does entropy have for the formation of complex structure (i.e. life) on this planet? Is this one of those low-probability instances where the entropy of a system seems to decrease? Could we say that life on this planet is a constant struggle against entropy (which we will eventually lose)?
    Actually what happens on Earth is fully consistent with the laws of entropy. The light from the Sun is the energy coming in. The black body radiation coming out has a higher entropy than the Sunlight coming in. Whatever happens in between doesn't really matter. Life is just a whole lot of available low entropy energy getting concentrated into one place.
    Some clocks are only right twice a day, but they are still right when they are right.
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    Quote Originally Posted by kojax View Post
    The thing about Time and Entropy, is information is the only part of reality that makes time a relevant concept. Entropy is a measure of how much useful energy is available, but you can also think of it as the amount of order in the system, or better yet, as the degree to which the system's order makes sense from the perspective of the order that exists in our larger system. Maybe another system based on another order would see a different amount of entropy?

    Our larger system has a certain defined arrow of time, but what if some other systems might have that arrow pointing in another direction?



    Quote Originally Posted by Incision View Post
    I hope this isn't far off-topic, but what implications does entropy have for the formation of complex structure (i.e. life) on this planet? Is this one of those low-probability instances where the entropy of a system seems to decrease? Could we say that life on this planet is a constant struggle against entropy (which we will eventually lose)?
    Actually what happens on Earth is fully consistent with the laws of entropy. The light from the Sun is the energy coming in. The black body radiation coming out has a higher entropy than the Sunlight coming in. Whatever happens in between doesn't really matter. Life is just a whole lot of available low entropy energy getting concentrated into one place.
    Again quoting Planck: "If a system is completely guarded against all external thermal and mechanical actions, then any ensuing chaneg in the entropy of the system will increase." Therefore, the entropy does change in a completely isolate system in which the energy does NOT change. Did I misread your statements?
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