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"When all systems taking part in a process are included, the entropy either remains constant or increases.... no process is possible in which the total entropy decreases, when all systems taking part in the process are included." This is a statement of the 2nd law of thermodynamics in terms of entropy from my physics textbook.
I am wondering if the 2nd law is invariant. If it is invariant, why is the universe in its current state, and not in a state of maximal disorder. In other words, why are there small "clusters" of low entropy in the universe (galaxies, planets, life forms, the human brain), when the state of maximal entropy would have clusters of matter and energy strewn randomly about.
Is the 2nd law of thermodynamics correct, or just an oversimplification (similar to Newton's laws of motion) that is only applicable to what we can experimentally observe? Is there an "easy" and well accepted solution to this question that I'm unaware of?
If you claim to "know" the answer, please state that it is a well accepted theory. If you have an opinion, feel free to state it as well (i.e. I would just like to know that it's an opinion, and not that 99% of physicists believe it to be true).
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This is a
profound question that nobody fully understands at present. You will hear people talk about inflationary cosmology from time to time, but this does not solve the problem, and confusion on this issue is common even among physics Ph.D.'s and practicing relativists!
Basically, gravity complicates thermodynamics on several levels. First and most important, if you simply treat the thermodynamics of a simple idealized box of gas, the most probable state (the one with highest entropy) is the one in which the gas is uniformly spread throughout the box. However, if you make the box big enough and allow gravity to interact, it turns out that this is
not the state of highest entropy. Matter can collapse (as it is observed to do in "star factory" nebulas -- note that this would not be possible if it violated the 2nd law!) and form stars, which eventually collapse further and form black holes -- these are states of
enormous entropy. The fact that we are alive here is actually a consequence of the universe starting out in a very "special" state in which the "matter" degrees of freedom were "thermalized" but gravitational degrees of freedom were somehow not. As gravitational systems slowly move toward states of higher entropy (our sun, for example), life on earth effectively mooches off the entropy imbalance. Obviously, if the universe were in a state of maximal entropy (consisting of black holes and little else), life would not be possible. But
why the universe should start out in a hugely improbable state is a truly baffling puzzle!
Here is a very recent paper on the subject by one of the world's foremost relativists:
http://xxx.lanl.gov/abs/gr-qc/0507094