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Entropy from the Big Bang

  1. Sep 28, 2004 #1
    I have a very limited knowledge about entropy, just the usual layman view, e.g. entropy reflects the number of ways a system can be arranged, or in even more layman language, the degree of order / disorder of the system.
    A system which can be subdivided in groups according to different conditions of its constituent parts (e.g. cold area + warm area) has less entropy than the same system when all of its parts have the same conditions.

    And we also know 2nd Law states that entropy always increases with time.

    This is all fine at our present times, systems are actually seen to behave towards increasing entropy.

    But I don't understand properly how the arguments goes from the early universe.
    Surely, there was a time when it all was the same, a soup of energy, then elementary particles, which later on combined to form atoms, and gradually recombine to form the vast diversity we see today.

    Besides the fact that temperature was then huge, it appears that energy was at the maximum disorder, all of it having the same specifications. Since then it has become into differentated classes, clustered into matter particles, objects .........

    Can someone clear for me how entropy increases despite the apparent self ordering and differentiation since the big bang?
    Is it just because it all gets colder?
    After the big bang when it all was in maximum disorder, was it just the huge temperature that amounts to a lowest total entropy? (is just temperature strong enough to overule the degree of material / spatial organisation?)
  2. jcsd
  3. Sep 29, 2004 #2


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    Hi Barcelona!. You are not as layman as you said.

    All the same, you seem to be dissapointed about the 2nd principle. You have to know the 2nd principle is not violated globally at all, only can violated locally. The fact that there are goups of ordered matter does not mean the 2nd principle is violated. Do not forget the 2nd principle states an increment of total entropy, also that one of the surroundings. Perhaps the matter of a planet has suffered some apparent ordering, but surely it has influenced on the surroundings. It is similar to what happens in a fluid. Some parts of the fluid seem to violate the 2nd principle, but globally the 2nd principle is validated.
  4. Sep 29, 2004 #3


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    I do not try to become myself a philosopher, but this local violation of the 2nd principle has a known proof: our proper life. We are alone in some trillions of kilometers around (or it seems so to scientist). Maybe we are the matter with the highest grade of order in the known Universe.

    On the other hand, we have experimental proofs that small adiabatic systems has always an increasing of entropy due to internal irreversibilities. Therefore, we make a "mental jump" and extrapolate it to the complete Universe, treating it as an adiabatic system. So that, the 2nd principle is something like a postulate, it has no mathematic proof (or yet I don't know it.)

  5. Sep 29, 2004 #4
    Hi Madrid ! nice to hear from so close !

    Well, I surely understand your point, the entropy of a partial system may decrease as long as the total entropy increases. This is how we can increase the order of things (e.g. a refrigerator or building a cathedral) or how life and self-organising systems work.

    But my question was about the entropy of the total system = the universe.

    I don't see how the entropy of the early universe could be lower than now, if the energy in it was totally disordered compared to now.
    Last edited by a moderator: Sep 29, 2004
  6. Sep 29, 2004 #5


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    Entropy always increases over time. The early universe was very smooth and uniform. This is a highly ordered [low entropy] state. It is now wrinkled and lumpy, a lower ordered [high entropy] state. Think of it as a perfectly made bed sheet [high order]. Once you sleep on it the sheet gets wrinkled [low order]. There are many more wrinkled states available than perfectly smooth states.
  7. Sep 29, 2004 #6
    Thanks Chronos, sorry if I'm stuborn but I still don't get it .....

    In the typical example of warm and cold, the entropy of the system is said to be maximum when the system is in thermodynamical equilibrium = when all particles are at the same temperature and heat doesn't flow anymore. the system remains "thermodynamically dead".

    The state where we have separated warm and cold parts is considered as the ordered state, and the state where all particles are at the same temperature, indistinguishable from eachother, is considered as the disordered state.

    Now, you say that the early universe (when everything was at the same temperature, undistinguishable from each other) was a highly ordered, low entropy state.

    ?? what's wrong ?!!!??
    Last edited by a moderator: Sep 29, 2004
  8. Sep 30, 2004 #7


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    Think of it as a checkerboard. On a smooth checkerboard, every shared boundary square [at vertical and horizontal orientation] is opposite colored and every diagonal boundary square [only having point contact at the intersections] is the same color. Randomize this pattern and you increase the entropy [the same as creating hot spots and clumps].
  9. Oct 18, 2004 #8
    It's how you look at it

    Seems to me that you have answered your own question (see italics).
    As the early universe expanded and cooled quarks and other exotic particles began to form and then matter & antimatter. This from a perfect, in your terms, soup of energy.
    As these processes continued, entropy increased and will continue to increase in line with increasing complexity or disorder.
    It's all relative and depends on your perspective.
  10. Sep 5, 2010 #9
    I know that this is a very old thread, but for what it's worth:

    Literally, the same qestion popped up in my head when I watched this talk: origin-universe-and-arrow-time by sean-carroll
    . I appreciated all the answers given so far, but I still did not get it. So I continued my search. Finally, these thoughts brought my mind to ease. Before you read this, I have to make it clear that I am not a physicist or anything similar.

    It is not so much about Randomness. If you think of it more in terms of possibilities, it makes more sense. At the beginning, the universe was small, dense and hot; not so many different particle types that are so restricted in their interactions as it is now. Literally, endless possibilities for every particle. Fortunately for us, not so much possibilities survived through the ages. Today, an electron cannot transmit gravitation, even if it wanted to. In the future space will continue to grow and separate galaxies from each other, reducing the possibilities for each particle even more.

    Considering your gas example, predictability is another key to understanding. At the beginning every particle is exploring the new space on its own; no boundaries, no constraints. At the end however, they are distributed over the hole space and believe me, these particles are lazy as hell. Once a particle is in one corner of space, it is unlikely that it will move its *** and end up at the other side. Only if you give incentives, like putting a bunch of cooler particles at the other side, it will move its *** (you know, everybody wants to stay with the cool kids, ... ähm particles).

    Last edited: Sep 5, 2010
  11. Sep 5, 2010 #10
    Why the Bang produced a low entropy state, from which "we" are slowly producing high entropy, is an open question. It is tied up with the issue of Universal Inflation (no, not economics...but....hmmm...maybe). I think the basic answer is that energy was very concentrated right after the Bang, with pockets of high and low concentration, and they are now tending to spread and even out. Why the unevenness? That's the main issue, so I'm really just begging the question. I tried a google of "big bang entropy" and found some interesting tidbits to chase down to find a better answer.
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