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I have a naive question here (I know I'm missing something obvious and fundamental).

The second law of thermodynamics tells us that entropy always increases, but the universe is cooling down;

Both statements are true, but do they not contradict one another?

I thought heat is what causes entropy to increase...

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I think you're getting your entropy confused.

Your cooling universe with gas spreading out, large stars exploding and becoming smaller etc

All show a change in entropy from low to high.

The universe at the big bang started out in a low entropy state and is increasing in entropy as it ages.

I'm reading this at the moment so only have a general understanding as i haven't looked at the maths yet.

Edited by ncjunk
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I don't understand it fully but i don't think there are ever any stupid questions and the high and low entropy is a bit confusing as we sometimes expect things to start out high and go to a lower state.

Reminds me of the Paul Weller song that goes " The more i see - the more i know - the more I know - the less I understand. "

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Entropy is related to gravity as well as heat: the tendency of matter to clump together creates increasing entropy. A smooth soup of particles (like the very early universe), with no large centres of gravitation, is a low entropy state. One with large gravitational centres (e.g. stars) is a higher entropy state. A black hole is the most entropy you can have in a given volume.

But we still need to think about heat energy. Think of a high-energy ultra-violet photon hitting the Earth, warming it, and causing the production of lots of lower-energy infra-red photons. This is a change from lower entropy to higher entropy. In the future, once all the black holes radiate away, there will be a very dilute soup of very low energy particles: a higher entropy state than the initial soup of high-energy particles.

Is the current entropy of the universe due mainly to heat or gravity? Some recent work suggests it may be dominated by the gravitation of super-massive black holes.

[0909.3983] A Larger Estimate of the Entropy of the Universe

These links might also be of interest:

Heat death of the universe - Wikipedia, the free encyclopedia

Black hole thermodynamics - Wikipedia, the free encyclopedia

BTW, responding to the initial comment, "I thought heat is what causes entropy to increase...", it would be more accurate to say that when heat gets moved around, this leads to an increase in entropy.


Edited by acey
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