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Comment by acqq | Hacker News Reader

From the comments of the author:

"You can puzzle your group by asking whether entropy increases as clouds of gas and dust in the early Universe collapse and form stars and planets. If it doesn’t increase, this process violates the Second Law. So it must increase. But why is it increasing? It seems that order is increasing… so doesn’t that mean entropy is decreasing?

If someone can’t answer this puzzle correctly, they don’t understand how thermodynamics works in our Universe."

So, who knows the right answer to the puzzle: where and how the increase of the entropy happens? (I also suggest that those who really know should wait for some hours... I'd first actually like to see some wrong answers which try to be right but don't manage to be -- just to get some better picture of the readers here -- I don't know if this will work).

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Not an expert - just guessing. If entropy is a measure of overall degrees of freedom, then even though gas collapsing into stars and planets might seem more “ordered” - the total amount of degrees of freedom is increased. Hence no laws of thermodynamics are violated. Planets and starts are more “complex” systems compared to gas - so there must be much more entropy aka degrees of freedom contained within them.

Something like this maybe?

"Ordered" actually means "has a wide variety", not "everything is the same." The self-attractive potential energy of a primordial gas cloud is highly directed (it has a center) and the range of distances from that center is great. When the cloud collapses the range collapses, there's less variation, and so entropy has increased.

Also, a gas cloud isn't a closed system. As it collapses it heats up and emits infrared photons that escape.

[edit: Actually, I think the important variation is "center" vs. "particle's position." A large sphere, with all particles at the same distance from the center, would still have a lot of potential energy, and a small sphere less so.]

The entropy in the various particles (photos, neutrinos, gravitons etc) the stars are emitting from being formed probably is more than the entropy lost from the gas clouds ordering themselves into stars would be my guess.

I’ll venture a guess.

I suppose that if we encode the state of a cloud of gas, we need relatively fewer bits to do so than something that requires more bits like a star. A star is like white noise, to a cold cloud of gas.

At the limit, you have gas particles all with zero velocity, so you can encode the velocity of each particle with no bits (I.E. constant of zero). Compare that to a star with every particle flying around at high temperatures. You’d need a number of bits to encode the velocity (state) of each particle.

Black holes require even more bits to encode the state of massive amount of particles they contain.

Am I close?

According to Baez, all the matter fields and radiation fields were in equilibrium at the big bang, so by elimination the answer has to have something to do with gravity (which is, after all, the force driving the formation of the gas clouds, stars, etc). So here's my guess: Evenly distributed matter has gravitational potential energy. By collapsing, a cloud of gas can convert this into thermal energy, raising its entropy. This compensates for the fact that the cloud's volume has reduced, which would reduce entropy on its own.

How did I do? :)

Here's another thing: I remember learning that all the nice usable energy we get from the sun is due to the fact that the big bang produced lots of hydrogen, but fusing four hydrogens into a helium yields a bunch of energy. (And you can still get some energy out by fusing further, all the way down to iron.) So if matter and radiation were at equilibrium at the big bang like Baez says, why wasn't more helium (and heavier elements down to iron) formed, leaving nothing for the stars to burn?

I think the answer has to be that the temperature was incredibly high at the time of the big bang: At really high temperatures, it's more favourable for nucleons to be flying around separate from each other as protons. At lower temperatures, it starts being better for them to be things like helium and carbon, and at lower temperatures still, iron. So if the universe's temperature had lowered very very gradually, we would be stuck with nothing but iron at this point, leaving nothing for the stars to burn. So it seems we owe our sunlight, in some sense, to the fact that the universe cooled fairly quickly?

> At lower temperatures, it starts being better for them to be things like helium and carbon, and at lower temperatures still, iron.

It actually doesn’t work that way: we don’t expect that the whole universe will ever be all iron.

Entropy increases because its much more likely than for it to decrease. And the reason higher entropy states are more likely than low entropy states are that there's more of them.

Now that enough time passed, the link with enough technical details but written to be accessible. Take care to read it whole, as the details matter.

https://www.forbes.com/sites/startswithabang/2017/04/15/ask-...

It does not actually explain why black holes have significantly more entropy than gas they formed from. In fact, there is not much difference between the question why do stars form considering the second law vs why do black holes form.

The question is based on a false premise. What the Second Law of Thermodynamics states is that the entropy of an isolated system is never decreasing. Is the Universe an isolated system (meaning it cannot exchange matter or energy with its environment across its boundary)? We don't know anything about the boundaries of the Universe, or if it has any at all, therefore the 2nd law doesn't apply. QED.

Based on our observations we can see a lot of galaxies as in practice isolated systems, and for each separate galaxy second law still holds.

So this attempt for explanation fails to answer the question.

No galaxies are particle isolated, as all of them emit radiation.

Alright it's been a few hours. I'll bite. The particles the stars formed out of where in a higher energy state or something. Ok now what's the true answer.

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