# Entropy in a blackhole

1. Jan 26, 2009

### Bacat

In thermodynamics we have a law that says that the entropy of the universe must always increase, though the entropy of sub-system can temporarily decrease. This increase of entropy generally happens through work -> heat processes (ie friction). We also think of the diffusion of atoms in a disorganized way as an increase of entropy. So if I have 1 mole of gas in a box and I open the box, the gas expands freely and entropy is increased.

I was thinking about the theory of blackholes the other day; the immense gravity of a blackhole condenses matter so completely that light cannot escape. If this is the case, then it is reasonable to assume that matter with mass certainly cannot escape either, therefore heat cannot escape (which is transferred by the kinetic motion of matter). Therefore, a blackhole condenses matter without heat loss. This seems to be a decrease in entropy (of the blackhole system). But what about the surroundings?

Let a gas cloud collide with the blackhole. Now the mole of gas which had expanded freely in a vaccuum is condensed into a much smaller volume without heat loss. The pressure will increase so much that the gas will condense into a solid, thus we have lost 1 mole of gas. This implies a decrease in entropy in the surroundings.

$$\Delta S_{total} = \Delta S_{surr} + \Delta S_{sys}$$

So the total entropy of the universe has decreased due to the blackhole, which seems to violate the second law of thermodynamics.

Can anyone shed light on this problem?

2. Jan 26, 2009

### George Jones

Staff Emeritus
3. Jan 26, 2009

### Bacat

That's very interesting and I think offers a way out of the paradox. I appreciate the time you took to write all of that out and now to have linked to it. It looks as if Hawking Radiation manages to re-balance the entropy so that thermodynamics still hold.

Do you have information critical mass and the expansion of the universe also?

Cheers.

4. Jan 26, 2009

### Naty1

There are all sorts of interesting implications...black holes have maximal entropy (in a given region of space) while the other major singularity, the big bang, has low entropy. I just know there is something that hasn't been fully appreciated yet.

Further, since the entropy of a black hole is proportional to its event horizon (rather than its volume) that leads into the holographic principle as well as a minimal size to space...suggesting planck size nuggets of one planck unt area carry one unit of entropy (information).