# Does a blackhole decrease entropy in the universe?

Asked by timothykinney (2743) January 26th, 2009

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.

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

Can you shed light on this problem?

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## 3 Answers

I can’t shed too much light on the problem, beyond this one thought: scientists are always discovering new things about how our universe got to where it is and how it continues progressing. There is likely no definitive answer to your question because we simply do not know at this time, we don’t have the tools or equipment to do in-depth scientific study of such phenomenon. Some work to look into would be research and theories presented by Stephen Hawking, you may also find some good leads from this Wikipedia article.

dynamicduo (14392)

Super interesting.

The wikipedia entry on black holes has a good explanation, as well as the special entry on black hole thermodynamics that dynamic duo linked to.

I really don’t understand the math (or the science, for that matter). Key points: black holes emit Hawking radiation which corresponds to their temperature. Black holes have entropy, and when a black hole swallows an object, the increase of entropy within the black hole is greater than the corresponding decrease in entropy.

bob (3223)

Given that entropy is a mathematical concept, such as “the quantitative measure of disorder in a system”, and as dynamicduo pointed out, we don’t know what happens inside the event horizon of black hole, except that the gravity effect seems to persist, the answer is essentially undefined and/or arbitrary.

If you’re interested in “order”, one might decide that it’s extremely “orderly” to have heaps of matter collected at a single point. Alternatively, coming from information theory, one might decide that there’s not much information at all in a black hole except gravitational strength and location, so if entropy is a decline in information, then a black hole represents an increase in entropy.

Zaku (30119)

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