A Conversation for Black Holes


Post 1

Joe Otten

The thing that has always puzzled me about black holes is what happens to the entropy of objects that fall into them?

By its description, a single infinitely dense point seems to have a very low entropy. But if we then let a high entropy object fall into a black hole, we appear to have a contradiction to the second law of thermodynamics.


Post 2


This topic is an active one in the field of astrophysics and quantum gravitation. In general, however, it is required of a black hole that its event horizon always increase, much like the total entropy of a closed system (i.e. - the universe). This thought lead to the hypothesis that a black hole's entropy is proportional to its event horizon's surface area. This came to be the Bekenstein-Hawking Formula:


If a black hole has an entropy, then it follows all the other laws of thermodynamics and has a temperature, also. So the black hole will radiate energy. This is where things start getting fuzzy. How can something that is impossible to escape radiate anything? I'm afraid I don't know much about what's new in that field of thought.


Post 3

Joe Otten

Thanks for that.

There is Hawking radiation, but I guess that is not what you are talking about. Could it be a mistake to consider a black hole demarcated by its event horizon to be an object, and thus to apply thermodynamic principles to that object.

After all the event horizon is not a physical structure and need not be in the same place from one moment to the next. (That episode of Voyager where the ship was stuck inside the event horizon of a black hole, looking for a crack to get out would have been hilarious if it had been slightly less obtuse.)


Post 4


The natural answer is that the laws of physics break down in a black hole. The entropy just vanishes. Entropy is a property of the universe, and all properties of the universe break down at the event horizo of a black hole.

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