Understanding Black Holes: Questions About Event Horizon

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SUMMARY

The forum discussion centers on the nature of black holes, specifically the behavior of objects as they approach and cross the event horizon. Participants clarify that an infalling observer experiences finite proper time while a distant observer perceives the crossing as taking an infinite amount of time due to extreme redshift effects. The conversation also touches on the implications of general relativity and the second law of thermodynamics in relation to black hole mechanics, including the concept of entropy and the formation of event horizons in closed universes. Key references include discussions on Hawking radiation and the Penrose-Hawking theorem.

PREREQUISITES
  • Understanding of general relativity and its implications for black hole physics
  • Familiarity with the concepts of event horizons and proper time
  • Knowledge of black hole thermodynamics and entropy
  • Basic grasp of cosmological models, including closed universes
NEXT STEPS
  • Research "Hawking radiation and black hole evaporation" for insights on black hole dynamics
  • Study "Penrose-Hawking theorem" to understand singularities and trapped surfaces
  • Explore "black hole thermodynamics" to learn about entropy and event horizon area
  • Investigate "closed universe models" to comprehend their implications for black hole formation
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Astronomers, physicists, and students of cosmology seeking to deepen their understanding of black hole mechanics and the interplay between general relativity and thermodynamics.

  • #31
JesseM said:
Again, in thermodynamics it is really isolated systems you should be talking about if you want to discuss the second law, not closed ones. ... Anyway, as I said, there is no violation of the second law as long as you say that a black hole's entropy is defined by the area of its event horizon, since when matter or energy falls in the event horizon grows by an amount enough to offset any decrease in entropy in the region outside the event horizon.

Are you sure that's what you want to say?

Regards,

Bill
 
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  • #32
JesseM said:
Again, in thermodynamics it is really isolated systems you should be talking about if you want to discuss the second law, not closed ones. ... Anyway, as I said, there is no violation of the second law as long as you say that a black hole's entropy is defined by the area of its event horizon, since when matter or energy falls in the event horizon grows by an amount enough to offset any decrease in entropy in the region outside the event horizon.
Antenna Guy said:
Are you sure that's what you want to say?
Yes. Weren't we discussing a larger isolated system that contains both a black hole and some matter/energy outside it? That's what you seemed to be talking about in the second paragraph of post #22 (which begins 'Now consider an arbitrarily larger closed system that contains a random distribution of bodies of mass - including a black hole'), and that's what I was responding to. So, for the larger system to qualify as "isolated" it must be true that no matter/energy is crossing the boundary of the larger system (say, a giant box containing a black hole and some other stuff), but matter/energy can certainly cross the event horizon.
 
  • #33
JesseM said:
Yes.

Consider how the entropy of a black hole changes within the context of the first bold.

Is a black hole an "isolated system"?

Regards,

Bill
 
  • #34
Antenna Guy said:
Is a black hole an "isolated system"?
No, it is not.
 
  • #35
Antenna Guy said:
Consider how the entropy of a black hole changes within the context of the first bold.

Is a black hole an "isolated system"?
On its own? Not if any matter/energy crosses the event horizon, by definition.
 

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