Understanding Black Holes: Questions About Event Horizon

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Discussion Overview

The discussion revolves around the nature of black holes, particularly focusing on the event horizon and the experiences of observers falling into a black hole versus distant observers. Participants explore theoretical implications, interpretations of existing models, and the conditions under which black holes might form in different types of universes.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants question the description of time experienced by an infalling object versus a distant observer, suggesting that the infalling observer would still be moving quickly across the event horizon despite the time dilation effects noted by distant observers.
  • It is noted that a distant observer cannot see inside the black hole, and as the infalling object approaches the horizon, the light from it becomes increasingly red-shifted, making it appear to slow down until it is no longer visible.
  • Some participants assert that there is no contradiction in the model of black holes assuming an asymptotically flat space at infinity, while others argue that the time taken to fall in from a finite radius is still perceived as infinite by an outside observer.
  • A debate arises regarding the possibility of black holes forming in a closed universe, with some asserting that there would not be enough time for an event horizon to form, while others reference theoretical models suggesting that black holes could exist in such a universe.
  • Participants discuss the implications of definitions of black holes and trapped surfaces, referencing literature that suggests black holes can be defined in closed universes, though some results may not extend to these scenarios.
  • There is a mention of the second law of thermodynamics in relation to black holes and energy concentration, indicating a desire for arguments based on thermodynamic principles.

Areas of Agreement / Disagreement

Participants express disagreement on several key points, particularly regarding the formation of black holes in closed universes and the interpretation of time experienced by observers near the event horizon. No consensus is reached on these issues.

Contextual Notes

Some arguments rely on specific definitions and assumptions about black holes and spacetime, which may not be universally accepted. The discussion includes references to theoretical models and literature that may not be accessible to all participants.

  • #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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