Observing a mass forever approaching a BH, and BH growth

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Discussion Overview

The discussion revolves around the apparent paradox of observing mass approaching a black hole (BH) and the implications for black hole growth. Participants explore the nature of event horizons, time dilation, and the mechanisms by which black holes accumulate mass over time, particularly in the context of supermassive black holes at galactic centers.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants assert that while one cannot observe an object crossing the event horizon, infalling mass can still contribute to the black hole's growth.
  • Others argue that the effects of infalling mass can be observed by remote observers long before the actual infall occurs.
  • A participant questions the implications of time dilation for an observer at the event horizon, suggesting that they would perceive significant cosmic events in a short time frame.
  • Another participant introduces the concept of Vaidya spacetime to illustrate how black holes can grow due to infalling radiation, indicating that the event horizon can expand before mass crosses it.
  • Some express confusion about the relationship between the inability to observe mass crossing the event horizon and the growth of black holes over billions of years.
  • A participant highlights that the formation of supermassive black holes remains a mystery, indicating that the process is not fully understood.
  • There is a discussion about the distinction between local and remote observations regarding time dilation and the perception of events near a black hole.

Areas of Agreement / Disagreement

Participants express a range of views, with no consensus reached on the implications of observing mass approaching a black hole versus the mechanisms of black hole growth. The discussion remains unresolved regarding the paradox of observing mass accumulation without witnessing it crossing the event horizon.

Contextual Notes

Participants note that the understanding of black hole growth and the effects of time dilation depend on the perspective of the observer, whether local or remote. The discussion also touches on the limitations of current observational tools in studying black holes.

  • #31
stefanbanev said:
It was a rhetorical figure to make the point that "entire spacetime" (not a "spacetime") is the way to imply that even if the information about "event" can not be acquired by any means still may belong to realm of observer

What does "realm of observer" mean?

A spacetime (whether you qualify it with "entire" or not) is a perfectly well-defined concept in GR: it's a 4-dimensional manifold whose geometry is obtained by solving the Einstein Field Equation. Some solutions of that equation, like the Schwarzschild solution describing a static black hole, contain horizons; that's why it's not always the case that all events in the spacetime are visible from everywhere. But the horizons, and the events inside them, and the crossings of those horizons by infalling objects, are predictions of GR, a well-confirmed physical theory. They aren't just pulled out of thin air. So if you want to deny that those events will be part of spacetime under appropriate conditions, you have to find some way of explaining why these predictions of GR are somehow not correct. Just saying "I can't observe them so I don't believe they're there" isn't enough.
 
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  • #32
PeterDonis said:
What does "realm of observer" mean?

A spacetime (whether you qualify it with "entire" or not) is a perfectly well-defined concept in GR: it's a 4-dimensional manifold whose geometry is obtained by solving the Einstein Field Equation. Some solutions of that equation, like the Schwarzschild solution describing a static black hole, contain horizons; that's why it's not always the case that all events in the spacetime are visible from everywhere. But the horizons, and the events inside them, and the crossings of those horizons by infalling objects, are predictions of GR, a well-confirmed physical theory. They aren't just pulled out of thin air. So if you want to deny that those events will be part of spacetime under appropriate conditions, you have to find some way of explaining why these predictions of GR are somehow not correct. Just saying "I can't observe them so I don't believe they're there" isn't enough.

Sure, agree on all accounts besides the last sentence that makes no sense for me. If I can not acquire information in principle about some events such events can not be qualified as "observables" yet, it does not preclude to include/incorporate such events into some model/theory as soon as such model/theory predicts correctly the outcome of experiments (the experiment's outcomes which I may qualify as observables).
 
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  • #33
stefanbanev said:
Sure, agree on all accounts besides the last sentence that makes no sense for me. If I can not acquire information in principle about some events such events can not be qualified as "observables" yet, it does not preclude to include/incorporate such events into some model/theory as soon as such model/theory predicts correctly the outcome of experiments (the experiment outcomes which I may qualify as observables).
So are 'you' the arbiter of what prediction is an observable? These GR predictions are readily observable to someone else. Consider my example of retridiction in Solar system mechanics. We can retrodict what a past observer ought to have seen but we can't communicate with them to verify this. However most would say these retrodictions are observables for a past observer, rather than being in different category than predictions. We believe such retrodictions to the extent we believe the physical theory. Similarly for GR predictions about the horizon crossing observers.
 
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  • #34
PAllen said:
So are 'you' the arbiter of what prediction is an observable? These GR predictions are readily observable to someone else. Consider my example of retridiction in Solar system mechanics. We can retrodict what a past observer ought to have seen but we can't communicate with them to verify this. However most would say these retrodictions are observables for a past observer, rather than being in different category predictions. We believe such retrodictions to the extent we believe the physical theory. Similarly for GR predictions about the horizon crossing observers.

It seems a rephrasing what I said above (I like my version more ;o)...
 
  • #35
stefanbanev said:
It seems a rephrasing what I said above (I like my version more ;o)...
No, I think we disagree. I say the observables of a theory are things that could be observed by some observer. You say the observables of a theory are things that could be observed by you.
 
  • #36
stefanbanev said:
If I can not acquire information in principle about some events such events can not be qualified as "observables" yet

You're not the only observer in the universe. An observer who falls into the black hole can see events behind the horizon just fine.
 
  • #37
The OP question has been answered. Thread closed.
 

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