4 ms·

How does that remain true after the black hole has evaporated and there is no longer a horizon?

lisper 5y ago
Disclaimer: we are at the hairy edge of my knowledge here, so what I am about to tell you could very well be wrong. Hawking radiation has never actually been observed. It is just something that pops out of the math if, as Sabine rightfully emphasizes in her video, you make certain assumptions. And one of those assumptions is that you have a fully-fledged black hole, i.e. an object that actually contains mass beyond the event horizon. We are used to thinking of this assumption as having actually been confirmed by observation, but it is not actually true. No one has ever actually observed a black hole, notwithstanding that we've ostensibly taken a picture of one. That image was of the radiation emitted by an accretion disk, not the black hole itself. Black holes themselves are, obviously, impossible to image. So we don't actually know whether black holes actually exist or not. The only thing we've directly observed is their gravitational effects, and the gravitational effects of an actual black hole are indistinguishable from having all of the mass of the hole actually resident just outside the event horizon. What actually happens at the horizon is beyond the reach of our current theories because there both gravity and quantum effects are significant, and we do not yet have a consistent theory of quantum gravity. Everything we think we know about black holes is actually the result of taking GR and QM and framming them together in some ad hoc way by adding simplifying assumptions which may or may not actually be true. This is the point Sabine was trying to make: the black hole information loss paradox is not a problem with physics, it's a problem with our current theories. We simply don't know how the universe actually behaves in the presence of extreme concentrations of mass/energy. The only thing that the BHILP actually tells us is that either GR or QM -- or both -- are wrong, mere approximations to the actual truth in the same way that Newtonian mechanics turned out to be an approximation to the actual truth (one that happens to work extremely well in weak gravitational fields). But no one has a clue which one is wrong or how despite 100 years of effort. And one of the reasons for this is that we have no data, and no reasonable prospects for obtaining it. So we may just have to make our peace with not knowing.
- platz 5y ago
  Wall of text, didn't answer the question.
  - lisper 5y ago
    The question assumes that black holes evaporate, and we don't actually know that they do. Is that better?
    - platz 5y ago
      That is a more transparent answer and direct answer, yes. Maybe if you had lead with that sentence, I wouldn't have been inclined to respond. But it also fails to address what I point out below. (as an aside, this answer is also not one that I find satisfactory on the actual topic.) Dllthomas correctly pointed out that given the two premises that (from the frame of the observer) (1) [matter takes infinite time to transit the horizon] and (2) [due to hawking radiation, black holes have a finite time span], then (2) resolves (1) when the black hole disappears. To recap, Instead of acknowledging that (2) resolves (1) you proceed to question the existence of hawking radiation and black hole evaporation, which compared to consensus is a radical view and is not warranted. Also the pivot was done in a way that seems to complicate and obfuscate rather than address the point directly (this is a common debate tactic; however I'm not sure if you were conscious of the behavior or if it was more subconscious/rationalization (more likely); you may not have even been aware you were doing it). It seems like you chose to reject consensus instead of simply accepting that (2) resolves (1), maybe because you seem to have a fixation on not ever conceding a point in a conversation. Sometimes it's ok just so say, "yeah, that's a good point". (btw if you're rejecting hawking radiation why say anything about black hole theory at all this point because it could all be wrong, no reason to speculate about it)
      - lisper 5y ago
        > you seem to have a fixation on not ever conceding a point That is quite the accusation coming from someone whose entry into the conversation was "Wall of text, didn't answer the question." But let's see... > Dllthomas correctly pointed out... dllthomas did not "point out" anything, correctly or otherwise. All he did was ask the following question: "How does that remain true after the black hole has evaporated and there is no longer a horizon?" This question assumes that black holes evaporate. That assumption may be incorrect. We do not know whether or not black holes actually evaporate. In fact, we do not even know whether or not black holes actually form at all. And therefore: > (2) resolves (1) when the black hole disappears That might be true if (2) were true. Even that is arguable, but it is neither here nor there because we do not know whether or not (2) is true > compared to consensus is a radical view Yes, of course. The consensus view leads to a paradox, and so we know that the consensus view cannot possibly be correct. We also know that decades of effort have not resolved this paradox, and so it is extremely unlikely that there is a simple straightforward solution that has simply been overlooked. So the correct solution will almost certainly be a radical departure from the current consensus.
daxfohl 5y ago
I think that's the point. From an outside perspective the black hole will evaporate before the thing falls in. Thus a thing can never fall in. From its perspective the hole will emit more and more intense radiation and finally evaporate just before it hits the horizon. If true, I think you can go even further and say no black hole can completely form; the collapsing matter just gets exponentially closer to being fully black until the effect of the Hawking radiation outweighs the gravitation, but it all evaporates before going fully black. No? (This latter part assumes there's some Hawking radiation or equivalent from pre-black holes as well. And I'm not sure whether that would be unitary or not, so it may not resolve the information paradox anyway). (Edit: I think the pre-Hawking radiation would be unitary, since the only reason Hawking radiation is not unitary is because BHs don't have information, but pre-black holes are not black holes. So doesn't that solve the info paradox? Without resorting to holographs and whatnot? Where's the error?)
- dllthomas 5y ago
  Maybe, but I think if it were easy to show that black holes (for whatever definition is important here) never actually form, the physicists would have noticed.
  - daxfohl 5y ago
    100% agree, but I'm still not seeing the flaw in the argument.
  - GoblinSlayer 5y ago
    When they are paid for research of black holes, it becomes easy to overlook their absence. They can also say they do hypothetical research, if black holes don't form by collapse maybe they form by other means, e.g. primordial black holes.

Disclaimer: we are at the hairy edge of my knowledge here, so what I am about to tell you could very well be wrong. Hawking radiation has never actually been observed. It is just something that pops out of the math if, as Sabine rightfully emphasizes in her video, you make certain assumptions. And one of those assumptions is that you have a fully-fledged black hole, i.e. an object that actually contains mass beyond the event horizon. We are used to thinking of this assumption as having actually been confirmed by observation, but it is not actually true. No one has ever actually observed a black hole, notwithstanding that we've ostensibly taken a picture of one. That image was of the radiation emitted by an accretion disk, not the black hole itself. Black holes themselves are, obviously, impossible to image. So we don't actually know whether black holes actually exist or not. The only thing we've directly observed is their gravitational effects, and the gravitational effects of an actual black hole are indistinguishable from having all of the mass of the hole actually resident just outside the event horizon. What actually happens at the horizon is beyond the reach of our current theories because there both gravity and quantum effects are significant, and we do not yet have a consistent theory of quantum gravity. Everything we think we know about black holes is actually the result of taking GR and QM and framming them together in some ad hoc way by adding simplifying assumptions which may or may not actually be true. This is the point Sabine was trying to make: the black hole information loss paradox is not a problem with physics, it's a problem with our current theories. We simply don't know how the universe actually behaves in the presence of extreme concentrations of mass/energy. The only thing that the BHILP actually tells us is that either GR or QM -- or both -- are wrong, mere approximations to the actual truth in the same way that Newtonian mechanics turned out to be an approximation to the actual truth (one that happens to work extremely well in weak gravitational fields). But no one has a clue which one is wrong or how despite 100 years of effort. And one of the reasons for this is that we have no data, and no reasonable prospects for obtaining it. So we may just have to make our peace with not knowing.

platz 5y ago
Wall of text, didn't answer the question.
- lisper 5y ago
  The question assumes that black holes evaporate, and we don't actually know that they do. Is that better?
  - platz 5y ago
    That is a more transparent answer and direct answer, yes. Maybe if you had lead with that sentence, I wouldn't have been inclined to respond. But it also fails to address what I point out below. (as an aside, this answer is also not one that I find satisfactory on the actual topic.) Dllthomas correctly pointed out that given the two premises that (from the frame of the observer) (1) [matter takes infinite time to transit the horizon] and (2) [due to hawking radiation, black holes have a finite time span], then (2) resolves (1) when the black hole disappears. To recap, Instead of acknowledging that (2) resolves (1) you proceed to question the existence of hawking radiation and black hole evaporation, which compared to consensus is a radical view and is not warranted. Also the pivot was done in a way that seems to complicate and obfuscate rather than address the point directly (this is a common debate tactic; however I'm not sure if you were conscious of the behavior or if it was more subconscious/rationalization (more likely); you may not have even been aware you were doing it). It seems like you chose to reject consensus instead of simply accepting that (2) resolves (1), maybe because you seem to have a fixation on not ever conceding a point in a conversation. Sometimes it's ok just so say, "yeah, that's a good point". (btw if you're rejecting hawking radiation why say anything about black hole theory at all this point because it could all be wrong, no reason to speculate about it)
    - lisper 5y ago
      > you seem to have a fixation on not ever conceding a point That is quite the accusation coming from someone whose entry into the conversation was "Wall of text, didn't answer the question." But let's see... > Dllthomas correctly pointed out... dllthomas did not "point out" anything, correctly or otherwise. All he did was ask the following question: "How does that remain true after the black hole has evaporated and there is no longer a horizon?" This question assumes that black holes evaporate. That assumption may be incorrect. We do not know whether or not black holes actually evaporate. In fact, we do not even know whether or not black holes actually form at all. And therefore: > (2) resolves (1) when the black hole disappears That might be true if (2) were true. Even that is arguable, but it is neither here nor there because we do not know whether or not (2) is true > compared to consensus is a radical view Yes, of course. The consensus view leads to a paradox, and so we know that the consensus view cannot possibly be correct. We also know that decades of effort have not resolved this paradox, and so it is extremely unlikely that there is a simple straightforward solution that has simply been overlooked. So the correct solution will almost certainly be a radical departure from the current consensus.

daxfohl 5y ago

I think that's the point. From an outside perspective the black hole will evaporate before the thing falls in. Thus a thing can never fall in. From its perspective the hole will emit more and more intense radiation and finally evaporate just before it hits the horizon. If true, I think you can go even further and say no black hole can completely form; the collapsing matter just gets exponentially closer to being fully black until the effect of the Hawking radiation outweighs the gravitation, but it all evaporates before going fully black. No? (This latter part assumes there's some Hawking radiation or equivalent from pre-black holes as well. And I'm not sure whether that would be unitary or not, so it may not resolve the information paradox anyway). (Edit: I think the pre-Hawking radiation would be unitary, since the only reason Hawking radiation is not unitary is because BHs don't have information, but pre-black holes are not black holes. So doesn't that solve the info paradox? Without resorting to holographs and whatnot? Where's the error?)

dllthomas 5y ago
Maybe, but I think if it were easy to show that black holes (for whatever definition is important here) never actually form, the physicists would have noticed.
- daxfohl 5y ago
  100% agree, but I'm still not seeing the flaw in the argument.
- GoblinSlayer 5y ago
  When they are paid for research of black holes, it becomes easy to overlook their absence. They can also say they do hypothetical research, if black holes don't form by collapse maybe they form by other means, e.g. primordial black holes.