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If no singularity, what’s inside a big black hole? 
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#55
Sep1211, 12:43 AM

P: 159

tom.stoer, I think what genneth is getting at is that near the event horizon, gravitational time dilation increases without bound.
http://en.wikipedia.org/wiki/Gravita...otating_sphere No matter how long it took for the black hole to evaporate, there is some finite distance from the event horizon where you would experience this time to be very short. Think about it this way. As you fall into the event horizon, the Hawking radiation from the black hole is blueshifted to such a high energy that it appears that the black hole is evaporating very quickly. The statement 'you would not notice anything while falling into a large black hole' is not technically true. I would hardly call being blasted by intense gamma radiation, increasing in energy to infinity, 'not noticing anything'. Of course, this is just a restating of the transPlanckian problem. Which indicates the difficulty current physics has with event horizons. There are proposed solutions of course, fuzzball being one of them. At the end of the day you need some form of quantum gravity to explain event horizons adequately. 


#56
Sep1211, 01:44 AM

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P: 5,366

Have you ever made a single calculation in general relativity by yourself? 


#57
Sep1211, 12:07 PM

P: 981

Consider the following statements, and tell me where the logic goes off the rails: 1. An asymptotic observer never sees an infalling observer cross the event/dynamical horizon. 2. The horizon evaporates in a finite time. 3. The asymptotic observer will see the infalling observer still there after the horizon evaporates. 4. Therefore from the asymptotic observer's point of view, she doesn't cross the horizon either, and will live to see it completely evaporate. This calculation can indeed be pushed all the way until the semiclassical approximation breaks down, and I think it's correct. I think this paper by Krauss (http://arxiv.org/abs/grqc/0609024 or Phys.Rev.D76:024005,2007) says the same thing, though I'm not sure I entirely agree with the details (event horizon vs. dynamical horizon, and therefore the interpretation). (Btw, I am in no way invested in the original genesis of this problem  I just think this scenario is worth thinking about as a thought experiment and might be informative on matters in general, not necessarily including the issue of what replaces a singularity...) 


#58
Sep1211, 12:32 PM

PF Gold
P: 6,118

Only thing that occurs to me is that is would appear this argument requires the the two observers see the same event as though it were happening at the same time for both of them. I'm not sure I've said that right, but my point is that it seems to merge the two reference frames in a way that is not correct.



#59
Sep1211, 01:50 PM

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P: 5,366

The general flaw is that you mix two scenarios, namely arguments for a static spacetime with arguments for a dynamic spacetime with an evaporating BH. The third flaw is that you don't calculate (or believe) what the infalling observer will actually see. The freefall time is much smaller than the evaporation time. 


#60
Sep1211, 02:41 PM

P: 407




#61
Sep1211, 05:38 PM

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P: 5,366

OK, maybe there is a "horizonissue", but only in the sense that there is an underlying microscopic structure to classical spacetime.
Or do you think that classical GR (to which I refer when I am talking about free fall, proper time etc.) will no longer be valid outside the horizon for large black holes? Of course we expect that the evaporation will change, but we do not expect any "quantum effects" for classical motian, do we? 


#62
Sep1211, 05:58 PM

P: 159

However, even though the blueshift would not be infinite, it would still happen. It still remains that a theory of quantum gravity is needed before a definitive answer can be given. This paper talks about this subject at length: http://arxiv.org/abs/0806.0628 


#63
Sep1211, 07:16 PM

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#64
Sep1311, 01:51 AM

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P: 1,677

Details about the small corrections due to quantum gravity cannot change this conclusion, which is why the information loss paradox is one of the greatest unresolved problems in theoretical physics. It isn't some mere detail of quantum gravity to be determined by future generations but rather the type of clash (like the UV catastrophe) that signals a theoretical underpinning must be altered (which qg must thereafter explain in detail microscopically) So in a sense the answer is yes, the classical theory most likely is incomplete (even macroscopically), or rather it appears necessary for there to be a complementarity between descriptions and/or a dual holographic formulation that rescues us from what would otherwise be an absurdity. 


#65
Sep1311, 02:02 AM

P: 407

At any rate, issues of singularity resolution at the center appear to be a red herring to this problem, it is not the relevant question to ask. 


#66
Sep1311, 02:15 AM

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P: 8,376

So how big roughly would the macroscopic effects be at the horizon? If say a 70 kg person fell through the event horizon of a large black hole, would he notice anything like a fuzzball?



#67
Sep1311, 02:24 AM

P: 407




#68
Sep1311, 09:48 AM

PF Gold
P: 1,960

EDIT.: Just noticed what suprised said above. So, what I mean is a killer fuzball. 


#69
Sep1311, 12:22 PM

P: 136

A lot of smart people decades ago questioned if black holes existed, and if they did exist doubted they were a point singularity. But black holes have now been confirmed by observation in our galaxy with a high degree of certainty. I think this talk about what happens at the horizon is too complicated; its different for smaller or bigger black holes anyway. The interesting issue is what's inside the event horizon and if it can be confirmed by observation.



#70
Sep1311, 12:24 PM

PF Gold
P: 1,960

I do not doubt that the objects seen are BH, classically. For all practical purposes we are at an infinite distance from all of them.



#71
Sep1311, 12:45 PM

P: 407




#72
Sep1311, 12:59 PM

P: 136

"The interesting issue is what's inside the event horizon and if it can be confirmed by observation."
Not necessarily. Observing gamma ray bursts from other galaxies, which might be clearly identified as neutron starBH mergers, might become routine in the future. Small BHBH mergers will be rarer, perhaps only once a year. If two small black holes merge, and IF the internal object is 75% of the Schwarzchild radius, perhaps one solar mass will be ejected. Very observable. 


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