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|    sci.physics.research    |    Current physics research. (Moderated)    |    17,516 messages    |
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|    Message 17,390 of 17,516    |
|    Stefan Ram to All    |
|    Falling into a black hole    |
|    26 May 24 16:46:52    |
      From: ram@zedat.fu-berlin.de              This might be a bit of a speculative question.               They say that if an object falls into a black hole, it would appear        to an observer at a great distance relative to the black hole that        the object slows down but never actually reaches the event horizon.               I'll refrain from pondering the idea that for an object near the        event horizon, quantum mechanically due to the uncertainty principle,        the probability of being inside the event horizon could already be        greater than 0. I'm sure experts have thought about that as well.               So, where was I? Ah yes, for an outside observer, the object reaches        the event horizon only after an infinite amount of time has passed.               On the other hand, from an outside perspective, the black hole        itself could have evaporated after a finite amount of time AFAIK.               What happens to the object, which from the outside appears        to be waiting just short of the event horizon, after the black        hole has evaporated from the point of view far from the black hole?        Would we find the object outside the event horizon then? - TIA!              [[Mod. note --       A few comments (I suspect the author already knows these things):              > for an outside observer, the object reaches the event horizon only       > after an infinite amount of time has passed              It's useful to think of the infalling object as sending out continuous       light (or radio) signals.              As the object approaches the event horizon, these signals take longer       and longer to reach a (fixed) outside observer, and the signals are more       and more redshifted. This results in the effect the author described,       and means that the outside observer never sees any signals emitted on       or inside the event horizon.              It may be useful to rephrase the above-quoted statement as:               Light/radio signals from the moment the infalling object crosses        the event horizon will take infinitely long to reach, and will        be infinitely redshifted when they arrive at, a (fixed) outside        observer.              This phrasing makes it clear(er) that the effects we're describing are       caused by light/radio signal propagation near the event horizon, rather       than the infalling object's motion being in any way arrested.              Indeed, an observer riding the infalling object itself would measure it       passing through the event horizon very quickly; it's "just" that after       passing through the event horizon her light/radio signals would never       reach the outside observer.              The time scales are interesting: For a 1-solar-mass black hole       (event horizon is a few kilometers in diameter):       * outside observer sees the object's radio signals infinitely redshift        and cut off on a time scale of microseconds; the last photon received        by the outside observer will be very low-enegy (upon reception) and        have originated just before the infalling object crossed the event        horizon       * infalling observer reaches r=0 singularity in microseconds        (but any messages from her about what it's like, will never        get to the outside observer)       * outside observer sees the black hole evaporate in something        on the order of 10^67 years              As for your actual question, what happens to things inside a black       hole when it (eventually) evaporates, it's a good question. Alas, I       don't think we (yet) understand quantum gravity well enough to know       the answer. :( There's a brief introduction to this topic at:        https://en.wikipedia.org/wiki/Hawking_radiation#Black_hole_evaporation       -- jt]]              --- SoupGate-Win32 v1.05        * Origin: you cannot sedate... all the things you hate (1:229/2)    |
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