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Black hole decay

  1. Apr 4, 2008 #1
    I have a question about someone falling into a black hole. General relativity predicts that the reference frame of an object falling into a black hole will reach the singularity in a finite amount of time. However it also predicts that the reference frame of an observer outside the event horizon will never observe the object crossing the event horizon.

    From what I understand, in the outside reference frame the object ACTUALLY never falls into the black hole (the fact that the time light takes to reach the observer approaches infinity as the location of the object at emission approaches the event horizon although a good explanation of this phenomenon is not the one general relativity predicts).

    However, taking Hawking radiation into account makes the black hole decay and evaporate in some finite amount of time. My question is what happens in the observers reference frame when the black hole evaporates? Does the object just dissapear (as would be the case if the above explanation were true)? Or does the object remain intact?
  2. jcsd
  3. Apr 6, 2008 #2
  4. Apr 6, 2008 #3
    Ha, that would be nice. Jump into a black hole with you radiation proof suit on. The journey will only take ten minutes by your watch but by the time you get to where the singularity was it has evaporated and you find yourself a billion years in the future because that is how long it takes a typical black hole to evaporate according to an external observer. (Only kidding ;)

    If you trawl through the forum threads you will see this subject has been discussed several times.

    Here are two recent ones:


  5. Apr 7, 2008 #4
    Although a black hole will loose mass by Hawking radiation it will most likely gain mass at a higher rate by absorbing radiation from the cosmic microwave background radiation and thus not decay at all. This may not apply for micro black holes mut I'm not exactly sure. Its been a very long time since I've worked with those equations.

  6. Apr 7, 2008 #5


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    At the relevant timescales, the universe will be colder than any black hole.
  7. Apr 7, 2008 #6
    ok well assume for the sake of argument that there is nothing for the black hole to suck in. I mean, eventually this will be true for all black holes (since there is a finite amount of energy in the universe).
  8. Apr 7, 2008 #7
    I checked out these topics and I didnt see my question either asked or answered. If somehow I missed it, could you please show me the answer given?

    Also, your hypothetical "joke" seems perfectly plausible to me. From how I currently understand the physics of black holes that must be what happens in order to keep all the reference frames in agreement as to what is happening. If this were not the case, it would imply that the only reason you never see someone fall into a black hole is because of the red shift of emitted photons. However this is not the case since GR predicts that the time of the object falling in approaches 0 and that the object ACTUALLY never falls in from the infinite rest frame.
    Last edited: Apr 7, 2008
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