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Black hole paradox?

  1. Feb 17, 2012 #1
    sorry if my understanding of high level physics is cursory, i didnt major in it at ptown even tho i always had a hard on for physics and slept through ap phys in high school because the concepts were infantile to me.

    from my understanding its conventional wisdom that a person falling into a black hole will fall through the event horizon of a supermassive black hole and feel that nothing has happened to him. but seeing as the lifespan of black holes is finite approx 10^100 years and for a person falling in to an enormous black hole whose tidal forces
    wont kill him as he nears the event horizon(dead or alive doesnt really matter), and since he will be
    infinitely old compared to the universe before he reaches the event
    horizon, wont the black hole, due to hawking radiation/ black hole evaporation and eventual explosion, simply explode in his face once he gets very close to the event horizon?

    this comesfrom my understanding as you near the event horizon, you near the speed of light, so if you were on the event horizon, your time will have stopped, while the rest of the universe continued forward in time, meaning the universe would have passed infinitely into the future by the time you reached the event horizon

    am i missing anything?
  2. jcsd
  3. Feb 18, 2012 #2
    I think that the slowing down aspect of the guy falling in belongs to the perception of an outside observer. It only looks like that to observer's hanging around outside and away from the black hole watching him fall in.
    For the guy falling in, local time and space will appear normal... until he gets spaghettified by tidal gravity differential... and of course nothing will be "normal" when what's left of him reaches the singularity, which he will do in a finite period of time by his clock.
  4. Feb 18, 2012 #3
    yeah but hawking radiation gives a lifespan on the largest black holes to be finite in the universe, even tho the black hole itself is not part of the universe, so to an observer outside who lived forever, he would see the black hole explode before the passenger crossed the event horizon. and since the lifespan of the blackhole is finite to the observer, the passenger as well will age longer than the black holes lifespan before reaching the event horizon.
  5. Feb 19, 2012 #4
  6. Feb 19, 2012 #5

    George Jones

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    Consider two observers, observer A that falls across the the event horizon and observer B that hovers at a finite "distance" above the event horizon, and two types of (uncharged) spherical black holes, a classical black hole that doesn't emit Hawking radiation and a semi-classical black hole that does.

    For the classical black hole case, B "sees" A on the event horizon at infinite future time, and B never sees the singularity.

    For the semi-classical black hole case, at some *finite* time B simultaneously "sees": A on the event horizon; the singularity. In other words, the singularity becomes naked, and A winks out of existence at some finite time in the future for B.

    In both cases, A crosses the event horizon, remains inside the event horizon, and hits the singularity. In both cases, B, does not see (even at infinite future time) A inside the event horizon, as this view is blocked by the singularity.

    These conclusions can be deduced from Penrose diagrams, FIGURE 5.17 and FIGURE 9.3 in Carroll's text, and Fig. 12.2 and Fig, 14.4 in Wald's text, or

  7. Feb 19, 2012 #6
    once the singularity is naked, it will be at the schwartchild radius and expand/explode wont it?
  8. Feb 19, 2012 #7
    Could someone breakdown the whole airplane approaching a black hole and the propellers expand, and expand, and expand, and spin faster, and faster.
  9. Feb 20, 2012 #8
    bump bump bump
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