Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Black Holes: Falling in and Looking out?

  1. Dec 13, 2011 #1
    I think we're all familliar with the old "What if you fell into a black hole?" Event horizons, time dialation (I think is the right term), and spagettification. There is however one thing these thought experiments never seem to cover that I'm really curious about.

    As I understand this scenario, once you cross the event horizon all of eternity has passed on the outside universe. Or rather, time is passing infinitely quickly outside. Correct?

    So what do you see if you look back out through the event horizon?

    Does space junk randomly appear as it is sucked past the event horizon and thus brought closer to your timeframe of referance? Is it just junk or the entire contents of the galaxy or even the universe?

    Or, would you look out and see the end of the universe (whatever that may be)?

    Or, would you be blinded by an eterneties worth of starlight from all the visible stars light suddenly being released from it's prison on the edge of this frozen star?
     
  2. jcsd
  3. Dec 13, 2011 #2

    PAllen

    User Avatar
    Science Advisor
    Gold Member

    Not correct. If you are watching a distant clock as you fall through the horizon and head for the singularity, you see that clock continuing to advance normally. Further, you will see it registering a short amount of time from when you crossed the horizon until you reach the singularity.

    You can see other objects that fell through the horizon shortly after you did. But not for long - you reach the singularity quite quickly (your time), even though the distant observer never sees anything cross the horizon.
     
  4. Dec 13, 2011 #3

    Chronos

    User Avatar
    Science Advisor
    Gold Member

    An observer free falling into a black hole would see external clocks slowing. The infalling photons stuggle to catch up with the infalling observer as that observer approaches light speed.
     
  5. Dec 13, 2011 #4

    PAllen

    User Avatar
    Science Advisor
    Gold Member

    Ah, that is even further away from the claim that the infalling observer sees time go infinitely fast outside. All I had figured out is that the proper time on a distant clock world line between the point whose photon reaches the infaller at the horizon, and the point on this clock world line whose photon reaches the infaller at the singularity - is finite and short. I never bothered to investigate the rate of this clock compared the infaller's as seen by the infaller. Thanks for the clarification.
     
  6. Dec 13, 2011 #5
    When you are free falling at escape velocity then exactly at the event horizon you will measure twice the emitted frequency of a beacon far removed.
     
  7. Dec 13, 2011 #6

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Half the frequency.
     
  8. Dec 13, 2011 #7
    Oops, you are correct, the measured frequencies are red shifted so it is half not double the frequency.

    Thanks for correcting me!
     
  9. Dec 14, 2011 #8
    There is a professor at University of Colorado who has put a great deal of effort into animated videos of the interiors of black holes. I heartily recommend his web site, which has some of the best explanations of relativity anywhere. In short, it is very complicated. He says much of the energy goes backward in time, which is has got to be the weirdest physics thing ever.
     
  10. Dec 14, 2011 #9

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Andrew Hamilton
    His site has spectacular animations and some good explanations, but it also has some explanations that are not so good. For example,
    from

    http://casa.colorado.edu/~ajsh/schwp.html#redshift

    is wrong.
    Can you give a specific reference for this?
     
  11. Dec 14, 2011 #10
    Last edited by a moderator: May 5, 2017
  12. Dec 14, 2011 #11

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Thanks. When I get some time, I will take a detailed look at this. Also, to be fair, I should explain why I think that the passage which I quoted is wrong.

    I'm not sure when I will get the required time. Papers at which I have to look are accumulating on my desk at such a rate that a black hole threatens to form.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Black Holes: Falling in and Looking out?
Loading...