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Question about black holes

  1. Jan 13, 2006 #1
    ok, so i used to think that when someone fell into a black hole several things would happen at the horizon
    1. the sky would light up because the microwave background radiation would
    be contracted into the visible range
    2. the outside universe would "age" to infinity (from the travelers point of view) because of the curving of space
    3. i would be unable to see my feet because light cannot escape a black hole

    but i realized there was a crucial flaw;
    all of these observations are from a shell (stationary) observers point of view.
    so now i have no idea what the traveler would see
    but more importantly what time would experience
  2. jcsd
  3. Jan 14, 2006 #2
    this is what happens...Journey to a Black Hole: A Thought Experiment

    Two observers: Jack & Jill

    Jack, in a spacesuit, is falling into a black hole. He is carrying a low-power laser beacon that flashes a beam of blue light once a second.

    Jill is orbiting the black hole in a starship at a safe distance away in a stable circular orbit. She watches Jack fall in by monitoring the incoming flashes from his laser beacon.

    Black Hole Thought Experiment

    He Said, She Said...

    From Jack's point of view:

    * He sees the ship getting further away.
    * He flashes his blue laser at Jill once a second by his watch.

    From Jill's point of view:

    * Each laser flash take longer to arrive than the last
    * Each laser flash become redder and fainter than the one before it.

    Near the Event Horizon...

    Jack Sees:

    * His blue laser flash every second by his watch
    * The outside world looks oddly distorted (positions of stars have changed since he started).

    Jill Sees:

    * Jack's laser flashing about once every hour.
    * The laser flashes are now shifted to radio wavelengths, and
    * the flashes are getting fainter with each flash.

    Down the hole...

    Jill Sees:

    * One last flash from Jack's laser after a long delay (months?)
    * The last flash is very faint and at very long radio wavelengths.
    * She never sees another flash from Jack...

    Jack Sees:

    * The universe appear to vanish as he crosses the event horizon
    * He gets shredded by strong tides near the singularity and crushed to infinite density.


    The powerful gravity of a black hole warps space and time around it:

    * Time appears to stand still at the event horizon as seen by a distant observer.
    * Time flows as it always does as seen by an infalling astronaut.
    * Light emerging from near the black hole is Gravitationally Redshifted to longer (red) wavelengths.
  4. Jan 14, 2006 #3


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    Well, skywolf, your are pretty much on target [the seeing your feet thing gets tricky, but it's safe to say they are redshifted]. The 'sky' not only blueshifts, it shrinks [a light cone thing].
  5. Jan 14, 2006 #4

    George Jones

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    The sky both blueshifts and redshifts, depending on viewing angle.

    Consider an observer that freely falls along a radial worldline feetfirst into a black hole. The stars directly above the observer are redshifted for the observer. Roughly, for these stars, Doppler redshift dominates gravitational blueshift.

    Also, due to the headlight effect, the portion of the sky taken up by the black hole is smaller than might be expected. At the the event horizon, it might be expected that the black hole takes up half the field of the view, i.e., that everything below horizontal is black, and that above horizontal the visible universe is seen. However, at the event horizon, the visible universe takes up more that half the field of view - everything above horizontal and some of the field of view below horizontal.

    In some sense, the sky expands.

    See the posts by pervect and me in https://www.physicsforums.com/showthread.php?t=104577".

    Last edited by a moderator: Apr 21, 2017
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