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What happens before crossing the Event Horizon

  1. Sep 16, 2008 #1
    Suppose this happens:

    10 billion people are taken to a large black hole.
    The people are thrown into the black hole as we fly around it in the large spaceship.

    I have read that when an object goes past the event horizon, we as observers away from the blackhole will see the people freeze just before crossing it.

    So does this mean that we would forever see 10 billion people frozen around this blackhole?

    If so, then why don't we see the tons of matter which the blackhole has previously devoured (other planets and stars etc) in blackholes today

    I would like to be told what is happening here... I don't understand why they would 'freeze' and many sources tell me things would indeed freeze before crossing the event horizon, to an on-looker's point of view. I would assume it is some kind of optical illusion, since the light cannot be frozen there...
  2. jcsd
  3. Sep 17, 2008 #2

    Jonathan Scott

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    Gold Member

    What happens is that as things approach the event horizon, then the light which is returned to an observer further away gets increasingly delayed. It would be infinitely delayed at the event horizon itself. This means that although the image appears to be slowing down to a halt as it approaches the event horizon, it also rapidly becomes extremely red-shifted, to the point where the frequency and intensity are so low that it is no longer observable.
  4. Sep 17, 2008 #3
    As Jonathan Scott said, it is not an instant freezing, it is a gradual red-shifting which is analogous to the doppler effect. Alternately, you can picture that it takes energy to "climb out" of the gravity well caused by the black hole, thus reducing the energy of each photon, leaving it with a longer wavelength.

    I suggest you read "The Black Hole Wars" by Leonard Susskind, as he goes into great detail about this very question.
  5. Sep 17, 2008 #4
    Thank you. I'll try to find that book online.
  6. Sep 18, 2008 #5
  7. Sep 21, 2008 #6
    Suppose a person sitting on the object falling into black hole were to emit signal at regular interval in his own reference frame.
    These signals would reach us at infinity at progressively increasing interval of time..
    As the object approaches event horizon , the timing between arrival of two signals would become infinite.
    Hence we would never see an object crossing event horizon.

    Now the object would fall into the black hole(and reach singularity) in finite proper time in it`s frame.
    This would result into increase in the mass of the black hole. Which would change the gravity. So with respect to us although we would never see object crossing even horizon we would see an increase in the gravity.

    Does it not in some sense violate causality.

    We see object that has not yet fallen into blackhole abut yet there is an increase in the gravity of the black hole ????????? :confused:
  8. Sep 21, 2008 #7
    Whyso? Light travels at c in all reference frames. All we would see is those signals changing in wavelength.
  9. Sep 21, 2008 #8
    Whyso? Light travels at c in all reference frames. All we would see is those signals changing in wavelength.

    Please read chapter on black holes from Sean Carroll`s book or free online notes on General relativity.
    This is true that signals would be redshifted. But also they would reach us with interval between two signals increasing as object approaches event horizon.

    You are right that light travels with speed c. But distance between two points with coordinate separation (dr) is not just (dr).

    But rather it is determined by metric as well.
    Scwarzschild metric has (1-2GM/r) factor in denominator accompanying (dr)**2 .
    Hence distance between two points with same coordinate separation would increase if we are close to event horizon.

    Hence separation between the signal sent by person sitting on object falling into blackhole would increase as object approaches event horizon.

    Because distance between two points is rendered infinite by the metric.
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