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A "Observation" of Black Hole Merger

  1. Apr 21, 2016 #1
    Premise 1: From the frame of reference of the earth, observing a thing falling into a black hole (crossing the Schwarzschild radius) is impossible -> the thing is observed falling toward but never reaching the radius, i.e. the process takes an infinite amount of time.

    Premise 2: Observation includes any manner of measurement and/or signal detection etc.

    Premise 3: Another black hole qualifies as "a thing" which can fall toward the black hole

    Conclusion: Actual black hole merger can never be observed, only a process leading toward it but taking an infinite amount of time.

    Question1: If the conclusion is correct, observation of black hole merger is impossible, then WHAT was observed by LIGO, and why is not still occurring (granted the signals may be weakening... but I assume that in our frame of reference the black holes are still merging.. and will continue to do so "forever") i.e. Did LIGO detect an actual merger or only an approach towards merger?

    Question 2: If the conclusion is incorrect which of the premises is false or how do the set of premises not logically necessitate the Conclusion?
  2. jcsd
  3. Apr 21, 2016 #2


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    Premise 1 is based on a static space-time with the "thing" not really perturbing the static Schwarzschild solution. This is definitely not the case for a black hole merger. The merger is very very far from being a static solution.

    Apart from that, the signal is generated in the inspiral phase and then settles down as the black holes merge into a single rotating black hole.
  4. Apr 21, 2016 #3
    Thank you!
  5. Apr 21, 2016 #4


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    @Orodruin, I have a question. My belief is that Premise 1 is false and I'd like to know why you seem to think that's not the case. My argument is that assuming Hawking Radiation is real then in the far far future the black hole will evaporate down to nothing. As it finishes evaporating, the view seen by the observer (clearly not on Earth since that will have long vanished) will change and the photons showing the in-falling object actually falling in will be released.

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