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Can I prevent a Black Hole from forming?

  1. Oct 13, 2014 #1
    If I were to travel towards a BH at, once again, close to c -- would I, in my frame of reference, prevent (observationally at least) the formation of a BH? In other words, can I match my speed with that of the collapsing object's light or even infalling matter (in an already formed BH) from disappearing by going very fast, thus keeping up with the redshift and stopping the event horizon from ever forming?
     
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  3. Oct 13, 2014 #2
  4. Oct 13, 2014 #3
    One more question: Is the energy gained by the object (the black hole) in my frame of reference do to the fact that as I speed up to close to c, I cover more time frames in a shorter period of time, thus catching up to a time when the light waves emitted by the BH had a higher frequency? In other words, when I travel at close to c, do I compress time in front of me?
     
  5. Oct 13, 2014 #4

    PeterDonis

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    zoki85's answer is correct, but perhaps a bit terse. ;)

    The key thing to realize is that your motion relative to the BH--or, more to the point, relative to the infalling matter that is forming a BH--in and of itself, does not affect the infalling matter and whether or not it forms a hole. The formation of the hole, if it happens, is frame-independent; all observers will agree that a black hole has formed. There's no way to keep it from happening by changing your own state of motion. You would have to do something to change the actual trajectories of the infalling matter before it got compact enough to form a black hole. (But doing that would mean that you, yourself, and whatever energy you were using to change the trajectories, would have to be counted in the total energy present in the spacetime, and therefore in the total mass present that could possibly form a black hole.)
     
  6. Oct 13, 2014 #5

    PeterDonis

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    This isn't really a good way of thinking about what happens as you travel faster.

    You can't "catch up to a time". You also can't catch up to light rays. As you speed up, you will see light rays have higher and higher frequencies if they are coming from in front of you (and lower and lower frequencies if they are coming from behind you), but that's purely an effect of your motion; it doesn't change where the actual light rays are in spacetime.

    Of course, if you end up driving yourself into the black hole, you will see all the light emitted by the matter that originally formed the hole, even after it crossed the horizon--but you will be inside the horizon yourself by that time.
     
  7. Oct 13, 2014 #6
    I just don't understand why black holes are frame-independent. The star or whatever formed it was luminous at some point right? In theory I should be able to shift its light to a point where I can measure it, no? Unless the collapse happens faster than light.



    I don't understand this, either. The infalling matter is emitting light, though redder and redder as the collapse continues. By going faster, I should be able to measure that light, right? And how does one change the trajectory of the infalling matter? What does that even mean lol? Thanks.
     
  8. Oct 13, 2014 #7

    PeterDonis

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    The fact that a black hole has formed is frame-independent. The particular frequencies of light that you observe being emitted from near its horizon are not; you can change those by changing your state of motion (just as with light emitted anywhere). Sorry if that wasn't clear from my previous post.

    "Redder and redder" is relative; this description is for an observer sitting at rest far away from the infalling matter and the black hole that forms from it. So changing your state of motion can change the frequencies you see (see above).

    Yes; but doing so won't change the fact that the infalling matter forms a black hole.

    It certainly wouldn't be easy. ;) But in principle you could, for example, attach rockets to many pieces of the infalling matter and fire the rockets to change the trajectories of the pieces in different ways, so that they weren't falling together any more.
     
  9. Oct 13, 2014 #8
    Yes, I'm still confused about space-time. I really don't have a grasp of that yet. But let me ask you this: if space and time are intertwined, and I were to shift the CMB for example, by going close to c, and then measuring the energy of those photons -- wouldn't that be the same thing as viewing them as they were in the past?
     
  10. Oct 13, 2014 #9

    PeterDonis

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    No. Perhaps you are being confused by time dilation. Let me set up a simple example to illustrate the distinction.

    Suppose you are sitting at rest on Earth, receiving light signals from your friend on Alpha Centauri. (Assume you and he are at rest relative to each other.) The light signals contain images of your friend's clock. You note that, at any given instant, the time shown on your friend's clock in the image you are receiving is 4.3 years earlier than the time shown on your own wall clock at that same instant. This makes sense because Alpha Centauri is 4.3 light years away from Earth.

    Now you get in your rocket and go speeding off to visit your friend at some speed close to the speed of light. You watch the images coming from your friend the whole time. What will you see?

    First, of course, you will have to feed the light signals into a computer or some other device to correct for the huge blueshift in the frequencies in order to extract the images. But once you do that, you will see the time on your friend's clock in the images advancing monotonically--that is, it always moves forward, never backward. For example, if, when you started out, his clock read 12:00 noon on January 1, 2100, then you would see his clock continually advance from that date, until, when you arrived at Alpha Centauri, you would see the final image of his clock, just before you landed, read some time in August, 2108 (4.3 years for the original light signal, the one you got just as you started out, to reach Earth, plus 4.3 years in the Earth/Alpha Centauri frame for you to travel, plus a little extra because you weren't quite traveling at the speed of light, after the time shown on his clock when you left Earth).

    Of course, your own clock, on board your ship, will read something quite different: it will have read some time in April, 2104, when you left Earth (4.3 years after January 1, 2100, because it took 4.3 years for that light signal to get to you, in the Earth/Alpha Centauri frame); but it will hardly have registered any elapsed time during the journey, because of time dilation. So when you arrive and meet your friend, your spaceship clock will read, say, some time in May, 2104, whereas his clock will read some time in August, 2108. Your clocks are out of sync because of the change in your relative motion during your trip. So you have not "gone back in time" by traveling fast; you've just changed the relationship between your clocks, while both of you continue to move forward in time.
     
  11. Oct 13, 2014 #10
    You totally got it, Peter. Of course one would never see a clock run backwards! What was I thinking lol. Thanks for that awesome reply.
     
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