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Black Hole Falling in / Observer Perspective Question

  1. Dec 16, 2011 #1
    These damn shows on the Science Channel seem to raise more questions for me than they answer, and maybe I'm understanding this wrong but here we go...

    If I were an unfortunate soul who fell into a black hole, the show claims that I will not feel a thing as I pass through the event horizon. Makes sense when I think of it like a waterfall, as the show wanted me to think. I would gradually gain speed and accelerate on my trajectory towards doom.

    If I were an observer that watched my friend fall into a black hole, time dilation would eventually bring him to a screeching halt after he passed through the event horizon and he would appear frozen in time. (This is where I think I am mistaken, but it's what the show said)

    Every single artist rendition of black holes show them as solid black spheres. How can this be if every object that's fallen into it is "frozen in time" (relative to an outside observer) shortly after crossing the event horizon. Wouldn't a two dimensional projection of the black hole's appetite be splashed all over it thus making it visible?

    This raises other questions I have but I'm going to go with this for now.

  2. jcsd
  3. Dec 16, 2011 #2
    The object falling into a black hole would seem to vanish as well as freeze at the event horizon because its light would be infinitely red-shifted. The show you are describing also seems to be neglecting tidal forces. Someone approaching a stellar-mass black hole would be gruesomely ripped apart long before crossing the EH, however with a super-massive BH a person could (in theory) indeed cross the event horizon and not have any sensation to that effect. I generally avoid those science shows on Discovery, Science, ect. They seem pretty universally terrible with the happy exception of Wonders of the Solar System / Wonders of the Universe with Brian Cox.
  4. Dec 16, 2011 #3
    Anything that passes the event horizon cannot return, because the escape velocity is greater than the speed of light, so even light cannot escape the black hole, hence why it is not visible and called a black hole.

    Now if you were observing someone falling into a black hole, you are seeing the photons being reflected off this person as they fall. The gravity distorts space-time and the closer and closer they get to the event horizon the slower the photons travel. We essentially view measure time interval in his frame to move slowly, time dilation. As he crosses the event horizon the photons he's emmiting or reflecting will stop and stay there at the horizon because they are slowed down so much. So he has actually passed into the event horizon and disappeared but the light stuck there will take an infinitely long time to reach you.
  5. Dec 16, 2011 #4


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    Would an object falling into a black hole, such as a flashlight, emit less light (fewer photons) over time in the frame of someone not falling in since time dilation slows it down?
  6. Dec 17, 2011 #5


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    Not fewer photons, just redshifted.
  7. Dec 17, 2011 #6


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    Why not few photons? What about time dilation effects?
  8. Dec 17, 2011 #7
    hmmm, can a black hole fall into a black hole?
  9. Dec 17, 2011 #8


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    If by that you are asking can two Black Holes merge together, then the answer is yes.
  10. Dec 19, 2011 #9
    The bold part seems to contradict your first paragraph depending on what you mean by "stuck".

    I seem to take from that the individual photons are stuck in a losing battle against the gravity of the BH. I'm imagining the last reflected photons taking an infinite amount of time to reach an outside observer.

    If they are stuck there though, they would be invisible no? Since they are unable to reach the observer, the fact that they exist at all is irrelevant because anyone beyond(outside) the EH would be unable to see them. The only photons that would be visible would be the last few reflected just before the person fell through the EH, and those would be distorted(red-shifted) as they fought against the gravity, though it seems to me that they would be just barely winning this battle, and slowly accelerate away from the BH.
    Last edited: Dec 19, 2011
  11. Dec 22, 2011 #10
    What the Universe would look like has been calculated: Falling into a Black Hole

    One wouldn't experience anything special at the event horizon; its being the event horizon is from the inability to escape, not anything directly local.

    One can easily estimate tidal forces.

    F ~ G*M*d/R3 ~ d/t2

    where t is the remaining time. Tidal forces won't be very strong until one has only about one second left. That's well outside of a solar-mass black hole, but inside a galaxy-center supermassive black hole. "Inside" and "outside" here are for the event horizon.

    This analysis also applies to orbits and being on surfaces.
  12. Dec 24, 2011 #11
    These shows on the Science Channel are mostly for entertainment purposes only.
  13. Dec 24, 2011 #12


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    That seems like a good question. My first thought was that in the frame of reference of the in-falling object, there is no time dilation but then I realized I'm only correct in that there's no time dilation due to acceleration of his velocity, but that doesn't cover the time dilation due to gravity. Hm ... this stuff really makes my head hurt.
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