Blueshift and Blackholes

  • #1
If you could safely stand inside a black hole about the size of a small room, filled it with smoke, and shined a laser - would you see the light not only bend towards the warping gravitational field but also blueshift?

My previous post was removed so if you responded I didn't get it. I guess there is specific topics you can't talk about in here. O' Well, Thanks for any response I get.
 

Answers and Replies

  • #2
Nabeshin
Science Advisor
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You can't stand (or float stationary) inside a black hole of any size. So as it stands your question is extremely ill-posed.
 
  • #3
Well considering the detailed post that I had previously was removed I didn't feel like spending my time typing up another. All I want to know is the effect on light from within a blackhole.
 
  • #4
Nabeshin
Science Advisor
2,205
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Ok. Let's say you are a freely falling observer and you let yourself fall into a black hole with a flashlight (by fall into I mean fall past the event horizon). Now, if you shine your flashlight, on a small enough distance/time scale, you will notice no difference from if you were just in normal flat spacetime.
 
  • #5
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Ok. Let's say you are a freely falling observer and you let yourself fall into a black hole with a flashlight (by fall into I mean fall past the event horizon). Now, if you shine your flashlight, on a small enough distance/time scale, you will notice no difference from if you were just in normal flat spacetime.
So do you claim an observer falling heads down with a flashlight who passed the event horizon can shine on objects below him?
 
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  • #6
Nabeshin
Science Advisor
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So do you claim an observer falling heads down with a flashlight who passed the event horizon can shine on objects below him?
Yes. After all, on a small enough distance/time scale the space is locally minkowski, or by the equivalence principle, he shouldn't be able to tell at all that he is in a BH on short enough distance/time scales. Of course this all takes place in his co-falling frame.
 
  • #7
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Yes. After all, on a small enough distance/time scale the space is locally minkowski, or by the equivalence principle, he shouldn't be able to tell at all that he is in a BH on short enough distance/time scales. Of course this all takes place in his co-falling frame.
Sorry but your answer does not say anything, you can answer any question about GR this way. It is like saying that a terminally ill patient is not really dying if you consider a short enough time scale.

Inside the event horizon light always moves inwards towards the singularity, even if you shine it in the direction of the event horizon.
 
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  • #8
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3
Sorry but your answer does not say anything, you can answer any question about GR this way. It is like saying that a terminally ill patient is not really dying if you consider a short enough time scale.

Inside the event horizon light always moves inwards towards the singularity, even if you shine it in the direction of the event horizon.
But also remember that in a Schwarzschild black hole the singularity is not a point in time at the center position of the black hole, but rather a point in time in the future of the observer. So any light beam fires from within a black hole will continue on until it hits the timelike singularity in the future.
 
  • #9
Nabeshin
Science Advisor
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Sorry but your answer does not say anything, you can answer any question about GR this way. It is like saying that a terminally ill patient is not really dying if you consider a short enough time scale.

Inside the event horizon light always moves inwards towards the singularity, even if you shine it in the direction of the event horizon.
So invoking the equivalence principle is insufficient to answer a question? It's perfectly legitimate to say that within the bounds of the EP, the falling observer will notice nothing different (including paths of light rays). I realize it's not very rigorous, but it's not incorrect either.

Here, I drew a terrible little picture to illustrate http://i50.tinypic.com/2hmet61.jpg", it's in Kruskal-Szekeres coordinates. The horizontal line represents our "man" falling into the black hole. The parallelogram represents his trajectory, and the future horizontal line is his position at a later time. I drew two light cones, both for his head and feet. Obviously, if he shines a light at his head (closer to the singularity), it can hit his feet (further from the singularity). I apologize for the picture being terrible, but hopefully you get the point?
 
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  • #10
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But also remember that in a Schwarzschild black hole the singularity is not a point in time at the center position of the black hole, but rather a point in time in the future of the observer. So any light beam fires from within a black hole will continue on until it hits the timelike singularity in the future.
Yes, but you mean spacelike singularity right?
 
  • #11
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3
Yes, but you mean spacelike singularity right?
Yes. My bad, typed too fast :P
 
  • #12
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So do you claim an observer falling heads down with a flashlight who passed the event horizon can shine on objects below him?
Yes. After all, on a small enough distance/time scale the space is locally minkowski, or by the equivalence principle, he shouldn't be able to tell at all that he is in a BH on short enough distance/time scales. Of course this all takes place in his co-falling frame.
... and if a freefalling observer falls feet first past the event horizon and shines a torch at his feet he should still be able to see the reflected light off his feet, despite the fact the light can not travel upwards towards his eyes. This suggests his head must somehow catch up with the light from his feet that is also heading towards the central singularity and has a head start. hmmm.. just something I'm pondering :yuck:
 
  • #13
1,538
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So invoking the equivalence principle is insufficient to answer a question? It's perfectly legitimate to say that within the bounds of the EP, the falling observer will notice nothing different (including paths of light rays). I realize it's not very rigorous, but it's not incorrect either.
If someone asks a question about strong fields it seems pretty obvious he is not interested in the limiting case. But no it is not wrong.
 

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