Do Falling Observers Experience Blueshift Inside a Black Hole?

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Discussion Overview

The discussion centers around the effects of light within a black hole, particularly whether a freely falling observer experiences blueshift when shining a flashlight after crossing the event horizon. Participants explore the implications of general relativity and the behavior of light in extreme gravitational fields.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant posits a scenario where a person stands inside a black hole and questions the behavior of light, specifically whether it would bend and blueshift.
  • Another participant asserts that one cannot stand inside a black hole, deeming the initial question ill-posed.
  • A participant suggests that a freely falling observer would not notice any difference in light behavior from normal spacetime on short time scales.
  • Some participants argue that light always moves inward towards the singularity once inside the event horizon, regardless of the direction it is initially pointed.
  • There is a discussion about the equivalence principle, with some participants claiming it allows the observer to perceive no difference in light paths, while others challenge this view, emphasizing the limitations of such reasoning in strong gravitational fields.
  • A participant introduces a diagram to illustrate the trajectory of a falling observer and the paths of light cones, attempting to clarify the discussion around light behavior in a black hole.
  • Another participant mentions the distinction between spacelike and timelike singularities, contributing to the technical depth of the conversation.
  • One participant speculates on the implications of a falling observer shining a light at their feet and whether they could perceive the light reflecting off their feet, raising questions about the nature of light's travel in such conditions.

Areas of Agreement / Disagreement

Participants express differing views on the behavior of light inside a black hole, particularly regarding the implications of the equivalence principle and the nature of light's movement towards the singularity. There is no consensus on the effects experienced by a falling observer.

Contextual Notes

Some discussions hinge on the interpretation of general relativity and the conditions under which light behaves in extreme gravitational fields. The conversation reflects various assumptions about the nature of spacetime and the experience of observers in black holes.

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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.
 
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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.
 
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 black hole.
 
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.
 
Nabeshin said:
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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Passionflower said:
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.
 
Nabeshin said:
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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Passionflower said:
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.
 
Passionflower said:
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
yenchin said:
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
Passionflower said:
Yes, but you mean spacelike singularity right?

Yes. My bad, typed too fast :P
 
  • #12
Passionflower said:
So do you claim an observer falling heads down with a flashlight who passed the event horizon can shine on objects below him?
Nabeshin said:
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
 
  • #13
Nabeshin said:
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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