What Does an Observer See When Falling Into a Black Hole?

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Discussion Overview

The discussion centers around the visual experiences of an observer falling into a black hole, particularly as they approach the event horizon (EH). It explores the implications of gravitational redshift and blueshift on the observer's perception of the universe, including how different initial conditions (such as falling from rest or with propulsion) affect what the observer sees. The conversation touches on theoretical aspects of general relativity and the nature of light in extreme gravitational fields.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants propose that an observer falling into a black hole would see the universe blueshifted as they approach the event horizon, suggesting a perception of the universe speeding up.
  • Others argue that if the observer falls from rest at a large distance, they would experience a redshift for light from the outside universe, indicating that they would see the universe slowing down, though not infinitely.
  • There is a discussion about the effects of different initial conditions, such as falling from a short distance or using an engine to approach the event horizon, which could lead to varying perceptions of redshift and blueshift.
  • Some participants express confusion about why an infalling observer would see the universe slow down, despite claims that they would perceive a blueshift.
  • A later reply questions whether light from the event horizon, such as Hawking radiation, would appear blueshifted to an infalling observer, suggesting implications for the event horizon's evaporation relative to the observer's crossing.
  • There is mention of the observer's perspective on the frequency of light received from distant sources, with explanations about how redshift affects the perceived motion of clocks in the universe.

Areas of Agreement / Disagreement

Participants express differing views on whether the observer sees the universe speed up or slow down as they approach the event horizon. There is no consensus on the exact nature of the visual experience, and the discussion remains unresolved regarding the implications of these observations.

Contextual Notes

Limitations in understanding arise from the complex nature of gravitational effects on light and the observer's frame of reference. The discussion also highlights the dependence on initial conditions and the observer's state of motion, which complicates the interpretations of redshift and blueshift.

Calimero
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Observer is falling into the BH. He looks at the stars. What does he see as he approaches EH? Universe in fast forward? Supernovas blasting at increasing rate, his galaxy merging with others...

At the very moment he crosses EH (he is still alive somehow) his last observation would be that universe is infinitely blueshifted, or not?
 
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If the observer fell from rest at a large distance, at the event horizon, the observer would see a red shift for stuff outside, i.e., the observer would see the outside universe slowing down (but not infinitely so). Also, the observer can still see parts of the outside universe even when the observer is inside the event horizon.
 
George Jones said:
If the observer fell from rest at a large distance, at the event horizon, the observer would see a red shift for stuff outside

And what if he is equipped with engine, and slowly approaches event horizon, or if he falls from the short distance?
 
George Jones said:
If the observer fell from rest at a large distance, at the event horizon, the observer would see a red shift for stuff outside, i.e., the observer would see the outside universe slowing down (but not infinitely so).

I wish you would expand on this a little. Perhaps I'm mistaken but it seems from other posts on this board the consensus is that the infalling observer would see the universe speed up. Why and how much would he see the universe slow down?
 
Calimero said:
And what if he is equipped with engine, and slowly approaches event horizon, or if he falls from the short distance?

Then the observer would see a blue shift.
skeptic2 said:
I wish you would expand on this a little. Perhaps I'm mistaken but it seems from other posts on this board the consensus is that the infalling observer would see the universe speed up. Why and how much would he see the universe slow down?

See

https://www.physicsforums.com/showthread.php?p=2425301#post2425301

and the posts in the thread to which I link there. I am not sure that I have explained things well, so ask any questions that you still have.
 
So stationary observer at the event horizon would see infinite blueshift?
 
I understand why photons would be red shifted instead of blue shifted. A freely falling observer looking back at an object falling after him would see the distance between him and the object continually increase. For photons it would be a red shift.

I do not understand why he would see the universe slow down, however.
 
Calimero said:
So stationary observer at the event horizon would see infinite blueshift?

Yes.
skeptic2 said:
I understand why photons would be red shifted instead of blue shifted. A freely falling observer looking back at an object falling after him would see the distance between him and the object continually increase. For photons it would be a red shift.

I do not understand why he would see the universe slow down, however.

For a red shift, the frequency of the light received by the falling observer is smaller than the frequency of the light emitted by a distant source. Imagine that the light is emitted from a clock, and that the clock hand whirls with the same frequency as the emitted light. The received light is used to watch the distant clock, so the falling observer sees the image of the clock hand whirl at the same frequency as the received light. But this frequency is lower than the emitted frequency, so the falling observer sees a slower moving clock hand.
 
Yes, that makes sense. So the stationary observer close to the horizon would see the universe speed up?
 
  • #10
skeptic2 said:
Yes, that makes sense. So the stationary observer close to the horizon would see the universe speed up?

Yes.
 
  • #11
From the above discussion one might presume that light from the event horizon, i.e. Hawking radiation, would be blue shifted to a free falling observer and thus would appear to age more quickly. If this is correct, does it mean that the event horizon would always evaporate before an observer were able to cross it?
 
  • #12
Hawking radiation is observer dependent: infalling observer sees hawking radiation from his own apparent horizon.
 

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