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

[Calimero]

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?

 

[George Jones]

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.

 

[Calimero]

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?

 

[skeptic2]

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?

 

[George Jones]

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.

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.

 

[Calimero]

So stationary observer at the event horizon would see infinite blueshift?

 

[skeptic2]

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.

 

[George Jones]

So stationary observer at the event horizon would see infinite blueshift?

Yes.

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.

 

[skeptic2]

Yes, that makes sense. So the stationary observer close to the horizon would see the universe speed up?

 

[George Jones]

Yes, that makes sense. So the stationary observer close to the horizon would see the universe speed up?

Yes.

 

[skeptic2]

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?

 

[Dmitry67]

Hawking radiation is observer dependent: infalling observer sees hawking radiation from his own apparent horizon.