What Would a Traveler See When Falling into a Black Hole?

In summary, the sky redshifts and shrinks depending on the angle you view it from. The stars above the black hole are blueshifted and the black hole itself takes up less of the sky than is expected.
  • #1
skywolf
81
0
ok, so i used to think that when someone fell into a black hole several things would happen at the horizon
1. the sky would light up because the microwave background radiation would
be contracted into the visible range
2. the outside universe would "age" to infinity (from the travelers point of view) because of the curving of space
3. i would be unable to see my feet because light cannot escape a black hole

but i realized there was a crucial flaw;
all of these observations are from a shell (stationary) observers point of view.
so now i have no idea what the traveler would see
but more importantly what time would experience
 
Physics news on Phys.org
  • #2
this is what happens...Journey to a Black Hole: A Thought Experiment

Two observers: Jack & Jill

Jack, in a spacesuit, is falling into a black hole. He is carrying a low-power laser beacon that flashes a beam of blue light once a second.

Jill is orbiting the black hole in a starship at a safe distance away in a stable circular orbit. She watches Jack fall in by monitoring the incoming flashes from his laser beacon.

Black Hole Thought Experiment

He Said, She Said...

From Jack's point of view:

* He sees the ship getting further away.
* He flashes his blue laser at Jill once a second by his watch.

From Jill's point of view:

* Each laser flash take longer to arrive than the last
* Each laser flash become redder and fainter than the one before it.

Near the Event Horizon...

Jack Sees:

* His blue laser flash every second by his watch
* The outside world looks oddly distorted (positions of stars have changed since he started).

Jill Sees:

* Jack's laser flashing about once every hour.
* The laser flashes are now shifted to radio wavelengths, and
* the flashes are getting fainter with each flash.

Down the hole...

Jill Sees:

* One last flash from Jack's laser after a long delay (months?)
* The last flash is very faint and at very long radio wavelengths.
* She never sees another flash from Jack...

Jack Sees:

* The universe appear to vanish as he crosses the event horizon
* He gets shredded by strong tides near the singularity and crushed to infinite density.

Moral:

The powerful gravity of a black hole warps space and time around it:

* Time appears to stand still at the event horizon as seen by a distant observer.
* Time flows as it always does as seen by an infalling astronaut.
* Light emerging from near the black hole is Gravitationally Redshifted to longer (red) wavelengths.
 
  • #3
Well, skywolf, your are pretty much on target [the seeing your feet thing gets tricky, but it's safe to say they are redshifted]. The 'sky' not only blueshifts, it shrinks [a light cone thing].
 
  • #4
Chronos said:
Well, skywolf, your are pretty much on target [the seeing your feet thing gets tricky, but it's safe to say they are redshifted]. The 'sky' not only blueshifts, it shrinks [a light cone thing].

The sky both blueshifts and redshifts, depending on viewing angle.

Consider an observer that freely falls along a radial worldline feetfirst into a black hole. The stars directly above the observer are redshifted for the observer. Roughly, for these stars, Doppler redshift dominates gravitational blueshift.

Also, due to the headlight effect, the portion of the sky taken up by the black hole is smaller than might be expected. At the the event horizon, it might be expected that the black hole takes up half the field of the view, i.e., that everything below horizontal is black, and that above horizontal the visible universe is seen. However, at the event horizon, the visible universe takes up more that half the field of view - everything above horizontal and some of the field of view below horizontal.

In some sense, the sky expands.

See the posts by pervect and me in https://www.physicsforums.com/showthread.php?t=104577".

Regards,
George
 
Last edited by a moderator:

1. What is a black hole?

A black hole is a region in space where the gravitational pull is so strong that nothing, including light, can escape from it. This strong gravitational force is created by a large amount of mass being packed into a very small space.

2. How are black holes formed?

Black holes are formed when a massive star dies and its core collapses under the force of its own gravity. This causes the star to become smaller and denser, eventually creating a black hole.

3. Can we see black holes?

No, black holes cannot be seen directly because they do not emit or reflect any light. However, we can observe the effects of a black hole on its surroundings, such as the distortion of light and the movement of stars and gas around it.

4. What happens if you fall into a black hole?

If you were to fall into a black hole, the intense gravitational force would cause your body to be stretched and torn apart. This is known as "spaghettification". As you approach the center of the black hole, called the singularity, time and space become infinite and you would ultimately be crushed to an infinitely small point.

5. Do black holes last forever?

According to current theories, black holes do not last forever. Over time, they slowly lose mass through a process called Hawking radiation. Eventually, a black hole will evaporate completely and disappear, leaving behind no trace of its existence.

Similar threads

  • Special and General Relativity
2
Replies
62
Views
3K
  • Special and General Relativity
Replies
4
Views
289
  • Special and General Relativity
2
Replies
57
Views
1K
  • Special and General Relativity
Replies
19
Views
1K
  • Special and General Relativity
2
Replies
67
Views
2K
  • Special and General Relativity
Replies
22
Views
1K
Replies
35
Views
1K
  • Special and General Relativity
Replies
11
Views
2K
  • Special and General Relativity
Replies
4
Views
788
  • Special and General Relativity
Replies
11
Views
639
Back
Top