# How would you see an Event horizon?

1. Nov 1, 2009

### Stark

As I understand it, an Event Horizon is a boundary surrounding a black hole where light is held 'still'. If the photons never get beyond this boundary, how can they be observed?

2. Nov 1, 2009

### Nabeshin

They don't get observed. That's why the black hole is black!

3. Nov 1, 2009

### DaveC426913

This is a misunderstanding.

The event horizon of a BH is not a boundary; it is simply a radius that defines a particular phenomenon. In simple terms:
- a light ray passing tangentially, but outside, this radius will be bent sharply around the black hole but will continue off into space.

- a light way passing tangentially, but inside, this radius will be bent so sharply that it will fall into the BH and not re-emerge. Likewise, any light rays that emanate from inside the EH will be bent around in a curve so that they will not exit the EH.

But ultimnjately, as Nabeshin points out, a photon inside the EH will not exit and thus will not be observed.

If a light ray happened to be perfectly aimed directly out from the centre of the BH, so that it could not be bent in any particular direction (like like balancing a pencil on its tip), the light ray does not stop moving, it is simply infinitely red-shifted.

4. Nov 3, 2009

### Ich

That's wrong. You're descrinbing the photon sphere, not the event horizon.
The EH is indeed defined as a boundary. From http://www.mathpages.com/home/kmath339.htm" [Broken]:
"The black hole region, B, of such a spacetime is
defined to be the points of M not contained in the
causal past of future null infinity. The boundary
of B in M is called the event horizon."
If we think of light as a classical particle, such a particle moving outwards at the EH would indeed have a constant r-coordinate, in this sense being "held still".
So the EH is the boundary of the region you can't observe.

Last edited by a moderator: May 4, 2017
5. Nov 3, 2009

### Chronos

Photons observed entering a black hole by an observer inside the event horizon will be infinitely blue shifted, as Ich noted.

6. Nov 3, 2009

### Nabeshin

Surely this is not true for a freely falling observer. Which class of observer are you talking about?

7. Nov 4, 2009

### Wallace

This is completely wrong. As far as an observer inside a black hole is concerned, the photons they observe don't look much different from when that particular observer was outside the black hole.

Note that you wouldn't even notice at what point you crossed 'inside' the event horizon if you fell into one (assuming you weren't ripped apart by tidal forces, lets assume you are very small and falling into a very large black hole).

As photons fall into a hole, they gain a graviational blueshift (for observers at a smaller radius than the emmitters). This is determined by the difference in radius and the relativistic version of the gravitationl potential, which depends on the black hole mass. There is no discontinuity in this process at the Schwarschild radius.

The only way to get an infinite gravitational blueshift is to have an infinite mass black hole, but that's just mathematical tomfoolery.

8. Nov 4, 2009

### George Jones

Staff Emeritus
Ich didn't note this.
Exactly.
Actually, for observers radially freely falling from infinity, Doppler trumps gravity for radial photons, i.e., a redshift. See post #9 and later posts in

9. Nov 4, 2009

### Nabeshin

I think the only way to get infinite blue shifted photons, besides the infinitely massive BH Wallace notes, is to position our observer just outside the event horizon, and have him attempt to remain stationary (i.e fire rockets radially outward). Such an observer finds he needs to expend a near infinite amount of energy to remain stationary, which is related to his discovery of infinitely blue shifted radiation.

10. Nov 17, 2009

### Chronos

Correct, Nabeshin. A stationary observer at the event horizon would perceive enormous blueshift. An observer in free fall would be less affected.

"What would you see from inside a black hole?"
http://curious.astro.cornell.edu/question.php?number=348 [Broken]
“... Eventually at the event horizon there is an infinite shift - so the light has an infinite amount of energy, but zero wavelength. ...” I concede Karen Masters assumes this is only true for a stationary observer at the event horizon, as was my original assumption.

"The edge of locality: visualizing a black hole from the inside"
http://arxiv.org/abs/0903.4717
“... Near the singularity, the observer's view is aberrated by the diverging tidal force into a horizontal plane. The view in the horizontal plane is highly blueshifted, but all directions other than horizontal appear highly redshifted. ...” This assumes an observer in free fall inside the event horizon. This paper is a good read for the curious.

"Journey into a Schwarzschild black hole"