Infalling, external observer

[BJC]

An external observer will never see an infalling observer cross the event horizon of a black hole - instead the infalling observer will appear to be stationary above the event horizon forever.

 

But how does the infalling observer see the external observer???

 

I contend that the infalling observer will see the external observer normally - just the way any observer would see another in somewhat flat space. Until the infalling observer crosses the event horizon and then all contact is lost.

Edited July 12, 2011 by BJC

[csmyth3025]

An external observer will never see an in-falling observer cross the event horizon of a black hole - instead the in-falling observer will appear to be stationary above the event horizon forever.

 

But how does the in-falling observer see the external observer???

 

I contend that the in-falling observer will see the external observer normally - just the way any observer would see another in somewhat flat space. Until the in-falling observer crosses the event horizon and then all contact is lost.

This is an interesting question because it involves several relativistic effects - the sum of which I'm totally unable to calculate.

 

The in-falling observer starting with zero tangential and radial velocity at an arbitrarily large distance from the event horizon (r>>>) will, I'm guessing, arrive at the event horizon with a radial velocity of c relative to his/her companion safely orbiting the black hole at the starting distance.

 

This presents several relativistic effects from the standpoint of the orbiting observer that will conspire to make the in-falling observer appear to both "freeze" and vanish as he/she reaches the event horizon:

 

1) The in-falling observer's accelerating velocity away from the orbiting observer will make it appear that his/her motions are increasingly slowed down and, simultaneously, the light by which the in-falling observer can be "seen" will be increasingly red shifted (special relativity effects).

 

2) The same "slowing down" and red shifting effects seen by the orbiting observer will be enhanced by the increasing gravitational intensity (the so-called gravity well) from which the in-falling observer's light is transmitted to the orbiting observer (general relativity effects).

 

It seems to me that the special relativity effects as seen be the in-falling observer looking out to his/her orbiting companion would be the same: the orbiting companion's motions would seem to be slowing down and the light by which the orbiting companion can be seen would be red shifted.

 

The general relativity effects as seen by the in-falling observer would be different. I believe that the light from the orbiting companion would be blue-shifted. I don't know how much this gravitational blue shifting would offset the special relativistic red shifting, though.

NOTE: I suspect that the two effects will cancel each other out.

 

The big question is: Would the in-falling observer see the motions of the orbiting observer slow down as he/she approached the event horizon? I don't have a clue.

 

Any comments from the more knowledgeable members here would be greatly appreciated.

 

Chris

 

Edited to add NOTE

Edited July 12, 2011 by csmyth3025

[SpeedFreek]

The in-faller can still see the outside universe after they have crossed the event horizon - the photons following the in-faller still move at c in relation to him. He sees the outside universe as redshifted.

 

At the link below is are a set of videos depicting the view of an in-faller. In the first one, the in-faller crosses the event horizon at t=35 and reaches the singularity at t=40 (those are the video timings, not from the onscreen clock).

 

http://jila.colorado...idebh/schw.html

Edited August 6, 2011 by SpeedFreek