Re: Can a photon bend back on itself? e.g. in an extreme gravitational field

Hallo, Paul!
Thanks for the question, it gave me a lot to think about. I shall try to
give an answer, but please do not take it as a definitelly correct - it's
just an attempt, as I am not really expert on the field (at least not yet!)

I am not sure I understand what the question means, so I'll give you few
of my interpretations, and the answers to these.

1)INTERPRETATION: The photon travells straight away from the center of the 
black hole (BH), so bending would mean that it is retarded to a halt by 
the extreme gravitational field, and cosecutively the direction of its 
motion is reversed.
  ANSWER: For as much as I know, no can do, photon can not be retarded, as 
it always travells at the speed of light. What should happen to it is that 
its wavelwngth is stretched, and the energy it lost due to the stretching 
is devoured by the gravitational field of the BH. But then again, I have 
never been to the interior of the BH, so some other things might happen 
there of which neither I, nor anyone else know about...

2)INTERPRETATION: The photon travells through the BH on a trajectory that 
is at some angle to the line that connects its current position and the 
centre of the black hole. The question is then whether this photon can 
achieve some sort of stable eliptical or circular orbit, and so to 'byte 
its own tail'.
  ANSWER: Well, the event horizon (EH) of the BH is defined as the 
distance from the BH centre at which the escape velocity is the speed of 
light. So if you emit a photon at this distance, and do it in such a way 
that photon's initial trajectory is at right angles to the BH's center, it 
might continue circling at this distance arround the BH indefinetely. 
However, BHs real have a nasty urge to gobble up the surrounding stuff, 
which makes them grow and so the EH continuously grows. Hence I guess it 
would be hard to get a REALLY stable orbit for the photon.
         Now, about byting the tail part, and also about the smallest 
possible radious of the circle. The trouble is that, if you reduce the EH 
(you do this by reducing the mass of your black hole) to very small 
distances, let say about the size of the atom, quantum effects come into 
play, and the whole classical picture of a point particle photon circling 
around the point particle BH gets blurred. All in all, I'm not really sure 
what to answer to this.

By the way, you can check these sites and their links for some additional 
info:
- antwrp.gsfc.nasa.gov/htmltest/rjn_bht.html 
- www.astro.ku.dk/~cramer/RelViz/text/exhib4/coordinates.html 

Hope I've been helpfull!
Duje