Can you see the back of your head

I just use a mirror.

 

Or two?

 

Quite so... except that you don't need to be quite that close.

Orbits around a black hole are ... different. The "photon sphere" is the distance from a point mass at which light moving tangentially will follow a perfect circle around the mass. For a non-rotating black hole, this is 1.5 times the Schwartzchild radius (event horizon).

Cheers -- sylas

 

This doesn't work, as the light will just fall into the black hole. You have to be at the photon sphere, which is a bit farther from the black hole. See

https://www.physicsforums.com/showthread.php?t=246751.

Maybe the original poster is thinking about closed universes.

[edit]Didn't notice that sylas posted while I was searching.[/edit]

 

Fun to think about:
http://books.google.com/books?id=6VD2mo-r_SEC&printsec=frontcover#PPA187,M1

Maybe?
http://www.independent.co.uk/news/s...ists--it-is-shaped-like-a-trumpet-560046.html
http://arxiv.org/abs/astro-ph/0412407

Probably not:
http://arxiv.org/abs/astro-ph/0603449

 

Especially if you see your back :rofl: