Can you see the back of your head

[jamesabc]

Main Question or Discussion Point

if you stood on a mountain would you be able to look straight ahead with a powerful telescope and see the back of your head? or would you just look out into space?

 

[Division]

I just use a mirror.

 

[russ_watters]

You would just look out into space - light doesn't curve around the earth like that.

 

[billiards]

I just use a mirror.

Or two?

 

[Borek]

Depends on where the mountain is. Assuming your eyes lie on the events horizon you should be able to see your back.

 

[sylas]

Depends on where the mountain is. Assuming your eyes lie on the events horizon you should be able to see your back.

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

 

[George Jones]

Depends on where the mountain is. Assuming your eyes lie on the events horizon you should be able to see your back.

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]

 

[atyy]

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

Maybe?
http://www.independent.co.uk/news/science/the-universe-is-not-round-say-scientists--it-is-shaped-like-a-trumpet-560046.html
http://arxiv.org/abs/astro-ph/0412407

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

 

[Borek]

Photon sphere... one learns all his life and dies stupid

 

[vanesch]

Photon sphere... one learns all his life and dies stupid

Especially if you see your back :rofl: