Can you see the back of your head

  • Thread starter jamesabc
  • Start date
  • #1
22
0
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?
 

Answers and Replies

  • #2
49
0
I just use a mirror.
 
  • #3
russ_watters
Mentor
20,262
6,854
You would just look out into space - light doesn't curve around the earth like that.
 
  • #5
Borek
Mentor
28,635
3,106
Depends on where the mountain is. Assuming your eyes lie on the events horizon you should be able to see your back.
 
  • #6
sylas
Science Advisor
1,646
7
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
 
Last edited:
  • #7
George Jones
Staff Emeritus
Science Advisor
Gold Member
7,414
1,054
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]
 
  • #9
Borek
Mentor
28,635
3,106
Photon sphere... one learns all his life and dies stupid :wink:
 
  • #10
vanesch
Staff Emeritus
Science Advisor
Gold Member
5,028
17
Photon sphere... one learns all his life and dies stupid :wink:
Especially if you see your back :rofl:
 

Related Threads on Can you see the back of your head

Replies
1
Views
6K
Replies
18
Views
12K
  • Last Post
Replies
17
Views
3K
  • Last Post
Replies
6
Views
1K
Replies
9
Views
2K
Replies
1
Views
1K
  • Last Post
Replies
7
Views
1K
Replies
4
Views
3K
Top