Can you see the back of your head

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Discussion Overview

The discussion revolves around the question of whether it is possible to see the back of one's head while standing on a mountain using a powerful telescope, with considerations of light behavior and gravitational effects, particularly in relation to black holes and photon spheres. The scope includes conceptual exploration and theoretical implications.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that using a powerful telescope from a mountain would only allow one to see out into space, as light does not curve around the Earth.
  • Another participant proposes using a mirror as a practical solution to see the back of one's head.
  • Some participants argue that if one were at the event horizon of a black hole, it might be possible to see the back of one's head, although this raises questions about the nature of light and gravity.
  • There is a discussion about the photon sphere around a black hole, where light can orbit the mass, suggesting that one would need to be at this distance rather than the event horizon to see the back of one's head.
  • Links to external resources are provided, indicating further reading on the shape of the universe and related theories.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility of seeing the back of one's head, with some asserting it is impossible under normal circumstances while others explore theoretical scenarios involving black holes. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Assumptions about light behavior near massive objects and the definitions of event horizons and photon spheres are critical to the discussion but are not fully explored or agreed upon.

jamesabc
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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?
 
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I just use a mirror.
 
You would just look out into space - light doesn't curve around the Earth like that.
 
Division said:
I just use a mirror.

Or two?
 
Depends on where the mountain is. Assuming your eyes lie on the events horizon you should be able to see your back.
 
Borek said:
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:
Borek said:
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]
 
Photon sphere... one learns all his life and dies stupid :wink:
 
  • #10
Borek said:
Photon sphere... one learns all his life and dies stupid :wink:

Especially if you see your back :smile:
 

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