# Photon and mass questions

1. Jul 16, 2008

### bassplayer142

I know minimal information on the theory behind GR. I know that light is bent from mass. Say a photon is launched perpendicular to a point source mass in a closed system. Does this photon ever come back? If it were at a slight angle would it eventually come back but in a long long time and distance? Thanks

2. Jul 17, 2008

### George Jones

Staff Emeritus

Last edited by a moderator: Apr 23, 2017
3. Jul 17, 2008

### DaveC426913

A photon could be deflected back toward the observer, but it would require a black hole-sized mass, and the photon would loop around in a very tight loop. No, you would not get a long-period "orbit" like what you're describing.

4. Jul 17, 2008

### bassplayer142

So your saying that it is impossible for a photon to be fired in any direction to escape a point source mass?

5. Jul 17, 2008

### DaveC426913

What? How did you conclude that?

Quite the contrary, you'd have to be very careful to get it to loop around, let alone orbit the mass. Read the thread George posted for how to do that.

Photons travelling near a large mass will follow a geodesic, which will be genrally like a hyperbola - it'll bend but then continue on straight.

6. Jul 17, 2008

### bassplayer142

Ok I understand now. Would it be safe to say that there is photons that being on the edge of the universe will never come back to collide with any mass?

7. Jul 17, 2008

### HallsofIvy

First, is it safe to talk about "the edge of the universe"?

Notice that edge of the universe is itself is made into a link. If you click on it you will find that there is no "edge of the universe".

8. Jul 17, 2008

### George Jones

Staff Emeritus
Did you read the link to the edge of the univers brought up?

There are closed universe models in which it is possible to send light out and have the light return the same place in space (same comoving spatial coordinates) and open universe models in which light sent out will never return.

9. Jul 17, 2008

### DrGreg

According to classical (i.e. non-quantum) general relativity, if a photon is radially launched at a distance exceeding $2Gm/c^2$ from a large symmetric non-rotating mass $m$ it will escape, otherwise it will not.

I say "large mass" because the formula above considers only the effect of the mass on the photon and not the effect of the photon on the mass. Also, if the mass is small (say an electron), then the distance above will be extremely tiny, so we can't ignore quantum theory. In quantum theory neither the mass nor the photon can be point sources -- they each must occupy some volume in some sense. And we don't have a theory that combines quantum theory and general relativity.

P.S. George's answer refers to photons launched in the other direction, i.e. tangentially.

10. Jul 17, 2008

### George Jones

Staff Emeritus
I wasn't quite sure what bassplayer142 meant in his original post, but I interpreted
to mean (spatially) perpendicular (according to hovering observer) to a radial vector.

11. Jul 17, 2008

### DrGreg

"perpendicular to a point source" is meaningless, so I interpreted it the other way!

12. Jul 17, 2008

### bassplayer142

I see what you mean now. But when I say perpendicular to a point source I mean launched away from the surface of the point perpendicularly. Which in itself may not make much sense :). My question has been answered. Thanks

13. Oct 8, 2008

### bassplayer142

Came back to this after more thought... If the photon was shot out "radially" then gravity would not affect the photon. When I say this I mean that a photon is falling towards a mass at acceleration g. But this g can't exist if the photon is heading straight out because the photon will only go the speed of light. What happens to the acceleration due to gravity?

14. Oct 8, 2008

### George Jones

Staff Emeritus
If a baseball is fired straight up, its speed decreases. If light is fired straight up, its (local) speed doesn't change, but its wavelength increases.

15. Oct 8, 2008

### bassplayer142

That sounds exactly what I was looking for. Is there an equation that states this in general terms? thanks