# GRAVITY can BIND LIGHT?

1. Aug 24, 2007

### orgthingy

i remember in middle school that the light can NEVER be bend! ive just read a book " a briefer history of time" by : s.hawking...... (im in high school now) but i really didn't understand y!!! :P

2. Aug 24, 2007

### orgthingy

i read in " a briefer history of time" that light can be bend! i rly didn't understand how :P

3. Aug 24, 2007

### DaveC426913

Gravity warps spacetime. Light follows the shortest path from point to point, which, in curved space is not a straight line.

4. Aug 24, 2007

### orgthingy

and y the hell "the universe is curved" ?

5. Aug 24, 2007

### HallsofIvy

Staff Emeritus
Why does that make you so angry?

6. Aug 24, 2007

### Staff: Mentor

Wait until he gets to college--then he'll really be pissed. :tongue2:

7. Aug 24, 2007

### rewebster

(you are funny, Doc-----)

and then he'll still come back to the PF and will still get these same types of answers

8. Aug 24, 2007

### Sicewa

Actually, gravity is caused by warped spacetime, which is caused by mass.

9. Aug 24, 2007

### StatusX

Light does go straight, but only from the point of view of what are called inertial observers (IOs). Standing still on the surface of the earth doesn't make you an IO, but being in free fall does (approximately and, of course, temporarily).

So if someone was falling towards the earth, they would see nearby beams of light travelling along straight lines. But if you watched this from the ground, these beams look bent, in the same way the path of a stone through the air, which the falling person sees as moving in a straight line, appears to you to be bent (although the light bends to a much smaller degree due to its much higher speed). So that's how light can "look" bent, even though, in an important sense, you're right to say it never really bends.

Actually, because of the curvature of space, even IOs can see distant light bending, because the curvature means roughly that an IO at one point won't look like an IO to an IO in another location.

10. Aug 24, 2007

### A.T.

Is it possible to send out a light beam, that returns to the emitter due to space curvature?

11. Aug 24, 2007

### cristo

Staff Emeritus
Yes. If you were on the surface of a massive object (a star, say) you could emit a light beam with an angle such that it circles the star and returns to you.

12. Aug 24, 2007

### genneth

Yes -- around any strong gravitational field, such as around a black hole. However, the orbit is unstable -- if you were not exactly on the correct orbit, you'd either decay into the hole or escape. I can't actually remember, but it's something like twice the Schwartzchild radius or something -- can an astrophysicist correct me?

13. Aug 25, 2007

### daniel_i_l

I'm pretty sure that it's 1.5 times that Schwartzchild radius.

14. Aug 25, 2007

### pervect

Staff Emeritus
Yes, the photon sphere is at 1.5 times the Schwarzschild radius, and at the photon sphere light can orbit a massive body. In theory, by looking in the right direction, you could see the back of your head at the photon sphere (the image might be dim and distorted, howeer). The orbit is unstable against small pertubations.

15. Aug 25, 2007

### cyruskoras

Surely gravity can bend the light. this is already proved by einstein in his theory on bending effect of light

16. Aug 25, 2007

### cyruskoras

surely light will have bending effect

17. Aug 25, 2007

### A.T.

Thanks for your answers. The reason that I asked this, is the statement that "light follows the shortest path from point to point".

This is maybe true locally, but if a photon send out from point A orbits a massive star and arrives back close to point A, then this is certainly not the shortest path between two points globally. The shortest path between two points, is always a geodesic. But not every geodesic is the shortest path.

One could say: "Light follows the straightest path from a point in a given direction"

18. Aug 25, 2007

### daniel_i_l

The path that a photon (or anything else) travels between two events is the path with the smallest distance through spacetime - not the shortest distance through space.
Lets look at an example - throwing a ball in the air. The two events are:
A: the ball is thrown in the air, h=0, t=0.
B: the ball is on the ground again, h=0, t=10.
Now lets see what path gives us the shortest ST interval between those two events. If the ball stays in the same place for the whole time then the space interval is 0 and the time interval is 10. But according to the SC metric the time interval (dt) contributes more to the interval the closer it is to the earth - so if during the 10 seconds the ball is a little higher in the air (h>0) the total interval will be smaller. But if it's too high the interval will get bigger (because it goes a long way in the 10 seconds). It turns out that the best path to take is one where the ball goes up and comes down in a ballistic trajectory.

19. Aug 28, 2007

### proton

so, gravity can BIND light!

Last edited: Aug 28, 2007
20. Aug 28, 2007

### genneth

To be honest, lots of things bend light. Changes of refractive index being the obvious one. It's certainly not true that light travels in straight lines... The full story behind going from working out the principle behind the path that light takes (Fermat's Principle) to modern day mechanics is jam-packed with the most fun kind of maths.

Now that I look at that sentence, it's not as much of an endorsement as I wanted... :(