# Photons stuck in an orbit

1. Apr 24, 2008

### nuby

What would be required to cause photons to orbit an object?

Last edited: Apr 24, 2008
2. Apr 24, 2008

### Nabeshin

A lot of gravity.

3. Apr 24, 2008

### nuby

Is that the only force that interacts with photons?

4. Apr 24, 2008

### Nabeshin

Well certainly electromagnetism can't interact with it because well... photons carry electromagnetism. Don't know about the strong or weak nuclear forces though.

5. Apr 24, 2008

gravity

Gravity would more than likely dominate, unless you worry about an atmosphere and then scattering or density variations, as well as refractive index variations (optical fiber) could trap the light. Black holes etc.. trap light through gravitational force

6. Apr 25, 2008

### dst

Anything that makes the path of least time in the form of a circle. I wonder if that's even possible since then it becomes essentially infinite length.

7. Apr 25, 2008

### George Jones

Staff Emeritus
Photons can orbit black holes.

A photon orbit around a spherical black hole has r coordinate 50% greater than the r coordinate of the event horizon.

8. Apr 25, 2008

### Crosson

I just want to point out that the reason photons don't interact with the electromagnetic force is not because they are the carriers of that force but rather because they are themselves charge neutral.

Compare this to the color force, which gives rise to the strong force in the nucleus, where the gluons that carry the force between variously colored quarks are themselves colored which gives rise to pure gluon interactions called glueballs and making the theory nonlinear.

I agree that a ray of light can orbit a blackhole, so can even a light packet, but getting it to happen to a single photon would likely require carefully controlled laboratory conditions.

9. Apr 25, 2008

### nuby

I'm curious how much force would be required for this, any ideas?

10. Apr 27, 2008

### nuby

Is this known, or impossible to figure out?

11. May 1, 2008

### nuby

Do photons interact with the nuclear strong force?

12. May 3, 2008

### lightarrow

To inject light into a circular glass fiber?

13. Jul 6, 2008

### nuby

Seems like the gravity to hold a photon in orbit around a black hole should be known? Is it?

14. Jul 7, 2008

### DrGreg

George Jones has already given the answer, if only you'd realise it, in post #7.

Find yourself a non-rotating spherical black hole of mass M, and carefully launch a photon into a circular orbit with circumference

$$\frac {6 \pi G M} {c^2}$$ ​

If you aim the photon in exactly the right direction, and if it never collides with anything and if there is nothing else in the Universe, then the photon will go into orbit.

However, photon orbits are unstable, so if you can't meet all the above conditions (and in practice you can't), the photon will eventually spiral out of orbit (inwards or outwards).

Reference: "Black hole: Photon sphere" on Wikipedia.

15. Jul 7, 2008

### DaveC426913

Why would a photon's orbit be any less stable than a regular mass orbiting a regular body?

16. Jul 7, 2008

### kanato

Because a regular mass can change speed. If it moves closer to the other object, the gravitational force will increase but its speed will also increase, so it won't end up crashing into the massive body. Photons can't increase or decrease speed, so if a photon is not at the ideal radius, then it won't get into a circular orbit because the attraction to the massive body is increased at a radius smaller than the circular orbit radius.

17. Jul 7, 2008

### DaveC426913

Right. Photons cannot hold elliptical orbits.

18. Jul 7, 2008

### George Jones

Staff Emeritus
For spherical black holes, there are unstable circular orbits of massive particles, as well as stable circular orbits.

For spherical black holes, "photon" orbits occurs only at $r = 1.5R_S$, where $R_S$ is the Schwarzschild radius of the event horizon. These orbits all are unstable.

Unstable circular orbits for massive particles occurs for $1.5R_S < r < 3R_S$.

Stable circular orbits for massive particles occurs for $r > 3R_S$.

The class of unstable circular orbits for massive particles can be divided into orbits that are very unstable, and orbits that are somewhat unstable.

19. Jul 7, 2008

### MeJennifer

As an aside, all orbits of objects of mass decay under GR.

20. Jul 7, 2008

### George Jones

Staff Emeritus
I'm not sure what you mean.