# From a photon's reference frame

1. Sep 6, 2006

### nealh149

Does light still travel at c?
Thank you.

2. Sep 6, 2006

### Put a quark in it

According to special relativity, something going less than c will have light speed away from it at c. That property doesn't apply to something that is also going C, so I would say no.

3. Sep 6, 2006

### Danger

Actually, the basis of Einstein's theories is that no matter what reference frame you observe from, light still travels at 'c'. A photon would therefore 'see' a parallel photon travelling at 'c' relative to itself.

4. Sep 7, 2006

### Staff: Mentor

An "object's reference frame" is a reference frame in which the object is at rest. But there is no inertial reference frame in which a photon (any photon) is at rest. Therefore questions about what things look like from the point of view of a photon don't have meaningful answers in the context of SR, strictly speaking.

Some people approach such questions by imagining the limiting case of an endless series of particles, each one moving faster than the preceding one, but I doubt that this is always valid.

5. Sep 7, 2006

### HallsofIvy

Actually, a photon can't "see" anything so the problem does not arise!

More specifically, the question of motion of one photon relative to another does not arise because time is stopped at speed c. There is no "motion" relative to a photon.

6. Sep 7, 2006

### WhyIsItSo

This seems to be one of the hairier issues of relativity (to me anyway).

I would have guessed two photons would view each other as at rest. Since a photon *IS* travelling at the speed of light, from its frame of reference (considering itself the "at rest" observer), wouldn't the universe have infinite mass? And does time have any meaning to a photon?

7. Sep 7, 2006

### Danger

I misspoke regarding the photon seeing anything. Forgot about the time factor. Anything that's even a teeny bit slower, though, will measure light travelling at c.

8. Sep 7, 2006

### rcgldr

If time is stopped, then how do different photons have different frequencies?

9. Sep 7, 2006

### AntiMagicMan

Because you cannot look at things from the point of view of the photon (remember the inertial frame does not exist) then this question doesn't really make sense. The frequency of a photon is a quantum mechanical property that is related to it's energy. So photons have different frequencies because they have different energies.

10. Sep 7, 2006

### rcgldr

My point here that if time has stopped for a photon, then how can it change states at a fixed rate (it's frequency)?