# Phase Velocity

1. Sep 16, 2011

### byron178

When something has a phase velocity greater than the speed of light,will it travel backwards in time in one frame?

2. Sep 16, 2011

### ghwellsjr

Nope.

3. Sep 16, 2011

### byron178

but relativity says that if something were to travel faster than light than in one frame it will travel backwards in time.

4. Sep 16, 2011

### Passionflower

Hmm, how do you conclude that?

5. Sep 16, 2011

### HallsofIvy

No, it doesn't. It says the normal matter cannot travel faster than light (in any frame).

6. Sep 16, 2011

### Passionflower

I would agree with that but do you think that that would imply that the phase does not go backwards in time in another frame?

7. Sep 17, 2011

### byron178

ive read that its like an illusion the phase velocity traveling faster than light.

8. Sep 17, 2011

### HallsofIvy

Apparently then the problem is that you do not understand what "phase" velocity means. "Phase" velocity is a property of a wave. There is nothing with mass moving at the "phase" velocity of a wave.

9. Sep 17, 2011

### Passionflower

Yes, and I implied that that was not the case?

Phase velocity can be observed, and thus it can also be observed in many frames of reference.

10. Sep 18, 2011

### Phrak

The propagation of emf in a waveguide has a wave velocity greater than c.

What does "backwards in time" mean?

11. Sep 19, 2011

### mathfeel

Plenty of things can be observed to be faster than speed of light. But they don't transmit information or matter.

Consider two very very long straight rulers, both moving at uniform speed (<c of course) perpendicular to their elongated direction. The point at which they cross has speed proportional to 1/tan(theta), where theta is the angle between them. So this point can travel at any arbitrarily high speed. Just like any point of the wave front of a dispersive wave.

12. Sep 19, 2011

### Passionflower

Yes I am aware of that.

Did anyone claim the contrary?

Not every single posting with a question about relativity is designed to question its validity as some apparently seem to think.