# Photon acceleration

1. Jul 4, 2008

### sinebar

When a photon is emitted from a particle, is it traveling at the speed of light upon being emmited or does it have to accerate from a zero velocity to the speed of light? If it starts out from zero velocity does it have rest mass?

2. Jul 4, 2008

### G01

Photons always travel at c in a vacuum. So, no. They don't accelerate. They are traveling at the speed of light from the moment they are emitted.

And no, a photon has no rest mass. This is why it travels at speed c, and also why it must travel at speed c.

3. Jul 4, 2008

### Mr Noblet

In refraction, what causes the change of direction and speed of the photon? This speed change is also discontinuous, right?

4. Jul 4, 2008

5. Jul 4, 2008

### Staff: Mentor

No. Why "must" it have zero velocity? There is no physical reason I can think of, your preconception is not a physical reason.

First, photons mediate the electromagnetic force, as far as I know there is no force that acts on photons, therefore there is nothing that could accelerate them.

Second, photons are massless, so if they have any v<c they don't exist.

Third, if they don't begin at c then energy and momentum (i.e. four-momentum) are not conserved.

Those are all physical reasons that they don't accelerate. I urge you to put aside your preconceptions.

6. Jul 4, 2008

### sinebar

Ok so at no point in the life of a photon does it ever have zero velocity? Even at the absolute instance of creation it is traveling at C? In other words a photon has never been at rest? If it has never been at rest then there must be absolutely no passage of time from creation to C?

7. Jul 4, 2008

### Hootenanny

Staff Emeritus
Photons always travel at C irrespective of the medium through which light is travelling. What is seen as refraction is actually the decrease or increase in the group velocity.

Last edited: Jul 4, 2008
8. Jul 4, 2008

### Hootenanny

Staff Emeritus
Special relativity requires that any massless particle always travels at C. So yes, for a photon to exist it must be travelling at C.

9. Jul 4, 2008

### Mr Noblet

So the photons themselves travel at c in all mediums, but the light propagates at different speeds? Is it because some photons are absorbed or deflected, so it slows the total rate at which the light moves through the medium? I'm not too familiar with a lot of optics, so maybe I'm completely wrong.

10. Jul 4, 2008

### Staff: Mentor

Correct. Otherwise it would violate all sorts of conservation laws.

11. Jul 4, 2008

### Hootenanny

Staff Emeritus
Correct.
Whilst this analogy is commonly used to explain this phenomenon it isn't an accurate description of the process. Perhaps the FAQ Do Photons Move Slower in a Solid Medium? which is part of the Physics Forums FAQ may help.

12. Jul 5, 2008

### Mr Noblet

Oh I see. So my thinking was basically correct, except that the photons are absorbed by vibrations in the lattice structure and not the atoms themselves. Thanks a lot.