# Photons and mass

1. Jan 3, 2006

### Mr.Socrates

They Say Photons Move In The Velocity Nearly Equal To Light ....
If A Body Approaches Velocity Of Light The Mass Increases ....

If U Think The Photons As Packets Of Energy Then E=mc^2 Or
E/c^2 Is M Then..... Will Energy Should Have Mass...

2. Jan 3, 2006

### Staff: Mentor

"They" should say that photons, being light, move exactly at the speed of light.

That is one way of looking at massive particles (particles with a non-zero rest mass). But it doesn't apply to photons, which have zero rest mass.
A better formula to use is this:
$$E^2 = p^2 c^2 + m^2 c^4$$, where "m" is the rest mass of the particle (which is zero for a photon) and "p" is its momentum.

3. Jan 3, 2006

### Mr.Socrates

then what do u think is momentum related with .... i think there is a small conection with mass.... photon whose " rest mass " is only 0 not the mass when it moves..... so the momentum should vary in each case where the velocity varies like in refraction...
+

4. Jan 3, 2006

### Staff: Mentor

The momentum of a photon is related to its energy. (E = pc)

Note: The effective (group) velocity of light varies during refraction, but not the photon speed.

5. Jan 3, 2006

### Mr.Socrates

ok if the group velocity changes wont there be a change in phase velocity with some uncertainity..... if the v changes the p changes.. what will be the velocity change in the relativistic frame of referance and if so there is a v change it is approaching c so what will happen to mass

6. Jan 3, 2006

### Tx

Anything that moves at the speed of light has no point-mass, so it is always in motion so it cannot have mass. Even if it had a tiny-tiny-tiny amount of mass, with the speed of light multiplied to it it would be gigantic.

Sorry Doc, Didn't see your explanation.

7. Jan 3, 2006

### Mk

Has anybody noticed that Socrates posts with all words capitalized, or all lower-case?

8. Jan 4, 2006

### spacetime

Photons are actually just light. Photon is a model, a short name for the fact that light has quanta of energy that sometimes show particle like beahvior on interaction with matter.

They are not separate particles that happen to travel at the speed of light.

Good observation, Mk!

Physics