# How can a photon exist if it has no mass?

1. Apr 13, 2009

### ScienceNerd36

How can a photon exist if it has no mass? I've always found this concept hard to grasp. (Rhyming not intended)

2. Apr 13, 2009

### Staff: Mentor

Re: Mass?

We're not in classical physics any more. Photons are quintessentially relativistic and quantum-mechanical. The general relation for energy, mass and momentum is

$$E^2 = (mc^2)^2 + (pc)^2$$

(This uses what is often called "rest mass", not "relativistic mass".) An object can have zero mass but still have nonzero energy and momentum.

Note that even in classical electrodynamics, electromagnetic waves carry energy and momentum without having mass.

3. Apr 13, 2009

### malawi_glenn

Re: Mass?

Why does everything that exists have to have mass? Is there an a priori reason??

4. Apr 13, 2009

### tiny-tim

Hi ScienceNerd36!

(erm … rhyming not achieved)

mass is energy, energy is mass

a photon does have mass

(it has no rest-mass, but why should that matter when a photon is never at rest? )

5. Apr 13, 2009

### malawi_glenn

Re: Mass?

oohh now you have started this war again Timmy, prepare for battle