Do Photons Have Mass and Why Does it Matter?

[Dr_Zinj]

We think the mass of a photon at rest is zero because that matches the mathematics of the equations we have that model the experimental and experiential data we have.

I don't know if there are other equations that can accurately describe the relationship between mass, energy, and velocity that yield resting photons with an actual mass. I certainly wasn't exposed to any rumor of such in Physics 101 or 102 in college.

 

[Aidyan]

Mass is a measure of the inertia of an object, that is the resistance of it against a change of motion. Since light always travels with the same speed for any observer it has no mass, per definition.

 

[Mark Harder]

I've had trouble with this concept, too. After all, light waves are bent by the gravitational curvature of space and they impart momentum to objects on which they impinge. How can they be affected by gravity and have momentum, if they have no mass? Chemistry students I tutor have trouble with this as well. The key is the distinction between rest mass and inertial/gravitational mass. This is how I think of it, physically:

To say that photons have a nonzero rest mass is to imply that they can exist at rest. However, QM assures us that photons are the particles that are equivalent to light waves. Like like all waves, light and photons of light, must be in motion. There's no such thing as a wave at rest, so photons at rest cannot exist. Their energy and mass derive solely from their motion. In other words, the notion that photons have a nonzero rest mass is as fictional as light that doesn't move.

 

[Mark Harder]

To say that photons have a nonzero rest mass is to imply that they can exist at rest.

Come to think of it, that's not a logical statement by itself, is it? Hypothetically, you could have a particle, system or whatever that does not exist at rest, but might have mass at rest. Also, must anything that exists, at rest or otherwise, have mass?
Sorry 'bout that.

 

[jerromyjon]

Also, must anything that exists, at rest or otherwise, have mass?

There are obviously many cases where gravity can be ignored, but everything I know anything about has an attraction to ANYTHING else, especially energy which most of baryonic mass consists of.