# Do photons violate our understanding of physics?

Reading one of the other threads got me thinking. If photons have momentum (as several people mentioned in the thread) this seems to indicate that they have mass, albeit very small. Since P = m*v and P > 0, m > 0. But wait, I thought that nothing with mass could go the speed of light, since it would require an infinite amount of energy to do so. Please correct me where I’m wrong! Thanks.

Here is a previous thread on this subject. In short, a photon is massless, but it’s momentum is equal to it’s frequency multiplied by Planck’s constant.

Photons do not have a rest mass.

Nothing with a non-zero rest mass can reach c. Photons have zero rest mass but non-zero rest mass, but they have energy. Energy is the same thing as mass, so they do have mass (i.e. relativistic mass).

Uh, let me try again:

Photons have zero rest mass but they have energy. Energy is the same thing as mass (relativistic mass), so they have non-zero relativistic mass.

ok I think I understand. How do we know that photons don’t have a rest mass though? There’s no way to catch a photon and mass it, so it seems that scientists have just created a loophole to explain photons. Is it because they are a form of energy at all times that they have no mass? I’m really confusing myself with this E=mc^2 stuff and I sound like an idiot now. Let me know if I’m on the right track.

I"m a long way from one of the cognoscente in quantum physics but I think the photon is demonstrated to have zero rest mass sort of by default. That it, there is no instance where assuming a photon to have zero rest mass gives a result that isn’t in agreement with experiment or violates any other physical principal. And I think there is no instance where assuming that a photon has rest mass is of value. And in fact, if it did have rest mass, then by relativisitic reasoning it couldn’t travel at the speed of light and so would violate the Theory of Realitivity which is well supported by evidence.

This isn’t all that unusual a situation. For example, there isn’t any more fundamental reason behind the conservation of energy that the fact that it works and by assuming that energy is conserved a lot of other things can be explained. And there isn’t any case where the law can be shown to be violated.

Let’s make this sentence read, “And in fact, if it did have rest mass, then by relativisitic reasoning it couldn’t travel at the speed of light because to do so would violate the Theory of Realitivity which is well supported by evidence.”

Can you stop a photon? Without having it be aborbed by matter, that is.

Through observation, physicists have placed a maxium value for the rest mass of photon, naturally this itself is very small.

The problem is if phtons don’t have zero rest mass then quantum physics doesn’t work.

toadspittle- I believe some scientists did freeze a photon, IIRC though this was to do with quantum entanglment, so they were freezing the information the proton contained.