Does Light Truly Have No Mass?

[kurious]

I think rest mass is conserved because momentum still exists for a photon as it does for rest masses.When a photon is reflected off a wall it exerts a force on the wall just like any other particle does.You might say this force is a relativistic momentum change but
in general relativity which deals with decceleration and acceleration forces as such don't exist.This seems to me inconsistent.

 

[jcsd]

I think rest mass is conserved because momentum still exists for a photon as it does for rest masses.When a photon is reflected off a wall it exerts a force on the wall just like any other particle does.You might say this force is a relativistic momentum change but
in general relativity which deals with decceleration and acceleration forces as such don't exist.This seems to me inconsistent.

Okay let's work out the momentum of a photon using the formula for a relatvistic particle with rest mass: p = \gamma mv (note m is rest mass). So for a photon we substitute in v = c:

p = \frac{mc}{\sqrt{1-\frac{c^2}{c^2}}} = \frac{mc}{0}

clearly we have a problem here, as if m is non-zero then we can interpret the momentum as infinite (if we set m as zero we could say that the momentum is indeterminate which is at least more physically realistic as indeed a photon can have have any value for p in special relatvity). So photons with rest masses just don't work in relativity, so the formula we should be using to find the momentum of a photon which can me derived from the formula for relativistic momentum-energy shown in my previous posts is p = \frac{E}{c} so a phtoon's (relativistic) momentum is dependent on it's energy not on it's mass (which is zero).

 

[jtolliver]

This does not prove that REST mass is not conserved.

The only way both restmass and energy-momentum can both be conserved is if all particles have the same four-velocity, which experimental evidence(actually everyday experience) shows is not the case.

 

[reilly]

The proof that a photon has no mass follows directly from the quantum conditions -- as in momentum p=h(1/wavelength), while energy E = h(frequency) -- so E=pc, and that's all she wrote.
Regards,
Reilly Atkinson

(You want details? They will be found in most any first-year physics text.)