How should look like the real experiment testing whether photon has mass?

Create empty spherical object with surface made of material reflecting photons in the all possible frequencies.

In the center of sphere place f.e. 1 gram of protons, and 1 gram of anti-protons or something like that which we really can measure.

Whole sphere should be extremely well shielded against any particles coming in and out, and against cosmic rays and neutrinos. Any particle outside should be recorded when going in, and when going out of sphere.

Inside of sphere there should be vacuum 100%.

We should be recording mass of sphere including content.

We're starting experiment. Protons and anti-protons are annihilating and producing photons which are bouncing forever due to reflective material.

If photon has mass 0, we should detect that whole sphere mass is smaller than before. They are no longer bound in regular particle and are now free.

If photon has mass >0, we should detect no change in mass of sphere.

We're opening "door", photons are finding exit, and sphere is made lighter.

You can invent your own version of above experiment using equivalent technique.

Experiment proposed by Przemyslaw Gruchala.