Was the mass of a single foton moving at the speed of light ever measured?

Question

If not how can we say they are massless?

Answer 1

It's close to impossible to measure the mass of a photon. According to Einstein, a particle that has velocity must have mass. So, a photon is not massless at all until someone disproves it.

Answer 2

It has been indirectly measured. Some very smart people figured out that adding mass to the photon would change the inverse square law that describes (to first order) the force between two charged particles. Two charged particles "feel" each other by exchanging photons. The classical force law F=k*q*Q/r^2, where F is the electrostatic (Coulomb) force, k is a constant of proportionality, q and Q are the charges for 2 different particles, and r is the seperation between the particles (r^2 for those not familiar with the notation is r*r = r raised to the power of 2).

If the photon had mass, the force law would look like this:

F = k*q*Q/r^(2+s), which means r raised to the (2+s) power, where s is a real number that we know to be very small.

To measure photon mass, one only has to measure the force between two charged particles at various distances and see if it follows the 1/r^2 law or the 1/r^(2+s). To date, all experiments to measure photon mass using this method (and there are others) gives s=0 to as many decimal places that we can measure accurately.

I do not know "why" the force law takes this form for photon mass, but it's related to Quantum Field Theory (so ask me after this semester and I should know since I'm taking that class now).

Some more info:
I do know that if there are extra dimensions, the force law takes that form as well due to Gauss's Law (the 1/r^2 is due to the surface area of a sphere in 3D space; adding dimensions affects the number the same was as adding mass to the photon). The fact that we see no deviation of the inverse square law for electrostatics suggests that extra dimensions or either extremely tiny (if they even exist) so that they only have an effect on the law at distances below what we've measured - smaller than an atom since we've measured it that small, OR photons do not travel in the extra dimensions. Gravitons, however, since they carry the force of gravity (also inverse square law to first order), which is, according to Einstein, the force that describes the geometry of spacetime, would travel in those extra-dimensions. Currently, we've measured gravity to roughly the millimeter level, and experiments are in progress to measure it at smaller scales. Gravitons are also massless. See how Nature loves symmetry?



PS: Photons do not escape a blackhole because of the extreme curvature of space around a blackhole. The mass of a photon NEVER enters the equations of General Relativity, and has nothing to do with blackholes. Furthermore, relativistic mass is sort of a meaningless thing to talk about since it varies so much with energy. It's an old-fashioned term that is no longer used in the physics community. Rest mass is an invariant and is what physicists mean when they talk about mass.

Answer 3

photon is not a particle like a quark or electron. A photon is a quanta or vessel of energy. It is (like Spock said) pure energy. The reason not even light can escape a black hole it because of the extreme warp or curvature of space around a black hole. Gravity, as explained by Einstein, is matter's ability to curve space. Example, the earth goes along in a straight line through the curved space around the Sun. So, a photon will goes along a straight path along the severely curved space into a black hole.

Answer 4

A photon does have mass because light cannot escape from a black hole and all a black hole is is a singularity with an infinite density.

Photons do not have REST MASS, but they do have relativistic mass!! Remember E=mc^2.

Until recently scientists did not believe that neutrinos had REST MASS, but it was always known that they had relativistic mass. However neutrinos also possess rest mass.

Answer 5

There is only one known Catholic photon and it goes to Mass every Sunday.

Answer 6

if it has any zero mass than if we assume the relativity be true, it will be of infinite mass at speed of light, which sure it is not.

Answer 7

you mean photon?