Curious? If photon has no mass does that mean it can't convert to any energy?

No. That is a common misconception. First of all E = mc^2 does not mean that mass can be converted into energy. That too is a common misconception. It means that the form of mass and the form of energy can change. But in all cases the total mass and the total energy remains the same. For example; when an electron and a positron annihilate they disappear and two photons are produced traveling in equal and opposite directions. The energy in the beginning was the sum of the kinetic energy and the sum of the rest energies of both particles. When the particles disappear then all the energy is in the form of photons. You can think of the energy of photons as being entirely kinetic energy. So the form of the matter went from electron/positron to photons. The form of the energy went from kinetic + rest energy to pure kinetic energy. So it was the form that changes. The conversion that physicists speak of is that the form of the mass and energy is converted from one form to another.

As far as what “mass” means; I assume that you’re familiar with the fact that many terms in the English language can at times have two different meanings, correct? The same thing happens in physics. For example; when you study quantum mechanics you have to keep in mind that when you’re talking about momentum you’re not talking about, mechanical momentum, you’re talking about canonical momentum. Therefore if someone asks you

*How is mechanical momentum defined?* you can’t just answer then with one or the other. You either have to ask for the context or given them both meanings. The same is true in relativity with mass. The term

*mass* has two meanings. It can refer either to relativistic mass (aka inertial mass) or proper mass (aka rest mass). The inertial mass of a photon is the m in p = mv. So if you know the momentum and since v = c for a photon we have m = p/c. Since for a photon E = pc -> p = E/c we have m = p/c = (E/c)/c = E/c^2. That’s the simplest way to obtain that relationship for a photon that I know of. For particles which have finite proper mass see

http://home.comcast.net/~peter.m.brown/sr/inertial_mass.htmhttp://home.comcast.net/~peter.m.brown/sr/einsteins_box.htmTo understand the mass-energy equivalence relationship E = mc^2 please see

http://home.comcast.net/~peter.m.brown/sr/mass_energy_equiv.htmI hope that was helpful? :)