As with most equations, the context of the equation is almost more important than the equation itself.  Einstein's famous formula is no different.  When we write

,

this is not the total energy of the particle; this is only the energy that is available in its mass for transformation into energy (its "rest-mass energy").  Or equivalently, it is the energy necessary to create that much mass.  A more general formula (also due to Einstein) is



where is the momentum of the particle.  This formula describes the kinetic and rest-mass energy of the particle (if there is additional energy, such as potential energy, you have to add that in too).  For the photon, as you have pointed out, the mass is zero.  Therefore there is no rest-mass contribution to the photon's energy.  Instead, its energy is due entirely to its momentum, which is given by

,

where is the photon's wavelength and is Planck's constant.  Thus, for a photon, the Einstein relation simplifies to



where is the frequency of the photon.


On another note, your question about the possible mass of the photon is interesting in itself.  We say that the mass of the photon is zero, but how sure are we?  How do we know that it is exactly zero, and not just very, very tiny (even compared to particles which already have tiny masses, like the electron or the neutrinos)?  Modern measurements quote a bound on the mass of the photon, saying, "it cannot have more mass than this."  I'm sure you can look up the modern bounds on the mass of the photon from the NIST table of physical constants on their website. 

Does that clarify it for you?

Last edited by M@Man (2008-04-22 04:01:46)