How can photons be massless

E=MC2 according to general relativity. But photons supposedly have no mass, so E=(0)C2=0. But E≠0, because the simple act of existing requires energy, and light interacts with normal matter via the electromagnetic force all the time, which again requires energy. Moreover, photons have been shown to oscillate between various different kinds/wavelengths, which apparently requires mass to occur (at least according to leading authorities on neutrinos). So how can photons be massless?

E = Mc^2 has nothing to do with the energy of a photon but rather how much energy it takes to form a lump of matter of a goven size (mass).

The energy of a photon = (Planck's constant * speed of light) / wavelength

http://en.wikipedia.org/wiki/Photon


Light has mass?
Light particles (Photons) are pure energy and exist only when in motion. We can debate if photons have mass. Because of Einstein's E = Mc^2 matter and energy are 2 forms of the same thing. Since photons have energy it can be argued that they have Relativistic Mass. On the other hand, Einstein was uncomfortable with the concept of Relativistic Mass because:

Many contemporary authors such as Taylor and Wheeler avoid using the concept of relativistic mass altogether:

"The concept of "relativistic mass" is subject to misunderstanding. That's why we don't use it. First, it applies the name mass - belonging to the magnitude of a 4-vector - to a very different concept, the time component of a 4-vector. Second, it makes increase of energy of an object with velocity or momentum appear to be connected with some change in internal structure of the object. In reality, the increase of energy with velocity originates not in the object but in the geometric properties of spacetime itself.

http://en.wikipedia.org/wiki/Mass_in_spe…

http://en.wikipedia.org/wiki/Mass%E2%80%…
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