One of the most amazing implications of Einstein’s relativity is the fact that the inertial mass of an object depends on its velocity. That sounds like a difficult thing to test, but that’s exactly what happened through a series of experiments performed by Kaufmann, Bucherer, Neumann and others.
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I have just witnessed a totally confusing amount of waffle from Pamela Gay. Anyone new to relativity would be completely baffled by her attempt to describe the notions of mass, momentum and energy.
First, you have to DEFINE what you mean by mass. It is also nonsense to describe the photon as a massless entity. The photon has energy, and so, by E=mc^2, it MUST have inertial mass. The concept of rest mass is not applicable to the photon as no observer can ever be at rest relative to the photon.
Right on. Too much waffle definitely !!
http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html