E=MC^2 question about the "M"

[Lazernugget]

So, energy equals mass times the speed of light squared.

Energy can determine speed, because the energy is transferred into the 3 spacial dimensions.

My question is with E=MC^2 itself, Energy of light would equal zero mass times the speed of light squared, right? But that would mean light had zero energy. Care to help explain? I really wanna figure this out, and I'm reading the Elegant Universe right now so it would help to understand this before going in too much further.

Thanks,

-Lazer

[WannabeNewton]

This question actually comes up a lot. E = m_{0}c^{2} is only valid for particles in their rest frames (the m_{0} is the rest mass). A photon has no rest frame so the equation does not apply to it. In general, E = \sqrt{(m_{0}c^{2})^{2} + (pc)^{2}} so for a photon E = pc where p is the momentum.