According to my physics textbook, the equation [tex]E^2=\left(mc^2\right)^2+\left(pc\right)^2[/tex] suggests that a particle may have energy and momentum even when it has no rest mass, and that the total energy then is [tex]E=pc[/tex]. This strikes me as odd, since the relativistic momentum of a particle is given by [tex]p=\gamma mv[/tex], which is zero when the rest mass is zero. But that must mean that for a particle with rest mass zero, [tex]E=pc[/tex] must also be zero, and that a particle with zero rest mass can not have energy and momentum, but this is false, because photons are particles with energy and no rest mass. If anyone would explain this to me, I would be grateful. Thank you in advance.