According to special relativity, the energy of an object with rest mass m and speed v is given by γmc, where γ is the Lorentz factor defined above. When v is zero, γ is equal to one, giving rise to the famous E = mc formula for mass–energy equivalence. The γ factor approaches infinity as v approaches c, and it would take an infinite amount of energy to accelerate an object with mass to the speed of light. The speed of light is the upper limit for the speeds of objects with positive rest mass, and individual photons cannot travel faster than the speed of light. This is experimentally established in many tests of relativistic energy and momentum.