Einstein's Famous Equation Isn't The Full Story
Einstein's famous equation only accounts for objects that have mass and that aren't moving. In fact, it's an abbreviated version of a larger equation that works for both massless and moving objects, including photons. Though this larger equation might look intimidating, it can be easily visualized using the Pythagorean theorem.
Key Facts In This Video
E=mc^2 only describes objects that have mass and that aren't moving. (0:06)
The equation for the energy of a moving, massless particle (such as a photon) is E=pc. (0:43)
An object's velocity is equal to the speed of light times the ratio of the object's momentum to energy. (1:08)