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.
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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)