MLA Citation: |
Bloomfield, Louis A. "Question 589"How Everything Works 17 Jul 2018. 17 Jul 2018 <http://howeverythingworks.org/print1.php?QNum=589>. |

589. If E=mc^{2} and we know light exists, why is it that light doesn't have infinite mass and consequently why aren't we all squashed? - M

The equation that you present is a simplification of the full relationship between energy, mass, momentum, and the speed of light, and is really only appropriate for stationary massive particles. In it, *E* is the particle's energy, *m* is the particle's rest mass, and *c* is the speed of light. Since light has no rest mass, the previous equation is simply not applicable to it. I should note that this equation is sometimes used to describe moving massive particles, in which case the *m* is allowed to increase to reflect the increasing energy of the moving particle. But the use of this equation for moving particles and the redefinition of mass as something other than rest mass often leads to confusion.

A better way to deal with moving particles, particularly massless particles, is to incorporate momentum into the problem. The full equation, correct for any particle, is *E ^{2}=m^{2}c^{4}+p^{2}c^{2}*. In this equation,

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