# What is the role of the speed of light in mass-energy equivalency? [duplicate]

Where does $c$ squared come into play in the equation $E=mc^2$. Multiplication obviously but how does energy equal mass times the speed of light?

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## marked as duplicate by DumpsterDoofus, Brandon Enright, Kyle Kanos, joshphysics, Qmechanic♦Mar 17 '14 at 8:36

I'm not sure this is actually a duplicate, since neither of the linked questions actually address the role of the constant $c$ in the equation. I thought we already had a question about that particular point, but I can't find it. – David Z Mar 17 '14 at 2:01
This mass–energy relation states that the universal proportionality factor between equivalent amounts of energy and mass is equal to the speed of light squared. This also serves to convert units of mass to units of energy, no matter what system of measurement units is used. To find out how much energy an object has, multiply the mass of the object by the square of the speed of light. Now we multiphy by $c^2$ because when mass is converted into energy, this energy is by definition moving at $c$. Pure energy is understood in terms of electromagnetic radiation which always moves at the speed of light