• Question: E=MC2, but why is the c squared?

    Asked by anon-201778 to Meirin on 4 Mar 2019.
    • Photo: Meirin Oan Evans

      Meirin Oan Evans answered on 4 Mar 2019:

      Great question! Maybe you’ve seen the formula for the kinetic energy of a moving object E=0.5mv^2 ? In this formula v is a speed. In E=mc^2 c is also a speed, the speed of light. In both cases the speed has to be squared to get from mass to energy!

Comments

  • Photo: Andy

    Andy commented on :

    As Meirin’s said, the c has to be squared to get the units right. But why c and not some other number with units of speed (squared)? It’s because this equation comes from Einstein’s theory of special relativity, which tells us that the speed of light, c, is just how fast light goes, but is the fastest that anything goes: it’s the natural link in our universe between time and space. Relativity is all about the relation between space and time, and so it’s very satisfying that exactly c is the number that appears, rather than c/2 or 0.3478c, or something like that: we’d then be asking “why that number in front?”!