# Q & A: Squaring the speed of light

Q:
My question relates to the equation E=Mc2My understanding is that no particle of matter can be accelerated to the speed of light, that the faster an object moves, the greater its mass.Therefore, how can the speed of light be squared ?
- Gary (age 64)
Carmel, IN, USA
A:

You are right that no particle with rest mass can be accelerated to the speed of light. But it does not mean that we cannot square the value of the speed of light because it is just a number. It is just a number that we are free to square, cube, multiply or divide.

By the way, you mentioned that mass gets greater as an object moves. Physicists try to avoid the term "relativistic mass" (the bigger mass in frames other than the particle's own rest frame) but say that energy gets greater as the object moves. It is an equivalent statement to E = mc2, but it is less confusing.

-SHC

To explain a little further, the "m" that increases with speed is the inertial mass, the m that appears in the equation p=mv, where p is momentum and v is velocity. Since that m obviously depends on the choice of reference frame, because speed depends on frame, we like to use the invariant rest mass to describe the mass of a particle. / Mike W.

(published on 01/19/2016)