= 0), we see that the energy,
of a, body at rest is equal to its mass. Had we chosen the second as our unit of time, we would have obtained
Mass and energy are therefore essentially alike; they are only different expressions for the same thing. The mass of a body is not a constant; it varies with changes in its energy.[11] We see from the last of equations (43) that
becomes infinite when
approaches 1, the velocity of light. If we develop