The Meaning of Relativity / Four lectures delivered at Princeton University, May, 1921

by Albert Einstein

In this,

is the density at rest, that is, the density of the ponderable matter, in the ordinary sense, measured with the aid of a unit measuring rod, and referred to a Galilean system of co-ordinates moving with the matter.

We observe, further, that in the co-ordinates we have chosen, we shall make only a relatively small error if we replace the

by

, so that we put