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

by Albert Einstein

General Statements about the Lorentz Transformation and its Theory of Invariants. The whole theory of invariants of the special theory of relativity depends upon the invariant

(23). Formally, it has the same rôle in the four-dimensional space-time continuum as the invariant

+

+

in the Euclidean geometry and in the pre-relativity physics. The latter quantity is not an invariant with respect to all the Lorentz transformations; the quantity