leaves the relation x 2 + y 2 + z 2 − − t 2 invariant. Einstein stressed the point that by this transformation a spherical light wave in one frame is transformed into a spherical light wave in another one. Poincaré showed that the Lorentz transformation can be seen as a rotation in four-dimensional space with time as fourth coordinate, with Minkowski deepening this insight much further (see History of special relativity).