‘Das hätte ich einfacher sagen können (I could have said that more simply).’ was Einstein’s sigh
when he had a chance to remark on his own derivation of the Lorentz transformation (LT) in the 1905
seminal paper. In fact, in a popular science exposition of the theory of relativity Einstein did
provide such a simple derivation of the LT. It is a curious historical fact that the latter
derivation was presented in 1916, while Einstein’s remark was made in 1943. Was the 1916 derivation
simple enough to relieve his sigh? Had he expected an even simpler derivation beyond his thoughts?
In this paper, Einstein’s simple derivation of the LT is revisited and analysed. We show that the LT
can be obtained from a symmetry principle hidden in Einstein’s logical reasoning. First, the
relativity principle can be restated as a mirror principle based on the space–time
exchange-inversion operation. Second, the assumed constancy of the speed of light (Einstein’s second
postulate) can be derived by u…