Engineering students occasionally wonder: why not $12mc2$? While most have no need for Einstein’s special theory of relativity, this theory nevertheless offers them a shining example of the power of mathematical deduction and beautiful simplicity. At its 100th anniversary in 2005, I gave my students a one-page pencil note explaining this glorious gem of a theory. The note became popular, and here, upon invitation, I give a simplified derivation, trying to waste no word and achieve utmost brevity, a boost to clarity.

Everything follows from three hypotheses: (I) The speed of light, c, is constant regardless of the speed of moving observer (as revealed in 1887 by Michelson–Morley experiment at Case Western, Cleveland), (II) momentum p = mv is conserved (though not F = ma, as erroneously assumed before Einstein), and (III) energy $E=∫F⋅...$