1 Einstein and the Formal Equivalence of Mass and Energy

This is a brief look at how Einstein explored formal symmetries between quantized matter and quantized radiation between 1903 and 1925. Specifically he employed thermodynamic comparisons between the ideal molecular gas and the photon gas. His achievements are tied in with a more general pattern in physics to explore formal symmetries between quantized mass and quantized energy. There is a priceless cartoon of a grade school Einstein handing in an assignment with nothing on the paper but his name and E = mc2. The second frame of the cartoon shows the paper after the teacher has graded it and her note at the bottom says: “Good effort Albert. Next time show your work, C minus.” Of course the grown up Einstein did show his work in his 1905 paper1 but he made no comments there on how his equation was to be applied. Succeeding generations of physicists have interpreted it strictly in quantitative terms: a certain quantity of mass is convertible to a certain quantity of energy and vice versa. But Einstein himself found a deeper, more fundamental connection between mass and energy, something akin to a formal symmetry. His application of this was usually within th