E = Mc^2 Wasn't There

It is of no real importance whatsoever, but amusing to note that the most famous equation of all time, E = Mc^2, did not actually appear in this form in Einstein's 1905 paper.

In his annus mirabilis Einstein published not three papers, as is sometimes suggested, but four. One paper concerned the quantum theory of light and one was on Brownian motion, but there were actually two papers on the theory of relativity. These two relativity papers appeared in the same volume of Annalen der Physik and therefore tend to be lumped together as a single entity, which indeed they are in logical terms. The second of the two papers was entitled (in translation) Does the Inertia Content of a Body Depend Upon its Energy Content?. It is less than three A5 pages long.

I must emphasise that the essential message that E = Mc^2 comes over loud and clear from the paper, which is a fine example of the simplicity and clarity of Einstein's reasoning. However, the equation in exactly that form does not appear. The nearest approach to it is given by the quote below

From this equation it directly follows that:-

If a body gives off the energy L in the form of radiation, its mass diminishes by L/c^2. The fact that the energy withdrawn from the body becomes energy of radiation evidently makes no difference, so that we are led to the more general conclusion that

The mass of a body is a measure of its energy-content; if the energy changes by L, the mass changes in the same sense by L/9 x 10^20, the energy being measured in ergs, and the mass in grammes.

A footnote in the 1920 translation (available below) explains the change in notation compared to the 1905 original, including the introduction of the symbol 'c' for the speed of light, which Einstein did not initially use.

I once had a paper rejected on the grounds that it did not deserve scientific attention since it had so few references. It is amusing to note that neither of the two 1905 papers on relativity had any references whatsover. The main paper on special relativity does not even reference Michelson & Morley, despite knowledge of the null result of this famous experiment being implicit in the argument. This would never get past a referee today.

Read Einstein's original 1905 paper: E=Mc^2

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