Catching up with my blog reading. Via Physics and Physicists, an ArXiv paper by L. B. Okun, The Einstein Formula: E₀=mc^2 “Isn’t the Lord Laughing?” detailing some history of “relativistic mass” and the confusion surrounding the term.

The article traces the way Einstein formulated the relation between energy and mass in his work from 1905 to 1955. Einstein emphasized quite often that the mass m of a body is equivalent to its rest energy E₀. At the same time he frequently resorted to the less clear-cut statement of equivalence of energy and mass. As a result, Einstein’s formula E₀ = mc² still remains much less known than its popular form, E = mc², in which E is the total energy equal to the sum of the rest energy and the kinetic energy of a freely moving body. One of the consequences of this is the widespread fallacy that the mass of a body increases when its velocity increases and even that this is an experimental fact.

[…]

Why is it that the weed of velocity-dependent mass is so resistant? First and foremost, because it does not lead to immediate mistakes as far as arithmetic or algebra are concerned. One can introduce additional ‘quasi-physical variables’ into any selfconsistent theory by multiplying true physical quantities by arbitrary powers of the speed of light. The most striking example of such a ‘quasi-quantity’ is the so-called ‘relativistic mass.’ If calculations are done carefully enough, their results should be the same as in the original theory. In a higher sense, however, after the introduction of such ‘quasi-quantities,’ the theory is mutilated because its symmetry properties are violated. (For example, the relativistic mass is only one component of a 4-vector, while the other three components are not even mentioned.)