During much of the 20th century it was widely believed that one of the significant insights of special relativity was “relativistic mass.” Today there are two schools on that issue: the traditional view that embraces speed-dependent “relativistic mass,” and the more modern position that rejects it, maintaining that there is only one mass and it's speed-independent. This paper explores the history of “relativistic mass,” emphasizing Einstein's public role and private thoughts. We show how the concept of speed-dependent mass mistakenly evolved out of a tangle of ideas despite Einstein's prescient reluctance. Along the way there will be previously unrevealed surprises (e.g., Einstein never derived the expression for “relativistic mass,” and privately disapproved of it).

## REFERENCES

*Physics with Modern Applications*(W.B. Saunders Co., Philadelphia, 1978), p. 456. Similarly, Marshall Burns,

*Modern Physics for Science and Engineering*(Harcourt Brace Javanovich Publishers, New York, 1988), p. 103 calls Eq. (1) “Einstein's mass equation.”

*Introduction to Atomic and Nuclear Physics*(Rinehart & Co., New York, 1958), p. 46.

*Einstein*(Simon & Schuster, New York, 2007), p. 130. Ronald Clark,

*Einstein: The Life and Times*(The World Pub. Co., New York, 1971), p. 100.

*Concepts of Mass in Contemporary Physics and Philosophy*(Princeton University Press, Princeton, 2000), p. 42 states that Einstein's first paper “introduced the notion of relativistic mass,” which Jammer earlier defined as $mr\u2009=\u2009m0(1\u2009\u2212\u2009v2/c2)\u22121/2$; it certainly did not do that. This is misleading even though Jammer goes on to point out that the expression is “not in its later accepted form.”

*The World of the Atom*(Basic Books, New York, 1966), Vol. I, p. 502.

*Concepts of Mass in Classical and Modern Physics*(Dover Publications, New York, 1997), p. 136.

*A History of the Theories of Aether & Electricity*Vol. 2 (Dover Publications, New York, 1989), p. 53, footnote 1. Lorentz's “transverse mass” is expressible as $m0(1\u2009\u2212\u2009v2/c2)\u22121/2$, but that's a far cry from being a statement of the direction-independent relativistic mass. That these two different ideas have the same mathematical form is no coincidence; the former helped convince people to accept the latter.

*Einstein's Theory of Relativity*(Dover Publications, New York, 1962).

*Theory of Relativity*(Pergamon Press, New York, 1958).

*The Evolution of Physics*by A. Einstein and L. Infeld (Simon and Schuster, New York, 1938), p. 205. This popularization was undertaken to make some money for Infeld, who was one of Einstein's near-destitute European protégés working at Princeton. The book was conceived by Infeld, who knew that it would surely succeed if Einstein's name was on it. It was written (in English) exclusively by Infeld, though he did discuss it periodically with Einstein (in German). Infeld, like most physicists of the day, seems to have embraced “relativistic mass“; nonetheless, that term is never used in the book. The exposition very cautiously tiptoes around the idea of inertia as if Einstein was watching over Infeld's shoulder as he wrote it. According to Infeld's autobiography, Einstein probably never even read the book.

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