information:classification, stylistic:tone, structural:expansion
Einstein's 1905 derivation

Already in his relativity paper "On the electrodynamics of moving bodies", Einstein derived the correct expression for the kinetic energy of particles:

Now the question remained open as to which formulation applies to bodies at rest. This was tackled by Einstein in his paper "Does the inertia of a body depend upon its energy content?", one of his Annus Mirabilis papers. Here, Einstein used V to represent the speed of light in vacuum and L to represent the energy lost by a body in the form of radiation. Consequently, the equation E = mc was not originally written as a formula but as a sentence in German saying that "if a body gives off the energy L in the form of radiation, its mass diminishes by L / V." A remark placed above it informed that the equation was approximated by neglecting "magnitudes of fourth and higher orders" of a series expansion. Einstein used a body emitting two light pulses in opposite directions, having energies of E before and E after the emission as seen in its rest frame. As seen from a moving frame, E becomes H and E becomes H. Einstein obtained, in modern notation:

He then argued that H − E can only differ from the kinetic energy K by an additive constant, which gives

Neglecting effects higher than third order in v / c after a Taylor series expansion of the right side of this yields:

Einstein concluded that the emission reduces the body's mass by E / c, and that the mass of a body is a measure of its energy content.

The correctness of Einstein's 1905 derivation of E = mc was criticized by German theoretical physicist Max Planck in 1907, who argued that it is only valid to first approximation. Another criticism was formulated by American physicist Herbert Ives in 1952 and the Israeli physicist Max Jammer in 1961, asserting that Einstein's derivation is based on begging the question. Other scholars, such as American and Chilean philosophers John Stachel and Roberto Torretti, have argued that Ives' criticism was wrong, and that Einstein's derivation was correct. American physics writer Hans Ohanian, in 2008, agreed with Stachel/Torretti's criticism of Ives, though he argued that Einstein's derivation was wrong for other reasons.