"Whittaker's credibility== 
 To Alvestrand. You probably know from the Poincare page that I think Whittaker is a tad one-sided on the E = mc^2 issue. On p.51 Whittaker writes  
 :::""In 1900, referring top the fact that in the free aether the electromagnetic momentum is 1/c^2 times the Poynting flux of energy, suggested that electromagnetic energy might possess density equal to 1/c^2 times the energy density: that is to say E = mc^2."" 
 Whittaker did not mention that Poincare did not believe this result and advanced it as a criticism of Lorentz's theory. He did not mention that Poincare called it a ""fictitious"" density and that Poincare had no idea where the mass came from; it was just created and if it was real mass this creation would violate the conservation of mass principle. Nor did Whittaker mention that Poincare retained this view until 1904 at least, since he discussed this problem a number of times in his popular science books from 1902-1904. (I am not knocking Poincare, just pointing out that Whittaker did not mention all this). On the next page Whittaker jumps to a paper by Lewis (1908) where he says that Lewis ""affirmed"" that a body absorbing energy dE increases its mass by dE/c^2, and  
 :""affirmed that the mass of a body is a direct measure of its total energy, according to the equation E = mc^2. As we have seen, Poincare had suggested this equation but had given practically no proof, while Einstein, who had also suggested it had given a proof (which however, was put forward only as approximate) [actually it was put forward as exact, Planck thought it was approximate and re-derived in 1907 - e4] for a particular case [if Einstein's case is particular, then Poincare's case is even more so - e4] (Whittaker 1953, p 52, comments in [] by me)"" 
 You will notice that Whittaker does not mention that Lewis was affirming Einstein’s (1905) and Planck’s (1907) result and suggests he was affirming Poincare’s result. Apart from the downplaying of Einstein's efforts, Whittaker is wrong to say that Poincare had suggested this equation in the sense that (Einstein and Planck and) Lewis had meant. Poincare (1900) had meant nothing of the kind, and had no idea of it in his book of 1904 (This result would have solved the whole problem of mass violation, energy violation, momentum violation  that Poincare couldn’t solve and which he still in 1904 thought cast doubt on Lorentz’s theory). Nor did Poincare mention E = mc^2 it in his 1905 paper or his 1906 paper (I may have missed it in the 1906 paper, can anybody else find it?). It seems very difficult to imagine that Whittaker couldn't see the difference between Poincare's ""ficititious fluid of radiation"" and the different idea that a body had less mass when it cooled down (lost heat by any means, so I conclude he sees what he wants to see.   

 Whittaker has little credibility on this issue, and one day it might be worth listing all the mistakes of facts, as well as the correct things, in his chapter on ""The relativity theory of Poincare and Lorentz"". It will be worth only when those who use Whittaker as an infallible published source are taken too seriously.    

 To forestall any possible flaming by anon69,I will mention that Ives J Opt Soc Am. V42, p540 1952 claimed Einstein’s “proof” was a tautology, J. Riesman and I. G. Young, JOSA, v43(7) July 1953, disputed Ives’ assertion, defended Einstein’s proof and his physical insight, Ives. H. E. JOSA, v43(7) July 1953 reasserted his claim that Einstein’s assumption was not a valid physical consideration.   

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