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Hello, Mary. The question you’ve asked seems straightforward: figure out when Einstein introduced the equation E=mc2 and to compare it to when he introduced relativity. I thought it would be easy to answer. Of course if it were that easy, you’d have figured it out by yourself! I also thought it was a fun question. After all I’m supposed to be a physicist, but at no time have I ever tracked down Einstein’s original work. As I quickly found out, it is difficult to find the original papers on relativity. There are many, many sites that explain Einstein’s ideas, but almost nobody actually ties their explanations to the original work. In fact, I was in the final stages of typing up this answer before I stumbled across full translations of the original relativity papers, made electronic by John Walker at www.fourmilab.ch. Enough of the preamble: on to answers! Q: If E=mc2 wasn't in the paper on relativity, does that mean it is not part of relativity? A: It is true that E=mc2 was not in “the” paper on relativity. Nevertheless, E=mc2 is definitely a part of relativity. Like most scientific theories, relativity is not confined to just one paper. In fact, much of what was in “the” paper had been developed by others in the field. As a groundbreaking idea, it took a lot of time and effort among many scientists before everyone agreed on exactly what the consequences of the theory were. “The” paper on relativity, the one usually associated with the introduction of special relativity, was Einstein’s “Zur Elektrodynamik bewegter Körper,” published in the Annalen der Physik, pp. 891-921, received in June, 1905. The title of the paper is frequently translated as, “On the electrodynamics of moving bodies”. You can view the paper as it was originally published if you wish – it’s located at: http://www3.interscience.wiley.com/cgi- bin/jabout/5006612/historic.html Even without knowing German one can see that there isn’t an equation that spells out E=mc2, though relativistic energy is introduced on p. 920 during what appears to be a discussion of the relativistic energy of electrons. You can also view a modern translation of the paper at: http://www.four milab.ch/etexts/einstein/specrel/www/ Q: ... did Einstein publish more than one paper on relativity that same year, and is this paper on Inertia part of Special Relativity? In September of 1905, Einstein submitted another, very short paper on relativity, “Ist die Trägheit eines Körpers von seinem Energiegehalt abhängig?” in the Annalen der Physik, vol. 18, p. 639, 1905. Its title is translated, “Does the Inertia of a Body Depend upon its Energy Content?” This contains the first mention of the equation E = mc2, but as you can see by reading the translation at http://www.fourmi lab.ch/etexts/einstein/E_mc2/www/ it’s not actually written that way. I encourage you to go to the link – at the very bottom of the web page is a boxed region describing the work, which contains the following sentence about the appearance of the relationship: 'In this paper Einstein uses L to denote energy; the italicised sentence in the conclusion may be written as the equation "m = L/c²" which, using the more modern E instead of L to denote energy, may be trivially rewritten as "E = mc²".' In other words, Einstein described his conclusion about energy and mass rather than presented it as the equation with which everyone is now familiar. Q: [natural followup question ...] So if it didn’t appear in either of the 1905 papers, when DID it first appear?? A: As for when the equation first appeared, I can only repeat what is said in the relativity FAQ at: http://www2.corepow er.com:8080/~relfaq/mass.html in which they claim, “The first record of the relationship of mass and energy explicitly in the form E = mc2 was written by Einstein in a review of relativity in 1907.” This would almost certainly refer to: “On the Inertia of Energy Required by the Relativity Principle (Über die vom Relativitätsprinzip geforderte Trägheit der Energie),” Annalen der Physik 23 (1907) 238 - Guy PS For a citation list of many of the original Einstein publications (and more) you can view: http://pup.princeton.edu/TO Cs/c4453.html

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