Re: is 'E=mc2' part of relativity, or some other work by Einstein?

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