Out of curiosity, I thought I'd have a look at the translation of Einstein's famous 5th paper of 1905, just to see if he ever really wrote "E=mc^{2}". Naturally, he didn't.

In that paper ("Does the Inertia of a Body Depend on It's Energy Content"), Einstein considered how two plane waves of light, each with energy L/2, emitted from a source sitting still, would look to a moving observer. The difference in energy between the two systems could only come from the "kinetic" energy of the body seen relative to the moving system. And when he worked out the math (in the Newtonian limit) he got this equation:

K_{0}-K_{1} = (L/V^{2})(v^{2}/2)

If you remember from freshman physics that kinetic energy is (mv^{2})/2, then you see that (L/V^{2}) is the mass which characterizes the loss of kinetic energy of the system. Thus, the energy radiated away actually changes the mass of the radiating system. Everything snaps into place when I tell you that he called the speed of light V, and not c, back in those days. That's the original "E=mc^{2}"!

So there's history in the making, but it never looks like you expect it to.

[Slight update: The reason why I went through this exercise was to do a little fact checking on the upcoming PBS show. They have the young Einstein apparently finishing a proof on his notepad, carefully writing "E=mc^{2}" on the last line. I hope I've shown that this didn't happen exactly like that, but to their credit they have him writing it below precisely the equation I feature above, so kudos to the PBS team for precision!]

I heard another take on that although I like yours as well. According to Joe Lykken, at a lecture he gave at Fermilab concerning the possibility of extra dimensions, the equation originally showed momentum in each of the dimensions being related to time (mass) with C^2 merely being a conversion factor between time and space.

Another old timer from my astronomy club told me after the lecture that he condensed it to convince politicians the need to invest in particle physics.

e=mc^2 was used to point out that one raisin has enough energy to power up New York City for a day.

I don't know if all of the above holds water, but I thought I'd mention it.

Posted by: Bob | October 06, 2005 at 08:14 AM

I like hearing all the different views on the origins of e=mc^2.

The one I heard is a touch different as well. From a lecture on hidden dimensions I attended at Fermilab about 16 months ago, Joe Lykken, the speaker, left me with another perception.

He claimed that e=mc^2 was not the original equation but an equation showing momentum in 3 dimensions is related to time ( mass) with a conversion factor between time and space being = c^2.

"c^2" apparently was merely the ratio that resulted and had nothing to do with light speed.

According to some old timers in my astronomy club, he then condensed it down to e=mc^2 in order to demonstrate to politicians that a raisin has enough energy to power up all of New York City. Particle accelerators became something to invest in.

Anyway, that is the way I heard it. I do like yours as well. Who knows? Just attach a probability arrow to each version.

Posted by: Bob | October 06, 2005 at 02:06 PM