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Newsletter: E=mc2 may well be the most famous of all equations. I think virtually everyone has seen it at least once. After all it’s everywhere — from coffee mugs to t-shirts. You might even have read it in a physics book. Yet, few might be comfortable explaining E=mc2 to their kids or grandkids. That’s a shame — do you really want to tell those inquiring young minds that you don’t understand something that they see so often? I believe everyone actually does understand the underlying principle of E=mc2, but in a quite different context: money. Not all of us are keen on numbers and equations, but a $ gets everyone’s attention. What Einstein said about mass and energy is the same as what we all know about money: there are many forms of money, and we can convert one form into another. We know dollars, euros, pesos, and yen are all forms of money. And, if we went to Japan, we could easily convert dollars into yen according to a simple rule: the number of dollars we wish to convert times an exchange rate equals the number of yen we would get, as illustrated in the upper half of this image. 
Similarly, physicists know there are many forms of energy, including light, heat, work, kinetic, and several types of potential energy. In 1905, Einstein made the startling claim that mass is another form of energy, a most tangible form. Mass can be converted into other forms of energy, just as $ can be converted into ¥. The amount of mass being converted times an exchange rate equals the amount of energy in another form, such as heat, as illustrated in the lower half of this image. It’s the same rule as converting money. The exchange rate for converting mass, Einstein said, is c2 — the square of c, which is the speed of light, the fastest speed at which any thing can move through space. In American units, c is a very large number, 671 million miles per hour, so c2 is a very, very large number. Einstein made the profound statement that even a small amount of mass can be converted into a fabulous amount of energy. Just how fabulous? The mass-energy in a single U.S. penny equals the amount of energy we now get by burning 2 million gallons of gasoline. A penny for 2 million gallons — perhaps we’re all paying too much for gas. Just for fun, I happen to have a picture to share with you of the moment Einstein discovered the proper exchange rate. 
He first tried E=ma2, but that was too small. He next tried E=mb2, but that was too big. When he got to E=mc2, he realized that was just right. Oh come on — you got to laugh! Physics jokes don’t get any better than that. 
Best Regards, Robert
Feb 2021
Note: Previous newsletters can be found on my website. | 
