Text: . . . suppose we have an object whose mass M is measured, and suppose something happens so that it flies into two equal pieces moving with speed w, so that they each have a mass mw. Now suppose that these pieces encounter enough material to slow them up until they stop; then they will have mass m0. How much energy will they have given to the material when they have stopped? Each will give an amount (mw-m0)c2 . . . This much energy is left in the material in some form, as heat, potential energy, or whatever. Now 2mw = M, so the liberated energy is E = (M-2m0)c2. This equation was used to estimate how much energy would be liberated under fission in the atomic bomb, for example. (Although the fragments are not exactly equal, they are nearly equal.) The mass of the uranium atom was known--it had been measured ahead of time--and the atoms into which it split, iodine, xenon, and so on, all were of known mass. By masses, we do not mean the masses while the atoms are moving, we mean the masses when the atoms are at rest. In other words, both M and m0 are known. So by subtracting the two numbers one can calculate how much energy will be released if M can be made to split in "half". For this reason poor old Einstein was called the "father" of the atomic bomb in all the newspapers. Of course, all that meant was that he could tell us ahead of time how much energy would be released if we told him what process would occur. The energy that should be liberated when an atom of uranium undergoes fission was estimated about six months before the first direct test, and as soon as the energy was in fact liberated, someone measured it directly (and if Einstein's formula had not worked, they would have measured it anyway), and the moment they measured it they no longer needed the formula. Of course, we should not belittle Einstein, but rather should criticize the newspapers and many popular descriptions of what causes what in the history of physics and technology. The problem of how to get the thing to occur in an effective and rapid manner is a completely different matter. ‹ Richard Feynman, Lectures on Terrorism, Volume 1

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