Sunday, 11 March 2012
The Einstein Misconception
As I originally stated it, the Mythconception concerns “the idea that Einstein’s 1905 theory of relativity, with its famous equation E = mc2, laid the theoretical groundwork for the atomic bomb -- which, the myth claims, works by converting small quantities of matter (atoms) into large quantities of energy. This view is repeated ad nauseam by the media, but it’s wrong. There is no annihilation of matter in a nuclear weapon, only the release of nuclear binding energy.”
To be even more pedantic, E = mc2 doesn’t describe the conversion of one thing into another, but the absolute equivalence of energy and mass... with c2 as the constant of proportionality. A cannonball may have a mass of 10 kg when it’s sitting in the cannon, but when it’s flying through the air it has a very slightly greater mass due to its additional kinetic energy. This extra energy/mass came from the chemical binding energy released by the exploding gunpowder. If you could collect all the combustion products together again, their combined mass would be just slightly less than that of the original gunpowder.
So even a mediaeval cannon obeys E = mc2 -- you don’t need an atom bomb! You might argue that a nuclear weapon is qualitatively different from a cannon, but it isn’t really. There’s a quantitative difference, in that the “slight” changes in mass in the case of a cannon are so slight as to be immeasurable, whereas they’re appreciably bigger in the nuclear case. But they’re still very small. The difference is that the energy released in this case is nuclear binding energy, not chemical binding energy.
The basic nuclear reaction involves the fission of an unstable heavy nucleus (such as Uranium 235) into lighter nuclei. This is commonly described as “splitting the atom”, which is accurate, and “annihilation of the atom”, which is journalistic bollocks. Nothing is annihilated. The fission products have just as many nucleons (protons and neutrons) as there were to start with. But because they’re stable, they have less binding energy -- hence less total mass. That missing energy/mass went into the X megatons of explosive energy.
The archetypal Cold War weapon was a “thermonuclear” device that produced additional energy from the fusion of heavy hydrogen into helium. Again, the number of nucleons remains the same, and the energy released comes from binding energy. If people want to say that matter is “destroyed” in a fission reaction, then they might as well say it is “created” in a fusion reaction!
I know I’m being pedantic -- it all comes down to what you mean by “matter”. I’m using the term to mean “nucleons”... and what I’m saying is that nucleons are neither created or destroyed in a nuclear reaction. But if by “matter” you mean the rest-mass of a macroscopic chunk of matter, then a measurable part of that comes from the nuclear binding energy, some of which is lost (in both fission and fusion reactions). But—coming back to Einstein—that’s a consequence of quantum theory, not the theory of relativity! E = mc2 merely describes one effect of the nuclear reaction, not its ultimate cause.
When you get down below the scale of nucleons, things get even more blurred. Protons and neutrons are made up of three quarks each, but if you could separate the quarks out they’d hardly have any mass at all -- most of the mass you’re aware of in the real world is the binding energy holding the quarks together!