Knocking on Heaven's Door - Lisa Randall [127]
Gell-Mann’s conjecture about the strong nuclear force was based on a brilliant insight about how the many particles that were constantly being discovered in the 1960s could be organized into sensible patterns that would explain their abundance and types. He hypothesized the existence of more basic elementary particles known as quarks, which he suggested carry a new type of charge. The strong nuclear force would then influence any object that carried the conjectured charge, and cause quarks to bind together to form neutral objects—much as the electric force binds electrons with charged nuclei to form neutral atoms. If true, all the particles being discovered could be interpreted as bound states of these quarks—aggregate objects that have no net charge.
Gell-Mann realized that if there were three different types of quarks, each of which carried a distinct color charge, many such combinations of neutral bound states would form. And these many combinations could (and did) correspond to the plethora of particles that were being found. Gell-Mann thereby had found a beautiful explanation for what seemed like an inexplicable mess of particles.
However, when Murray—as well as the physicist (and later neurobiologist) George Zweig—first proposed this idea, people didn’t even believe it was a proper scientific theory. The reason is somewhat technical but interesting. Particle physics calculations rely on particles not interacting when they are far apart, so that we can compute the finite effects of the interactions that occur when they are close together. With this assumption, any interaction can be entirely captured by the local forces that apply when the interacting particles are in close proximity.
The force that Gell-Mann had conjectured, on the other hand, was stronger when particles were farther apart. That meant that quarks would always interact, even when very distant. According to the then-reigning criteria, Gell-Mann’s guess didn’t even correspond to a true theory that could be used for reliable calculations. Because quarks always interact, even their so-called asymptotic states—the states involving quarks that are far away from everything else—are very complicated. In an apparent concession to ugliness, the asymptotic states they postulated weren’t the simple particles you’d like to see in a calculable theory.
Initially, no one knew how to organize calculations among these complicated strongly bound states. However, today’s physicists think quite oppositely about the strong force. We now understand it much better than we did when the idea was first proposed. David Gross, David Politzer, and Frank Wilczek won the Nobel Prize for what they called “asymptotic freedom.” According to their calculations, the force is strong only at low energies. At high energies, the strong force is not much more powerful than other forces, and calculations work just as they should. In fact, some physicists today think theories such as the strong force, which become much weaker at high energies, are the only well-defined theories, since the interaction strength won’t grow to infinite strength at high energy as it might otherwise do.
Gell-Mann’s theory of the strong force is an interesting example of the interplay between aesthetic and scientific criteria. Simplicity was his initial guide. But hard scientific calculations and theoretical insights were necessary before everyone could agree on the beauty of his suggestion.
This, of course, isn’t the only example. Many of our most trusted theories have aspects so superficially ugly and uncompelling that even respected and well-established scientists rejected them initially. Quantum field theory, which combines quantum mechanics and special relativity, underlies all of particle physics. Yet the Nobel Prize—winning Italian scientist Enrico Fermi (among others) rejected it at first. For him, the problem was that although quantum field theory formalizes and systematizes all calculations and makes many correct predictions, it involves calculaional techniques that