Knocking on Heaven's Door - Lisa Randall [45]
“Instead of a rather large number of ‘indivisible’ atoms of classical physics, we are left with only three essentially different entities; protons, electrons, and neutrons… Thus it seems we have actually hit the bottom in our search for the basic elements of which matter is formed.” 28
That was a little shortsighted. More precisely, it was not shortsighted enough. There does exist further substructure—more elementary components to the proton and neutron—but the more fundamental elements were challenging to find. One had to be able to study length scales smaller than the size of the proton and neutron, which required higher energies or smaller probes than existed when Gamow made his inaccurate prediction.
If we were to now enter inside the nucleus to see nucleons and protons with size about a fermi—about ten times smaller than the nucleus itself—we would encounter objects Murray Gell-Mann and George Zweig suspected existed inside nucleons. Gell-Mann creatively named these units of substructure quarks, in his telling inspired by a line from James Joyce’s Finnegans Wake (“three quarks for Muster Mark”). The up and down quarks inside a nucleon are the more fundamental objects of smaller size (the two up and one down quarks inside are shown in Figure 16) that a force called the strong nuclear force binds together to form protons and neutrons. Despite its generic name, the strong force is a specific force of nature—one that complements the other known forces of electromagnetism, gravity, and the weak nuclear force that we’ll discuss later.
The strong force is called the strong force because it is strong—that’s an actual quote from a fellow physicist. Even though it sounds pretty silly, it’s in fact true. That’s why quarks are always found bound together into objects such as protons and neutrons for which the direct influence of the strong nuclear force cancels. The force is so strong that in the absence of other influences the strongly interacting components won’t ever be found far apart.
[ FIGURE 16 ] The charge of a proton is carried by three valence quarks—two up quarks and a down quark.
One can never isolate a single quark. It’s as if all quarks carry a sort of glue that becomes sticky at long distances (the particles that communicate the strong force are for this reason known as gluons). You might think of an elastic band whose restoring force comes into play only when you stretch it. Inside a proton or neutron, quarks are free to move around. But trying to remove one of the quarks any significant distance away would require additional energy.
Though this description is entirely correct and fair, one should be careful in its interpretation. One can’t help but think of quarks as all bound together in a sack with some tangible barrier from which they cannot escape. In fact, one model of nuclear systems essentially treats the protons and neutrons in precisely this way. But that model, unlike others we will later encounter, is not a hypothesis for what is really going on. Its purpose was solely to make calculations in a range of distances and energies where forces were so strong our familiar methods don’t apply.
Protons and neutrons are not sausages. There is no synthetic caing that surrounds the quarks in a proton. Protons are stable collections of three quarks held together through the strong force. Because of the strong interactions, three light quarks concertedly act as one single object, either a neutron or proton.
Another significant consequence of the strong force—and quantum mechanics—is the ready creation of additional virtual particles inside a proton or neutron—particles permitted by quantum mechanics that don’t last forever but at any given time contribute energy. The mass—and hence, a la Einstein’s E = mc2, the energy—in a proton or neutron is not carried just by the quarks themselves but also by the bonds that tie them together. The