David Gross’ Nobel Prize-winning theory may not have everyday applications, but it continues to provide inspiration to physicists.
Gross performed extensive work in developing the nuts and bolts of Quantum Chromodynamics (QCD), a theory that explains the “strong force” of nature. Prior to his contributions, physicists were not sure of the strong-force mechanism that keeps the quarks inside protons and neutrons stable. Since then, QCD has maintained an important role in modern physics and has been related to parts of string theory, a proposed “theory of everything.”
Frank Wilczek and David Politzer shared the Prize with Gross for forming a team in the seventies with the goal to explain the strong force.
“I was following the roads that lead to this theory for about five years,” Gross said Wednesday, a day after learning he was UCSB’s most recent Nobel Prize winner. “This was a total mystery and I tried all sorts of things that were unsuccessful. When Frank started to work with me, we tried the last, the only, possibility I hadn’t tried.”
A key discovery of QCD is asymptotic freedom, the tenet that states the attractive force between quarks will decrease with decreasing separation.
“Once we made that discovery, it was absolutely clear, almost overnight, what the theory was because there really is only one kind of theory that does this,” Gross said.
For a while, Gross was worried that even the one remaining explanation would not be sufficient to account for his experimental data.
“Originally, I had a feeling it wasn’t going to work in this theory either, and then we would be in a real crisis,” Gross said. “But since it actually did work, there was no question. There was really only one theory you could write down.”
As the pieces fell into place, the first guesses that Gross and Wilczek formulated turned out to make more and more sense. By investigating the strong force, the team was looking into forces that affect particles called hadrons. Protons and neutrons, which reside in all atomic nuclei, are hadrons and were theorized to contain smaller particles called quarks.
“Almost everything we knew about hadrons fit in [with the theory],” Gross said. “It was clear to me, and maybe even more so Frank, who was younger, that it had to be the right theory.”
Not everyone agreed so freely that quarks had a role in the strong force, or even that quarks existed. In the 70s, the idea of a quark did not have the wide base of support that it does today. One such critic was Steven Weinberg, who was awarded the Nobel Prize in 1979 for his theory that united the electromagnetic force and the weak force.
“I was working with a colleague on a test of whether there were quarks. The data seemed to show that they behaved like quarks, that the proton was made out of quarks,” Gross said. “[Weinberg] said ‘I don’t believe any of that. I think quarks are nonsense.'”
Weinberg was not the only skeptic, as QCD brought some rather difficult conclusions. One necessary conclusion of asymptotic freedom is that quarks can never be extracted from the larger particles that contain them. This is because the strong force that pulls the quarks together gets stronger as they are pulled apart. An infinite force would be needed to isolate them. Gross sympathized with those who had trouble with this idea.
“How can you possibly believe that the fundamental constituents of matter are things that you can never see?” Gross said. “How do you have building blocks when you don’t have the blocks?”
The ‘chromo’ in Chromodynamics resulted from an arbitrary color system.
“Almost every time you make progress in physics, in calculating, it’s because you have some small parameter,” Gross said. “But QCD is a theory with no small parameters. So what do you do? You invent a parameter. In QCD you have the quarks come in three, what are called, colors.”
Some mathematical work showed that the colors were a parameter worth exploring.
“It turns out that when the number of colors becomes big, solving the theory becomes much simpler and much more like a string theory,” Gross said. “In fact, string theory was originally an attempt to construct a theory of the strong force; it was not an attempt to construct a unifying theory.”
This relation between theories arises in part because of the principle of asymptotic freedom. The strong force that holds quarks together is like a rubber band because it takes more force to stretch the farther the band is pulled.
“A rubber band is like a string,” Gross said. “And [the Kavli Institute for Theoretical Physics has] a program now called ‘QCD and string theory’.”
In addition to this relation with string theory, QCD has some very useful results of its own. Some of these results are printed in every high school physics textbook, namely, the mass and size of the proton.
“QCD is a remarkable theorem; it’s a theory with no scale,” Gross said. “So if you ask ‘what is the size of the proton in terms of the parameters of the theory?’ Well, the theory doesn’t have any parameters.”
Gross explained that the correct value for the size of the proton could be obtained by examining the mathematical function that governs the attractive force between quarks. When two quarks are on top of each other, the force is theoretically zero. When the quarks infinitely far apart, the force is infinite.
“Somewhere in between [zero and infinity], there’s a factor of one. That somewhere is the size and that is the size of the proton.”