Blog

Category
  • Asymptotic Freedom: David Gross

    The STARMUS Festival in the Canary Islands claims to make “the most universal science and art accessible to the public.” The speakers are some of the best in their field and include physicists, astrophysicists, chemists, biochemists, biologists, neuroscientists, and economists. Many of them are nobel laureates. Although the festival claims to be aimed at the “public”, I suspect that many of those brainy nobel laureates don’t have a good idea of where the intellect of the pubic lays. That’s why I am looking at the conference program now to investigate some of the conference speakers’ areas of expertise. I hope that by knowing a bit more about some of the speakers’ interests, I’ll get more out of the conference.

    On the first day David Gross, who won the Nobel Prize in Physics in 2004, will discuss the great challenges faced by physics. One of his challenges was explaining the behavior of quarks by introducing the property of asymptotic freedom, an explanation for which he (and two others) received the Nobel prize. 

    What exactly is asymptotic freedom, or AF as I’ll call it? It doesn’t sound too daunting. I know what freedom is—being free to do or think what you want without being constrained. I know what an asymptote is—a line that approaches a curve but does not meet is. How does that relate to physics? Why would someone get a nobel prize for that?

    To understand AF, you need to have a basic understanding of atoms, those tiny things that make up matter. Atoms in turn are made up of subatomic particles—protons, neutrons, and electrons. While most of us are worried about how to keep our lives togethers, people like David Gross are concerned with how an atom keeps itself together. Physicists know there is a strong nuclear force that holds protons and neutrons in place in an atom, but they wanted to know more about that force, as it is one of the four fundamental forces in the universe. (The other forces are electromagnetic, weak, and gravitational.) 

    How strong is the strong force? Over very tiny distances—atomic nucleus sized—the strong force is 100 times stronger than the electromagnetic force that repels positively charged protons. That’s why an atomic nucleus stays together under normal circumstances.

    Both protons and neutrons are themselves made up of quarks—precisely three quarks. These days, quarks are assigned “colors”, either red, green or blue. This might see like an homage to the pixel, but using color as a metaphor in physics helps to explain a lot of subatomic particle interactions that I’m not going to explain in this discussion. (I’ll also ignore quark “flavors.”) Suffice it to say that subatomic particle interactions have to result in white. Red, green, and blue combine to white.  (Image from Wiki Commons.)

    So far you know that the quarks are held in place by a strong force. The theory behind this strong force is named quantum chromodynamics (QCD) because of the arbitrary use of color. That finally brings us to AF.

    AF is important because it explains some baffling behavior of quarks. You can’t see quarks, which indicates they are trapped in matter by the strong force. If they weren’t confined, you’d be able to see them, right?  Yet a big smash up at the linear accelerator down the road from me—Stanford Linear Accelerator (SLAC)—showed that in a high-energy reaction  the force between the quarks weakens and the distance between them decreases asymptotically. That is, they get closer and closer, but don’t run into each other.

    What  it boils down to is that quarks have two phases—confinement and AF. Much like water and steam, the phases depend on temperature. In the case of quarks, temperature (which is really energy) is measured in Mega electron Volts, or MeV. The phase change occurs at 160 MeV. Quarks are mostly confined below that energy level, and mostly have asymptotic freedom above that level.

    So what’s the lesson for the lay person? Although your personal life may seem to be falling apart, take comfort in the fact that the atoms around you are quite stable. You might need to expend a lot of energy to keep things together, but atoms are just the opposite.  

    AF isn’t all that Dr. Gross is known for. He is one of the signers of the Humanist Manifesto.