Beautiful Science: ‘Asymptotic Freedom’ (Physics)

The electrical force pushing positively-charged protons apart and the strong force acting on both protons and neutrons inside of an atom’s nucleus.

Some of the greatest discoveries in science, like those that lead to Nobel Prizes, appear so alien and daunting to non-specialists that many people feel they would never be able to grasp even the gist of it. Which is a pity because these discoveries are not only hugely inspiring, they are fundamental to our existence.

Fortunately, the core ideas behind most discoveries are quite simple and intuitive (leaving aside the technical bits of course!). I wlll use just one example – from theoretical physics – to demonstrate my claim. It is the work that led to the award of the 2004 Nobel Prize for Physics to three theoretical physicists: David Gross, Frank Wilczek, and David Politzer, for the discovery that explained how quarks, the elementary constituents of the atomic nucleus, are bound together to form protons and neutrons.

From left: David Politzer, David Gross, and Frank Wilczek.

What is the big deal about quarks? Let’s just say quarks are the basic building blocks of everything observable in the universe – and that includes every flower, every leaf, every insect, and you and me. Without quarks, I wouldn’t exist to write this blog, and you wouldn’t exist to read it. Quarks are that fundamental. 

In 1973, Gross and Wilczek, working at Princeton, and Politzer, working independently at Harvard, showed that the attraction between quarks grows weaker as the quarks approach one another more closely, and correspondingly that the attraction grows stronger as the quarks are separated. This discovery, known as “asymptotic freedom,” established quantum chromodynamics (QCD) as the correct theory of the strong nuclear force, one of the four fundamental forces in Nature.

At the subatomic level, the nucleus of every atom is held together (i.e., preventing from falling apart) by what is known as the strong nuclear force, or simply the strong force. Of the four fundamental forces of nature – the others besides the strong force are electromagnetism, the weak nuclear force (responsible for the decay of radioactive nuclei and the sun’s energy), and gravitation — the strong force was by far the most poorly understood in the early 1970s.

It had been suggested in 1964 by Caltech physicist Murray Gell-Mann that protons and neutrons contain more elementary objects, which he called quarks. Yet isolated quarks are never seen, indicating that the quarks are permanently bound together by powerful nuclear forces. Meanwhile, studies of high energy collisions between electrons and protons performed at the Stanford Linear Accelerator Center (SLAC) had probed the internal structure of the proton, and Caltech’s Richard Feynman had suggested in 1969 that the results of these experiments could be explained if quarks inside a proton are nearly free, not subject to any force.

The weak force for example govern the interactions between subatomic particles and plays a crucial role in powering stars and creating elements. It is also responsible for much of the natural radiation present in the universe. Feynman’s suggestion, together with the observation that quarks are unable to escape from nuclear particles, posed a deep puzzle: how could nuclear forces be both strong enough to account for the permanent confinement of quarks and weak enough to account for the SLAC experiments? Gross, Politzer and Wilczek solved the puzzle by deriving a mathematical theory of the fundamental behavior of quarks. They propose that when quarks are bound closely together, the forces they attract them are strong (already this hints at the origin of the strong force). But when quarks are far apart from each other, they are “asymptotically free”, meaning their float around freely because the forces that hold them are now very weak. This is why the SLAC experiment, by separating the quarks effectively set them loose, consistent with Feynman’s conjecture.

So there you have it: in QCD, quarks are held together are strong but when separated, as in the SLAC bombardment, the attraction grows weak. The discovery of asymptotic freedom provided a highly satisfying and beautiful resolution of the puzzle posed earlier. It is interesting to note that all this work was done in the early early 1970s with pencil and paper – in other words, all in the mind! Recalling those grueling days, Gross, Wilczek, and Politzer later revealed that the methods they needed were newly developed and fraught with subtleties. Today, the calculation is routinely assigned to physics graduate students as a homework exercise, another “proof” that asymptotic freedom (and many other “deep ideas” in science) are not as alien and intractable as they seem.

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