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Leaping into the Grand Unknown

1.

Frank Wilczek is one of the most brilliant practitioners of particle physics. Particle physics is the science that tries to understand the smallest building blocks of earth and sky, just as biol-ogy tries to understand living creatures. Particle physics is running about two hundred years behind biology. In the eighteenth century, Carl Linnaeus started systematic biology by giving Latin names to species of plants and animals, Homo sapiens for humans and Pan troglodytes for chimpanzees. In the nineteenth century, Darwin created a unified theory for biology by explaining the origin of species. In the twentieth century, Ernest Rutherford laid the ground for particle physics by discovering that every atom has a nucleus that is vastly smaller than the atom itself, and that the nucleus is made of particles that are smaller still. In the twenty-first century, particle physicists are hoping for a new Darwin who will explain the origin of particles.

It is too soon to tell whether Wilczek will be the new Darwin. His book is not the new Origin of Species. It is more like Darwin’s Voyage of the Beagle, a popular account of a voyage of exploration, describing the landscape and the newly discovered creatures that still have to be explained. Wilczek is a theoretician and not an experimenter. His strength lies in leaps of the imagination rather than in heavy hardware or heavy calculations. He shared the 2004 Nobel Prize in physics for inventing the concept that he called “Asymptotic Freedom.”

He writes as he thinks, with a lightness of touch that can come only to one who is absolute master of his subject. He borrowed his title from Milan Kundera, the Czech writer whose novel The Unbearable Lightness of Being takes a gloomier view of lightness. For Wilczek, the lightness of being is not only bearable but exhilarating. He says:

There’s also a joke involved. A central theme of this book is that the ancient contrast between celestial light and earthy matter has been transcended. In modern physics, there’s only one thing, and it’s more like the traditional idea of light than the traditional idea of matter. Hence, The Lightness of Being.

Wilczek has undertaken a difficult task: to describe the central problems of particle physics to an audience ignorant of mathematics, using few equations and mostly colloquial language. His idiosyncratic jargon words, such as Core, Grid, and Jesuit Credo, are explained in an extensive glossary at the end of the book. The glossary is fun to read, full of jokes and surprises. The words Core, Grid, and Jesuit Credo are not to be found in other books about physics. They are jargon invented by Wilczek to express his personal view of the way nature works. Core is like the core curriculum which undergraduates majoring in physics are supposed to learn. It is a solidly established theory, confirmed by experiments but still obviously incomplete. It is incomplete because it describes what nature does but does not explain why. The glossary says, “The Core theory contains esthetic flaws, so we hope it is not Nature’s last word.”

Grid is Wilczek’s word for the stuff that exists in apparently empty space. According to his view of the universe, empty space is not a featureless void. It is a highly structured, powerful medium whose activity molds the world. He says, “Where our eyes see nothing, our brains, pondering the revelations of sharply tuned experiments, discover the Grid that powers physical reality.”

The Jesuit Credo refers not to a theory of the universe but to a way of approaching research: “It is more blessed to ask forgiveness than permission.” This is a rule propounded by the Jesuits for saints and sinners trying to find the right way to live. If you ask for permission, the authorities will probably say no. If you ask for forgiveness, they are more likely to say yes. Wilczek was brought up in a Catholic family with a proper respect for Jesuits. The Jesuit Credo is particularly helpful for a scientist trying to find the right way to think. It is more blessed for a scientist to make a leap in the dark, and afterward be proved wrong, than to stay timidly within the limits of the known.

The main part of Wilczek’s book, with the title “The Origin of Mass,” describes the Core theory, the part of particle physics that is firmly based on the weak and strong forces that we observe in nature. Atoms and nuclei are held together by forces acting between all the pairs of particles that they contain. Each force acts between two particles and its strength depends on the distance between the two particles. Weak forces hold atoms together and grow weaker at large distances. Strong forces hold nuclei together and grow stronger at large distances. Large distances mean distances larger than the nucleus of an atom, and small distances mean distances smaller than a nucleus. The doctrine of Asymptotic Freedom, which Wilczek discovered, says that the behavior of these forces at short distances is the opposite of their behavior at large distances. At large distances, the strong force is strong and the weak force is weak, but at short distances the opposite occurs: the weak force grows stronger and the strong force grows weaker.

He called this doctrine Asymptotic Freedom because it implies that at high energies the strongly interacting particles become almost free. Strongly interacting particles are called hadrons, from the Greek word hadros, meaning fat. The higher the energy of a collision, the shorter the distance between the colliding particles. In collisions between hadrons with very high energy, the strong forces paradoxically become weak and the probabilities of collisions become small.

Another consequence of Asymptotic Freedom is that we can calculate the masses of hadrons, starting from a knowledge of the strength of the strong force. Masses calculated in this way agree with the observed masses of known particles. This is what Wilczek means by “The Origin of Mass.” The masses of familiar objects like atoms arise from the peculiar symmetry of the strong forces. Modern theories of particle physics have the marvelous property, first pointed out by the Chinese-American physicist Frank Yang, that the strength of particle interactions is dictated by the symmetry of the theory. Since Wilczek finds the masses depending on the strength of forces, and Yang finds the strength of forces dictated by symmetry, the final result is to make mass a consequence of symmetry alone.

The last part of the book, with the title “Is Beauty Truth?,” is brief and speculative. It describes a Grand Unified Theory of particle physics going far beyond the Core, introducing a whole menagerie of hypothetical particles that are sisters to the known particles, and a symmetry principle known as Supersymmetry that interchanges each known particle with its sister. The word “interchange” here does not mean a physical replacement of one particle by another. It means the mathematical interchange of the entire assemblage of known particles with the assemblage of their hypothetical sisters. The hypothesis of Supersymmetry says that the equations describing the universe remain unchanged when all the known particles are interchanged with their unknown sisters. The interchange is a mathematical abstraction, not a physical action.

The Grand Unified Theory is a bold venture into the unknown. It is a mathematical construction of spectacular beauty, unsupported by any experimental evidence. All that we can say for sure is that this theory is possibly true and certainly testable. Wilczek believes that the basic laws of nature must be beautiful, and therefore a theory that is beautiful has a good chance of being true. He believes that the Grand Unified Theory is true because it is aesthetically pleasing. He points to several famous examples from the history of physics, when theories designed to be beautiful turned out to be true. The best-known examples are the Dirac wave-equation for the electron and the Einstein theory of General Relativity for gravity. If the Grand Unified Theory turns out to be true, it will be another example of beauty lighting the way to truth.

At the end of the book, a chapter entitled “Anticipating a New Golden Age” describes Wilczek’s hopes for the future of particle physics. He sees the Golden Age starting very soon. His hopes are based on the Large Hadron Collider (LHC), the biggest and newest particle accelerator, built by the European Center for Nuclear Research (CERN) in Geneva. The LHC is a splendid machine, accelerating two beams of particles in opposite directions around a circular vacuum pipe that has a circumference of twenty-seven kilometers. Particle detectors surround the beams where they collide, so that the products of the collisions are detected. The energy of each accelerated particle will be more than seven times the energy of a particle in any other accelerator. Wilczek confidently expects that supersymmetric sisters of known particles will be found among the debris from collisions in the LHC. By observing new particles and interactions in detail, he hopes to fill quickly the gaps in the Grand Unified Theory.

Incidentally, Wilczek expects the LHC to solve one of the central mysteries of astronomy by identifying the dark matter that pervades the universe. We know that the universe is full of dark matter, which weighs about five times as much as the visible matter that we observe in the form of galaxies and stars. We detect the dark matter by seeing its gravitational effect on visible matter, but we do not know what the dark matter is. If the supersymmetric sisters of known particles exist, they could be the dark matter. If all goes well, the LHC will kill two birds with one stone, observing the creation of dark matter in particle collisions, and at the same time testing the theory of Supersymmetry. Wilczek believes that all will go well. He sees the coming Golden Age as a culminating moment in the history of science:

Through patchy clouds, off in the distance, we seem to glimpse a mathematical Paradise, where the elements that build reality shed their dross. Correcting for the distortions of our everyday vision, we create in our minds a vision of what they might really be: pure and ideal, symmetric, equal, and perfect.

2.

Wilczek, like most scientists who are actively engaged in exploring, does not pay much attention to the history of his science. He lives in the era of particle accelerators, and assumes that particle accelerators in general, and the LHC in particular, will be the main source of experimental information about particles in the future. Since I am older and left the field of particle physics many years ago, I look at the field with a longer perspective. I find it useful to examine the past in order to explain why I disagree with Wilczek about the future. Here is a summary of the history as I remember it.

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