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.
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.
Before World War II, particle physics did not exist. We had atomic physics, the science of atoms and electrons, and nuclear physics, the science of atomic nuclei. Beyond these well-established areas of knowledge, there was a dimly lit zone of peculiar phenomena called cosmic rays. Cosmic rays were a gentle rain of high-energy particles and radiation that came down onto the earth from outer space. We called the high-energy particles mesons. Nobody knew what they were, where they came from, or why they existed. They appeared to come more or less uniformly from all directions, at all times of day or night, summer or winter. They were an enduring mystery, not yet a science.
Particle physics emerged unexpectedly in the 1940s, during the early postwar years, while the soldiers were still coming home from battlefields and prison camps. Particle physics started with makeshift equipment salvaged from the war to explore a new universe. The new field was a symbol of hope for a generation battered by war. It proved that former enemies could work together fruitfully on peaceful problems. It gave us reason to dream that friendly collaboration could spread from the world of science to the more contentious worlds of power and politics.
In 1947, Cecil Powell did a historic experiment in Bristol. He was an expert on photography and knew how to cook photographic plates so as to make them sensitive to cosmic rays. In his plates he could see tracks of cosmic rays coming to rest. When an object comes to rest in a known place at a known time, it is no longer a vague flow of unknown stuff. It is a unique and concrete object. It is accessible to the tools of science. After Powell detected a cosmic ray coming to rest, he knew where it was, and he could see what it did next. What it often did next was to produce a secondary particle moving close to the speed of light. When he started to study the secondary particles, the mystery of cosmic rays was transformed into the science of particle physics.
Powell trained an army of human image-scanners to examine with microscopes the tracks of cosmic rays coming to rest in his plates. His unique skill as an experimenter was to motivate people, not to build apparatus. His scanners worked long hours searching for rare needles in a haystack of photographic clutter. They worked together as a team. A scanner who found something new was given full credit for the discovery, but the others who worked equally hard and found nothing were given credit too. One of his scanners, Marietta Kurz, discovered a cosmic ray that came to rest twice. It stopped in a plate, then produced a secondary particle that moved a short distance before stopping again, then produced a tertiary particle that moved faster and escaped from the plate. Powell called the primary particle a pi-meson, and the secondary particle a mu-meson. The pi changed into a mu, and the mu changed into something else. This experiment revealed and named the first two species in the particle zoo.
After Powell, the pioneers of particle physics continued for five years to work with cosmic rays, finding several more species of particle. One of the particles that they failed to find was the antiproton. According to theory, every particle with an electric charge should have an antiparticle with the opposite charge. The proton, which is the positively charged nucleus of the hydrogen atom and a component of every other kind of atom, should have a negatively charged twin called the antiproton. Cosmic ray experiments failed to find the antiproton because it cannot be brought to rest in matter. Every antiproton stopped in matter immediately finds a proton and annihilates itself together with its twin. Cosmic ray experts hunted for the antiproton in vain. Meanwhile, builders of particle accelerators were developing a new set of tools. Ernest Lawrence, the original inventor of the cyclotron, built a large accelerator which he called the Bevatron. In 1955 two physicists at Berkeley in California, Emilio Segrè and Owen Chamberlain, used the new accelerator to produce antiprotons in quantity and detect their annihilation. They received the Nobel Prize in 1959 for discovering the antiproton.
After 1955, a few particle physicists continued to study cosmic rays and other kinds of natural radiation with passive detectors, but the new experimental tool, the high-energy accelerator, rapidly took over the field. Particle accelerators had many advantages over passive detectors. Accelerators provided particles in far greater numbers, with precisely known energies, under control of the experimenter. Accelerator experiments were more quantitative and more precise. But accelerators also had some serious disadvantages. They were more expensive than passive detectors, they required teams of engineers to keep them running, and they produced particles with a limited range of energies.
Nature provided among the cosmic rays a small number of particles with energies millions of times larger than the largest accelerator could reach. If the distribution of effort between accelerators and passive detectors had been rationally planned, particle physicists would have maintained a balance between the two types of instrument, perhaps three quarters of the money for accelerators and one quarter for passive detectors. Instead, accelerators became the prevailing fashion. The era of accelerator physics had begun, and big accelerators became political status symbols for countries competing for scientific leadership. For forty years after 1955, the United States built a succession of big accelerators and only two passive detectors. The Soviet Union and the European laboratory CERN followed suit, putting almost all their efforts into accelerators. Meanwhile, serious research using passive detectors continued in Canada and Japan, countries with high scientific standards and limited resources.
In the United States, Raymond Davis Jr. was a lonely pioneer who found a new way of doing experiments with natural radiation. He demonstrated that he could detect the appearance of a single atom of argon in a tank containing six hundred tons of a common industrial cleaning fluid. This cleaning fluid is cheap and available in big quantities. It consists of 13 percent carbon and 87 percent chlorine. Argon is a gas with properties totally different from chlorine. Davis put his tank full of cleaning fluid a mile underground in a mined-out cavity belonging to the Homestake gold mine in South Dakota, so as to get away from the confusing effects of cosmic rays. He was interested in observing natural radiation from the center of the sun. According to the standard model of nuclear energy generation in the sun, the sun produces particles called neutrinos, which arrive at the earth and very rarely cause chlorine atoms to change into argon atoms. The predicted rate of appearance of argon atoms in Davis’s tank was three per month. Davis claimed that he could reliably count the argon atoms. He counted them for many years and found only one per month instead of three. The deficiency of argon atoms was known as the “solar neutrino problem.”
The solar neutrino problem could be explained in three ways. Either Davis’s experiment was wrong, or the standard model of the sun was wrong, or the standard theory of the neutrino was wrong. For many years, most of the experts believed that the experiment was wrong, that Davis missed two thirds of the argon atoms because they slipped through his counters. Davis did some careful tests which convinced the experts that his counters were not to blame, and then they mostly believed that the model of the sun was wrong. The model of the sun was checked by accurate measurements of seismic waves traveling through the sun, and turned out to be correct. So the experts finally had to admit that their theory of the neutrino was wrong.
We now know that there are three kinds of neutrinos. Only one kind is produced in the sun, and only that kind was detected in Davis’s tank, but many switch smoothly from one kind to another while they are traveling from the sun to the earth. Two thirds of them are the wrong kind to be detected when they arrive at the tank, neatly explaining Davis’s result. This discovery was the first evidence for processes not included in the scheme that Wilczek calls the Core. Davis was awarded a belated Nobel Prize for it in 2002. During the years while Davis was working alone with his tank, larger teams of physicists and engineers were making discoveries at a rapid pace with accelerators. The accelerator era was in full swing. Particle physics as we know it today is largely the fruit of accelerators.
So much for the history. Now I turn from the past to the future. Wilczek’s expectation, that the advent of the LHC will bring a Golden Age of particle physics, is widely shared among physicists and widely propagated in the press and television. The public is led to believe that the LHC is the only road to glory. This belief is dangerous because it promises too much. If it should happen that the LHC fails, the public may decide that particle physics is no longer worth supporting. The public needs to hear some bad news and some good news. The bad news is that the LHC may fail. The good news is that if the LHC fails, there are other ways to explore the world of particles and arrive at a Golden Age. The failure of the LHC would be a serious setback, but it would not be the end of particle physics.
There are two reasons to be skeptical about the importance of the LHC, one technical and one historical. The technical weakness of the LHC arises from the nature of the collisions that it studies. These are collisions of protons with protons, and they have the unfortunate habit of being messy. Two protons colliding at the energy of the LHC behave rather like two sandbags, splitting open and strewing sand in all directions. A typical proton–proton collision in the LHC will produce a large spray of secondary particles, and the collisions are occurring at a rate of millions per second. The machine must automatically discard the vast majority of the collisions, so that the small minority that might be scientifically important can be precisely recorded and analyzed. The criteria for discarding events must be written into the software program that controls the handling of information. The software program tells the detectors which collisions to ignore. There is a serious danger that the LHC can discover only things that the programmers of the software expected. The most important discoveries may be things that nobody expected. The most important discoveries may be missed.
Another way to go ahead with particle physics is to follow the lead of Davis and build large passive detectors observing natural radiation. In the last twenty years, the two most ambitious passive detectors were built in Canada and Japan. Both these detectors made important discoveries, confirming and completing the work of Davis. In a well-designed passive detector deep underground, events of any kind are rare, every event is recorded in detail, and if anything unexpected happens you will see it.
There are also historical reasons not to expect too much from the LHC. I have made a survey of the history of important discoveries in particle physics over the last sixty years. To avoid making personal judgments about importance, I define an important discovery to be one that resulted in a Nobel Prize for the discoverers. This is an objective criterion, and it usually agrees with my subjective judgment. In my opinion, the Nobel Committee has made remarkably few mistakes in its awards. There have been sixteen important experimental discoveries between 1945 and 2008.
Each experimental discovery lies on one of three frontiers between known and unknown territory. It is on the energy frontier if it reaches a new range of energy of particles. It is on the rarity frontier if it reaches a new range of rarity of events. It is on the accuracy frontier if it reaches a new range of accuracy of measurements. I assigned each of the sixteen important discoveries to one of the three frontiers. In most cases, the assignments are unambiguous. For example, two of the three discoveries that I mentioned earlier, Powell’s discovery of double-stopping mesons and Davis’s discovery of missing solar neutrinos, lie on the rarity frontier, while only one, Segrè and Chamberlain’s discovery of the antiproton, lies on the energy frontier.
The results of my survey are then as follows: four discoveries on the energy frontier, four on the rarity frontier, eight on the accuracy frontier. Only a quarter of the discoveries were made on the energy frontier, while half of them were made on the accuracy frontier. For making important discoveries, high accuracy was more useful than high energy. The historical record contradicts the prevailing view that the LHC is the indispensable tool for new discoveries because it has the highest energy.
The majority of young particle physicists today believe in big accelerators as the essential tools of their trade. Like Napoleon, they believe that God is on the side of the big battalions. They consider passive detectors of natural radiation to be quaint relics of ancient times. When I say that passive detectors may still beat accelerators at the game of discovery, they think this is the wishful thinking of an old man in love with the past. I freely admit that I am guilty of wishful thinking. I have a sentimental attachment to passive detectors, and a dislike of machines that cost billions of dollars to build and inevitably become embroiled in politics. But I see evidence, in the recent triumphs of passive detectors and the diminishing fertility of accelerators, that nature may share my prejudices. I leave it to nature to decide whether passive detectors or the LHC will prevail in the race to discover her secrets.
Fortunately, passive detectors are much cheaper than the LHC. The best of the existing passive detectors were built by Canada and Japan, countries that could not afford to build giant accelerators. The race for important discoveries does not always go to the highest energy and the most expensive machine. More often than not, the race goes to the smartest brain. After all, that is why Wilczek won a Nobel Prize.
April 9, 2009