Dreams of Earth and Sky (16 page)

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:

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 along 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 that 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 the 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 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 that 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 of 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.