Developments

Theoretical physicist C. Llewellyn Smith discusses the discoveries that scientists have made to date about the electron and other elementary particles—subatomic particles that scientists believe cannot be split into smaller units of matter. Scientists have discovered what Smith refers to as sibling and cousin particles to the electron, but much about the nature of these particles is still a mystery. One way scientists learn about these particles is to accelerate them to high energies, smash them together, and then study what happens when they collide. By observing the behavior of these particles, scientists hope to learn more about the fundamental structures of the universe.

Electrons: The First Hundred Years

The discovery of the electron was announced by J. J. Thomson just over 100 years ago, on April 30, 1897. In the intervening years we have come to understand the mechanics that describe the behavior of electrons—and indeed of all matter on a small scale—which is called quantum mechanics. By exploiting this knowledge, we have learned to manipulate electrons and make devices of a tremendous practical and economic importance, such as transistors and lasers.

Meanwhile, what have we learned of the nature of the electron itself? From the start, electrons were found to behave as elementary particles, and this is still the case today. We know that if the electron has any structure, it is on a scale of less than 10­18 m, i.e. less than 1 billionth of 1 billionth of a meter.

However, a major complication has emerged. We have discovered that the electron has a sibling and cousins that are apparently equally fundamental. The sibling is an electrically neutral particle, called the neutrino, which is much lighter than the electron. The cousins are two electrically charged particles, called the mu and the tau, which also have neutral siblings. The mu and the tau seem to be identical copies of the electron, except that they are respectively 200 and 3,500 times heavier. Their role in the scheme of things and the origin of their different masses remain mysteries—just the sort of mysteries that particle physicists, who study the constituents of matter and the forces that control their behavior, wish to resolve.

We therefore know of six seemingly fundamental particles, the electron, the mu, the tau and their neutral siblings, which—like the electron—do not feel the nuclear force, and incidentally are known generically as leptons.

What about the constituents of atomic nuclei, which of course do feel the nuclear force? At first sight, nuclei are made of protons and neutrons, but these particles turned out not to be elementary. It was found that when protons and neutrons are smashed together, new particles are created. We now know that all these particles are made of more elementary entities, called quarks. In a collision, pairs of quarks and their antiparticles, called antiquarks, can be created: part of the energy (e) of the incoming particles is turned into mass (m) of these new particles, thanks to the famous equivalence e = mc². The quarks in the projectiles and the created quark-antiquark pairs can then rearrange themselves to make various different sorts of new particles.

Today, six types of quarks are known which, like the leptons (the electron and its relations) have simple properties, and could be elementary. In the past 30 years a recipe that describes the behavior of these particles has been developed. It is called the "Standard Model" of particle physics. However, we lack a real understanding of the nature of these particles, and the logic behind the Standard Model. What is wrong with the Standard Model?

First, it does not consistently combine Einstein's theory of the properties of space (called General Relativity) with a quantum mechanical description of the properties of matter. It is therefore logically incomplete.

Second, it contains too many apparently arbitrary futures—it is too baroque, too byzantine—to be complete. It does not explain the role of the mu and the tau, or answer the question whether the fact that the numbers of leptons and quarks are the same—six each—is a coincidence, or an indication of a deep connection between these different types of particles. On paper, we can construct theories that give better answers and explanations, and in which there are such connections, but we do not know which, if any, of these theories is correct.

Third, it has a missing, untested, element. This is not some minor detail, but a central element, namely a mechanism to generate the observed masses of the known particles, and hence also the different ranges of the known forces (long range for gravity and electromagnetism, as users of magnetic compasses know, but very short range for the nuclear and the so-called weak forces, although in every other respect these forces appear very similar). On paper, a possible mechanism is known, called the Higgs mechanism, after the British physicist Peter Higgs who invented it. But there are alternative mechanisms, and in any case the Higgs mechanism is a generic idea. We not only need to know if nature uses it, but if so, how it is realized in detail.

Luckily the prospects of developing a deeper understanding are good. The way forward is to perform experiments that can distinguish the different possibilities. We know that the answer to the mystery of the origin of mass, and the different ranges of forces, and certain other very important questions, must lie in an energy range that will be explored in experiments at the Large Hadron Collider, a new accelerator now under construction at CERN [also known as the European Laboratory for Particle Physics] near Geneva.

The fundamental tools on which experimental particle physics depends are large accelerators, like the Large Hadron Collider, which accelerate particles to very high energies and smash them together. By studying what happens in the collisions of these particles, which are typically electrons or protons (the nuclei of hydrogen atoms), we can learn about their natures. The conditions that are created in these collisions of particles existed just after the birth of the universe, when it was extremely hot and dense. Knowledge derived from experiments in particle physics is therefore essential input for those who wish to understand the structure of the universe as a whole, and how it evolved from an initial fireball into its present form.

The Large Hadron Collider will therefore not only open up a large new window on the nature of matter, when it comes into operation in 2005, but also advance our understanding of the structure of the universe. However, although it will undoubtedly resolve some major questions and greatly improve our knowledge of nature, it would be very surprising if it established a "final theory."

The only candidate theory currently known which appears to have the potential to resolve all the problems mentioned above—the reason for the existence of the mu and tau, reconciliation of Einstein's general relativity with quantum mechanics, etc.—describes the electron and its relatives and the quarks, not as pointlike objects, but as different vibrating modes of tiny strings. However, these strings are so small (10-35 m) that they will never be observed directly. If this is so, the electron and the other known particles will continue forever to appear to be fundamental pointlike objects, even if the—currently very speculative—"string theory" scores enough successes to convince us that this is not the case!

Developments