A Brief Introduction to Particle Physics

Superstring Theory

a type of theory of particle physics that treats elementary particles as extended one-dimensional "string-like" objects rather than as the dimensionless points in space-time used in other theories. Superstring theories became popular during the 1980s when Michael Green of Queen Mary College, London, and John Schwarz of the California Institute of Technology showed that certain types of such theories might provide a fully self-consistent quantum theory that describes gravity as well as the weak, strong, and electromagnetic forces. The development of such a unified quantum theory is a major goal in theoretical particle physics, but usually the inclusion of gravity has led to intractable problems with infinite quantities in the calculations.

The basic entities in superstring theory are one-dimensional massless strings only 10^-33 cm long. (This distance is the so-called Planck length, at which quantum effects in gravity can no longer be ignored.) The strings vibrate, and each different mode of vibration corresponds to a different particle. The strings can also interact in ways that correspond to the observed interactions of particles.

String theories of elementary particles were first introduced in the early 1970s in attempts to describe the strong force. Although quantum chromodynamics was soon accepted as the correct theory of the strong force, string theory was given new life with the incorporation of supersymmetry, the symmetry between fermions (particles with half-integral values of spin) and bosons (particles with integral values of spin). Not only did the resulting superstring theory successfully encompass all the fundamental forces, but only a limited type of the many possible versions seemed to be properly self-consistent, making it a leading candidate for a fully unified theory of particles and forces.

At first sight this particular type of superstring theory might seem to have an insurmountable drawback in that it deals with a space-time of ten dimensions instead of the three dimensions of space and one of time that are perceived in the everyday world. It seems, however, that six of the ten dimensions may be "compactified," or "curled up"--i.e., so small that they are unnoticeable. Other problems remain, however. The theory is still a long way from explaining the masses of the known particles. It also implies the existence of new particles in a form of "shadow matter," with which normal matter can interact only through gravity.

A Brief Introduction to Particle Physics

Physicist Clifford V. Johnson takes readers on a brief introductory tour of the world of particle physics. A leading theoretician in elementary particle physics, Johnson traces the history of this field from its beginnings to the present day. He explains why physicists are currently intrigued with the exotic ideas of superstrings and M-theory.

A Brief Introduction to Particle Physics

By Clifford V. Johnson

High-energy particle physicists are using particle accelerators measuring 8 km (5 mi) across to study something billions of times too small to see. Why? To find out what everything is made of and where it comes from. These physicists are constructing and testing new theories about objects called superstrings. Superstrings may explain the nature of space and time and of everything in them, from the light you are using to read these words to black holes so dense that they can capture light forever. Possibly the smallest objects allowed by the laws of physics, superstrings may tell us about the largest event of all time: the big bang, and the creation of the universe!

These are exciting ideas, still strange to most people. For the past 100 years physicists have descended to deeper and deeper levels of structure, into the heart of matter and energy and of existence itself. Read on to follow their progress.

Elementary, my dear: atoms and molecules

The world around us, full of books, computers, mountains, lakes, and people, is made by rearranging slightly more than 100 chemical elements. Oxygen, hydrogen, carbon, and nitrogen are elements especially important to living things; silicon is especially important to computer chips.

The smallest recognizable form in which a chemical element occurs is the atom, and the atoms of one element are unlike the atoms of any other element. Every atom has a small core called a nucleus around which electrons swarm. Electrons, tiny particles with a negative electrical charge, determine the chemical properties of an element—that is, how it interacts with other atoms to make the things around us. Electrons also are what move through wires to make light, heat, and video games.

In 1869, before anyone knew anything about nuclei or electrons, Russian chemist Dmitry Mendeleyev grouped the elements according to their physical qualities and discovered the periodic law. He was able to predict the qualities of elements that had not yet been discovered. By the early 1900s scientists had discovered the nucleus and electrons.

Atoms stick together and form larger objects called molecules because of a force called electromagnetism. The best-known form of electromagnetism is radiation: light, radio waves, X rays, and infrared and ultraviolet radiation.

Smaller: a century of quantum physics

Modern physics starts with light and other forms of electromagnetic radiation. In 1900 German physicist Max Planck proposed the quantum theory, which says that light comes in units of energy called quanta. As we will explain, these units of light are waves and they are also particles. Light is simultaneously energy and matter. And so is everything else.

It was Albert Einstein who first proposed (in 1905) that Planck's units of light can be considered particles. He named these particles photons. In the same year, Einstein published what is known as the special theory of relativity. According to this theory, the speed of light is actually the fastest that anything in the universe can go, and all forms of electromagnetic radiation are forms of light, moving at the same speed.

What differentiates radio waves, visible light, and X rays is their energy. This energy is directly related to the wave’s length. Light waves, like ocean waves, have peaks and troughs that repeat at regular intervals, and wavelength is the distance between each pair of peaks (or troughs). The shorter the wavelength, the higher the energy.

How does this relate to our story? It turns out that the process by which electrons interact is an exchange of photons (particles of light). Therefore we can study electrons by probing them with photons.

Tiny particles, big tools

To really understand what things are made of, we must probe them or move them around and thus learn how they work. In the case of electrons, physicists probe them with photons, the particles that carry the electromagnetic force.

While some physicists studied electrons and photons, others pondered and probed the atomic nucleus. The nucleus of each chemical element contains a distinctive number of positively charged protons and a number of uncharged neutrons that can vary slightly from atom to atom. Protons and neutrons are the source of radioactivity and of nuclear energy. In 1964 physicists suggested that protons and neutrons are made of still smaller particles they called quarks.

Probing protons and neutrons requires particles with extremely high energies. Particle accelerators are large machines for bringing particles to these high energies. These machines have to be big, because they accelerate particles by applying force many times, over long distances. Some particle accelerators are the largest machines ever constructed. This is rather ironic given that these are delicate scientific instruments designed to probe the shortest distances ever investigated!

The standard model: describing fundamental particles

The proposal and acceptance of quarks was a major step in putting together what is called the standard model of particles and forces. This unified theory describes all of the fundamental particles, from which everything is made, and how they interact. There are twelve kinds of fundamental particles: six kinds of quarks and six kinds of leptons, including the electron.

Four forces are believed to control all the interactions of these fundamental particles. They are the strong force, which holds the nucleus together; the weak force, responsible for radioactivity; the electromagnetic force, which provides electric charge and binds electrons to atomic nuclei; and gravitation, which holds us on Earth. The standard model identifies a force-carrying particle to correspond with three of these forces. The photon, for example, carries the electromagnetic force. Physicists have not yet detected a particle that carries gravitation.

Powerful mathematical techniques called gauge field theories allow physicists to describe, calculate, and predict the interactions of these particles and forces. Gauge theories combine quantum physics and special relativity into consistent equations that produce extremely accurate results. The extraordinary precision of quantum electrodynamics, for example, has filled our world with ultrareliable lasers and transistors.

A situation of some gravity

The mathematical rules that come together in the standard model can explain every particle physics phenomenon that we have ever seen. Physicists can explain forces; they can explain particles. But they cannot yet explain why forces and particles are what they are. Basic properties, such as the speed of light, must be taken from measurements. And physicists cannot yet provide a satisfactory description of gravity.

The basic behavior of gravity was taught to us by English physicist Sir Isaac Newton. After creating the basics of quantum physics in his theory of special relativity, Albert Einstein in 1915 clarified and extended Newton’s explanation with his own description of gravity, known as general relativity. Not even Einstein, however, could bring the two theories of relativity into a single unified field theory. Since everything else is governed by quantum physics on small scales, what is the quantum theory of gravity? No one has yet proposed a satisfactory answer to this question. Physicists have been trying to find one for a long time.

At first, this might not seem to be an important problem. Compared to other forces, gravity is extremely weak. We are aware of its action in everyday life because its pull corresponds to mass, and Earth has a huge amount of mass and hence a big gravitational pull. Fundamental particles have tiny masses and hence a miniscule gravitational pull. So couldn’t we just ignore gravity when studying fundamental particles? The ability to ignore gravity on this scale is why we have made so much progress in particle physics over so many years without possessing a theory of quantum gravity.

There are several reasons, however, why we cannot ignore gravity forever. One reason is simply that scientists want to know the whole story. A second reason is that gravity, as Einstein taught us, is the essential physics of space and time. If this physics is not subject to the same quantum laws that any other physics are subject to, something is wrong somewhere. A third reason is that an understanding of quantum gravity is necessary to deal with some important questions in cosmology—for example, how did the universe get to be the way it is, and why did galaxies form?

From the smallest to the largest

Gravitation has been shown to spread in waves, and physicists theorize the existence of a corresponding particle, the graviton. The force of gravity, like everything else, has a natural quantum length. For gravity it is about 10^-31m. This is about a million billion times smaller than a proton.

We can't build an accelerator to probe that distance using today’s technology, because the proportions of size and energy show that it would stretch from here to the stars! But we know that the universe began with the big bang, when all matter and force originated. Everything we know about today follows from the period after the big bang, when the universe expanded. Everything we know indicates that in the fractions of a second following the big bang, the universe was extremely small and dense. At some earliest time, the entire universe was no larger across than the quantum length of gravity. If we are to understand the true nature of where everything comes from and how it really fits together, we must understand quantum gravity!

These questions may seem almost metaphysical. Physicists now suspect that research in this direction will answer many other questions about the standard model—such as why are there are so many different fundamental particles. Other questions are more immediately practical. Our control of technology arises from our understanding of particles and forces. Answers to physicists’ questions could increase computing power or help us find new sources of energy. They will shape the 21st century as quantum physics has shaped the 20th.

Superstring theory: good vibrations?

Among the most promising new theories is the idea that everything is made of fundamental “strings,” rather than of another layer of tiny particles. The best analogy for these minute entities is a guitar or violin string, which vibrates to produce notes of different frequencies and wavelengths. Superstring theory proposes that if we WERE able to look closely enough at a fundamental particle—at quantum-length distances—we would see a tiny, vibrating loop!

In this view, all the different types of fundamental particles that we find in the standard model are really just different vibrations of the same string, which can split and join in ways that change its evident nature. This is the case not only for particles of matter, such as quarks and electrons, but also for force-carrying particles, such as photons.

This is a very clever idea, since it unifies everything we have learned in a simple way. In its details, the theory is extremely complicated but very promising. For example, the superstring theory very naturally describes the graviton among its vibrations, and it also explains the quantum properties of many types of black holes. There are also signs that the quantum length of gravity is really the smallest physically possible distance. Below this scale, points in space and time are no longer connected in sequence, so distances cannot be measured or described. The very notions of space, time, and distance seem to stop making sense!

Recent discoveries have shown that the five leading versions of superstring theory are all contained within a powerful complex known as M-Theory. M-Theory says that entities mathematically resembling membranes and other extended objects may also be important. The end of the story has not yet been written, however. Physicists are still working out the details, and it will take many years to be confident that this approach is correct and comprehensive. Much remains to be learned, and surprises are guaranteed. In the quest to probe these small distances, experimentally and theoretically, our understanding of nature is forever enriched, and we approach at least a part of ultimate truth.

Standard Model

Standard Model, in physics, theory that summarizes scientists' current understanding of elementary particles and the fundamental forces of nature.

In relativistic quantum field theory (QFT), matter consists of particles called fermions, and forces are mediated by the interaction or exchange of other particles called bosons. In the standard model, the basic fermions come in three families, with each family made up of certain quarks and leptons. The first family, which consists of low-mass quarks and leptons, consists of the up and down quarks, the electron and its neutrino, and an antiparticle corresponding to each (see Antimatter). The quarks bind into triplets to form neutrons and protons, which bind together to form nuclei, which bind to electrons to form atoms. The electron neutrinos participate in the radioactive “beta” decay of neutrons into protons (see Radioactivity). The particles that make up the other two families of fermions are not present in ordinary matter, but can be created in powerful particle accelerators. The second family consists of the charm and strange quarks, the muon and muon neutrino, and an antiparticle corresponding to each. The third family consists of the top and bottom quarks, the tau and tau neutrino, and an antiparticle corresponding to each.

The basic bosons are the gluons, which mediate the strong nuclear force; the photon, which mediates electromagnetism; the weakons, which mediate the weak nuclear force; and the graviton, which some physicists believe mediates the gravitational force, though its existence has not yet been experimentally confirmed. The QFT of the strong interaction is called quantum chromodynamics; the QFT of the electromagnetic and weak nuclear interactions is called electroweak theory.

Although the standard model is consistent with all experiments performed so far, it has many shortcomings. It does not incorporate gravity, the weakest force; it does not explain the spectrum of particle masses; it has many arbitrary parameters; and it does not completely unify the strong and electroweak interactions. Grand Unification Theories attempt to unify the strong and electroweak interactions by assuming these interactions are equivalent at sufficiently high energies. The ultimate goal in physics is to formulate a Theory of Everything that would unify all interactions—electroweak, strong, and gravitational.

Contributed By: John Lindner