There's Plenty of

In String Theory

By Gordon Kane

In July, more than 350 string theorists from around the world will meet at the University of Michigan. In 1984, the world contained only a few people who could be called string theorists in today's sense. Why did this explosive growth occur?

String theory is an extremely ambitious activity. For the first time in history, we have a scientific attempt to ask what are the most basic laws that govern the natural universe, and where the laws of nature that allow our universe to exist come from. The theory can address what space and time are and why they exist. It allows gravity and quantum theory to work consistently together in one theory, which has been a notoriously difficult problem. It incorporates all the forces that affect the basic structure of the world. Clearly, formulating and testing any approach that promises so much is worth a great deal of effort.

Another area of physics where exciting developments have occurred, and where remarkable discoveries are hoped for in the next few years at upgraded and new experimental facilities, is called "supersymmetry." Supersymmetry is an idea that was formulated in the early 1970s, so it had a few years lead on string theory. There is good reason to believe that supersymmetry and string theory together provide a far more successful description of the world than either does separately.

What Is at Stake for Nonexperts?
What are string theory and supersymmetry? How do they fit together? What could convince us that they are indeed the correct description of our world? What stake might non-physicists have in this quest? Let's address the last question first. As science increasingly comprehended the phenomena of the world, we learned the causes of lightning and earthquakes and volcanoes, and we learned that they are not the tools and punishments of the gods. We explain the real causes to children so they will be awed but not frightened by lightning and thunder.

Before Isaac Newton, there was no understanding of why the Sun rose every day. After Newton's work people who learned of it understood that the Sun did not rise because they had prayed for it to rise, and that changed our worldview. Learning about the immensity of our universe, that our Sun is but one of billions and billions of stars, has had a similar profound effect on how we think and feel about our place in it. If we do learn the answers to our questions about the primary laws and the origins of the universe, even the fact of learning them will change how we think and feel about our universe and its meaning.

The knowledge derived from supersymmetry and string theories may change the worldview of many people, but it isn't likely to have practical impact. The experiments involved in such research, however, have always necessarily yielded so-called spin-offs that have major practical impact. That is because frontier research requires new experimental techniques. The new techniques lead in turn to innovations and new technology and products. In practice, all investments in research in particle physics are more than paid back to society economically. The World Wide Web, for example, is the result of a communication system invented to enable particle physics groups around the world to transmit data and analyses efficiently among international research partners. On the medical front, 80,000 people benefit daily from "accelerator medicine," that is, from medical therapies resulting from techniques originally developed for high-energy accelerator experiments exploring the elementary particles of matter. These are just two examples from a very long list.

The Standard Model
Today we have a remarkably successful description of the observed physical world. We know the five basic forces that act in nature: they are called the gravitational, electrical, magnetic, weak and strong forces. Currently, they are somewhat unified into a simpler picture so that at most three of them are independent; further understanding of unification is expected when supersymmetry and string theory are included.

We also know the basic particles the forces act on: all that we see in the universe is composed simply of three kinds of particles, electrons that we have all learned a little about, and two others called quarks.

The quarks are similar to electrons, but also interact via the strong force (which electrons don't). The strong force binds the quarks into protons and neutrons. Protons and neutrons then combine into nuclei—they form the 92 long-lived nuclei. Those nuclei interact with electrons and form the 92 atoms of the chemical elements. The atoms form molecules. From such simple ingredients all the complexity and variety of our world is built up.

Quantum theory and Einstein's special theory of relativity give the rules that govern how all the interactions occur. This picture, combining the particles and forces and rules, is called the Standard Model of particle physics. It gives us a complete description of how the natural world works (though not why it works that way). It includes all that has been learned about the physical world over four centuries and explains a huge number of observations.

Matter is made of molecules, which are in turn made of atoms. Salt, for example, is a molecule containing one sodium and one chlorine atom bound together. Each atom has a nucleus to which they are bonded by photons. The nucleus is made of protons and neutrons bound together. Neutrons and protons are made of quarks bound by gluons. A typical small molecule has a diameter of about one-millionth of a centimeter; an atom is a tenth of that, and the atom's components many thousand times smaller still.

The Frontier beyond the Standard Model
There are, however, a number of phenomena that the Standard Model does not explain. They are not questions about how things work, or outcomes of experiments, but rather why certain aspects of the world are the way they are, or what certain things are. Physics has not tended to ask these questions, because they were too remote from what was understood scientifically. Until the past decade or so, almost all physics focused on how things work, not on why they are the way they are. The boundaries of science have been moving, however, and even though they are rather technical, I'll describe a few examples of what scientists are looking for at the latest frontier outposts.

Over 90 percent of the universe is dark; that is, it doesn't make light like our Sun and other stars do. We deduce that the dark matter is there much the same way we could figure out we had a moon even if the atmosphere was so thick we couldn't see it—from the dark matter's effects. (For the Moon, the biggest effect is the tides.) It is impossible for the Standard Model to explain what this dark matter is, but supersymmetry provides a natural candidate, the lightest of the new particles predicted by supersymmetry. Experiments are under way to test whether this is indeed the explanation. Although the electron and two quarks do make up all that we see, there are actually more particles. The additional ones have been discovered at particle accelerator facilities, where beams of particles are accelerated to very high energies and then collided. Sometimes previously unknown particles emerge from such collisions. Most of the new particles decay into the more familiar ones very quickly, the most stable one existing for only a millionth of a second. A few of them don't decay but interact so little they do not enter into what we see. There are two more particles like the electron, and also two more like each quark. Why the additional particles exist is a mystery. They do not play any known role in how the world behaves. We can describe their behavior completely, but we do not know any reason they should exist. String theory can address this issue. To be complete the Standard Model requires a new particle to exist, called a Higgs boson (named after Peter Higgs, a physicist involved in the invention of the mechanism that leads to the prediction of so-called Higgs bosons). Interactions with the Higgs boson allow particles to have mass. In the Standard Model the Higgs boson is introduced in an arbitrary and conceptually unsatisfactory way, though technically it is all right. Supersymmetry accounts for Higgs bosons naturally, and allows one to derive their properties, so it puts this whole "Higgs physics" area on a firm footing. Detecting Higgs bosons in experiments is thus a crucial test of supersymmetry. (See Michigan Today, October 1990, "Material Witnesses," by Madeleine Strong Diehl.)