Stephen Hawking (36 page)

“What do you think of it?” Linde asked.

“Fascinating,” said the businessman. “I can't put it down.”

“Oh, that's interesting,” the scientist replied. “I found it quite heavy going in places and didn't fully understand some parts.”

At which point the businessman closed the book on his lap, leaned across with a compassionate smile, and said, “Let me explain. . . .”

^*
A cult success of the seventies.

15

THE END OF PHYSICS?

S
tephen Hawking is fond of suggesting that the end may be in sight for theoretical physics. Hearing Hawking tell you that physics may be coming to an end became something of a cliché in the trade in the 1980s, as at the beginning of that decade he used his inaugural lecture as Lucasian Professor to pose that question. Thirty years on, the end doesn't look any closer than it did then, but he is still optimistically proclaiming it. But even if theoretical physics really did reach the “end” Hawking so eagerly predicts, there would still be plenty of work left for physicists to do.

In an interview in
Newsweek
in 1988, Hawking said that after discovering a theory of everything, “there would still be lots to do,” but physics would then be “like mountaineering after Everest.”
¹

Other cosmologists, including Martin Rees, prefer a slightly different analogy. They point out that learning the rules of chess is only the first step on a long and fascinating path to becoming a grand master. The long-sought-after theory of everything, they say, would be no more than the physics equivalent of the rules of chess, with grand-master status still far away over the horizon.

The immediate goal of physics—the Holy Grail that Hawking and a few other researchers believe lies just around the corner—is a complete, consistent, unified theory in which all physical interactions are described by one set of equations. To see what this means and how daunting the search for such a theory must be, we shall look at the modern understanding of the way the Universe works, which requires four separate theories to explain different features of the world.

Back in the nineteenth century, only two theories were needed (so in a way physics has gotten more complicated in the past hundred years). Newton's theory of gravity described the force that holds planets in their orbits around the Sun or makes an apple fall from a tree; Maxwell's equations of electromagnetism described the behavior of radiation, including light, and the forces that operate between electrically charged particles or between magnets.

As we explained in
Chapter 2
, though, these two theories were incompatible. Maxwell's equations set a speed for light that is the same for all observers, while Newtonian mechanics said
that the speed measured for light would depend on the motion of the observer. This dichotomy was one of the principal reasons why Einstein developed first the special theory of relativity and then the general theory—an improved theory of gravity that is compatible with Maxwell's equations. Both the general theory and Maxwell's theory are, however, “classical” theories in the strict sense of the term. That is, they treat the Universe as a continuum. Space, in the classical view, can be subdivided and measured in units as small as you wish, while electromagnetic energy can come in a quantity as small as you wish.

The quantum revolution changed the way physicists view the world. They now regard the Universe as discontinuous, with an ultimate limit on how small a “piece” of electromagnetic energy can be, and even on how small a unit of time or a measure of distance can be. It was discoveries concerning the nature of light that led to the quantum revolution, and electromagnetism was eventually superseded by a new theory, quantum electrodynamics (QED), that incorporates the best of Maxwell's theory with the new quantum rules.

But QED did not become established until the 1940s, by which time two “new” forces were on the agenda. Both these forces have only
very
short range and operate only within the nucleus of an atom (which is why they were unknown in the nineteenth century before the nucleus was discovered). One is called the strong force and acts as the glue that holds the particles in the nucleus together; the other is known as the weak force (because, logically enough, it is weaker than the strong force), and it is responsible for radioactive decay.

In many ways, however, the weak force resembles the electromagnetic force. Building from the success of QED, in the
1950s and 1960s physicists developed a mathematical theory that could describe both the weak force and electromagnetism with one set of equations. It was called the “electroweak” theory, and it made one key prediction: with the weak force there should be associated three types of particles which, between them, play much the same role that the photon (the particle of light) does in QED. Unlike the photon, however, these particles (known as W
₊
, W
_−
, and Z
₀
) should, according to the new theory, have mass. Not just any old mass, either, but very well-determined masses—about nine times the mass of a proton for the two W particles and eight times the mass of the proton for the Z
₀
. In 1983, the particle accelerator team at CERN in Geneva found traces of particles with exactly the right properties. The electroweak theory was a proven success, and physicists were back to just three theories needed to explain the workings of the Universe.

With this success under their belts, theorists have developed a theory similar to QED to describe the strong force. We now know that nuclear particles (protons and neutrons) are actually made of fundamental entities known as quarks. Quarks come in different varieties, and physicists whimsically give these the names of colors—red, green, and blue. This doesn't mean that quarks really are red, green, or blue any more than the fact that a drink is called a rusty nail means that it really does contain oxidized iron. They are just names. But, extending the whimsy, physicists call the quantum theory that describes how quarks interact, and which is responsible for the strong force, “quantum chromodynamics” (from the Greek word for color) or QCD. There are several promising ways now being investigated that might lead to a single theory that
encompasses both QCD and the electroweak theory. Such sets of equations are known, rather pretentiously, as Grand Unified Theories, or GUTs. But QCD is not yet as well established as the electroweak theory, and the GUTs themselves are only indicative of the form a future definitive theory might take.

Even worse, the pretentiousness of calling these Grand Unified Theories is highlighted by the fact that none of this progress toward unification takes any account of gravity at all! The first force of nature to be investigated, and at least partially understood, it has proved the most intractable when it comes to trying to fit it into the quantum mold. Without gravity included in their mesh, it seems fair to say that—paraphrasing Hawking's famous comment about black holes—Grand Unified Theories ain't so grand after all. In spite of Hawking's success in using a partial unification of quantum theory and general relativity in his investigations of black holes and the beginning of time, gravity is still best described by the general theory of relativity—a classical continuum theory.

The prospect of incorporating gravity into what, we suppose, would have to be called a “super-unified theory” has been “just around the corner” for decades. Logically, we might guess that first we need to develop a quantum theory of gravity and then build from this to a unification with the other three forces. One feature of any such quantum theory of gravity is that it, too, must incorporate particles that are associated with the gravitational force, again reminiscent of the way photons are associated with electromagnetism. (In case you are wondering, yes, there are similar particles involved in QCD, the theory of the strong force; they are called “gluons,” but nobody has yet detected one.) Physicists even have a name for
these hypothetical particles of gravity—“gravitons.” But just as calling a quark “red” does not mean that it is actually colored red, so giving the quantum gravity particle a name does not mean that anybody has yet found one or even that anybody has come up with a satisfactory quantum theory of gravity.

At the time of Hawking's inaugural lecture in 1980, researchers were getting excited about a family of possible quantum gravity theories that together go by the name of supergravity. One version of supergravity is called “
N
= 8” because as well as predicting the existence of one type of graviton, it also requires an additional eight varieties of particles known as gravitinos (together with a further 154 varieties of other as yet undiscovered particles). The plethora of particles associated with this favored version of supergravity may seem unwieldy—and it is, but it does represent a considerable advance on previous attempts to find a quantum theory of gravity, which seemed to require an infinite number of “new” particles. Indeed, out of all the variations on the supergravity theme,
N
= 8 is the only one that operates naturally in four dimensions (three of space plus one of time) and contains a finite number of particles. It certainly got Hawking's vote as the theory most likely to succeed in 1980.

In the next few years, everything changed. By the mid-1980s, enthusiasm for supergravity had been swept away in a rising tide of support for a completely different kind of idea, known as string theory. The central idea of string theory is that entities that we are used to thinking of as points (such as electrons and quarks) are actually linear—tiny “strings.” Such strings would be very small indeed: it would take 10
²⁰
of them, laid end to end, to stretch across the diameter of a proton.
Such strings might be open, with their ends waving free, or closed into little loops. Either way, some theorists believe, the way they vibrate and interact with one another could explain many features of the physical world.

String theory actually dates back to the late 1960s, when it was invoked to describe the strong force. The success of QCD left this early version of string theory by the wayside, although a few mathematicians dabbled with it out of interest in the equations, rather than in any expectation of making a major breakthrough in unifying our understanding of the forces of nature. In the mid-1970s, two of those researchers, Joël Scherk in Paris and John Schwarz at Caltech, actually found a way to describe gravity using string theory. But the response of their colleagues was, essentially, “Who needs it?” At that time, most gravity researchers were more interested in supergravity. String theory wasn't needed to explain the strong force, supergravity looked promising, so why bother with strings at all?

Their attitude changed when it turned out to be horrendously difficult to do any calculations at all using the
N
= 8 supergravity theory. Even if there were no infinities to worry about, 154 types of particles, in addition to the graviton and eight gravitinos, were almost too much of a handful to keep mathematical tabs on. Hawking says that it was generally reckoned in the early 1980s that, even using a computer, it would take four years to complete a calculation, checking that all the particles in the theory were accounted for, with no infinities still hidden away somewhere, and that it would be almost impossible to carry out the calculation without making a mistake. So nobody was prepared to give up his career to do the calculation.

The main reason for the revival of interest in string theory in the mid-1980s, however, was the realization that in their most satisfactory form, these theories
automatically
include the graviton. In other attempts to build a quantum theory of gravity, researchers had started out knowing the properties a graviton ought to have and tried to build a theory around it, even if that meant taking 162 other particles on board as well. With string theory, they were working with the quantum equations in a general way, playing mathematical games, and found that the closed loops of string described by some of the equations have just the properties required to provide a description of gravity—they are, indeed, gravitons. Inevitably, the new variation on the string theme was dubbed “superstring theory.” By 1988, with the publication of
A Brief History of Time
, it was this road toward superunification that Hawking was enthusiastically endorsing.

But there are still snags. One is that people are still unsure what all the equations mean. As the example of the graviton illustrates, the equations have come first, with physical insight into their significance lagging, and there are still plenty of equations for which, as yet, there is no physical insight. This is quite different from the way the great developments in physics were made earlier in the twentieth century and, indeed, in the centuries back to Newton's time. For example, Einstein used to tell how he was sitting in his office in Berne one day when he was suddenly struck by the thought that a man falling from a roof would not feel the force of gravity while he was falling. That insight into the nature of gravity led directly to the general theory of relativity—physical insight first and then the equations. Exactly the same process was at work
when Newton watched the apple fall from a tree and went on to develop his theory of gravity.

But it seems that science, or at least physics, no longer works like that. One of the pioneers of superstring theory is Michael Green, of Queen Mary College in London. In an article in
Scientific American
in 1986, he pointed out that with string theory

details have come first; we are still groping for a unifying insight into the logic of the theory. For example, the occurrence of the massless graviton . . . appears accidental and somewhat mysterious; one would like them to emerge naturally in a theory after the unifying principles are well established.
²

Another oddity of superstring theory does not seem to trouble the mathematicians but demonstrates all too clearly to lesser mortals how far these ideas have strayed from everyday reality. What appeared to be the best versions of superstring theories, the ones in which gravitons seem to emerge naturally (if mysteriously) from the equations, only work in a little matter of twenty-six dimensions. So if superstrings really do describe the workings of the Universe, where are all the extra dimensions hidden?