Parallel Worlds (30 page)

Two die-hards
who continued to work on the theory during those bleak years were John Schwarz
of Cal Tech and Joel Scherk of the Ecole Normale Superieure in Paris. Until
then, the string model was supposed to describe just the strong nuclear
interactions. But there was a problem: the model predicted a particle that did
not occur in the strong interactions, a curious particle with zero mass that
possessed 2 quantum units of spin. All attempts to get rid of this pesky
particle had failed. Every time one tried to eliminate this spin-2 particle,
the model collapsed and lost its magical properties. Somehow, this unwanted
spin-2 particle seemed to hold the secret of the entire model.

Then Scherk and
Schwarz made a bold conjecture. Perhaps the flaw was actually a blessing. If
they reinterpreted this worrisome spin-2 particle as the graviton (a particle
of gravity arising from Einstein's theory), then the theory actually
incorporated Einstein's theory of gravity! (In other words, Einstein's theory
of general relativity simply emerges as the lowest vibration or note of the
super- string.) Ironically, while in other quantum theories physicists
strenuously try to avoid including any mention of gravity, string theory
demands it. (That, in fact, is one of the attractive features of string
theory—that it must include gravity or else the theory is inconsistent.) With
this daring leap, scientists realized that the string model was incorrectly
being applied to the wrong problem. It was not meant to be a theory of just the
strong nuclear interactions; it was instead a theory of everything. As Witten
has emphasized, one attractive feature of string theory is that it demands the
presence of gravity. While standard field theories have failed for decades to
incorporate gravity, gravity is actually obligatory in string theory.

Scherk and
Schwarz's seminal idea, however, was universally ignored. For string theory to
describe both gravity and the subatomic world, it meant that the strings would
have to be only 10
^-33
cm long (the Planck length); in other words,
they were a billion billion times smaller than a proton. This was too much for
most physicists to accept.

But by the
mid-1980s, other attempts at a unified field theory had floundered. Theories
that tried to naively attach gravity to the Standard Model were drowning in a
morass of infinities (which I shall explain shortly). Every time someone tried
to artificially marry gravity with the other quantum forces, it led to
mathematical inconsistencies that killed the theory. (Einstein believed that
perhaps God had no choice in creating the universe. One reason for this might
be that only a single theory is free of all these mathematical
inconsistencies.)

There were two
such kinds of mathematical inconsistencies. The first was the problem of
infinities. Usually, quantum fluctuations are tiny. Quantum effects are usually
only a small correction to Newton's laws of motion. This is why we can, for the
most part, ignore them in our macroscopic world—they are too small to be noticed.
However, when gravity is turned into a quantum theory, these quantum
fluctuations actually become infinite, which is nonsense. The second
mathematical inconsistency has to do with "anomalies," small
aberrations in the theory that arise when we add quantum fluctuations to a
theory. These anomalies spoil the original symmetry of the theory, thereby
robbing it of its original power.

For example,
think of a rocket designer who must create a sleek, streamlined vehicle to
slice through the atmosphere. The rocket must possess great symmetry in order
to reduce air friction and drag (in this case, cylindrical symmetry, so the
rocket remains the same when we rotate it around its axis). This symmetry is
called O(2). But there are two potential problems. First, because the rocket
travels at such great velocity, vibrations can occur in the wings. Usually,
these vibrations are quite small in subsonic airplanes. However, traveling at
hypersonic velocities, these fluctuations can grow in intensity and eventually
tear the wing off. Similar divergences plague any quantum theory of gravity.
Normally, they are so small they can be ignored, but in a quantum theory of
gravity they blow up in your face.

The second
problem with the rocket ship is that tiny cracks may occur in the hull. These
flaws ruin the original O(2) symmetry of the rocket ship. Tiny as they are,
these flaws can eventually spread and rip the hull apart. Similarly, such
"cracks" can kill the symmetries of a theory of gravity.

There are two
ways to solve these problems. One is to find Band- Aid solutions, like patching
up the cracks with glue and bracing the wings with sticks, hoping that the
rocket won't explode in the atmosphere. This is the approach historically
taken by most physicists in trying to marry quantum theory with gravity. They
tried to brush these two problems under the rug. The second way to proceed is
to start all over again, with a new shape and new, exotic materials that can
withstand the stresses of space travel.

Physicists had
spent decades trying to patch up a quantum theory of gravity, only to find it
hopelessly riddled with new divergences and anomalies. Gradually, they realized
the solution might be to abandon the Band-Aid approach and adopt an entirely
new theory.

STRING BANDWAGON

In 1984, the
tide against string theory suddenly turned. John Schwarz of Cal Tech and Mike
Green, then at Queen Mary's College in London, showed that string theory was
devoid of all the inconsistencies that had killed off so many other theories.
Physicists already knew that string theory was free of mathematical
divergences. But Schwarz and Green showed that it was also free of anomalies.
As a result, string theory became the leading (and today, the only) candidate
for a theory of everything.

Suddenly, a
theory that had been considered essentially dead was resurrected. From a theory
of nothing, string theory suddenly became a theory of everything. Scores of
physicists desperately tried to read the papers on string theory. An avalanche
of papers began to pour out of research laboratories around the world. Old
papers that were gathering dust in the library suddenly became the hottest
topic in physics. The idea of parallel universes, once considered too outlandish
to be true, now came center stage in the physics community, with hundreds of
conferences and literally tens of thousands of papers devoted to the subject.

(At times,
things got out of hand, as some physicists got "Nobel fever." In August,
1991,
Discover
magazine even
splashed on its cover the sensational title: "The New Theory of
Everything: A Physicist Tackles the Ultimate Cosmic Riddle." The article
quoted one physicist who was in hot pursuit of fame and glory: "I'm not
one to be modest. If this works out, there will be a Nobel Prize in it,"
he boasted. When faced with the criticism that string theory was still in its
infancy, he shot back, "The biggest string guys are saying it would take
four hundred years to prove strings, but I say they should shut up.")

The gold rush
was on.

Eventually,
there was a backlash against the "superstring bandwagon." One
Harvard physicist has sneered that string theory is not really a branch of
physics at all, but actually a branch of pure mathematics, or philosophy, if
not religion. Nobel laureate Sheldon Glashow of Harvard led the charge,
comparing the superstring bandwagon to the Star Wars program (which consumes
vast resources yet can never be tested). Glashow has said that he is actually
quite happy that so many young physicists work on string theory, because, he
says, it keeps them out of his hair. When asked about Witten's comment that
string theory may dominate physics for the next fifty years, in the same way
that quantum mechanics dominated the last fifty years, he replies that string
theory will dominate physics the same way that Kaluza-Klein theory (which he
considers "kooky") dominated physics for the last fifty years, which
is not at all. He tried to keep string theorists out of Harvard. But as the
next generation of physicists shifted to string theory, even the lone voice of
a Nobel laureate was soon drowned out. (Harvard has since hired several young
string theorists.)

COSMIC MUSIC

Einstein once
said that if a theory did not offer a physical picture that even a child could
understand, then it was probably useless. Fortunately, behind string theory
there is a simple physical picture, a picture based on music.

According to
string theory, if you had a supermicroscope and could peer into the heart of an
electron, you would see not a point particle but a vibrating string. (The
string is extremely tiny, at the Planck length of 10
^-33
cm, a
billion billion times smaller than a proton, so all subatomic particles appear
pointlike.) If we were to pluck this string, the vibration would change; the
electron might turn into a neutrino. Pluck it again and it might turn into a
quark. In fact, if you plucked it hard enough, it could turn into any of the
known subatomic particles. In this way, string theory can effortlessly explain
why there are so many subatomic particles. They are nothing but different
"notes" that one can play on a superstring. To give an analogy, on a
violin string the notes A or B or C sharp are not fundamental. By simply
plucking the string in different ways, we can generate all the notes of the
musical scale. B flat, for example, is not more fundamental than G. All of
them are nothing but notes on a violin string. In the same way, electrons and
quarks are not fundamental, but the string is. In fact, all the subparticles of
the universe can be viewed as nothing but different vibrations of the string.
The "harmonies" of the string are the laws of physics.

Strings can
interact by splitting and rejoining, thus creating the interactions we see
among electrons and protons in atoms. In this way, through string theory, we
can reproduce all the laws of atomic and nuclear physics. The
"melodies" that can be written on strings correspond to the laws of
chemistry. The universe can now be viewed as a vast symphony of strings.

Not only does
string theory explain the particles of the quantum theory as the musical notes
of the universe, it explains Einstein's relativity theory as well—the lowest
vibration of the string, a spin- two particle with zero mass, can be
interpreted as the graviton, a particle or quantum of gravity. If we calculate
the interactions of these gravitons, we find precisely Einstein's old theory of
gravity in quantum form. As the string moves and breaks and reforms, it places
enormous restrictions on space-time. When we analyze these constraints, we
again find Einstein's old theory of general relativity. Thus, string theory
neatly explains Einstein's theory with no additional work. Edward Witten has
said that if Einstein had never discovered relativity, his theory might have
been discovered as a by-product of string theory. General relativity, in some
sense, is for free.

The beauty of
string theory is that it can be likened to music. Music provides the metaphor
by which we can understand the nature of the universe, both at the subatomic
level and at the cosmic level. As the celebrated violinist Yehudi Menuhin once
wrote, "Music creates order out of chaos; for rhythm imposes unanimity
upon the divergent; melody imposes continuity upon the disjointed; and harmony
imposes compatibility upon the incongruous."

Einstein would
write that his search for a unified field theory would ultimately allow him to
"read the Mind of God." If string theory is correct, we now see that
the Mind of God represents cosmic music resonating through ten-dimensional
hyperspace. As Gottfried Leibniz once said, "Music is the hidden
arithmetic exercise of a soul unconscious that it is calculating."

Historically,
the link between music and science was forged as early as the fifth century
b.c.
, when the Greek
Pythagoreans discovered the laws of harmony and reduced them to mathematics.
They found that the tone of a plucked lyre string corresponded to its length.
If one doubled the length of a lyre string, then the note went down by a full
octave. If the length of a string was reduced by two- thirds, then the tone
changed by a fifth. Hence, the laws of music and harmony could be reduced to
precise relations between numbers. Not surprisingly, the Pythagoreans' motto
was "All things are numbers." Originally, they were so pleased with
this result that they dared to apply these laws of harmony to the entire
universe. Their effort failed because of the enormous complexity of matter. However,
in some sense, with string theory, physicists are going back to the Pythagorean
dream.

Commenting on
this historic link, Jamie James once said, "Music and science were [once]
identified so profoundly that anyone who suggested that there was any essential
difference between them would have been considered an ignoramus, [but now]
someone proposing that they have anything in common runs the risk of being labeled
a philistine by one group and a dilettante by the other—and, most damning of
all, a popularizer by both."

PROBLEMS IN HYPERSPACE

But if higher dimensions actually exist in nature and not
only in pure mathematics, then string theorists have to face the same problem
that dogged Theodr Kaluza and Felix Klein back in 1921 when they formulated the
first higher-dimensional theory: where are these higher dimensions?