Parallel Worlds (13 page)

What was also historic
about Guth's discovery was that it represented the application of elementary
particle physics, which involves analyzing the tiniest particles found in
nature, to cosmology, the study of the universe in its entirety, including its
origin. We now realize that the deepest mysteries of the universe cannot be
solved without the physics of the extremely small: the world of the quantum
theory and elementary particle physics.

SEARCH FOR UNIFICATION

Guth was born in
1947 in New Brunswick, New Jersey. Unlike Einstein, Gamow, or Hoyle, there was
no instrument or seminal moment that propelled him into the world of physics.
Neither of his parents graduated from college or showed much interest in
science. But by his own admission he was always fascinated by the relationship
between math and the laws of nature.

At MIT in the
1960s, he seriously considered a career in elementary particle physics. In
particular, he was fascinated by the excitement generated by a new revolution
sweeping through physics, the search for the unification of all fundamental
forces. For ages, the holy grail of physics has been to search for unifying
themes that can explain the complexities of the universe in the simplest, most
coherent fashion. Since the time of the Greeks, scientists have thought that
the universe we see today represents the broken, shattered remnants of a
greater simplicity, and our goal is to reveal this unification.

After two
thousand years of investigation into the nature of matter and energy,
physicists have determined that just four fundamental forces drive the
universe. (Scientists have tried to look for a possible fifth force, but so far
all results in this direction have been negative or inconclusive.)

The first force
is gravity, which holds the Sun together and guides planets in their celestial
orbits in the solar system. If gravity were suddenly "turned off,"
the stars in the heavens would explode, Earth would disintegrate, and we would
all be flung into outer space at about a thousand miles an hour.

The second great
force is electromagnetism, the force that lights up our cities, fills our world
with TV, cell phones, radio, laser beams, and the Internet. If the
electromagnetic force were suddenly shut down, civilization would be instantly
hurled a century or two into the past into darkness and silence. This was
graphically illustrated by the great blackout of 2003, which paralyzed the
entire Northeast. If we examine the electromagnetic force microscopically, we
see that it is actually made of tiny particles, or quanta, called photons.

The third force
is the weak nuclear force, which is responsible for radioactive decay. Because
the weak force is not strong enough to hold the nucleus of the atom together,
it allows the nucleus to break up or decay. Nuclear medicine in hospitals
relies heavily on the nuclear force. The weak force also helps to heat up the
center of Earth via radioactive materials, which drive the immense power of
volcanoes. The weak force, in turn, is based on the interactions of electrons
and neutrinos (ghost-like particles that are nearly massless and can pass
through trillions of miles of solid lead without interacting with anything).
These electrons and neutrinos interact by exchanging other particles, called
W- and Z-bosons.

The strong
nuclear force holds the nuclei of the atoms together. Without the nuclear
force, the nuclei would all disintegrate, atoms would fall apart, and reality
as we know it would dissolve. The strong nuclear force is responsible for the
approximately one hundred elements we see filling up the universe. Together,
the weak and strong nuclear forces are responsible for the light emanating from
stars via Einstein's equation
E = mc
²
.
Without the nuclear force, the entire universe would be
darkened, plunging the temperature on Earth and freezing the oceans solid.

The astonishing
feature of these four forces is that they are entirely different from each
other, with different strengths and properties. For example, gravity is by far
the weakest of the four forces, 10
³⁶
times weaker than the
electromagnetic force. The earth weighs 6 trillion trillion kilograms, yet its
massive weight and its gravity can easily be canceled by the electromagnetic
force. Your comb, for example, can pick up tiny pieces of paper via static electricity,
thereby canceling the gravity of the entire earth. Also, gravity is strictly
attractive. The electromagnetic force can be both attractive or repulsive,
depending on the charge of a particle.

UNIFICATION AT THE BIG BANG

One of the
fundamental questions facing physics is: why should the universe be ruled by
four distinct forces? And why should these four forces look so dissimilar, with
different strengths, different interactions, and different physics?

Einstein was the
first to embark upon a campaign to unify these forces into a single,
comprehensive theory, starting by uniting gravity with the electromagnetic
force. He failed because he was too far ahead of his time; too little was known
about the strong force to make a realistic unified field theory. But Einstein's
pioneering work opened the eyes of the physics world to the possibility of a
"theory of everything."

The goal of a
unified field theory seemed utterly hopeless in the 1950s, especially when
elementary particle physics was in total chaos, with atom smashers blasting
nuclei apart to find the "elementary constituents" of matter, only
to find hundreds more particles streaming out of the experiments.
"Elementary particle physics" became a contradiction in terms, a
cosmic joke. The Greeks thought that, as we broke down a substance to its basic
building blocks, things would get simpler. The opposite happened: physicists
struggled to find enough letters in the Greek alphabet to label these
particles. J. Robert Oppenheimer joked that the Nobel Prize in physics should
go to the physicist who did not discover a new particle that year. Nobel
laureate Steven Weinberg began to wonder whether the human mind was even
capable of solving the secret of the nuclear force.

This bedlam of
confusion, however, was somewhat tamed in the early 1960s when Murray Gell-Mann
and George Zweig of Cal Tech proposed the idea of quarks, the constituents that
make up the protons and neutrons. According to quark theory, three quarks make
up a proton or a neutron, and a quark and antiquark make up a meson (a particle
that holds the nucleus together). This was only a partial solution (since today
we are flooded with different types of quarks), but it did serve to inject new
energy into a once dormant field.

In 1967, a
stunning breakthrough was made by physicists Steven Weinberg and Abdus Salam,
who showed that it was possible to unify the weak and electromagnetic forces.
They created a new theory whereby electrons and neutrinos (which are called
leptons) interacted with each other by exchanging new particles called the W-
and Z-bosons as well as photons. By treating the W- and Z-bosons and photons on
the very same footing, they created a theory which unified the two forces. In
1979, Steven Weinberg, Sheldon Glashow, and Abdus Salam were awarded the Nobel
Prize for their collective work in unifying two of the four forces, the
electromagnetic force with the weak force, and providing insight into the
strong nuclear force.

In the 1970s,
physicists analyzed the data coming from the particle accelerator at the
Stanford Linear Accelerator Center (SLAC), which fired intense beams of
electrons at a target in order to probe deep into the interior of the proton.
They found that the strong nuclear force that held the quarks together inside
the proton could be explained by introducing new particles called gluons, which
are the quanta of the strong nuclear force. The binding force holding the
proton together could be explained by the exchange of gluons between the
constituent quarks. This led to a new theory of the strong nuclear force called
Quantum Chromodynamics.

So by the mid
1970s, it was possible to splice three of the four forces together (excluding
gravity) to get what is called the Standard Model, a theory of quarks,
electrons, and neutrinos, which interact by exchanging gluons, W- and Z-bosons,
and photons. It is the culmination of decades of painfully slow research in
particle physics. At present, the Standard Model fits all the experimental data
concerning particle physics, without exception.

Although the
Standard Model is one of the most successful physical theories of all time, it
is remarkably ugly. It is hard to believe that nature at a fundamental level
can operate on a theory that seems to be so cobbled together. For example,
there are nineteen arbitrary parameters in the theory that are simply put in
by hand, without any rhyme or reason (that is, the various masses and interaction
strengths are not determined by the theory but have to be determined by
experiment; ideally, in a true unified theory, these constants would be
determined by the theory itself, without relying on outside experiments).

Furthermore,
there are three exact copies of elementary particles, called generations. It
is hard to believe that nature, at its most fundamental level, would include
three exact copies of subatomic particles. Except for the masses of these
particles, these generations are duplicates of each other. (For example, the
carbon copies of the electron include the muon, which weighs 200 times more
than the

These are
the subatomic particles contained within the Standard Model, the most
successful theory of elementary particles. It is built out of quarks, which
make up the protons and neutrons, leptons like the electron and neutrino, and
many other particles. Notice that the model results in three identical copies
of subatomic particles. Since the Standard Model fails to account for gravity
(and seems so awkward), theoretical physicists feel it cannot be the final
theory.
electron, and the tau particle, which weighs 3,500 times more.) And last, the
Standard Model makes no mention of gravity, although gravity is perhaps the
most pervasive force in the universe.

Because
the Standard Model, notwithstanding its stunning experimental successes, seems
so contrived, physicists tried to develop yet another theory, or the grand
unified theory (GUT), which put the quarks and leptons on the same footing. It
also treated the gluon, the W- and Z-boson, and the photon on the same level.
(It could not be the "final theory," however, because gravity was
still conspicuously left out; it was considered too difficult to merge with the
other forces, as we shall see.)

This program of
unification, in turn, introduced a new paradigm to cosmology. The idea was
simple and elegant: at the instant of the big bang, all four fundamental forces
were unified into a single, coherent force, a mysterious
"superforce." All four forces had the same strength and were part of
a larger, coherent whole. The universe started out in a state of perfection.
However, as the universe began to expand and cool rapidly, the original
superforce began to "crack," with different forces breaking off one
after the other.

According to
this theory, the cooling of the universe after the big bang is analogous to the
freezing of water. When water is in liquid form, it is quite uniform and
smooth. However, when it freezes, millions of tiny ice crystals form inside.
When liquid water is totally frozen, its original uniformity is quite broken,
with the ice containing cracks, bubbles, and crystals.

In other words,
today we see that the universe is horribly broken. It is not uniform or
symmetrical at all but consists of jagged mountain ranges, volcanoes,
hurricanes, rocky asteroids, and exploding stars, without any coherent unity;
moreover, we also see the four fundamental forces without any relationship to
each other. But the reason why the universe is so broken is that it is quite
old and cold.

Although the
universe started in a state of perfect unity, today it has gone through many
phase transitions, or changes of state, with the forces of the universe
breaking free of the others one by one as it cooled. It is the job of
physicists to go backward, to reconstruct the steps by which the universe
originally started (in a state of perfection) and which led to the broken
universe we see around us.

The key,
therefore, is to understand precisely how these phase transitions occurred at
the beginning of the universe, which physicists call "spontaneous
breaking." Whether it is the melting of ice, the boiling of water, the
creation of rain clouds, or the cooling of the big bang, phase transitions can
connect two entirely different phases of matter. (To illustrate how powerful
these phase transitions can be, the artist Bob Miller has asked the riddle:
"How would you suspend 500,000 pounds of water in the air with no visible
means of support? The answer: build a cloud.")

FALSE VACUUM

When one force
breaks off from the other forces, the process can be compared to the breaking
of a dam. Rivers flow downhill because water flows in the direction of the
lowest energy, which is sea level. The lowest energy state is called a vacuum.
However, there is an unusual state called the false vacuum. If we dam a river,
for example, the dam appears to be stable, but it is actually under tremendous
pressure. If a tiny crack occurs in the dam, the pressure can suddenly burst
the dam and release a torrent of energy from the false vacuum (the dammed
river) and cause a catastrophic flood toward the true vacuum (sea level).
Entire villages can be flooded if we have spontaneous breaking of the dam and a
sudden transition to the true vacuum.