Parallel Worlds (37 page)

THE END?

If M-theory is
successful, if it is indeed a theory of everything, is it the end of physics as
we know it?

The answer is
no. Let me give you an example. Even if we know the rules of chess, knowing the
rules does not make us a grand master. Similarly, knowing the laws of the
universe does not mean that we are grand masters in terms of understanding its
rich variety of solutions.

Personally, I
think it still might be a bit premature to apply M-theory to cosmology,
although it gives a startling new picture of the way the universe might have
begun. The main problem, I think, is that the model is not in its final form.
M-theory may very well be the theory of everything, but I believe that it is
far from finished. The theory has been evolving backward since 1968, and its
final equations have still not been found. (For example, string theory can be
formulated via string field theory, as Kikkawa and I showed years ago. The
counterpart of these equations for M-theory is unknown.)

Several problems
confront M-theory. One is that physicists are now drowning in p-branes. A
series of papers has been written trying to catalog the bewildering variety of
membranes that can exist in different dimensions. There are membranes shaped
like a doughnut with a hole, a doughnut with multiple holes, intersecting membranes,
and so forth.

One is reminded
of what happens when the fabled blind wise men confront an elephant. Touching
the elephant in different places, each comes up with his own theory. One wise
man, touching the tail, says that the elephant is a one-brane (a string).
Another wise man, touching the ear, says that the elephant is a two-brane (a
membrane). Finally, the last says that the other two wise men are wrong.
Touching the legs, which feel like tree trunks, the third wise man says that
the elephant is really a three-brane. Because they are blind, they cannot see
the big picture, that the sum total of a one- brane, two-brane, and three-brane
is nothing but a single animal, an elephant.

Similarly, it's
hard to believe that the hundreds of membranes found in M-theory are somehow
fundamental. At present, we have no comprehensive understanding of M-theory. My
own point of view, which has guided my current research, is that these membranes
and strings represent the "condensation" of space. Einstein tried to
describe matter in purely geometrical terms, as some kind of kink in the fabric
of space-time. If we have a bed sheet, for example, and a kink develops, the
kink acts as if it has a life of its own. Einstein tried to model the electron
and other elementary particles as some kind of disturbance in the geometry of
space-time. Although he ultimately failed, this idea may be resurrected on a
much higher level in M-theory.

I believe
Einstein was on the right track. His idea was to generate subatomic physics via
geometry. Instead of trying to find a geometric analog to point particles,
which was Einstein's strategy, one could revise it and try to construct a
geometric analog of strings and membranes made of pure space-time.

One way to see
the logic of this approach is to look at physics historically. In the past,
whenever physicists were confronted with a spectrum of objects, we realized
that there was something more fundamental at the root. For example, when we
discovered the spectral lines emitted from hydrogen gas, we eventually realized
that they originated from the atom, from quantum leaps made by the electron as
it circled the nucleus. Similarly, when confronted by the proliferation of
strong particles in the i950s, physicists eventually realized that they were
nothing but bound states of quarks. And when confronted with the proliferation
of quarks and other "elementary" particles of the Standard Model,
most physicists now believe that they arise out of vibrations of the string.

With M-theory,
we are confronted with the proliferation of p-branes of all type and varieties.
It's hard to believe that these can be fundamental, because there are simply
too many p-branes, and because they are inherently unstable and divergent. A
simpler solution, which agrees with the historical approach, is to assume that
M-theory originates from an even simpler paradigm, perhaps geometry itself.

In order to
settle this fundamental question, we need to know the physical principle
underlying the theory, not just its arcane mathematics. As physicist Brian
Greene says, "Currently, string theorists are in a position analogous to
an Einstein bereft of the equivalence principle. Since Veneziano's insightful
guess in 1968, the theory has been pieced together, discovery by discovery,
revolution by revolution. But a central organizing principle that embraces
these discoveries and all other features of the theory within one overarching
and systematic framework—a framework that makes the existence of each
individual ingredient absolutely inevitable—is still missing. The discovery of
this principle would mark a pivotal moment in the development of string theory,
as it would likely expose the theory's inner workings with unforeseen
clarity."

It would also
make sense of the millions of solutions so far found for string theory, each
one representing a fully self-consistent universe. In the past, it was thought
that, of this forest of solutions, only one represented the true solution of
string theory. Today, our thinking is shifting. So far, there is no way to
select out one universe out of the millions that have been discovered so far.
There is a growing body of opinion that states that if we cannot find the
unique solution to string theory, it's probably because there is none. All
solutions are equal. There is a multiverse of universes, each one consistent
with all the laws of physics. This then leads us to what is called the
anthropic principle and the possibility of a "designer universe."

Numerous
universes might have been botched and bungled throughout an eternity, ere this
system was struck out; much labor lost, many fruitless trials made, and a slow
but continual improvement carried out during infinite ages in the art of
world-making.

—David Hume

When
I
was a child
in second grade, my teacher made a
casual remark that I will never forget. She said, "God so loved the earth,
that He put the earth just right from the sun." As a child of six, I was
shocked by the simplicity and power of this argument. If God had put Earth too
far from the Sun, then the oceans would have frozen. If He had put Earth too
close, then the oceans would have boiled off. To her, this meant that not only
did God exist, but that He was also benevolent, so loving Earth that He put it
just right from the Sun. It made a deep impact on me.

Today,
scientists say that Earth lives in the "Goldilocks zone" from the
Sun, just far enough so that liquid water, the "universal solvent,"
can exist to create the chemicals of life. If Earth were farther from the Sun,
it might become like Mars, a "frozen desert," where temperatures have
created a harsh, barren surface where water and even carbon dioxide are often
frozen solid. Even beneath the soil of Mars one finds permafrost, a permanent
layer of frozen water.

If Earth were
closer to the Sun, then it might become more like the planet Venus, which is
nearly identical to Earth in size but is known as the "greenhouse
planet." Because Venus is so close to the Sun, and its atmosphere is made
of carbon dioxide, the energy of sunlight is captured by Venus, sending
temperatures soaring to 900 degrees Fahrenheit. Because of this, Venus is the
hottest planet, on average, in the solar system. With rains of sulfuric acid,
atmospheric pressures a hundred times greater than those found on Earth, and scorching
temperatures, Venus is perhaps the most hellish planet in the solar system,
largely because it is closer to the Sun than is Earth.

Analyzing my
second grade teacher's argument, scientists would say that her statement is an
example of the anthropic principle, which states that the laws of nature are
arranged so that life and consciousness are possible. Whether these laws are
arranged by some greater design or by accident has been the subject of much
debate, especially in recent years, because of the overwhelming number of
"accidents" or coincidences that have been found which make life and
consciousness possible. To some, this is evidence of a deity who has
deliberately arranged the laws of nature to make life, and us, possible. But to
other scientists, it means we are the by-products of a series of lucky
accidents. Or perhaps, if one believes the ramifications of inflation and
M-theory, there is a multiverse of universes.

To appreciate
the complexity of these arguments, consider first the coincidences that make
life on Earth possible. We live not just within the Goldilocks zone of the Sun,
we also live within a series of other Goldilocks zones. For example, our Moon
is just the right size to stabilize Earth's orbit. If the Moon were much
smaller, even tiny perturbations in Earth's spin would slowly accumulate over
hundreds of millions of years, causing Earth to wobble disastrously and
creating drastic changes in the climate so as to make life impossible. Computer
programs show that without a large Moon (about a third the size of Earth),
Earth's axis might have shifted by as much as 90 degrees over a period of many
millions of years. Since scientists believe the creation of DNA required
hundreds of millions of years of climactic stability, an Earth that periodically
tips on its axis would create catastrophic changes in the weather, making the
creation of

DNA impossible.
Fortunately, our Moon is "just right" in size to stabilize the orbit
of Earth, so that such a disaster will not happen. (The moons of Mars are not
large enough to stabilize its spin. As a result, Mars is slowly beginning to
enter another era of instability. In the past, astronomers believe, Mars might
have wobbled on its axis by as much as 45 degrees.)

Due to small
tidal forces, the Moon is also moving away from Earth at the rate of about 4
centimeters per year; in about 2 billion years, it will be too far to stabilize
Earth's spin. This could be disastrous for life on Earth. Billions of years
from now, not only will the night sky be moonless, we might see an entirely
different set of constellations, as Earth tumbles in its orbit. The weather on
Earth will become unrecognizable, making life impossible.

Geologist Peter
Ward and astronomer Donald Brownlee of the University of Washington write,
"Without the Moon there would be no moonbeams, no month, no lunacy, no
Apollo program, less poetry, and a world where every night was dark and
gloomy. Without the Moon it is also likely that no birds, redwoods, whales,
trilobite, or other advanced life would ever grace the earth."

Similarly,
computer models of our solar system show that the presence of the planet
Jupiter in our solar system is a fortuitous one for life on Earth, because its
immense gravity helps to fling asteroids into outer space. It took almost a
billion years, during the "age of meteors," which extended from 3.5
billion to 4.5 billion years ago, to "clean out" our solar system of
the debris of asteroids and comets left over from its creation. If Jupiter were
much smaller and its gravity much weaker, then our solar system would still be
full of asteroids, making life on Earth impossible, as asteroids plunged into
our oceans and destroyed life. Hence, Jupiter, too, is just the right size.

We also live in
the Goldilocks zone of planetary masses. If Earth were a bit smaller, its
gravity would be so weak that it could not keep its oxygen. If it were too
large, it would retain many of its primordial, poisonous gases, making life
impossible. Earth has "just the right" weight to keep an atmospheric
composition beneficial to life.

We also live in
the Goldilocks zone of permissible planetary orbits. Remarkably, the orbits of
the other planets, except for Pluto, are all nearly circular, meaning that
planetary impacts are quite rare in the solar system. This means that Earth
won't come close to any gas giants, whose gravity could easily disrupt Earth's
orbit. This is again good for life, which requires hundreds of millions of
years of stability.

Likewise, Earth
also exists within the Goldilocks zone of the Milky Way galaxy, about
two-thirds of the way from the center. If the solar system were too close to
the galactic center, where a black hole lurks, the radiation field would be so
intense that life would be impossible. And if the solar system were too far
away, there would not be enough higher elements to create the necessary
elements of life.

Scientists can
provide scores of examples where Earth lies within myriad Goldilocks zones.
Astronomers Ward and Brownlee argue that we live within so many narrow bands or
Goldilocks zones that perhaps intelligent life on earth
is
unique to the galaxy, maybe even to the universe. They
recite a remarkable list of ways that Earth has "just the right"
amount of oceans, plate tectonics, oxygen content, heat content, tilt of its
axis, and so on to create intelligent life. If Earth were outside just one
these very narrow bands, we would not be here to discuss the question.

Was Earth placed
in the middle of all these Goldilocks zones because God loved it? Perhaps. We
can, however, arrive at a conclusion that does not rely on a deity. Perhaps
there are millions of dead planets in space that
are
too close to their suns, whose moons are too small, whose Jupiters are too
small, or that are too close to their galactic center. The existence of
Goldilocks zones with respect to Earth does not necessarily mean that God has
bestowed a special blessing on us; it might simply be a coincidence, one rare
example among millions of dead planets in space that lie outside Goldilocks
zones.