Parallel Worlds (39 page)

But if gravity
were a bit stronger, this would cause stars to heat up too fast, and they would
burn up their fuel so quickly that life could never get started. Also, a
stronger gravity would mean that galaxies would form earlier and would be quite
small. The stars would be more densely packed, making disastrous collisions
between various stars and planets.

Third is Omega,
the relative density of the universe. If Omega were too small, then the
universe would have expanded and cooled too fast. But if Omega were too large,
then the universe would have collapsed before life could start. Rees writes,
"At one second after the big bang, Omega cannot have differed from unity
by more than one part in a million billion (one in 10
¹⁵
) in order
that the universe should now, after io billion years, be still expanding and
with a value of Omega that has certainly not departed wildly from unity."

Fourth is
Lambda, the cosmological constant, which determines the acceleration of the
universe. If it were just a few times larger, the antigravity it would create
would blow the universe apart, sending it into an immediate big freeze, making
life impossible. But if the cosmological constant were negative, the universe
would have contracted violently into a big crunch, too soon for life to form.
In other words, the cosmological constant, like Omega, must also be within a
certain narrow band to make life possible.

Fifth is Q, the
amplitude of the irregularities in the cosmic microwave background, which
equals 10
^-5
. If this number were a bit smaller, then the universe
would be extremely uniform, a lifeless mass of gas and dust, which would never
condense into the stars and galaxies of today. The universe would be dark,
uniform, featureless, and lifeless. If Q were larger, then matter would have
condensed earlier in the history of the universe into huge supergalactic
structures. These "great gobs of matter would have condensed into huge
black holes," says Rees. These black holes would be heavier than an entire
cluster of galaxies. Whatever stars can form in these huge cluster of gas would
be so tightly packed that planetary systems would be impossible.

Last is D, the
number of spatial dimensions. Due to interest in M-theory, physicists have
returned to the question of whether life is possible in higher or lower
dimensions. If space is one-dimensional, then life probably cannot exist
because the universe is trivial. Usually, when physicists try to apply the
quantum theory to one- dimensional universes, we find that particles pass
through one other without interacting. So it's possible that universes existing
in one dimension cannot support life because particles cannot "stick"
together to form increasingly complex objects.

In two space
dimensions, we also have a problem because life forms would probably
disintegrate. Imagine a two-dimensional race of flat beings, called
Flatlanders, living on a tabletop. Imagine them trying to eat. The passage
extending from its mouth to its rear would split the Flatlander in half, and he
would fall apart. Thus, it's difficult to imagine how a Flatlander could exist
as a complex being without disintegrating or falling into separate pieces.

Another argument
from biology indicates that intelligence cannot exist in fewer than three
dimensions. Our brain consists of a large number of overlapping neurons
connected by a vast electrical network. If the universe were one- or
two-dimensional, then it would be difficult to construct complex neural
networks, especially if they short-circuit by being placed on top of each
other. In lower dimensions, we are severely limited by the number of complex
logic circuits and neurons we can place in a small area. Our own brain, for
example, consists of about i00 billion neurons, about as many stars as in the
Milky Way galaxy, with each neuron connected to about 10,000 other neurons.
Such complexity would be hard to duplicate in lower dimensions.

In four space
dimensions, one has another problem: planets are not stable in their orbits
around the Sun. Newton's inverse square law is replaced by an inverse cube law,
and in 1917, Paul Ehrenfest, a close colleague of Einstein, speculated about
what physics might look like in other dimensions. He analyzed what is called
the Poisson-Laplace equation (which governs the motion of planetary objects as
well as electric charges in atoms) and found that orbits are not stable in four
or higher spatial dimensions. Since electrons in atoms as well as planets
experience random collisions, this means that atoms and solar systems probably
cannot exist in higher dimensions. In other words, three dimensions are
special.

To Rees, the
anthropic principle is one of the most compelling arguments for the
multiverse. In the same way that the existence of Goldilocks zones for Earth
implies extrasolar planets, the existence of Goldilocks zones for the universe
implies there are parallel universes. Rees comments, "If there is a large
stock of clothing, you're not surprised to find a suit that fits. If there are
many universes, each governed by a differing set of numbers, there will be one
where there is a particular set of numbers suitable to life. We are in that
one." In other words, our universe is the way it is because of the law of
averages over many universes in the multiverse, not because of a grand design.

Weinberg seems
to agree on this point. Weinberg, in fact, finds the idea of a multiverse
intellectually pleasing. He never did like the idea that time could suddenly
spring into existence at the big bang, and that time could not exist before
that. In a multiverse, we have the eternal creation of universes.

There is
another, quirky reason why Rees prefers the multiverse idea. The universe, he
finds, contains a small amount of "ugliness." For example, Earth's
orbit is slightly elliptical. If it were perfectly spherical, then one might
argue, as theologians have, that it was a by-product of divine intervention.
But it is not, indicating a certain amount of randomness within the narrow
Goldilocks band. Similarly, the cosmological constant is not perfectly zero but
is small, which indicates that our universe is "no more special than our
presence requires." This is all consistent with our universe being
randomly generated by accident.

EVOLUTION OF
UNIVERSES

Being an
astronomer, rather than a philosopher, Rees says that the bottom line is that
all these theories have to be testable. In fact, that is the reason why he
favors the multiverse idea rather than competing, mystical theories. The
multiverse theory, he believes, can be tested in the next twenty years.

One variation of
the multiverse idea is actually testable today. Physicist Lee Smolin goes even
further than Rees and assumes that an "evolution" of universes took
place, analogous to Darwinian evolution, ultimately leading to universes like
ours. In the chaotic inflationary theory, for example, the physical constants
of the "daughter" universes have slightly different physical constants
than the mother universe. If universes can sprout from black holes, as some
physicists believe, then the universes that dominate the mul- tiverse are those
that have the most black holes. This means that, as in the animal kingdom, the
universes that give rise to the most "children" eventually dominate
to spread their "genetic information"— the physical constants of
nature. If true, then our universe might have had an infinite number of
ancestor universes in the past, and our universe is a by-product of trillions
of years of natural selection. In other words, our universe is the by-product
of survival of the fittest, meaning it is the child of universes with the
maximum number of black holes.

Although a
Darwinian evolution among universes is a strange and novel idea, Smolin
believes that it can be tested by simply counting the number of black holes.
Our universe should be maximally favorable to the creation of black holes.
(However, one still has to prove that universes with the most black holes are
the ones that favor life, like ours.)

Because this
idea is testable, counterexamples can be considered. For example, perhaps it
can be shown, by hypothetically adjusting the physical parameters of the
universe, that black holes are most readily produced in universes that are
lifeless. For example, perhaps one might be able to show that a universe with a
much stronger nuclear force has stars that burn out extremely quickly,
creating large numbers of supernovae that then collapse into black holes. In such
a universe, a larger value for the nuclear force means that stars live for
brief periods, and hence life cannot get started. But this universe might also
have more black holes, thereby disproving Smolin's idea. The advantage of this
idea is that can be tested, reproduced, or falsified (the hallmark of any true
scientific theory). Time will tell whether it holds up or not.

Although any
theory involving wormholes, superstrings, and higher dimensions is beyond our
current experimental ability, new experiments are now being conducted and
future ones planned that may determine whether these theories are correct or
not. We are in the midst of a revolution in experimental science, with the full
power of satellites, space telescopes, gravity wave detectors, and lasers being
brought to bear on these questions. The bountiful harvest from these
experiments could very well resolve some of the deepest questions in cosmology.

Remarkable claims require
remarkable proof.

—Carl Sagan

Parallel universes,
dimensional
portals, and higher dimensions, as spectacular as they are, require airtight
proof of their existence. As the astronomer Ken Croswell remarks, "Other
universes can get intoxicating: you can say anything you want about them and
never be proven wrong, as long as astronomers never see them." Previously,
it seemed hopeless to test many of these predictions, given the primitiveness
of our experimental equipment. However, recent advances in computers, lasers,
and satellite technology have put many of these theories tantalizingly close
to experimental verification.

Direct
verification of these ideas may prove to be exceedingly difficult, but
indirect verification may be within reach. We sometimes forget that much of
astronomical science is done indirectly. For example, no one has ever visited
the Sun or the stars, yet we know what the stars are made of by analyzing the
light given off by these luminous objects. By analyzing the spectrum of light
within starlight, we know indirectly that the stars are made primarily of
hydrogen and some helium. Likewise, no one has ever seen a black hole, and in
fact black holes are invisible and cannot be directly seen. However, we see
indirect evidence of their existence by looking for accretion disks and
computing the mass of these dead stars.

In all these
experiments, we look for "echoes" from the stars and black holes to
determine their nature. Likewise, the eleventh dimension may be beyond our
direct reach, but there are ways in which inflation and superstring theory may
be verified, in light of the new revolutionary instruments now at our disposal.

GPS AND RELATIVITY

The simplest
example of the way satellites have revolutionized research in relativity is
the Global Positioning System (GPS), in which twenty-four satellites
continually orbit Earth, emitting precise, synchronized pulses which allow one
to triangulate one's position on the planet to remarkable accuracy. The GPS has
become an essential feature of navigation, commerce, as well as warfare.
Everything from computerized maps inside cars to cruise missiles depends on the
ability to synchronize signals to within 50 billionths of a second to locate an
object on Earth to within 15 yards. But in order to guarantee such incredible
accuracy, scientists must calculate slight corrections to Newton's laws due to
relativity, which states that radio waves will be slightly shifted in frequency
as satellites soar in outer space. In fact, if we foolishly discard the
corrections due to relativity, then the GPS clocks will run faster each day by
40,000 billions of a second, and the entire system will become unreliable.
Relativity theory is thus absolutely essential for commerce and the military.
Physicist Clifford Will, who once briefed a U.S. Air Force general about the
crucial corrections to the GPS coming from Einstein's theory of relativity,
once commented that he knew that relativity theory had come of age when even senior
Pentagon officials had to be briefed on it.

GRAVITY WAVE
DETECTORS

So far, almost
everything we know about astronomy has come in the form of electromagnetic
radiation, whether it's starlight or radio or microwave signals from deep
space. Now scientists are introducing the first new medium for scientific
discovery, gravity itself. "Every time we have looked at the sky in a new
way, we have seen a new universe," says Gary Sanders of Cal Tech and
deputy director of the gravity wave project.

It was Einstein,
in 1916, who first proposed the existence of gravity waves. Consider what
would happen if the Sun disappeared. Recall the analogy of a bowling ball
sinking into a mattress? Or better, a trampoline net? If the ball is suddenly
removed, the trampoline net will immediately spring back into its original
position, creating shock waves that ripple outward along the trampoline net. If
the bowling ball is replaced by the Sun, then we see that shock waves of
gravity travel at a specific speed, the speed of light.