Parallel Worlds (20 page)

But in 2003, new
evidence scientists collected suggested that gamma ray bursters are the result
of a "hypernova" that creates a massive black hole. By rapidly
focusing telescopes and satellites in the direction of gamma ray bursters,
scientists found that they resembled a massive supernova. Since the exploding
star has an enormous magnetic field and ejects radiation via its north and
south polar directions, it might appear as if the supernova is more energetic
than it actually is—that is, we observe these bursters only if they are pointed
directly at Earth, giving the false impression that they are more powerful than
they really are.

If indeed gamma
ray bursters are black holes in formation, then the next generation of space
telescopes should be able to analyze them in great detail and perhaps answer some
of our deepest questions about space and time. Specifically, if black holes
can bend space into a pretzel, can they also bend time?

VAN
STOCKUM'STIME MACHINE

Einstein's
theory links space and time into an inseparable unity. As a result, any
wormhole that connects two distant points in space might also connect two
distant points in time. In other words, Einstein's theory allows for the
possibility of time travel.

The concept of
time itself has evolved over the centuries. To Newton, time was like an arrow;
once fired, it never changed course and traveled unerringly and uniformly to
its target. Einstein then introduced the concept of warped space, so time was
more like a river that gently speeded up or slowed down as it meandered through
the universe. But Einstein worried about the possibility that perhaps the river
of time can bend back on itself. Perhaps there could be whirlpools or forks in
the river of time.

In 1937, this
possibility was realized when W. J. Van Stockum found a solution to Einstein's
equations which permitted time travel. He began with an infinite, spinning
cylinder. Although it's not physically possible to build an infinite object, he
calculated that if such a cylinder spun around at or near the speed of light,
it would drag the fabric of space-time along with it, much like molasses is
dragged along with the blades of a blender. (This is called frame- dragging,
and it has now been experimentally seen in detailed photographs of rotating
black holes.)

Anyone brave
enough to travel around the cylinder would be swept along, attaining fantastic
speeds. In fact, to a distant observer, it would appear that the individual was
exceeding the speed of light. Although Van Stockum himself did not realize it
at the time, by making a complete trip around the cylinder, you could actually
go back in time, returning before you left. If you left at noon, then by the
time you returned to your starting point, say, it might be 6 p.m. the previous
night. The faster the cylinder spun, the further back in time you would go (the
only limitation being that you could not go further back in time than the
creation of the cylinder itself).

Since the
cylinder is like a maypole, every time you danced around the pole, you would
wind up further and further back in time. Of course, one could dismiss such a
solution because cylinders cannot be infinitely long. Also, if such a cylinder
could be built, the centrifugal forces on the cylinder, because it spins near
the speed of light, would be enormous, causing the material that made up the
cylinder to fly apart.

GODEL UNIVERSE

In 1949, Kurt
Godel, the great mathematical logician, found an even stranger solution to
Einstein's equations. He assumed that the entire universe was rotating. Like
the Van Stockum cylinder, one is swept up by the molasses-like nature of
space-time. By taking a rocket ship around the Godel universe, you return to
your starting point but shift back in time.

In Godel's
universe, a person can, in principle, travel between any two points in space
and time in the universe. Every event, in any time period, can be visited, no
matter how distant in the past. Because of gravity, there is a tendency for
Godel's universe to collapse on itself. Hence, the centrifugal force of
rotation must balance this gravitational force. In other words, the universe
must spin above a certain speed. The larger the universe, the greater the tendency
to collapse, and the faster the universe would have to spin to prevent
collapse.

For a universe
our size, for example, Godel calculated that it would have to rotate once every
70 billion years, and the minimum radius for time travel would be 16 billion
light-years. To travel back in time, however, you would have to travel just
below the speed of Hg
^ht
.

Godel was well
aware of the paradoxes that could arise from his solution—the possibility of
meeting yourself in the past and altering the course of history. "By
making a round trip on a rocket ship in a sufficiently wide course, it is
possible in these worlds to travel into any region of the past, present, and
future, and back again, exactly as it is possible in other worlds to travel to
distant parts of space," he wrote. "This state of affairs seems to
imply an absurdity.

For it enables
one to travel into the near past of those places where he has himself lived.
There he would find a person who would be himself at some earlier period of
life. Now he could do something to this person which, by his memory, he knows
has not happened to him."

Einstein was deeply
disturbed by the solution found by his friend and neighbor at the Institute for
Advanced Study at Princeton. His response is quite revealing:

Kurt Godel's essay constitutes, in my opinion, an important
contribution to the general theory of relativity, especially to the analysis
of the concept of time. The problem here involved disturbed me already at the
time of the building up of the general theory of relativity, without my having
succeeded in clarifying it . . . The distinction "earlier- later" is
abandoned for world-points which lie far apart in a cosmo- logical sense, and
those paradoxes, regarding the direction of the causal connection, arise, of
which Mr. Godel has spoken . . . It will be interesting to weigh whether these
are not to be excluded on physical grounds.

Einstein's
response is interesting for two reasons. First, he admitted that the
possibility of time travel bothered him when he first formulated general
relativity. Since time and space are treated like a piece of rubber that can
bend and warp, Einstein worried that the fabric of space-time would warp so
much that time travel might be possible. Second, he ruled out Godel's solution
on the basis of "physical grounds"—that is, the universe does not
spin, it expands.

When Einstein
died, it was widely known that his equations allowed for strange phenomena
(time travel, wormholes). But no one gave them much thought because scientists
felt they could not be realized in nature. The consensus was that these
solutions had no basis in the real world; you would die if you tried to reach
a parallel universe via a black hole; the universe did not spin; and you cannot
make infinite cylinders, making time travel an academic question.

THORNETIME
MACHINE

The issue of
time travel lay dormant for thirty-five years until 1985, when the astronomer
Carl Sagan was writing his novel
Contact
and wanted to incorporate a way in which the heroine could
travel to the star Vega. This would require a two-way journey, one in which the
heroine would travel to Vega and then return to Earth, something that would not
be allowed by black hole-type wormholes. He turned to the physicist Kip Thorne
for advice. Thorne shocked the physics world by finding new solutions to
Einstein's equations that allowed for time travel without many of the previous
problems. In 1988, with colleagues Michael Morris and Ulvi Yurtsever, Thorne
showed that it was possible to build a time machine if one could somehow obtain
strange forms of matter and energy, such as "exotic negative matter"
and "negative energy." Physicists were at first skeptical of this new
solution, since no one had ever seen this exotic matter before, and negative
energy only exists in minute quantities. But it represented a breakthrough in
our understanding of time travel.

The great
advantage of negative matter and negative energy is that they make a wormhole
transversable, so you can make a two- way trip through it without having to
worry about event horizons. In fact, Thorne's group found that a trip through
such a time machine might be quite mild, compared to the stress found on a commercial
airline.

One problem,
however, is that exotic matter (or negative matter) is quite extraordinary in
its properties. Unlike antimatter (which is known to exist and most likely
falls to the ground under Earth's gravitational field), negative matter falls
up, so it will float upward in Earth's gravity because it possesses
antigravity. It is repelled, not attracted, by ordinary matter, and by other
negative matter. This means that it is also quite difficult to find in nature,
if it exists at all. When Earth was first formed 4.5 billion years ago, any
negative matter on Earth would have floated away into deep space. So negative
matter might possibly be floating in space, far away from any planets.
(Negative matter will probably never strike a passing star or planet, since it
is repelled by ordinary matter.)

While negative
matter has never been seen (and quite possibly does not exist), negative energy
is physically possible but extremely rare. In 1933, Henrik Casimir showed that
two uncharged parallel metal plates can create negative energy. Normally, one
would expect that two plates would remain stationary because they are uncharged.
However, Casimir showed that there is a very small attractive force between
these two uncharged parallel plates. In 1948, this tiny force was actually
measured, showing that negative energy was a real possibility. The Casimir
effect exploits a rather bizarre feature of the vacuum. According to the quantum
theory, empty space is teeming with "virtual particles" which dance
in and out of nothingness. This violation of the conservation of energy is
possible because of the Heisenberg uncertainty principle, which allows for
violations of cherished classical laws as long as they occur very briefly. For
example, an electron and antielectron, due to uncertainty, have a certain
small probability of being created out of nothing and then annihilating each
other. Because the parallel plates are very close to each other, these virtual
particles cannot easily come between the two plates. Thus, because there are
more virtual particles surrounding the plates than there are between them,
this creates an inward force from the outside that pushes the parallel plates
together slightly. This effect was precisely measured in 1996 by Steven
Lamoreaux at the Los Alamos National Laboratory. The attractive force he
measured was tiny (equal to the weight of 1/30,000 of an insect like an ant).
The smaller the separation of the plates, the greater the force of attraction.

So here is how
the time machine Thorne dreamed up might operate. An advanced civilization
would start with two parallel plates, separated by an extremely small gap.
These parallel plates would then be reshaped into a sphere, so the sphere
consists of an inner and outer shell. Then they would make two such spheres and
somehow string a wormhole between them, so a tunnel in space connects both
spheres. Each sphere now encloses a mouth of the wormhole.

Normally, time
beats in synchronization for both spheres. But if we now put one sphere into a
rocket ship that is sent speeding near the speed of light, time slows down for
that rocket ship, so that the two spheres are no longer synchronized in time.
The clock on the rocket ship beats much slower than the clock on Earth. Then if
one jumps into the sphere on Earth, one may be sucked through the wormhole
connecting them and wind up in the other rocket ship, sometime in the past.
(This time machine, however, cannot take you back before the creation of the
machine itself.)

PROBLEMS WITH NEGATIVE ENERGY

Although
Thorne's solution was quite sensational when announced, there were severe
obstacles to its actual creation, even for an advanced civilization. First,
one must obtain large quantities of negative energy, which is quite rare. This
type of wormhole depends on a huge amount of negative energy to keep the
wormhole's mouth open. If one creates negative energy via the Casimir effect,
which is quite small, then the size of the wormhole would have to be much
smaller than an atom, making travel through the wormhole impractical. There
are other sources of negative energy besides the Casimir effect, but all of
them are quite difficult to manipulate. For example, physicists Paul Davies and
Stephen Fulling have shown that a rapidly moving mirror can be shown to create
negative energy, which accumulates in front of the mirror as it moves.
Unfortunately, one has to move the mirror at near light speed in order to
obtain negative energy. And like the Casimir effect, the negative energy
created is small.

Another way to
extract negative energy is to use high-powered laser beams. Within the energy
states of the laser, there are "squeezed states" in which positive
and negative energy coexist. However, this effect is also quite difficult to
manipulate. A typical pulse of negative energy might last for 10
^-15
seconds, followed by a pulse of positive energy. Separating positive energy
states from negative energy states is possible, although extremely difficult. I
discuss this more in chapter 11.