Time Travel Research Center © 2005 Cetin BAL - GSM:+90 05366063183 -Turkey/Denizli
The science and the fiction of time travel are weird. But the science is weirder.
by Scott Mowbray
When H.G. Wells sent the hero of The Time Machine
into what Wells called "futurity," it was on a grim 30-million-year round-trip
to pretty much the end of Earth time, when the last, poorest excuses for
life were flopping around like squid under a darkening sun. Wells wasn't the
first writer to imagine time travel, but he advanced the idea that a machine,
rather than an angel or a bonk on the head, could accomplish it, and he
pushed his machine to the limit. It moved through futurity like a bucking
bronco: "I have already told you of the sickness and confusion that comes
with time travelling," Wells' hero remarks. "And this time I was not seated
properly in the saddle ..."
To Wells in 1895, time was a dimension much like forward and back, or up and
down, but he gave no clue as to how the machine might move a human being
through the fourth dimension into the future: He just wanted to get there.
Einstein offered an answer seven years later, in 1905, with his Special
Theory of Relativity. Time, by Einstein's equations, was not a fixed
property of the universe (moving in one direction at the same rate for
everyone, which was Newton's view), but a relative property of things in
motion. A clock in motion ticked slower than a stationary clock; a moving
clock traveled into the future relative to the clock at rest. It turned out
that we'd been time traveling all along, we just didn't have clocks precise
enough to show it. (Later we built such clocks, and they did.)
So began a century of strong, almost gravitational, attraction between
physics and fiction, as both orbited around an idea that seems fantastic
whether tackled by Rod Serling or by Einstein's heirs. The neatest, and
certainly the most famous, example of the synergy may be that of Carl Sagan
and his novel Contact. In the early 198os, Sagan turned to a
physicist friend, Kip Thorne of Caltech, when he needed to jump a character
through space. Thorne developed a theory whose byproduct was, essentially, a
blueprint for a time machine that would require a "supercivilization" to
build. Sagan's book became a movie starring Jodie Foster. Thorne's so-called
wormhole theory was published in the eminent journal Physical Review
Letters.
Scientists tell us it's technically possible. Here's a how-to guide for the
ambitious tinkerer.by Michael Moyer
Start with a Black Hole ...
The physical possibility of time traveL is something of a catch-22. Any
object that's surrounded by the twisted space-time that time travel requires
must by its very nature be fantastically perilous, a maelstrom that would
inevitably tear apart the foolhardy traveler. So physicists have labored to
create a theoretically acceptable time machine that's free from nasty side
effects like certain death. Their starting point: black holes.
Black holes are famous for sucking in everything around them—including light—and
never letting go. But black holes have other characteristics, namely the way
they bend nearby space-time. A black hole is infinitely dense, which means
that it pulls the fabric of space-time to the breaking point—creating a deep
pockmark, complete with a tiny rip at the bottom.
Many have wondered what lies on the other side of this rip. In 1935,
Einstein and his colleague Nathan Rosen developed a scenario in which the
tiny rip in a black hole could be connected to another tiny rip in another
black hole, joining two disparate parts of space-time via a narrow channel,
or throat. The Einstein-Rosen bridge, as the notion was then called, looks
like a black hole attached to a mirror image of itself.
This bridge—a sort of back door leading from the interior of one black hole
into another—is today known as a wormhole. Such a portal could in theory
create a shortcut through space-time—just the thing a time traveler would
need if he wanted to cheat Father Time out of a few million years.
Next, Modify the Wormhole ...
The problem with wormholes is that the channel created between two black
holes is minuscule, smaller than the center of a single atom, and remains
open for only a fraction of a second. Even light, the fastest entity in the
universe, would not have enough time to pass through. And no matter how
sturdy his spacecraft, our traveler would inevitably be ripped apart by the
black hole's immense gravitational forces. Because of these and other
problems, the Einstein-Rosen bridge was for many years thought of as a
geometric curiosity, a theoretical quirk that could never be of use to even
a fictional time traveler. Einstein's equations might allow for wormholes,
but the universe certainly did not. All that changed in the 1980s, however,
when a physicist at the California Institute of Technology devised a better
way to use wormholes as time machines.
If Einstein and Rosen are the architects of the
space-time shortcut, then Kip Thorne of Caltech is its structural engineer.
Starting from the rough sketch that Einstein and Rosen left behind, Thorne
created an algorithm that describes in strict mathematical terms the physics
of a working time machine. Of course, actually building Thorne's time portal
would require a technological prowess that is at least many centuries away.
But his work proves that time travel is possible—at least in theory.
Thorne's problem was finding a way to hold open the wormhole's channel, or
throat, long enough for an explorer to pass through. Ordinary matter won't
do: No matter how strong it is, any scaffolding made of matter cannot brace
against the crush of space-time. Thorne needed a substance that could
counteract the squeeze of a black hole. Thorne needed antigravity.
Instead of contracting the space around it, as ordinary matter does,
antigravity—or negative energy, as it is sometimes called—pushes it apart.
In theory, antigravity would be placed inside a wormhole's throat, opening
it wide enough for an astronaut, or possibly even a spaceship, to pass
through. Antigravity does the trick; the problem is finding it. Einstein
first postulated the existence of antigravity on cosmic scales in 1915, a
conjecture proven correct eight decades later. But Einstein's antigravity is
wispy and dilute, a spoonful of sugar dissolved in the Pacific Ocean.
Opening a wormhole requires a regular torrent of antigravity.
The best current candidate for creating concentrated antigravity is called
the Casimir effect. Because of the quirks of quantum mechanics, two flat
metal plates held a hair's width apart generate a small amount of negative
energy. That energy, multiplied many times over, could in principle be used
to create a traversable wormhole. The widening, meanwhile, would dilute the
strength of nearby gravity, preventing the traveler from being torn apart.
Once the antigravity scaffolding is holding open the
portal, the traveler passing through would emerge in a distant place. But
time travelers, of course, want to journey not just geographically but
temporally. So Thorne's next step was to desynchronize the two regions on
either side of the wormhole.
To do this, he applied an old trick of Einstein's. A major consequence of
Einstein's Special Theory of Relativity is that time slows for objects that
move quickly. Thorne applied this principle to one of the two black holes
that make up a wormhole. Imagine lassoing one of the black holes—perhaps by
trapping it inside a cage of negative energy—and towing it around the
universe at close to the speed of light. That black hole, and therefore that
end of the wormhole, would age more slowly than the stationary end of the
wormhole. Over time, the black holes would become desynchronized, two
objects connected through the wormhole but existing in different eras. An
explorer who entered the stationary end of the wormhole would exit the
moving end, many years earlier than when he departed, making the wormhole a
true time portal.
Or Try It on a Shoestring
The most recent development in the physics of time travel came in 1991, when
Princeton astrophysicist J. Richard Gott III suggested that hypothetical
objects called cosmic strings might enable an astronaut to travel backward
in time. Cosmic strings are long, thin objects that some cosmologists
believe coalesced out of the universe's very earliest days. They are
infinitely long, no wider than a single atom, and so dense that a few miles
of a single cosmic string would outweigh Earth itself.
Gott's proposal relies on idealized versions of cosmic strings. In order to
be em-ployed in the service of a time traveler, two cosmic strings,
perfectly parallel and traveling at nearly the speed of light, must whiz
past one another like two cars traveling in opposite directions on a highway.
As the strings pass each other, space-time would become profoundly distorted
by the influence of these fast-moving filaments. A savvy time traveler,
waiting in a nearby spaceship, could exploit those distortions by flying
around the coupled strings. If he timed it just right, the twists in space-time
would enable him to return to his starting point before he began—making the
voyage a one-way trip back in time. Which means that, according to the laws
of physics, journeys through time are conceivable, if rather difficult to
arrange. It may be only a matter of time.
It takes a lot of gravity to significantly warp
time. A black hole has such enormous gravitational force that it creates a
tear in space-time itself, but a black hole is no portal because it will
suck a would-be time traveler into the cramped quarters of infinite density,
forever. A properly engineered wormhole, however, theoretically creates a
passage between two black holes that leads to another place in the universe
through the space-time tear; a bit of galactic-scale fiddling with one end
of the wormhole turns it into a time machine (more
on wormhole engineering).
"What Kip Thorne and his colleagues noted," says Gott, "was that if you
moved the wormhole mouth correctly, Jodie Foster (in the film version of
Contact) could have come back before she left... . Jodie Foster would
have been waiting at the same spot to shake hands with Jodie Foster when she
arrived." The notion of creating a hopeless causal loop in time is
childishly easy to understand. In Back to the Future, Michael J. Fox finds
himself fading from existence after journeying back in the souped-up
DeLorean and attracting his mother's romantic interest at a time when she
was supposed to be falling in love with Fox's future father. (According to
one academic paper, this "is the first science fiction film to make explicit
the incestuous possibilities that have always been at the heart of our
fascination with time travel.")
Davies says the paradoxes of time travel have repelled some physicists, who
were afraid of being ridiculed. Igor Novikov, a Russian astrophysicist who
has written extensively on the subject, says that for decades, "very serious
mathematicians, very serious physicists were not brave enough to declare
that time travel is possible."
Many resolutions to the paradox have been proposed. One simply maintains
that the universe won't let paradoxes be created: If you try to kill your
grandfather, you won't be able to. You'll change your mind, or the gun won't
go off, or you'll be a lousy shot. This notion, it will be noted, has
serious implications for the existence of free will. Other approaches say
there really are no paradoxes; every problem can be solved mathematically
without producing a paradoxical inconsistency.
But the paradox problem does not bother David Deutsch, a theoretical
physicist at Oxford University. Deutsch dislikes the violent aspect of the
grandfather paradox, which he says is only confused "by the issue of human
conflict that people put into the story to make it more interesting."
Deutsch has instead created a gentler paradox.
In this experiment, a person watches a time machine to see whether a copy of
himself emerges on, say, Tuesday. If it does not, on Wednesday he journeys
back in time one day—emerging from the time machine on the same Tuesday when
he had not emerged before. The experiment can be reversed: If, in the
opposite scenario, he does emerge on the Tuesday, he simply waits until
Wednesday and chooses not to get in the time machine. In either case, a
paradox is created: The time traveler is there on Tuesday and not there at
the same time—a phenomenon that, intriguingly, echoes some of the
fundamental mysteries of particle behavior at the quantum level.
Indeed, Deutsch reaches into quantum mechanics for an explanation of the
paradox. He is a leading proponent of the many worlds theory, in which
multiple new universes are triggered by each quantum event. If two subatomic
particles collide, for example, one of the particles does not go either
right or left, but rather it goes right in one universe and left in another
universe that is instantly created in the so-called multiverse. In the
Deutsch experiment, one universe contains the lab in which the experimenter
did not journey back, another the lab in which he did: paradox resolved.
In any case, a hundred years after the Special Theory of Relativity, time
travel is theoretically possible, and therefore theoretically in the future
of human development. So the big question is: When will we have a machine to
test the theories? Wormhole engineering doesn't offer much promise, since it
involves near-light-speed travel, antigravity, and the like. But Paul Davies
is hopeful. He says we would begin by sending a particle, rather than a
human being, back in time to test the paradoxes. Emerging theories about the
nature of gravitation already suggest, he says, that "it might be possible
to reconfigure space-time with energies that could be accessible by the next-generation
of particle accelerator." If so, "We're talking about more like 100 years,"
Davies says, "than about who knows how many years."
> Reported by Peter Kobel
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