Time Travel Research Center
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How to Build a Time Machine
It wouldn't be easy, but it might be possible
By Paul Davies
September 2002 issue
WORMHOLE GENERATOR/TOWING MACHINE
is imagined by futurist
artist Peter Bollinger. This painting depicts a gigantic space-based
particle accelerator that is capable of creating, enlarging and moving
wormholes for use as time machines.
Image: PETER BOLLINGER
forward in time is easy enough. If you move close to the speed of
light or sit in a strong gravitational field, you experience time more
slowly than other people do--another way of saying that you travel
into their future.Traveling into the past is rather trickier.
Relativity theory allows it in certain spacetime configurations: a
rotating universe, a rotating cylinder and, most famously, a wormhole--a
tunnel through space and time. Time travel has been a popular science-fiction
theme since H. G. Wells wrote his celebrated novel The Time Machine in
1895. But can it really be done? Is it possible to build a machine
that would transport a human being into the past or future? For
decades, time travel lay beyond the fringe of respectable science. In
recent years, however, the topic has become something of a cottage
industry among theoretical physicists. The motivation has been partly
recreational--time travel is fun to think about. But this research has
a serious side, too. Understanding the relation between cause and
effect is a key part of attempts to construct a unified theory of
physics. If unrestricted time travel were possible, even in principle,
the nature of such a unified theory could be drastically affected. Our
best understanding of time comes from Einstein's theories of
relativity. Prior to these theories, time was widely regarded as
absolute and universal, the same for everyone no matter what their
physical circumstances were. In his special theory of relativity,
Einstein proposed that the measured interval between two events
depends on how the observer is moving. Crucially, two observers who
move differently will experience different durations between the same
Time Machine in Three Not So Easy Steps
The effect is often described using the "twin paradox." Suppose
that Sally and Sam are twins. Sally boards a rocket ship and travels
at high speed to a nearby star, turns around and flies back to Earth,
while Sam stays at home. For Sally the duration of the journey might
be, say, one year, but when she returns and steps out of the spaceship,
she finds that 10 years have elapsed on Earth. Her brother is now nine
years older than she is. Sally and Sam are no longer the same age,
despite the fact that they were born on the same day. This example
illustrates a limited type of time travel. In effect, Sally has leaped
nine years into Earth's future.
The effect, known as time dilation, occurs whenever two observers
move relative to each other. In daily life we don't notice weird time
warps, because the effect becomes dramatic only when the motion occurs
at close to the speed of light. Even at aircraft speeds, the time
dilation in a typical journey amounts to just a few nanoseconds--hardly
an adventure of Wellsian proportions. Nevertheless, atomic clocks are
accurate enough to record the shift and confirm that time really is
stretched by motion. So travel into the future is a proved fact, even
if it has so far been in rather unexciting amounts.
A Wormhole Time Machine in Three Not So Easy Steps
Image: PHILIP HOWE
observe really dramatic time warps, one has to look beyond the realm
of ordinary experience. Subatomic particles can be propelled at nearly
the speed of light in large accelerator machines. Some of these
particles, such as muons, have a built-in clock because they decay
with a definite half-life; in accordance with Einstein's theory, fast-moving
muons inside accelerators are observed to decay in slow motion. Some
cosmic rays also experience spectacular time warps. These particles
move so close to the speed of light that, from their point of view,
they cross the galaxy in minutes, even though in Earth's frame of
reference they seem to take tens of thousands of years. If time
dilation did not occur, those particles would never make it here.
Speed is one way to jump ahead in time. Gravity is another. In his
general theory of relativity, Einstein predicted that gravity slows
time. Clocks run a bit faster in the attic than in the basement, which
is closer to the center of Earth and therefore deeper down in a
gravitational field. Similarly, clocks run faster in space than on the
ground. Once again the effect is minuscule, but it has been directly
measured using accurate clocks. Indeed, these time-warping effects
have to be taken into account in the Global Positioning System. If
they weren't, sailors, taxi drivers and cruise missiles could find
themselves many kilometers off course.
Mother of All Paradoxes
Click here for interactive demonstration
the surface of a neutron star, gravity is so strong that time is
slowed by about 30 percent relative to Earth time. Viewed from such a
star, events here would resemble a fast-forwarded video. A black hole
represents the ultimate time warp; at the surface of the hole, time
stands still relative to Earth. This means that if you fell into a
black hole from nearby, in the brief interval it took you to reach the
surface, all of eternity would pass by in the wider universe. The
region within the black hole is therefore beyond the end of time, as
far as the outside universe is concerned. If an astronaut could zoom
very close to a black hole and return unscathed--admittedly a fanciful,
not to mention foolhardy, prospect--he could leap far into the future.
My Head Is Spinning
So far I have discussed travel forward in time. What about going
backward? This is much more problematic. In 1948 Kurt Gödel of the
Institute for Advanced Study in Princeton, N.J., produced a solution
of Einstein's gravitational field equations that described a rotating
universe. In this universe, an astronaut could travel through space so
as to reach his own past. This comes about because of the way gravity
affects light. The rotation of the universe would drag light (and thus
the causal relations between objects) around with it, enabling a
material object to travel in a closed loop in space that is also a
closed loop in time, without at any stage exceeding the speed of light
in the immediate neighborhood of the particle. Gödel's solution was
shrugged aside as a mathematical curiosity--after all, observations
show no sign that the universe as a whole is spinning. His result
served nonetheless to demonstrate that going back in time was not
forbidden by the theory of relativity. Indeed, Einstein confessed that
he was troubled by the thought that his theory might permit travel
into the past under some circumstances.
Other scenarios have been found to permit travel into the past. For
example, in 1974 Frank J. Tipler of Tulane University calculated that
a massive, infinitely long cylinder spinning on its axis at near the
speed of light could let astronauts visit their own past, again by
dragging light around the cylinder into a loop. In 1991 J. Richard
Gott of Princeton University predicted that cosmic strings--structures
that cosmologists think were created in the early stages of the big
bang--could produce similar results. But in the mid-1980s the most
realistic scenario for a time machine emerged, based on the concept of
Image: EVERETT COLLECTION
science fiction, wormholes are sometimes called stargates; they offer
a shortcut between two widely separated points in space. Jump through
a hypothetical wormhole, and you might come out moments later on the
other side of the galaxy. Wormholes naturally fit into the general
theory of relativity, whereby gravity warps not only time but also
space. The theory allows the analogue of alternative road and tunnel
routes connecting two points in space. Mathematicians refer to such a
space as multiply connected. Just as a tunnel passing under a hill can
be shorter than the surface street, a wormhole may be shorter than the
usual route through ordinary space.
Click here for a sidebar on existing forms of forward time travel
The wormhole was used as a fictional device by Carl Sagan in his
1985 novel Contact. Prompted by Sagan, Kip S. Thorne and his co-workers
at the California Institute of Technology set out to find whether
wormholes were consistent with known physics. Their starting point was
that a wormhole would resemble a black hole in being an object with
fearsome gravity. But unlike a black hole, which offers a one-way
journey to nowhere, a wormhole would have an exit as well as an
wormhole to be traversable, it must contain what Thorne termed
exotic matter. In effect, this is something that will generate
antigravity to combat the natural tendency of a massive system to
implode into a black hole under its intense weight. Antigravity, or
gravitational repulsion, can be generated by negative energy or
pressure. Negative-energy states are known to exist in certain quantum
systems, which suggests that Thorne's exotic matter is not ruled out
by the laws of physics, although it is unclear whether enough
antigravitating stuff can be assembled to stabilize a wormhole [see "Negative
Energy, Wormholes and Warp Drive," by Lawrence H. Ford and Thomas A.
Roman; Scientific American, January 2000].
Soon Thorne and his colleagues realized that if a stable wormhole
could be created, then it could readily be turned into a time machine.
An astronaut who passed through one might come out not only somewhere
else in the universe but somewhen else, too--in either the future or
The wormhole was used as a fictional device by Carl Sagan in
his novel Contact.
To adapt the wormhole for time travel, one of its mouths could be
towed to a neutron star and placed close to its surface. The gravity
of the star would slow time near that wormhole mouth, so that a time
difference between the ends of the wormhole would gradually accumulate.
If both mouths were then parked at a convenient place in space, this
time difference would remain frozen in.
Suppose the difference were 10 years. An astronaut passing through
the wormhole in one direction would jump 10 years into the future,
whereas an astronaut passing in the other direction would jump 10
years into the past. By returning to his starting point at high speed
across ordinary space, the second astronaut might get back home before
he left. In other words, a closed loop in space could become a loop in
time as well. The one restriction is that the astronaut could not
return to a time before the wormhole was first built.
formidable problem that stands in the way of making a wormhole time
machine is the creation of the wormhole in the first place. Possibly
space is threaded with such structures naturally--relics of the big
bang. If so, a supercivilization might commandeer one. Alternatively,
wormholes might naturally come into existence on tiny scales, the so-called
Planck length, about 20 factors of 10 as small as an atomic nucleus.
In principle, such a minute wormhole could be stabilized by a pulse of
energy and then somehow inflated to usable dimensions.
Assuming that the engineering problems could be overcome, the
production of a time machine could open up a Pandora's box of causal
paradoxes. Consider, for example, the time traveler who visits the
past and murders his mother when she was a young girl. How do we make
sense of this? If the girl dies, she cannot become the time traveler's
mother. But if the time traveler was never born, he could not go back
and murder his mother.
Paradoxes of this kind arise when the time traveler tries to change
the past, which is obviously impossible. But that does not prevent
someone from being a part of the past. Suppose the time traveler goes
back and rescues a young girl from murder, and this girl grows up to
become his mother. The causal loop is now self-consistent and no
longer paradoxical. Causal consistency might impose restrictions on
what a time traveler is able to do, but it does not rule out time
travel per se.
Even if time travel isn't strictly paradoxical, it is certainly
weird. Consider the time traveler who leaps ahead a year and reads
about a new mathematical theorem in a future edition of Scientific
American. He notes the details, returns to his own time and
teaches the theorem to a student, who then writes it up for
Scientific American. The article is, of course, the very one that
the time traveler read. The question then arises: Where did the
information about the theorem come from? Not from the time traveler,
because he read it, but not from the student either, who learned it
from the time traveler. The information seemingly came into existence
from nowhere, reasonlessly.
The bizarre consequences of time
travel have led some scientists to reject the notion outright. Stephen
W. Hawking of the University of Cambridge has proposed a "chronology
protection conjecture," which would outlaw causal loops. Because the
theory of relativity is known to permit causal loops, chronology
protection would require some other factor to intercede to prevent
travel into the past. What might this factor be? One suggestion is
that quantum processes will come to the rescue. The existence of a
time machine would allow particles to loop into their own past.
Calculations hint that the ensuing disturbance would become self-reinforcing,
creating a runaway surge of energy that would wreck the wormhole.
Chronology protection is still just a conjecture, so time travel
remains a possibility. A final resolution of the matter may have to
await the successful union of quantum mechanics and gravitation,
perhaps through a theory such as string theory or its extension, so-called
M-theory. It is even conceivable that the next generation of particle
accelerators will be able to create subatomic wormholes that survive
long enough for nearby particles to execute fleeting causal loops.
This would be a far cry from Wells's vision of a time machine, but it
would forever change our picture of physical reality.
It is conceivable that the next generation of particle
accelerators will be able to create subatomic wormholes
More to Explore:
Time Travel in Einstein's Universe: The Physical Possibilities of
Travel through Time. J. Richard Gott III. Houghton Mifflin, 2001.
The Quantum Physics of Time Travel. David Deutsch and Michael
Lockwood in Scientific American, Vol. 270, No. 3, pages 6874;
March 1994. Available at the Archive
Black Holes and Time Warps: Einstein's Outrageous Legacy. Kip S.
Thorne. W. W. Norton, 1994.
How to Build a Time Machine. Paul Davies. Viking, 2002.
Time Machines: Time Travel in Physics, Metaphysics, and Science
Fiction. Paul J. Nahin. American Institute of Physics, 1993.
PAUL DAVIES is a theoretical physicist at the Australian Center for
Astrobiology at Macquarie University in Sydney. He is one of the most
prolific writers of popular-level books in physics. His scientific
research interests include black holes, quantum field theory, the
origin of the universe, the nature of consciousness and the origin of
life. Davies is also the winner of the 2002 Michael Faraday Prize,
given annually by the Royal Society to the scientist who "has done
most to further public communication of science, engineering or
technology in the U.K."
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Reproduction in whole or in part without permission is prohibited
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