Transcript Time Travel in Physics and Science Fiction John G. Cramer Professor of Physics Department of Physics University of Washington Seattle, Washington 98195-1560 November 3, 2003 John G.

Time Travel in
Physics and Science Fiction
John G. Cramer
Professor of Physics
Department of Physics
University of Washington
Seattle, Washington 98195-1560
November 3, 2003
John G. Cramer
1
Time Travel and Communication
• We all can travel through time (but always
forward).
• We also can communicate through time (but
always from past to future).
• Relativity tells us how to move forward through
time more slowly (go fast or go to a highgravity region), and even how to stop time
altogether (go to a black hole’s event horizon).
• The trick would be to move backwards through
time, or to communicate from future to past.
• Perhaps those things are possible. That’s what
we will consider tonight.
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The Law of Causality
Here’s what we are up against: The Law of Causality.
Causality, sometimes called “The Arrow-of-Time
Problem”, is the least understood of the laws of physics.
Classical: A cause must precede all of its effects.
Special Relativity: Matter and information
cannot be transmitted across spacelike
or negative timelike intervals. (v < c)
General Relativity: No such prejudices
about the direction of time or the
construction of timelike loops. The
speed of light is only a local speed limit.
Maybe there’s a way around Causality using General Relativity.
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Time Travel Paradoxes in SF
1.
2.
There are two basic time travel paradoxes
described in science fiction:
Killing your Grandmother - you use a
time machine to go into the past and kill
your grandmother when she was a small
child.
Question: What happens to you?
The Invention from the Future - you
send the plans for an important invention to
your past self and become rich and famous.
Question: Where did the invention come
from?
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Resolving Time Paradoxes in SF
To answer, you need a model of time, relating past and future.
1. Block Universe: the past is fixed and
cannot be changed – By His Bootstraps, All
You Zombies (Robert A. Heinlein)
2. Mutable Past: the past can be changed,
and it changes the future – Terminator,
Back to the Future (Movies)
3. Branching Universes – Changing the past
creates a new branch of the universe –
Timescape (Gregory Benford)
4. Timelike Loops Unravel and Reknit the
Fabric of Time – Einstein’s Bridge (John
Cramer)
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Time Travel in Physics
Until 1988, time travel was considered to be
outside the realm of physics.
Then Kip Thorne and his student Michael
Morris published a paper describing how
wormholes, a construct of general
relativity, could be localized, stabilized,
and converted into time machines.
Now the physics literature contains hundreds of papers on
time travel.
But is it really possible?
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Wormhole Physics - 1
In 1935, A. Einstein and N. Rosen
published a paper in Physical
Review 48, 73 (1935) showing that
implicit in the general relativity
formalism is a curved-space
structure that can join two distant
regions of space-time through a
tunnel-like curved spatial shortcut.
The purpose of the paper of was not
to promote faster-than-light or interuniverse travel, but to attempt to explain fundamental particles like electrons as
space-tunnels threaded by electric lines of force. Their particle model was
subsequently shown to be invalid when it was realized that the smallest possible
mass-energy of such a curved-space topology is a Planck mass, far larger than the
mass-energy of an electron. Their spatial short-cut subsequently became known
as an Einstein-Rosen Bridge, rechristened “wormhole” by John Wheeler.
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Wormhole Physics - 2
In 1962 John Wheeler and a
collaborator discovered that the
Einstein-Rosen bridge space-time
structure, which Wheeler re-christened
as a "wormhole," was dynamically
unstable in field-free space.
They showed that if such a
wormhole somehow opened, it would
close up again before even a single
photon could be transmitted through it,
thereby preserving Einsteinian
causality. Physicists were relieved.
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Wormhole Physics - 3
In 1989 Kip Thorne and his graduate
student Mike Morris showed that a
wormhole might be snatched from the
quantum foam and stabilized by a
region of space containing negative
mass-energy.
They suggested that an "advanced
civilization" capable of manipulating
planet-scale quantities of mass-energy
might use the Casimir effect to
produce such a region of negative
A Wormhole on Times Square
mass energy and, starting with
vacuum fluctuations, might create
stable wormholes.
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Wormhole Physics - 4
Thorne and Morris also told us how to convert
A
a wormhole into a Time Machine. To do this:
1. Put Wormhole Mouth A on a space ship while
B
Wormhole Mouth B remains on Earth
2. Launch the space ship with Wormhole Mouth A to travel at
near-light velocities on a voyage of several light years.
3. Bring Wormhole Mouth A back to Earth.
Because of relativistic time dilation, Wormhole
Mouth A will now be younger than Wormhole
Mouth B by several years. They now connect
the past to the present, even if they are in the
same room. The wormhole now spans a timelike interval and has become a time machine.
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Construction Problems?
In quantum field theory, the interaction between two particles
is typically requires division by the space-time interval separating the
particles. If this interval goes to zero (as it would along many paths
threading a timelike wormhole), the interaction become singular (1/0).
Steven Hawking has suggested that
in any attempt to create a Morris-Thorne
timelike wormhole, this singularity
condition would occur and would prevent
creation of a time machine. He suggests that
Nature Abhors a Time Machine, that the
quantum vacuum would rise up and smite
the would-be time machine builder.
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Einstein’s Bridge (Avon-1997)
In my novel, Einstein’s Bridge, set at the never-built
Superconducting Super Collider (SSC), I deal with these
issues as follows:
• Collisions at the SSC produces extradimensional signals that propagate
to other bubble-universes.
• Intelligent aliens in another universe
use such signals to establish wormhole
contact with physicists at the SSC to
warn them of an impending disaster.
• SSC Physicists use a time-like wormhole
to destroy the universe (a la Hawking)
back to start of the wormhole, so that
the universe can re-evolve from that
point (without the SSC).
(now in 5th paperback printing)
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Conclusions
• It may be possible to construct a time
machine without violating the laws of
physics.
• We have some ideas (wormholes, non-linear quantum
nonlocality, …) but no technology that could
accomplish it at the moment. Thorne and Morris (and
I) invoked an “advanced civilization” to do the job.
• Finally, there is also “The Fermi Paradox of Time
Travel”:
If there are Time Travelers, why aren’t they here?
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Afterword: T2 and Twistor
Twistor (Morrow/Avon-1989)
In 1989, I was on a committee with Glenn F. Knoll,
a Professor of Nuclear Engineering at the
University of Michigan. He told me that his son
worked for Industrial Light and Magic and had
done the “water-snake” effects in the movie The
Abyss. I gave Glenn a hardcover copy of Twistor,
and asked that after he read it, he pass it along to
his son to have a look at movie possibilities.
I never hear from the younger Knoll or IL&M, but I
did notice that the scene in Terminator 2 in which
the Terminator first appears at the center of a large
sphere, with things cut off at the edges, looks a lot
like one of my scenes in Twistor.
If you’re going to be ripped off, I guess its preferable
to be ripped off by the best.
November 3, 2003
John G. Cramer
(now in 4th paperback printing)
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The
QUESTIONS?
End
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