November 3, 2003John G. Cramer1 Time Travel in Physics and Science Fiction John G. Cramer Professor of Physics Department of Physics University of Washington Seattle, Washington

November 3, 2003John G. Cramer2 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 high- gravity 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.

November 3, 2003John G. Cramer3 The Law of Causality 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. Causality, sometimes called “The Arrow-of-Time Problem”, is the least understood of the laws of physics. Here’s what we are up against: The Law of Causality.

November 3, 2003John G. Cramer4 Time Travel Paradoxes in SF There are two basic time travel paradoxes described in science fiction: 1.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? 2.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?

November 3, 2003John G. Cramer5 Resolving Time Paradoxes in SF 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) To answer, you need a model of time, relating past and future.

November 3, 2003John G. Cramer6 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?

November 3, 2003John G. Cramer7 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 inter- universe 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.

November 3, 2003John G. Cramer8 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. Wormhole Physics - 2

November 3, 2003John G. Cramer9 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 mass energy and, starting with vacuum fluctuations, might create stable wormholes. Wormhole Physics - 3 A Wormhole on Times Square

November 3, 2003John G. Cramer10 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 time- like interval and has become a time machine. Wormhole Physics - 4 Thorne and Morris also told us how to convert a wormhole into a Time Machine. To do this: 1.Put Wormhole Mouth A on a space ship while 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. A B

November 3, 2003John G. Cramer11 Construction Problems? 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. 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).

November 3, 2003John G. Cramer12 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 extra- dimensional 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 5 th paperback printing)

November 3, 2003John G. Cramer13 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?

November 3, 2003John G. Cramer14 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. (now in 4 th paperback printing)

November 3, 2003John G. Cramer15 The End The End QUESTIONS?