Dr. Friedman's Response

Dear Andreea, Brian, Helen, Lindsey, and Todd,

Here are quick replies to the last two questions:

3) Is the concept of time travel inextricably linked to metaphysics?

Causality is a non-scientific term, yet it appears consistently in any discussion of time travel. Causality has a technical meaning in physics: It means first that no signal can travel faster than the speed of light (which might more accurately be called the speed of information). In particular, no object can move faster light. When this first condition is satisfied (and it is), then a model of the universe is said to be causal if one can consistently define a direction of time and if no future directed particle trajectory can pass twice through the same point.

4) What are the reasons for your personal fascination with time?

General relativity, the precise mathematical description of gravity interms of the geometry of spacetime, is now well tested. Highly accurate solar system tests were carried out more than a decade ago;and the last 15 years has seen the dramatic verification of its prediction of gravitational waves by observation of the spiraling together of two neutron stars (the Hulse-Taylor binary pulsar), with accuracy in the rate of energy loss to gravitational waves tested to 0.1%.

These equations have solutions that include, in addition to the gravitational waves and black holes that have been seen, a variety of objects that are not observed: These include, in particular, wormholes and more exotic topological structures, and closed timelike curves, regions of spacetime that violate causality and permit signals to loop back to the past. The deep question here is what prevents these objects from being part of our universe. The work that many of us have done on the "Chronology protection conjecture" has gone some distance to answering why one does not encounter macroscopically large regions with closed timelike curves. The work I did with Kristin Schleich and Don Witt proved the "Topological Censorship Theorem", showing that topological structures collapse too quickly for light to traverse them; this is part of the reason why the topology of space that we see is trivial. (Although space is dull in this way from distances much smaller than nuclei to distances much larger than galaxies, on the smallest scale and on the largest scale, the universe is likely to have interesting topology.)

Regards,

John Friedman