Friday, December 17, 2004

On Godel and time travel

It's time for more general relativity blogging.

I just read an interesting article about the Einstein and Godel, who were friends while concurrently working at the Institute for Advanced Study. The author touches on a lot of important ideas of physics and math, but I wanted to focus on his take on Godel's discovery of solutions to Einstein's equations of general relativy that correspond to universes with time loops (closed timelike curves in current technical jargon). Equivalently, Godel showed that GR does not prohibit time machines. The author suggests that Godel's results indicates that time breaks down, just as Godel's Incompleteness Theorem proves that Hilbert's program breaks down. But as Stephen Hawking noted, "we have not been invaded by hordes of tourists from the future." I think it far, far more plausible that Godel's discovery of time loop solutions to GR provides additional evidence that GR is an incomplete description of spacetime, quantum physics already providing more than enough reasons to think this.

Take a look at Matt Visser's slides from a talk on the physics of time travel, skipping the parts you don't understand. The bottom line is that the physical possibility of time travel depends on how gravity behaves on quantum scales.

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