Daniel sends me this Physorg article titled: “Professor Predicts Human Time Travel This Century.” (Rule of thumb: never believe any prediction whose time span streches beyond the retirement age of the person making the prediction!) The physicist in this title is University of Connecticut’s Ronald Mallett and the prediction is based, in some ways, on his paper , “The gravitational field of a circulating light beam,” Foundations of Physics 33, 1307 (2003). In this paper the Mallett solves Einstein’s equation for an infinitely long cylinder of rotating light and finds that this solution contains closed timelike curves. A similar construction of an infinite cylinder of rotating dust also produces closed timelike curves (this solution actually predates Godel’s universe historically, although the closed time like curves were not pointed out until after Godel constructed his crazy universe.) I’m always skeptical about these solutions as it is generally the case that there is something unphysical about these solutions. In the above papers case this appears to be the fact the solution is not assymptotically flat. It is also not clear that the solution is robust to perturbations or will be valid for a cyllinder of finite length (although apparently for the dust case the finite length solution has closed time like curves.) But what is fascinating to me is how simple it sometimes appears to be to make general relativistic systems which have closed time like curves. Okay, simple is perhaps not the best word, and of course the real question is whether it is possible to make solutions which don’t have physically bad properties. For some excited crazy optimism along these lines, check out gr-qc/0211051.
Of course in any popular science article about time travel, the question of grandfather violence comes up. Interestingly Mallett deals with the issues of causality for systems with closed time like curves a la David Deutsch:
“The Grandfather Paradox [where you go back in time and kill your grandfather] is not an issue,” said Mallett. “In a sense, time travel means that you’re traveling both in time and into other universes. If you go back into the past, you’ll go into another universe. As soon as you arrive at the past, you’re making a choice and there’ll be a split. Our universe will not be affected by what you do in your visit to the past.”
Which makes me think that this process should not be called “time travel” but should be called rewind, as in “pushing rewind on the VCR and start recording a new program over the old one.” When I was growing up I used to wonder what made us think that if we traveled back in time whether there would actually be anything back there. What a bummer to build a time machine, take a trip into the past, and find that you are the only thing in existence in this past!
Finally, the article ends in a very sad manner:
In light of this causal “safety,” it’s kind of ironic that what prompted Mallett as a child to investigate time travel was a desire to change the past in hopes of a different future. When he was 10 years old, his father died of a heart attack at age 33. After reading The Time Machine by H.G. Wells, Mallett was determined to find a way to go back and warn his father about the dangers of smoking.
Which should convince you that physicists too are only human.
I remember that when I read Bonnor’s paper from 2002 that I thought his conclusions around equ. (2) are flawed. The solution requires a ‘strut’ at r=0, in a real experiments this would be some sort of a ‘stick’ of finite radius, which would make it impossible to experience the CTC at small r=0.
(If one assumes that the ‘stick’ can be arbitrarily thin, one is back at the usual question if matter as we know it allows for this…)
As you wrote already, there is always something unphysical about these examples and I am afraid we have to wait for a true quantum theory of gravitation to answer the question of CTCs.
ah boundary conditions and the abuse or ignorance thereof……..
When I was in grad school Frank Shu used to tell us that what distinguished all great physicists was their ability to deal with boundary conditions.
The AP reported this afternoon that there is already a chiropractor who can time travel. And, in doing so, he can treat patience by reaching back to when the injury occurred and undoing the events that caused the injury. Cool! Proof that time travel is possible and that chiropractors aren’t quacks.
– or not –
It’s entirley possible to travel back in time and fix an injury, though only in another branch of the wave function. In fact this happens routinely. You don’t think so? prove me wrong…
More to the point, if there is a circle in the spacetime geometry, it seems to me one is forced to impose some sort of periodicity. This is simply what it *means* to have a circle, though one can always change the question and discuss the universal cover. Now, such conditions will forbid any kind of complexity, so maybe ultimately reasoning could just be anthropic…
Chronology protection (can CTCs develope in the future) is still up for grabs, all Hawking showed is that GR necessarily breaks down in such situations.
Grr. I always hate it when people link to the paper rather than the abstract. And I’m not sure that cache link will always be valid.
Oops, thanks Aaron. Fixed.
Well, things like this always bring up the sticky question of the nature of space and time. Historically, of course, Heisenberg attempted to abolish the very notion of space-time within the atom (rendering it meaningless in such instances) – changing his mind a year later, but hey, he was entitled. But it brings up the following curious question: how can space-time be meaningless on the quantum level but not on the macroscopic level? Hmmm…