'About time: Is time travel possible?' http://preview.tinyurl.com/TimeTravelRealitiy
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- 13 October 2011 by Marcus Chown
- Magazine issue 2833
IT IS easy to dismiss time travel as nothing more than science fiction. After all, H. G. Wells wrote The Time Machine in the late 1800s, but still no one has built one that works. Don't give up yet, though: we are continuing to make discoveries that may show us the way forward - or back.
Time travel is inherent in the basics of general relativity. Einstein's theory predicts that time runs more slowly in strong gravity, so you grow old more slowly living in a bungalow than in a skyscraper: being closer to the ground, you are in marginally stronger gravity (see "Personal time warps"). So to make a time machine, you simply have to connect two regions where time flows at different rates.
Take, for instance, the Earth and the immediate vicinity of a black hole, where strong gravity makes time flow extremely slowly. Say you start two clocks ticking on Monday at the two locations. When Friday comes around on Earth, it will still be only Wednesday by the black hole. So if you could travel instantaneously from Earth to near the black hole, you could travel from Friday back to Wednesday. Hey presto: time travel.
The question is, can you? Yes - in principle. According to quantum theory, the fabric of space-time is a tangle of sub-microscopic shortcuts through space and time known as wormholes. A few steps along such a tunnel and you might emerge light years away on the other side of the galaxy, or years in the past or future. It is possible that ghostly particles called neutrinos might already be performing such a feat (New Scientist, 1 October, p 6).
For the rest of us, however, there are a few practical problems to sort out first. To use a wormhole for time travel, it has to link the times and places you want to travel between: that might mean somehow towing one end to the nearest black hole.
Manage that and you've still got issues: you would need to inflate the quantum-scale wormhole to macroscopic size and find a way to keep its entrance and exit open. Quite some challenge, because wormholes are terminally unstable and snap shut in the blink of an eye. To prop them open, you will need a hypothetical type of matter with repulsive gravity. We do not know whether such exotic matter with sufficient strength exists. But what we do know is that to create a tunnel with a mouth about a metre across - wide enough for someone to crawl through - you would have to use the total energy pumped out by a large fraction of the stars in our Milky Way in a year.
For all that effort, such a time machine will never take us back to great moments in history. If we find a wormhole, it is by definition the first moment that time travellers to the past will be able to reach. So if you want to go on a dinosaur safari, you have only one option: find a time machine abandoned on Earth by extraterrestrials at least 65 million years ago.
Nonetheless, we could do some interesting things with our own time machine. As soon as we have made one, for instance, future civilisations will be able to come back and visit us. That opens up an interesting possibility: could someone come back and kill a direct ancestor, making their own existence impossible? This is time travel's most famous conceptual puzzle, the "grandfather paradox". And it turns out that quantum physics may have an answer.
For years now, quantum physicists have been "teleporting" particles by copying the information that describes a particle and pasting it onto another, distant one. In January, Seth Lloyd of the Massachusetts Institute of Technology and Aephraim Steinberg of the University of Toronto, Canada, showed that quantum rules allow this kind of teleportation to be done in time as well as space. Because the quantum states of particles such as photons and electrons can be affected by measurements that will be done in their futures, time travel comes naturally to the quantum world.
Lloyd and Steinberg's experiments showed that, with photons at least, the mechanics of time travel conspire to uphold familiar notions of cause and effect. They set up photons to travel backwards in time and then flip their polarisation state. This flip corresponded to the photon entering a state that meant it could not have travelled back in time in the first place; the new state "kills" the earlier one.
Because of the probabilities involved in quantum measurements there was always a chance of either process failing to happen. Lloyd and Steinberg found that when they set up the photon to kill its "grandfather", either the time travel or the polarisation flip always failed.
It's an example of what Stephen Hawking at the University of Cambridge calls chronology protection. As the difficulty of creating a wormhole time-machine also shows, the laws of physics seem determined to maintain common-sense rules of cause and effect. Nonetheless, the door to time travel is still firmly open.
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