Movement into the past gets one step less improbable.
One of the major difficulties of travelling backwards in time has just been solved, according to an Israeli theoretical physicist. And the solution, he says, is doughnut-shaped.
Trips in time have been theoretically possible ever since Einstein worked out that heavy masses can warp both time and space, and that objects travelling close to the speed of light tend to experience the passage of time more slowly.
Moving forwards in time is therefore easy. Certain short-lived cosmic particles, for example, can be seen on Earth. Their journey looks to us as if it has taken thousands of years, but the particle feels as though it has whipped across space in just a few minutes, and arrives on Earth before it has had time to decay. In effect, the particle has travelled into the future, living beyond its years.
But getting back to the past is more problematic. Researchers thought you would need all kinds of strange things to do this, including a neutron star (which we know to exist), worm holes (which we don't), and a kind of exotic matter that we can only imagine.
Time present and time past
This is where Amos Ori from Technion, the Israel Institute of Technology in Haifa, comes in. He says that according to Einstein's theories, space can be twisted enough to create a local gravity field that looks like a doughnut of some arbitrary size. The gravitational field lines circle around this doughnut, so that space and time are both tightly curved back on themselves. Crucially, this does away with the need for any hypothetical exotic matter.
Although it is difficult to describe what this would look or be like in real life, Ori says the mathematics reveal that every period of time after the time machine was created would be somewhere in the vacuum inside the doughnut. All you need to do is work out how to get there.
In theory, it should be possible to travel back to any point in time after the time machine was built, reports Ori in Physical Review Letters1. One slight snag is that he has not worked out how to generate the gravitational doughnut, although he has some ideas. "It's wild speculation, but you may need to move large masses rapidly in a circular motion," Ori says.
"The paper is a welcome addition to the subject, and it does look like an improvement on the previous models," says Paul Davies, a theoretical physicist at Macquarie University in Sydney, Australia, and author of How to Build a Time Machine.
The leading model of travel into the past involves zipping through a wormhole, which offers a shortcut between two distant points in space. If you could connect a wormhole between Earth and something very heavy, such as a neutron star, this would set up a time difference between the two ends. This is thanks to the fact that mass can warp space and time, such that a clock on the surface of a dense neutron star would run about 30% slower than it does on Earth.
But wormholes are tricky beasts, and need something to stop them collapsing under their own intense gravity. Kip Thorne, a theoretical physicist at the California Institute of Technology, Pasadena, has speculated that some form of exotic antigravity matter would be needed to keep the wormhole open. Unfortunately for eager time lords, physicists have never seen anything like this.
A perpetual possibility
There are still difficulties to overcome with the doughnut model, however. Davis thinks that the instability of the compact vacuum core might be an insurmountable problem. "Closed time-like curves are inherently unstable against quantum fluctuations," he says. He expects a huge energy surge inside the doughnut would probably destroy it.
Ori agrees that energy fluctuations might be problematic, but thinks this could be soluble. "Unfortunately it's not going to be in existence in our generation, or maybe ever," he says. Still, he puts the chances of ever being able to construct a time machine at 50:50.
OriA., et al. Phys. Rev. Lett., 95. 021101 (2005).