|
|
Time travel: is it really possible?
Time travel had always been a staple of science fiction but with the advent of general relativity it has been contemplated by some serious physicists. According to Einstein, one can travel in time only by opening up a hole in the field of space-time and jump through a gateway or “wormhole” to the fourth dimension. It might be one of our greatest dreams, but for many physicists, it is their worst nightmare.
The problem is that no one has yet come up with the ultimate explanation for why time machines can’t work. The best we have so far is Stephen Hawking’s Chronology Protection Conjecture (CPC) which, in short, suggests that the universe has a built-in time-cop. So whenever anyone is on the threshold of constructing a working time machine the time-cop interferes, shutting the operation down before it has a chance to cause chaos in the past. However, there are no time-cops apparent in the laws of physics so, at the moment, the CPC is simply a wishful thought.
But things are about to change. In the past year, a variety of new approaches to the time travel mystery have appeared. Although different from one another, these approaches have one thing in common: they refer to the string theory, the theory which is the topmost contender for a “theory of everything”. String theory is based on the principle that the basic components of matter are not described correctly when we model them as point-like objects. Rather, according to this theory, the basic “particles” are actually tiny closed loops of string with radii. So with strings, it appears that the time travel loophole may finally be sewn shut.
Actually, physicists wouldn’t have to worry about time travel if it weren’t for the most famous physicist of all. When Albert Einstein came up with his general theory of relativity in 1915, “he unknowingly threw open the doors for chronological violation. General relativity is completely infested with time machines,” says Matt Visser, professor of mathematics in New Zealand. “It certainly seems to permit all of these weird solutions in which time travel is theoretically possible.”
Before general relativity came along, such solutions were unimaginable. According to Newton the direction of time is firm and unalterable. But general relativity —which includes Einstein’s idea which describes gravity as the result of matter distorting space and time — is a very different story. Unlike earlier theories it does not start with a supposed global framework for time, but simply offers the rules for how time is assumed in local circumstances.
“It turns out general relativity relates the distribution of matter ‘here’ to the curvature of space and the flow of time ‘here’, but it doesn’t give you much long-distance information,” says Visser.
Thus, because of its generality, general relativity disturbs no extra assumptions on the nature of space and time as a whole. For example, cosmologists have no way of knowing whether the universe is never-ending simply by reading Einstein’s equations. Additional information is needed to reveal if space stretches out to infinity or curves back on it self.
It is a fact that closed time-like curves are not easily manufactured or exploited. If time travel is possible it can only be achieved with technology on a scale far beyond 21st century civilization. Take Gödel’s classic solution to Einstein’s equations, for instance. It describes a universe that spins rapidly to avoid contraction under gravity. One of the side-effects is that light travels in looping paths instead of straight lines inside the universe and this is how a traveller can time travel, by outrunning light.
The most popular time machine is the Kerr black hole, which, by rotating, stretches the vision into a ring. By passing through the ring in just the right way, one can travel back in time. The trouble is that there is no escape from the black hole. A five-dimensional counterpart, the BMPV black hole, allows closed time-like curves outside the black hole’s boundaries if it is rotating fast enough. It is called BMVP black hole after the physicists Jason Breckenridge, Myers, Cumrun Vafa and Peet.
But that’s not the point; the point is that general relativity doesn’t declare it out, it just tells us that time travel is difficult and expensive. Just because time appears to flow one way in our part of the universe there is nothing explicit in general relativity that says it cannot behave differently elsewhere. In particular, some solutions of Einstein’s equations lead to “closed time-like curves”, unbroken pathways through space-time that allows travellers to loop back in time and bump into earlier versions of them. “Physicists tend to get upset about that,” Visser says.
So one can only wonder about the possibility that Hawking’s CPC may turn out to be wrong. But the fact remains that no one yet knows whether time travel is just a technical challenge or a basic impossibility.
Concerned physicists are taking to mean relativity’s non-judgemental attitude to time travel as a sign that the theory must be incomplete. “In some sense, general relativity is an amazing theory in that it predicts its own demise,” says Rob Myers of the Perimeter Institute in Canada. “Einstein’s theory is telling you it can only bring you so far and then you’re going to need a better theory to understand how physics proceeds from there.”
Lisa Dyson, a graduate student in theoretical physics at MIT also agrees with the fact. “We know that relativity is not the whole story,” she says. “General relativity is a theory of gravity, but there are other forces that govern the world: the strong, weak and electromagnetic forces. Once we understand how all the forces are unified, we may find that time travel is inconsistent with this unified theory.”
At present, forces other than gravity are understood through quantum mechanics. Physicists have been striving for decades to unite quantum mechanics with relativity to produce a theory of “quantum gravity.” And the best candidate so far is the string theory.
String theory is an extensive, multidimensional way of describing the universe. Many physicists expect that string theory will, in some way, clearly rule out time travel. “String theory shows promise as the theory that unifies gravity with the other fundamental forces of nature,” Dyson says. “If our usual notion of chronology is built into our universe, then chronology should be protected in string theory.”
In some specific cases, this expectation is now being borne out. One particularly useful development came recently, when a group led by physicist Jerome Gauntlett, at Queen Mary University of London, was working on a simpler approximation to string theory known as “five-dimensional super-gravity”. Although it is a close family member of the string theory, the group discovered that many solutions to super-gravity allow for time travel in the same way that general relativity does. “What surprised me was that such solutions turn out to be rather common,” says Gauntlett.
Petr Horava, a theorist at the University of California, was teaching a graduate course in string theory when Gauntlett’s paper appeared online. He decided to give over a lecture for dealing with the issues it raised, and then assigned chronology protection as a problem for his students to solve. The idea was to use the tools of string theory to get rid of some or all of the time travel set-ups that showed up in five-dimensional super-gravity.
However, among the other solutions listed in Gauntlett’s analysis is a five-dimensional spinning black hole known as a BMPV black hole. BMPV black holes can become time machines, but only when they are spinning fast enough. They are also the super-gravity equivalent of the Kerr black holes, well known as time machines in general relativity. Dyson, one of Horava’s female students, began to ponder how difficult it would it be to build such an object and spin it up to the right speed. Using only paper and pencil she set about a task that would have made the universe tremble - constructing a whirling five-dimensional black hole from scratch!
How did she do that? “It’s not so hard,” said Dyson, “it’s similar to constructing an ordinary black hole. You start with empty space, and then bring in matter from all directions until there is a sufficient amount in a small enough region, and the black hole will form. In the same away,” she says, “a BMPV black hole is made of constituents brought in from an infinite distance, like a contracting shell. But instead of ordinary matter, the necessary constituents turn out to be gravitational waves and D-branes.” D-branes are creatures of string theory most easily understood as multidimensional membranes or hyper-surfaces that occupy a 10-dimensional space-time. In the mathematically simpler world of the BMPV black hole, D-branes appear as particles and the gravitational waves as ripples in the gravitational field. D-branes can also be considered as the result of particles called gravitons.
When Dyson’s calculations brought these ingredients together to make a theoretical BMPV black hole, she discovered an interesting phenomenon. At the point in the construction process when the black hole is on the verge of becoming a time machine, the building blocks no longer behave as planned. Instead of everything meeting at the same point, the system creates a shell of gravitons with the D-branes inside. No amount of mathematical planning can get the gravitons to come any closer. The end result of this is that the BMPV black hole’s spin never gets fast enough to make an accessible closed time-like curve.
Dyson’s result suggests that the way the D-branes and gravitons affect each other — and the space they occupy — creates an obstacle to time travel around BMPV black holes. “It’s as though you’re trying to build that last little bit of your time machine and there’s a force that stays your hand,” Myers says.
As with Horava’s group, Dyson’s result only applies to a specific situation, but it could not be taken as a universal proof of Hawking’s conjecture. On the other hand, it showed that the tools of string theory can interfere to prevent the existence of some of the kinds of time machines that general relativity allows. “Whether or not string theory uses the same mechanism to prohibit other kinds of time travel is something that is currently being investigated,” claims Dyson.
But because string theory remains incomplete itself, new tools to attack problems like chronology protection continue to emerge. So is this the beginning of the end for time travel? Theorists are still cautious about formally announcing its downfall. “I think a lot of string theorists would be happy if we could find a concrete mechanism that just disallows all closed time-like curves,” Myers says. “On the other hand, perhaps in other situations closed time-like curves do exist and we have to come to grips with all the paradoxes and problems that brings.”
Peet is similarly reserved. “So far we’ve been pretty lucky but I think there are some nagging doubts among people who work on this,” she says. “Maybe there are examples of chronology violation that string theory can’t cure. We hope that’s not true.”
Science fiction writers, of course, hope otherwise. Even Peet, a devoted Star Trek fan, does admit a brief regret at the prospect of time travel’s fall. “If someone made a spaceship that was capable of time travel I’d be one of the first to go on a tourist mission,” she says. “When I realised it might not be possible, I was kind of sad,” she adds. “But I got over it.”
So this is how it will be in the future. Forget about that excellent adventure where you visit the ancient Greeks or give your great-grandfather the shock of his life. The idea of travelling through time is suddenly beginning to fall apart.
The writer contributes regularly to Dawn ScienceDotcom on science-related issues
|
|
