Thursday, January 19, 2012

Death-defying time crystal could outlast the universe

 In 4D, they'd survive anything <i>(Image: Benedict Redgrove/Image Bank/Getty)</i>
In 4D, they'd survive anything (Image: Benedict Redgrove/Image Bank/Getty)

We don't have to take the heat death of the universe lying down – a time crystal, symmetrical in time rather than space, would have the power to survive

IT IS a chilling prospect. At some point, the universe faces a cold and haunting future. Its entropy, or disorder, is always increasing and when it peaks, energy will no longer be able to flow, making life and almost everything else impossible. By this time, black holes will have evaporated and ever-accelerating expansion will have blown galaxies apart. All that remains will be a uniformly cold, dark, diffuse expanse.
According to the laws of thermodynamics, this dismal picture, called the heat death of the universe, is unavoidable. Now Nobel laureate Frank Wilczek, a theoretical physicist at the Massachusetts Institute of Technology, has hatched a survival plan. He reckons a hypothetical device he calls a time crystal could power a computer that would keep on running long after everything else has succumbed to the pull of entropy.
"It's not the most immediate problem in the world, but the point is, we don't have to take the heat death of the universe lying down," Wilczek says. "We can put up a pretty good fight for a very long time." Such a device might even be able to simulate someone's brain - giving a form of life extension.
Wilczek came up with this bizarre idea while studying solid crystals, three-dimensional structures in which the atoms are arranged in regular, repeating patterns. These patterns arise because they cost atoms the least energy to maintain, and so are most comfortable. If you add more energy, the crystal might disappear - ice crystals will melt to liquid water, for instance. Eventually, though, the heat will dissipate and the ice will refreeze in the same pattern.
Thanks to Einstein's special theory of relativity, physicists are used to thinking of time as a dimension, a simple extension to the three dimensions of space. So if you can have crystals in three dimensions, Wilczek wondered, why not in four?
He reasoned that the periodic rows of atoms in an ordinary crystal could translate to periodic motion in a time crystal (see diagram). Any object that moves in a circle and returns to the same spot at regular intervals, like a planet orbiting a star, has the same sort of symmetry in time that crystals have in space. But to truly count as a time crystal, that orbit would have to represent the object's lowest energy state - just as the periodic arrangement of atoms in a crystal is their lowest energy state.
In other words, the crystal would be able to keep on orbiting forever without needing any extra energy. That makes it look "perilously close to a perpetual motion machine", Wilczek says. Normally forbidden by thermodynamics, there is at least one instance of perpetual motion that is allowed: the electrons flowing in a superconductor.
Unlike electrons travelling through an ordinary conductive wire, where resistance erodes their flow, those in superconductors have normally been cooled close to absolute zero, share the same quantum state and so flow without resistance. This makes superconductors very efficient at producing huge currents.
Wilczek points out that superconductors' lack of resistance also means that their electrons could, in theory, flow forever without any energy input - just what is needed to create a time crystal. "A superconductor is close [to a time crystal], in the sense that it has currents that flow forever, even in its lowest energy state," he says. "If you add more energy, it'll eventually shed that energy and go back to doing what it wants to do."
In a time crystal, electrons will have to flow in a loop not a line as they do in an ordinary superconductor. What's more, they will have to bunch up rather than flowing as a smooth stream. This is to ensure that the charge repeats periodically over time, echoing the way that atoms repeat in space in an ordinary crystal.
At the recent State of the Universe symposium convened in Cambridge, UK, to mark Stephen Hawking's 70th birthday, Wilczek showed mathematically that a lump of looping charge can zip around forever in its lowest energy state. "It is perpetual motion, but not in the forbidden sense," he says.
It should be possible to build such a device, although Wilczek admits he doesn't know how difficult it would be. "At present they're mathematical models," he says. "I don't have specific designs or proposals."
Assuming a time crystal can be made, Wilczek reckons it could be made into a computer. Because the time crystal's charge changes its position in the loop, it can occupy different states that are analogous to an ordinary computer's 1s and 0s. These superconducting bits could then be linked together to form a computer. By contrast, an ordinary superconductor couldn't do this because its electrons travel in a continuous stream.
Crucially, such a computer should be perfectly placed to survive the heat death of the universe. For one, it would already be in its lowest energy state, so it wouldn't need extra energy input to keep on running. "It doesn't need a power source," Wilczek says. To boot, superconductors tend to work only at temperatures close to absolute zero. So as the universe gets colder and darker, and all other processes cease, this computer would simply get more comfortable. "It could dodge the heat death of the universe for a very long time," he says.
Some doubt there would be any point to such a device. "It's probably not revolutionary," says Craig Hogan, a physicist at Fermilab in Batavia, Illinois.
It's not hard to dream up some possibilities, though. Like an ultimate time capsule, Wilczek's computer could store information about the universe's former life, the only legacy of what the universe once was. More ambitiously, if the right science and technology ever becomes available, humans might upload their brains to the computer, in a sense living on while the universe outside dissolves.
It wouldn't be much of a life. As the computer wouldn't be able to accept new inputs, it would run the same brain-simulating program on a loop. "My best idea of what we'd want to do is to find some kind of experience that we really like, and program it in, so that experience would happen over and over again," says Wilczek.
Milan Cirkovic, a physicist at the Institute for Ethics and Emerging Technologies, doesn't relish that repetition. Luckily for him, others have come up with some equally wacky alternatives.
Freeman Dyson of the Institute for Advanced Study in Princeton, New Jersey, reckons conscious beings on the verge of a heat death might slow down their sense of subjective time. Andrei Linde of Stanford University in California suggested that if our universe is just one of many in a multiverse, we could tunnel through a black hole to a neighbouring universe that is still warm and humming. The latter appeals to Cirkovic.
It's not clear that we do live in a multiverse, however. So Wilczek remains excited about the time crystal. "If the options were to have pleasant dreams forever, or just fade out, I guess I'd rather have the dreams."

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