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Tom Siegfried, Randomness

To build a clock that ticks forever, you need a spacetime crystal blueprint

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SN Prime | October 22, 2012 | Vol. 2, No. 40

Nothing lasts forever, although some things seem to. Speeches at political conventions, the NBA play-offs and those fight scenes in the Matrix movies just go on and on and on. Sometimes life itself seems like one never-ending wallpaper pattern, duplicated over and over again at regular intervals.

Whether life is really like that or not, lots of other things are. Take crystals, regular organizations of atoms assembled in repeating patterns. Snowflakes, for instance, are a particular type of ice crystal, with six identical arms. Turn a flake by one-sixth of a full rotation and it looks the same as before. In other words, snowflakes exhibit symmetry in space.

Of course, simpler objects also look the same when rotated, such as a perfectly smooth sphere. But that symmetry is continuous — the sphere looks the same no matter how much you turn it. Snowflakes and other crystals possess discrete symmetry — their appearance repeats itself only at specific distances in space. So you can think of them as space crystals.

Not long ago, physics Nobel laureate Frank Wilczek of MIT pondered the symmetry of space crystals and wondered: If there’s such a thing as space crystals and time is just another dimension like the three of space, then why can’t there be time crystals?

A time crystal would exhibit symmetry in time analogous to a snowflake’s symmetry in space. Just as rotating a snowflake repeats its patterns at specific regular distances in space, a time crystal would return to its initial configuration at regular intervals of time. Wilczek worked out a little math and discovered that there is, in fact, no theoretical reason not to have time crystals, but figuring out how to make one might be difficult.

Now, though, physicists at the University of California, Berkeley and collaborators say they’ve found a way. In a paper published in the October 19 Physical Review Letters, the Berkeley group proposes a design for a “spacetime crystal” — both a space crystal and a time crystal at the same time. (And at different times.)

This scheme calls for trapping ions — atoms carrying an electrical charge due to some missing electrons — in electric fields to form a ring, sort of like a bagel. Because these ions, being identically charged, repulse each other, they’d assume positions at equal distances around the ring, just like the regularly spaced atoms of an ordinary crystal. A nearby magnetic field would induce the ring to rotate. One of the ions in the ring could be flagged in a detectable way (think of a raisin embedded in the bagel) allowing scientists to detect the repeating periodic motion, demonstrating time crystal status. Since it’s in its “ground state,” meaning it requires no input of energy to sustain itself, the rotating atom-bagel would constitute a clock that ticked forever.

Don’t worry, it’s not a perpetual motion machine. The ground state is the condition of lowest possible energy. You can’t get any energy out of this rotating bagel and it can do no work, so the laws of thermodynamics remain unbroken.

It’s a cool idea — literally, as the spacetime crystal relies on quantum processes that work at temperatures of nanokelvins. (That’s like a billionth of the temperature found in the depths of outer space.)

“In order to experimentally realize such a space-time crystal with trapped ions, we need to confine ions tightly to have a small diameter, and cool the ions to a very low temperature,” Tongcang Li and colleagues write in their paper.

Ion traps — devices using an electrical field to confine the ions in a small space — are already widely used for a variety of experimental purposes. And achieving the necessary ultracold temperatures in the lab isn’t easy, but methods do exist.

So the Berkeley blueprint for a spacetime crystal is far from a science fiction fantasy. It might even be within the reach of current technology. It’s plausible to imagine a ring containing perhaps 100 beryllium-9 ions cooled to 1.1 nanokelvins that would permit a spacetime crystal demonstration. A laser pulse would be needed to tweak one of the ions so that it could be detected by a second “probe” laser to verify the ring’s rotation.

Besides offering a spectacular example of a totally unexpected scientific finding, time crystals might even be good for something. “If they can be created, time crystals may have intriguing applications, from precise timekeeping to the simulation of … quantum computing schemes,” physicist Jakub Zakrzewski writes in a commentary on the new work.

Perhaps even more intriguing, time crystals supply a novel way for conceptualizing other processes in physics. A clock that ticks forever might just describe other phenomena in the universe — or perhaps even the whole universe itself, Zakrzewski speculates.

In other words, the universe may exist as a periodic departure from the sameness of time implied by the symmetry of a clock that never ticks (like the surface of a sphere, changeless no matter how much it turns). Knowing whether time crystals help make sense of such vague ideas will take some time. But maybe not forever.

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