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How to Build A Space-Time Crystal

Nobody has ever seen a space-time crystal let alone made one. But now physicists think they know how

A couple of months back, we looked at the notion of time crystals, an idea put forward by Nobel-prize winning physicist Frank Wilczek and his pal Al Shapere.

These guys examined the fundamental properties of ordinary spatial crystals and asked why similar objects couldn’t exist in the dimension of time instead. 

One of the basic properties of spatial crystals is that they form when a system drops to its lowest possible energy state.  They are not the result of adding energy to a system, but of taking it away. All of it.

Another basic property is that when these objects reach their lowest energy configuration, their symmetry breaks down. Instead of being the same in all directions, like the laws of physics, these objects become the same in only a few directions. It is this symmetry-breaking and the periodic structure it produces that defines crystals.

Wilczek and Shapere persuasively argued that there’s no reason why similar periodic structures couldn’t exist in time. And they said that finding them would give physicists a new way to study the process of symmetry-breaking and the laws of physics behind it.

There was just one problem, however. These guys hadn’t worked out how to build a time crystal.

That changes today with the work of Tongcang Li at the University of California, Berkeley and a few buddies who say they’ve worked out how to do it. These guys say they know how to create an object in its lowest energy state that shows periodic structure both in space and time–a space-time crystal.

Their idea is remarkably simple. Their space-time crystal consists of a cloud of beryllium ions trapped in an circular electromagnetic field.  The ions naturally repel each other and so spontaneously form a circle. That’s a type of spatial ionic crystal, something physicists have played with for years.

However, if Wilczek and Shapere are right, this ring of ions ought to rotate, even when cooled almost to absolute zero. Such a rotating ring is periodic both in space and in time and so becomes a space-time crystal.

The idea of a permanently rotating ring might have uncomfortable parallels with a perpetual motion device. But a space-time crystal does not violate any laws of physics. That’s because it exists in its lowest energy state and so cannot do work–energy cannot be extracted from this system even though it is moving. 

That’s more than a mere curiosity. One reason why space-time crystals are interesting is that their periodicity in time makes them natural clocks. So there should be plenty of people with more than a passing interesting in making one.

And that should be sooner rather than later. Tongcang and co’s space-time crystal ought to be possible to make now using state of the art ion traps.  They won’t want anybody stealing a march on them so there’s a good chance Tongcang and co are building one now. Perhaps they have a space-time crystal in their lab now. 

If that’s the case, we ought to be reading the details about the first space-time crystal sometime in the days and weeks to come. 

Ref: arxiv.org/abs/1206.4772: Space-Time Crystals Of Trapped Tons 

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