September 25, 2012
Physicists Create Eternal Clock Using 4-Dimensional ‘Space-Time Crystals’
Alan McStravick for redOrbit.com - Your Universe Online
Theoretical Physics made a huge leap from concept to reality yesterday thanks to collaboration between an international team of scientists and researchers with the U.S. Department of Energy´s (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab). The team has proposed the experimental design of a space-time crystal that is based on an electric-field ion trap and the Coulomb repulsion of particles that carry the same electrical charge.
“The electric field of the ion trap holds charged particles in place and Coulomb repulsion causes them to spontaneously form a spatial ring crystal,” says Xiang Zhang, a faculty scientist with Berkeley Lab´s Materials Sciences Division who led this research.
“Under the application of a weak static magnetic field, this ring-shaped ion crystal will begin a rotation that will never stop. The persistent rotation of trapped ions produces temporal order, leading to the formation of a space-time crystal at the lowest quantum energy state.”
Professor Zhang, the Ernest S. Kuh Endowed Chair Professor of Mechanical Engineering at the University of California, Berkeley (UC), is also the director of Harvard´s Nanoscale Science and Engineering Center. He authored the paper describing this work in the Physical Review Letters. The paper, entitled “Space-time crystals of trapped ions” was also co-authored by Tongcang Li, Zhe-Xuan Gong, Zhang-Qi Yin, Haitao Quan, Xiaobo Yin, Peng Zhang and Luming Duan.
To understand Zhang´s contribution, it is important to review what was known about crystals in the physical world up until yesterday. Traditional crystals are solid, three-dimensional structures made up of atoms or molecules that bond together in an orderly and repeating pattern. A few common examples of crystals include ice, salt and snowflakes. Crystallization only takes place when heat is removed from a molecular system and it is allowed to reach a lower energy state. At a certain point, continuous spatial symmetry breaks down the crystal and assumes what is known as “discrete symmetry”, which means that instead of the structure being the same in all directions, it is the same in only a few directions.
As mind-boggling as the concept may be for most of us, these researchers were able to harness the properties of crystal formation to devise a clock that operates in 4 dimensions, needs no power source and will keep time even after the universe as we know it comes to an end.
“While a space-time crystal looks like a perpetual motion machine and may seem implausible at first glance, keep in mind that a superconductor or even a normal metal ring can support persistent electron currents in its quantum ground state under the right conditions,” explained Tongcang Li, lead author of the study and a post-doc in Zhang´s research group.
“Of course, electrons in a metal lack spatial order and therefore can´t be used to make a space-time crystal.”
Li wants there to be no mistake that their proposed space-time crystal is not a perpetual motion machine because being at the lowest quantum energy state, there is no energy output. There are a great many scientific studies for which a space-time crystal would be invaluable.
“The space-time crystal would be a many-body system in and of itself,” Li says. “As such, it could provide us with a new way to explore classic many-body problem physics questions. For example, how does a space-time crystal emerge? How does time translation symmetry break? What are the quasi-particles in space-time crystals? What are the effects of defects on space-time crystals? Studying such questions will significantly advance our understanding of nature.”
The space-time crystal, once constructed, will operate at its lowest quantum energy state. Therefore, its temporal order — or timekeeping — will theoretically persist even after the rest of our universe reaches entropy, also known as thermodynamic equilibrium or “heat-death.”
“Great progress has been made over the last few decades in exploring the exciting physics of low-dimensional crystalline materials such as two-dimensional graphene, one-dimensional nanotubes, and zero-dimensional buckyballs,” says Li. “The idea of creating a crystal with dimensions higher than that of conventional 3D crystals is an important conceptual breakthrough in physics and it is very exciting for us to be the first to devise a way to realize a space-time crystal.”
Beyond the ability to create a device that will keep time even more accurately than the atomic clock, the space-time crystal will also offer other practical applications for the study of physics.
It is important to note that a crystal, with its added dimension, has a periodic structure in time as well as space. This provides scientists with a more effective means by which to study how complex physical properties and behaviors emerge from the collective interactions of large numbers of individual particles, or the so-called many-body problem of physics. Entanglement, another hot issue in modern physics where an action on one particle impacts another particle even if the two particles are separated by vast distances, is another phenomena that could be delved into with this new device.
The theory that a crystal has discrete order in time was only proposed this year by Nobel-prize winning physicist Frank Wilczek of the Massachusetts Institute of Technology (MIT). Wilczek was able to mathematically prove that a time crystal could exist, although he was unable at the time to postulate how one could be created. It took Zhang and his researchers, who had been working on issues regarding temporal order in a different system since September of last year, to come up with the experimental design that would allow them to build a crystal that is discrete in both space and time — a space-time crystal. You can find papers on both of these proposals in this month´s issue of Physical Review Letters.