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BY
NATALIE WOLCHOVER, SIMONS SCIENCE NEWS
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04.30.13
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9:30
AM
Physicists
plan to create a "time crystal" - a theoretical object that
moves in a repeating pattern without using energy - inside a device
called an ion trap.
age:
H
In
February 2012, the Nobel Prize-winning physicist Frank Wilczek
decided to go public with a strange and, he worried, somewhat
embarrassing idea. Impossible as it seemed, Wilczek had developed an
apparent proof of "time crystals" - physical structures
that move in a repeating pattern, like minute hands rounding clocks,
without expending energy or ever winding down. Unlike clocks or any
other known objects, time crystals derive their movement not from
stored energy but from a break in the symmetry of time, enabling a special
form of perpetual motion.
"Most
research in physics is continuations of things that have gone
before," said Wilczek, a professor at the Massachusetts
Institute of Technology. This, he said, was "kind of outside the
box."
Wilczek's
idea met with a muted response from physicists. Here was a brilliant
professor known for developing exotic theories that later entered the
mainstream, including the existence of particles called axions and
anyons, and discovering a property of nuclear forces known as asymptotic
freedom (for which he shared the Nobel Prize in physics in
2004). But perpetual motion, deemed impossible by the
fundamental laws of physics, was hard to swallow. Did the work
constitute a major breakthrough or faulty logic? Jakub Zakrzewski, a
professor of physics and head of atomic optics at Jagiellonian
University in Poland who wrote a perspective on the research that
accompanied Wilczek's publication, says: "I simply don't
know."
Now,
a technological advance has made it possible for physicists to test
the idea. They plan to build a time crystal, not in the hope that
this perpetuum mobile will generate an endless supply of
energy (as inventors have striven in vain to do for more than a
thousand years) but that it will yield a better theory of time
itself.
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Nobel Prize Physicist Frank Wilczek
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A Crazy Concept
The
idea came to Wilczek while he was preparing a class lecture in 2010.
"I was thinking about the classification of crystals, and then
it just occurred to me that it's natural to think about space and
time together," he said. "So if you think about crystals in
space, it's very natural also to think about the classification of
crystalline behavior in time."
When
matter crystallizes, its atoms spontaneously organize themselves into
the rows, columns and stacks of a three-dimensional lattice. An atom
occupies each "lattice point," but the balance of forces
between the atoms prevents them from inhabiting the space between.
Because the atoms suddenly have a discrete, rather than continuous,
set of choices for where to exist, crystals are said to break the spatial symmetry of nature - the
usual rule that all places in space are equivalent. But what about
the temporal symmetry of nature - the rule that stable objects stay
the same throughout time?
Wilczek
mulled over the possibility for months. Eventually, his equations
indicated that atoms could indeed form a regularly repeating lattice
in time, returning to their initial arrangement only after discrete
(rather than continuous) intervals, thereby breaking time symmetry.
Without consuming or producing energy, time crystals would be stable,
in what physicists call their "ground state," despite
cyclical variations in structure that scientists say can be
interpreted as perpetual motion.
"For
a physicist, this is really a crazy concept to think of a ground
state which is time-dependent," said Hartmut Häffner, a quantum
physicist at the University of California, Berkeley. "The
definition of a ground state is that this is energy-zero. But if the
state is time-dependent, that implies that the energy changes or
something is changing. Something is moving around."
How
can something move, and keep moving forever, without expending
energy? It seemed an absurd idea - a major break from the accepted
laws of physics. But Wilczek's papers on quantum andclassical time crystals (the latter
co-authored by Alfred Shapere of the University of Kentucky) survived
a panel of expert reviewers and were published in Physical Review
Letters in October 2012. Wilczek didn't claim to know whether objects
that break the symmetry of time exist in nature, but he wanted
experimentalists to try to make one.
"It's
like you draw targets and wait for arrows to hit them," he said.
"If there's no logical barrier to this behavior being realized,
then I expect it will be realized."
n June, a group of
physicists led by Xiang Zhang, a nanoengineer at Berkeley, and
Tongcang Li, a physicist and postdoctoral researcher in Zhang's
group, proposed creating a time crystal in the form of a persistently
rotating ring of charged atoms, or ions. (Li said he had been
contemplating the idea before reading Wilczek's papers.) The group's article was published with
Wilczek's in Physical Review Letters.
Since
then, a single critic - Patrick Bruno, a theoretical physicist at the
European Synchrotron Radiation Facility in France - has voiced dissent
in the academic literature. Bruno thinks Wilczek and company
mistakenly identified time-dependent behavior of objects in excited
energetic states, rather than their ground states. There is nothing
surprising about objects with surplus energy moving in a cyclical
fashion, with the motion decaying as the energy dissipates. To be a
time crystal, an object must exhibit perpetual motion in its ground
state.
Bruno's
comment and Wilczek's reply appeared in Physical Review Letters in March
2013. Bruno demonstrated that a lower energy state is possible in a
model system that Wilczek had proposed as a hypothetical example of a
quantum time crystal. Wilczek said that although the example is not a
time crystal, he doesn't think the error "calls into question
the basic concepts."
"I
proved that example is not correct," Bruno said. "But I
have no general proof - so far, at least."
The
debate will probably not be settled on theoretical grounds. "The
ball is really in the hands of our very clever experimental
colleagues," Zakrzewski said.
An
international team led by Berkeley scientists is preparing an
elaborate lab experiment, although it may take "anywhere between
three and infinity years" to complete, depending on funding or
unforeseen technical difficulties, said Häffner, who is co-principal
investigator with Zhang. The hope is that time crystals will push
physics beyond the precise but seemingly imperfect laws of quantum
mechanics and lead the way to a grander theory.
"I'm
very interested in seeing if I can make a new contribution following
Einstein," Li said. "He said that quantum mechanics is not
complete."
To Build an Ion Ring
In
Albert Einstein's theory of general relativity (the body of laws
governing gravity and the large-scale structure of the universe), the
dimensions of time and space are woven together into the same fabric,
known as space-time. But in quantum mechanics (the laws governing
interactions on the subatomic scale), the time dimension is
represented in a different way than the three dimensions of space - "a
disturbing, aesthetically unpleasant asymmetry," Zakrzewski
said.
The
different treatments of time may be one source of incompatibility
between general relativity and quantum mechanics, at least one of
which must be altered for there to be an all-encompassing theory of
quantum gravity (widely viewed as a major goal of theoretical
physics). Which concept of time is right?
If
time crystals are able to break time symmetry in the same way that
conventional crystals break space symmetry, "it tells you that in
nature those two quantities seem to have similar properties, and that
ultimately should reflect itself in a theory," Häffner said.
This would suggest that quantum mechanics is inadequate, and that a
better quantum theory might treat time and space as two threads of
the same fabric.
The
Berkeley-led team will attempt to build a time crystal by injecting
100 calcium ions into a small chamber surrounded by electrodes. The
electric field generated by the electrodes will corral the ions in a
"trap" 100 microns wide, or roughly the width of a human
hair. The scientists must precisely calibrate the electrodes to
smooth out the field. Because like charges repel, the ions will space
themselves evenly around the outer edge of the trap, forming a
crystalline ring.
At
first, the ions will vibrate in an excited state, but diode lasers
like those found in DVD players will be used to gradually scatter
away their extra kinetic energy. According to the group's
calculations, the ion ring should settle into its ground state when
the ions are laser-cooled to around one-billionth of a degree above
absolute zero. Access to this temperature regime had long been
obstructed by background heat emanating from trap electrodes, but in
September, a breakthrough technique for cleaning
surface contaminants off electrodes enabled a 100-fold reduction in
ion trap background heat. "That's exactly the factor we need to
bring this experiment into reach," Häffner said.
Next,
the researchers will switch on a static magnetic field in the trap,
which their theory says should induce the ions to start rotating (and
continue doing so indefinitely). If all goes as planned, the ions
will cycle around to their starting point at fixed intervals, forming
a regularly repeating lattice in time that breaks temporal symmetry.
To
see the ring's rotation, the scientists will zap one of the ions with
a laser, effectively tagging it by putting it into a different
electronic state than the other 99 ions. It will stay bright (and
reveal its new location) when the others are darkened by a second laser.
If
the bright ion is circling the ring at a steady rate, then the
scientists will have demonstrated, for the first time, that the
translational symmetry of time can be broken. "It will really
challenge our understanding," Li said. "But first we need
to prove that it does indeed exist."
Until
that happens, some physicists will remain deeply skeptical. "I
personally think it's not possible to detect motion in the ground
state," Bruno said. "They may be able to make a ring of
ions in a toroidal trap and do some interesting physics with that,
but they will not see their ever-ticking clock as they claim."
Original story reprinted with permission
from Simons Science News, an editorially independent
division of SimonsFoundation.org whose mission is to
enhance public understanding of science by covering research
developments and trends in mathematics and the physical and life
sciences.
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