It is
nice to know that we have a theory of time? Yet a crystal that is
apparently stable at the ground state is interesting as it is neither
releasing or absorbing energy. It may actually be an excellent tool
to test out ideas of time. This is not so much perpetual motion as
frictionless load free rotation. It is certainly not free energy.
I
wonder how it might behave in low gravity.
I
have not found anyone doing high level research on gravity and time
using a pure empirical approach without the built in assumptions of
our theory. If you are aware of such I could use a pointer.
Time Crystals’
Could Upend Physicists’ Theory of Time
BY NATALIE WOLCHOVER,
SIMONS SCIENCE NEWS
04.30.13
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.
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 at 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 and classical 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.”
The Big Test
In 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.”
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