Atom & Cosmos
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Crystals in time
may be possible
By Alexandra Witze
What sounds like the title of a bad fan
tasy movie — time crystals — could be the
next big thing in theoretical physics.
In two new papers, Nobel Prize–
winning physicist Frank Wilczek lays
out the mathematics of how an object
moving in its lowest energy state could
experience a sort of structure in time.
Such a “time crystal” would be the tem
poral equivalent of an everyday crystal,
in which atoms occupy positions that
repeat periodically in space.
The work, done partly with University
of Kentucky physicist Alfred Shapere,
appeared February 12 on arXiv.org.
“We don’t know whether such things
do exist in nature, but the surprise is that
they can exist,” says physicist Maulik
Parikh of Arizona State University.
Scientists can’t predict how impor
tant time crystals may turn out to be, or
whether they have any practical appli
cation. But Wilczek, of MIT, says the
concept reminds him of the excitement
he felt when he helped describe a new
class of fundamental particles, called
anyons, in the early 1980s. “I had very
much the same kind of feeling as I’m
having here,” he says, “that I had found
a new logical possibility for how mat
ter might behave that opened up a new
world with many possible directions.”
Wilczek dreamed up time crystals
after teaching a class about classify
ing crystals in three dimensions and
wondering why that structure couldn’t
extend to the fourth dimension — time.
To visualize a time crystal, think of
Earth looping back to its same location
in space every 365¼ days; the planet
repeats itself periodically as it moves
through time. But a true time crystal is
made not of a planet but of an object in
its lowest energy state, like an electron
stripped of all possible energy.
Lunar lights made by molten blobs
Mysterious moon flashes sparked by minimeteorite impacts
By Nadia Drake
Meteorites colliding with the moon
sometimes set off tiny lights danc
ing across its surface. Now scientists
think they know what powers these
lunar lightbulbs, in the absence of any
atmosphere that would otherwise set
incoming meteors ablaze: The flashes
result from superhot material released
as tiny rocky objects strike the moon’s
surface.
“You have just a small piece of
cometary material or asteroid, about 10
centimeters, that can do a very bright
flash visible from the Earth,” says study
coauthor Sylvain Bouley, a planetary
scientist at the Paris Observatory.
Bouley and colleagues settle an old
debate about where the twinkling lunar
lights come from in the March Icarus.
Observed for more than half a millen
nium, lunar impacts occur hundreds of
times each year. Meteor showers, such
as the Leonids in November, can dump
as many as 20 objects on the moon in
one night.
Initially, scientists didn’t think the
flashes necessarily came from the
moon; they might have been reflections
from tumbling satellites or some other
kind of phenomenon. Then, debate
revolved around whether impacts or
something in the moon such as volca
nism produced the transient flashes.
Most recently, researchers couldn’t
decide between hot, charged particles
or liquid droplets kicked up by impacts
as the culprit.
To answer the question, Bouley and
his colleagues looked at lunar flashes
recorded from 1999 to 2007. The
researchers calculated the brightness of
each flash, plus the probable sizes and
speeds for 54 collisions. Most impactors
were around 10 centimeters in diameter
and traveled at speeds of up to 72 kilo
meters per second, Bouley says.
