of Newcastle University in England; it
takes just about 10 seconds for the turbulent fluid to become quiet. A pendulum,
released from up high, will move back and
forth, back and forth before eventually
swinging to rest. It slows because of friction created as it passes by air molecules.
But liquid helium is basically frictionless,
so any added energy should take quite a
while to dissipate.
Computer simulations suggest a solution to the puzzle: Reconnecting vortices
could get rid of the energy
quickly by adding some wobbling of their own, Barenghi
has suggested in several
recent papers. Quickly after
vortex lines snap away from
each other, they could jerk
and wiggle and form what
are called “Kelvin waves.” If
these wiggly lines loop up,
they can cascade into even tinier rings.
Each time the rings get smaller, sound
particles called phonons should be
emitted, Barenghi says. No one has yet
listened for the ping of phonons coming
from a quantum liquid, though, so the
case hasn’t been closed.
Swirling superfluid helium presents
other puzzles. In 2008 Lathrop, graduate
student Matthew Paoletti and colleagues
reported in Physical Review Letters that
many more quantum vortices sweep
around at high speeds than would be
expected for classical turbulence, a pace
that may be explained by reconnections.
Lathrop would also like an explanation
for filaments observed in the fluid that
look like lines with equally spaced dots
on them, an unpredicted quantum pearl
necklace of sorts.
By studying turbulence in other quan-
tum fluids, such as an ultracold gaslike
state of matter called a Bose-Einstein
condensate, scientists might get a more
complete picture of quantum turbulence
and answer some lingering questions.
Vortices in a Bose-Einstein condensate
may be easier to visualize than in helium
because the spaghetti strands can be
more than a thousand times
thicker than those in liquid
helium. Plus, the conden-
sate balloons to 40 times its
size when allowed to freely
expand, magnifying the
vortices, Jamil Abo-Shaeer
says. Abo-Shaeer, now with
the defense research agency
DARPA in Arlington, Va.,
was a member of the MIT team that first
spotted vortices in the condensates back
in 2001. Another advantage of studying
turbulence in this ultracold gas, says Abo-
Shaeer, is that it is easy to get nearly all of
the atoms acting in unison. In some cases,
even in superfluid helium cooled to just
above absolute zero, not all the atoms
want to participate.
Though quantum vortices had already
been spotted in the condensates, a team
reported first seeing the vortices tan-
gle in 2009 in Physical Review Letters.
Vanderlei Bagnato of the University of
Sao Paulo in Brazil, a coauthor on the
paper, thinks the same turbulent wiggle
that appears to dissipate energy in the
Reconnecting
vortices could
get rid of the
energy quickly
by adding
some wobbling
of their own.
superfluid helium may be doing so in the
Bose-Einstein condensate as well.
Heavenly twists
As Bagnato and colleagues watch turbulence play out in the lab, others are
turning to traces of the phenomenon
elsewhere in the cosmos.
In another University of Maryland
building, James Drake tackles the problem of why the sun violently spews particles, an ongoing process that can interfere
with satellites. Drake has dedicated much
of his career to the idea that hot plasma
is ejected from the sun because magnetic
field lines cross and twist.
He was working on his laptop in his
office in 2006 when Lathrop, a longtime
colleague and friend, rushed in. Lathrop,
who had just observed the liquid helium
turbulence the day before, commandeered the keyboard and typed in a web
address. As Drake watched a full-screen
movie of lines bumping into each other
in liquid helium, he knew in an instant
that he was seeing reconnection.
The way that vortices snapped away
from each other is similar to how Drake
imagined magnetic field lines twisting in
the sun. “It was incredible,” Drake says.
“It’s exactly what we’ve been studying
for decades.”
Though the processes aren’t exactly
alike, both quantum vortices and mag-
netic field lines can be thought of as
strings with tension, like a stretched
guitar string or rubber band. Lathrop
and colleagues had found that, in helium,
smaller vortices reconnect faster, and
Getting connected physicist richard feynman predicted that vortices that behave like strings of spaghetti would show up in a quantum fluid
as energy is added — say by swirling. interactions among these vortices may help dissipate energy in such systems.
sources: M.s. paoletti et al/Physica D: NoNliNear PheNomeNa 2010, M.s. paoletti and d.p. lathrop/aNNu. rev. coNDeNs. matter Phys. 2011
when the system is cut off
from added energy, vortices
fight to avoid tangling. still,
some cross over each other
to form the letter X.
after forming an X, the vortices
can swap ends in what’s called
a “reconnection event.” this
meet-up can cause them to fly
away from each other.
as the vortices snap away,
they can develop ripples called
kelvin waves. in some cases,
the new vortices will loop back
on themselves to form circles.
such circles can cascade into
even smaller circles, a process
that releases particles of
sound, or phonons, and may
also dissipate energy.
