zipping points one minute and wiggle
like waves the next. And particles can’t
have just any amount of energy; they are
confined to particular levels, called quantized states.
Usually quantum weirdness appears
only in the microworld (SN: 11/20/10, p.
15), but larger systems sometimes display such bizarre behavior too. In 1937
physicists discovered that helium cooled
to just above absolute zero flows with
almost no viscosity, an exotic property
called superfluidity. Russian condensed-matter theorist Lev Landau explained
that superfluidity occurs when the atoms
join up in a quantum state, which forces
them to lose their separate identities and
act as one, and he developed a mathematical explanation of that behavior.
So what exactly would happen to the
united particles if someone casually
swirled a mug of quantum liquid?
Landau’s math suggested that the particles, because they are in a low energy
state, would want to remain stationary.
So convincing the superfluid to actually
rotate like a spinning tornado would
require an enormous amount of energy.
Feynman imagined, in a paper published in 1955, that the fluid would ripple
in ways unfamiliar to coffee drinkers,
cleverly skirting Landau’s requirement
and still absorbing the energy from
the rotating mug. Holes in the liquid,
or quantum vortices, would form, but
nothing would swirl within them. As
long as the mug kept rotating, the vortices would line up into an orderly lattice.
What would happen when the mug
stopped rotating, though, would be most
fascinating. The holes, which are actually
3-D voids that behave something like
strings of spaghetti, would fall against
the wall of the mug or tangle in complex
ways. Any added energy would have to
go somewhere: A quick escape, Feynman
speculated, would be for two vortices to
collide head-on and snap apart. Sometimes they could loop back on themselves
to make SpaghettiOs and shed energy by
cascading into smaller rings.
Creating these tubelike vortices
wouldn’t take much energy, Feynman
speculated, and liquid helium would be
the best place to see
them. But spotting
turbulent activity in
the superfluid turned
out to be a tall order.
When turbulence is
invisible, like in blustering wind, scientists
can plant a sock on a
stick to watch how it
flaps. When visualizing how fluids rotate,
scientists might add food coloring and
watch how the colors move. But finding a way to see quantum vortices on the
surface of liquid helium was a challenge
because helium is so light. Just about any
tracer sinks to the bottom.
Scientists tried to visualize the curving lines and circles that Feynman had
pictured. Theorists derived equations
for such vortices and experimentalists
even captured snapshots of the quantum hubbub. But quantum vortices that
connect, snap and shrink remained a figment of the imagination for decades.
fog of ice particles. He
sprinkled the hydrogen particles like
snow onto the helium.
physicists who had
been thinking about
the dynamics of
time, got wind of the
new technique. They urged Bewley and
his adviser, Maryland’s Dan Lathrop, to
try the same experiment but with much
colder helium, at 2 degrees Celsius above
On a late night in the lab, Bewley
shined a laser onto the supercold liquid
with the hydrogen snow. He was shocked
to see Feynman’s vortices pop into existence and bump into each other. A few
days later, he and his adviser caught the
whole dance on tape, publishing the new
techniques and observations in Nature
in 2006. Physicists rushed back to the
problem of quantum vortices when they
saw movies from Lathrop’s lab. They
wanted to see what would happen as the
On his laptop in his Maryland office,
Lathrop plays a movie of a quantum
fluid. Energy is steadily added, this time
by a heater instead of spinning. White
dots and lines sway on a black backdrop
as if stars in the night sky were floating
down a stream. But then the liquid is
abruptly cut off from its heat source. The
dance speeds up to a rave: Lines collide.
Lines snap away. Then everything calms
But the calming appears to occur too
quickly, says physicist Carlo Barenghi
A computer simulation shows
how quantum vortices might look
in a turbulent material.
Of serendipity and snaps
In 2006, a physics graduate student at
the University of Maryland unwittingly
stirred the dreams into reality. Gregory
Bewley, transplanted from a lab at Yale,
was finishing his thesis on how fluids such
as the ocean, the atmosphere and Earth’s
molten core experience turbulence while
rotating with the Earth. His experiments
involved spinning a cylinder the size of a
skateboard and watching how the liquid
helium sloshed inside.
Frustrated that none of the tracer particles he could buy would float, he created
a new technique to freeze hydrogen, the
only element lighter than helium, into a
By sprinkling ice particles made of hydrogen (white) into supercooled helium
(black), researchers have been able to watch quantum vortices meet up and then
fly away from each other in real time (four movie stills shown).