Quantum Whirls

How turbulence plays out in exotic materials

4:58pm, February 25, 2011
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Grab a mug and slosh the morning coffee around and around and a spinning vortex appears. The swirling rings, with their eddies and choppy waves, obey the laws of classical turbulence, which engineers and applied physicists routinely invoke to study how air flows over an airplane wing or how blood flows through tiny vessels.

Shake up a cup of quantum fluid instead and you still get vortices, but nothing like the tornado in your morning brew.

Quantum vortices can look like tiny rings that shatter into even more minuscule rings and then shrink away altogether. Connected one moment, in the next they appear to flex into curved lines — as if snipped with scissors. Sometimes these lines tangle like a ball of cat hair on a rug. And they can cross over each other into a letter X, swap ends and then shoot away with the gusto of a rubber band flinging from the finger of a mischievous third-grader.

Such scenarios go far beyond breakfast-table turbulence. This is the strange world of quantum turbulence, which pops up not in coffee cups, but in supercold helium, other types of strange cold matter and, some now think, the fabric of the universe.

Physicist Richard Feynman predicted the existence of quantum vortices more than half a century ago. But only in the last few years have physicists actually been able to watch the vortices’ behavior with the naked eye. Researchers are now stirring up quantum liquids to see whether Feynman’s ideas were correct, and going a step beyond to find out in fuller detail what happens as a sloshing quantum fluid rocks to rest. Understanding turbulence in these fluids may also offer clues to astrophysical mysteries — violent ejections of gas from the sun may obey the same strange dynamics, and thinking of the early universe as a fabric riddled with such vortices might help explain unexpected voids in the cosmos.

Theoretical swirlings

The rules of physics changed dramatically with the arrival of quantum mechanics, which describes how tiny particles such as electrons play. In the quantum world, particles can act like 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.

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 fog of ice particles. He sprinkled the hydrogen particles like snow onto the helium. They floated.

Two colleagues, physicists who had been thinking about the dynamics of quantum turbulence since Feynman’s 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 absolute zero.

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 energy dissipated.

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 down again.

But the calming appears to occur too quickly, says physicist Carlo Barenghi 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 quantum 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 condensate 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 tangle 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 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 magnetic 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 Drake thought the finding might help explain why reconnections happen faster among magnetic field lines in the sun than classical magnetic theory would predict. Drake and colleagues also argued, in a paper published last year in the Astrophysical Journal, that similar reconnections may be happening among magnetic field lines at the edge of the solar system.

Real quantum turbulence, not just an analog, may even have occurred at more distant reaches. Kerson Huang, a physicist at MIT, thinks that quantum turbulence could explain a lot about the cosmos. Astronomers studying the heavens have found that there are gaps of relatively empty space billions of light-years across, where few galaxies are found. But space in general looks uniform, so why these gaps exist has remained a mystery.

Huang thinks if the early universe were like a superfluid punctured by quantum vortices, these vortices could have created the gaps. If so, this phenomenon might be seen in a particle accelerator designed to imitate the vacuum of the early universe, such as the Large Hadron Collider outside Geneva. Colliding beams of protons at the LHC generate exotic particles and release a lot of energy.

“It’s a fireball that could burn a hole in the vacuum, so to speak,” Huang says. Such holes would have been pinpricks in the early universe, but would have bloated as the universe expanded, creating the cosmic voids, wrote Huang and colleagues in a paper posted online in November at arXiv.org.

However, Huang says, people haven’t thought deeply enough about how to detect hints of vortices in collider data: “We might create it, but we might not recognize it.” He is now doing calculations to figure out whether vortices would grow to a size that matches the size of voids found in the cosmos.

“His ideas are interesting because they connect quantum theory to vortices to the early universe,” Lathrop says. If Huang turns out to be right, his work could help rule out some theories for the birth of the universe.

For now, his cosmic void idea is speculative. But sometimes speculation turns out to be true. That was the case with Feynman’s predictions about quantum vortices. After suggesting that they would not only pop up, but also reconnect to dissipate energy, Feynman wrote in his 1955 paper: “Having travelled so far making one unverified conjecture upon another we may have strayed very far from the truth.” Surely you were joking, Mr. Feynman — your predictions were spot-on.

Marissa Cevallos is a former science writer intern at Science News.

Credits: clockwise from top left: airportrait/istockphoto; MedicalRF/photoresearchers; gomezdavid/istockphoto; © Wei Seng Chen/dreamstime

All shook up Though you won’t spot quantum turbulence in your coffee mug or kitchen sink, you probably experience some form of classical turbulence every day. The phenomenon occurs when fluid attempts to flow past an object or another fluid, bouncing and twisting to create chaotic patterns.

Buckle up Rough air in the form of strong updrafts in storms, downdrafts on the leeward side of a mountain and shifting jet stream boundaries can shake unbuckled airline passengers from their seats. Though bumps caused by air moving against air can injure a person onboard, they rarely damage planes.

Flowing through When doctors bring a stethoscope to your heart, they are listening for the lubb-dupp that comes with a healthy beat. Obstructions resulting from valve malfunctions or clogged arteries can cause a whooshing or blowing sound because they interrupt blood’s streamlined flow to create an audible, turbulent one.

Shine down As the sun’s core burns, energy is radiated outward. This results in a constant, turbulent overturning of hot and cold gases in the sun’s outer layer, called the convection zone. Computer simulations are helping scientists understand how small-scale turbulence can result in the large-scale order seen in the convection zone.

What a drag The flow of air around a moving car takes on a turbulent quality and increases drag. Though the effect may not matter much at city speeds, Formula 1 race car designers have put a lot of effort into reducing turbulence in air passing over their vehicles. At the same time, organizers want turbulence behind the car reduced because it makes overtaking another car more difficult.

Seaside surf As a wave approaches the beach, the gradual slope of the ocean bottom causes the wave to steepen until its crest becomes unstable, resulting in white water. Similar disruptions can occur mid-ocean anytime water’s flow is interrupted by an obstruction, such as a reef.

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