Lauren Fuge
“When I meet God,” physicist Werner Heisenberg allegedly once said, “I’m going to ask him two questions: why relativity? And why turbulence? I really believe he’ll have an answer for the first.”
Although the quote is almost certainly fictional, it captures the sheer frustration many physicists feel about turbulence: the complex, chaotic, unpredictable flows in fluids.
This phenomenon surrounds us: swirling gases in the atmosphere disrupting our flights; the movement of rivers around rocks; the flow of blood through our arteries. We also see it on cosmic scales, explains quantum physicist Warwick Bowen from the University of Queensland (UQ), from gas flowing in galaxy clusters to the Great Red Spot – a massive cyclone on Jupiter.
“You could fit our planet within this one storm, and it’s existed for many hundreds of years – for the whole time that we’ve been able to observe Jupiter,” Bowen says.
This long-term stability is typical of turbulent phenomena but is utterly perplexing to physicists like Bowen, who are used to seeing order dissipate into disorder.
“There’s a natural tendency in physics for structures that are large to break down into smaller structures and eventually disappear,” he says. “But it seems that in the Great Red Spot of Jupiter, that doesn’t happen – these large structures are stable over very long periods of time.”
And we still don’t know why. Turbulence has always been too complex to accurately analyse or even measure. Even after centuries of study, physicists have no general theoretical description of it – it’s been described as the last great outstanding problem of classical physics.
According to Bowen, who wrestles with very tiny turbulent systems in his lab in Brisbane, this gaping hole in theory is “kind of crazy”.
The most commonly used equations to describe fluid flow were first developed by Swiss polymath Leonhard Euler in 1757. But in the intervening 300 years, no one has managed to solve the equations to describe realistic conditions. They rapidly become unstable and intractably tangled, for the same reason it’s difficult to precisely predict the weather: very small changes have enormous effects, so an infinitesimal inaccuracy could throw off predictions of the system’s evolution.
“We don’t even know if there are unique solutions to the problem of turbulence at all, or whether it can be solved,” Bowen admits.
That doesn’t mean researchers haven’t tried. It is, after all, one of the Clay Institute’s seven unsolved “Millennium Prize” problems, meaning there’s a cool million dollars waiting for the first scientist to solve these equations.
But Bowen’s team isn’t so interested in taking a pen-and-paper approach – instead, they use lasers to observe turbulence in an ultra-cold quantum fluid in their lab.
His lab was one of three Australian teams who produced a suite of landmark papers in Science in 2019, describing the very first experimental demonstration of the microscopic origins of turbulence. Specifically, they showed that vortices can emerge on the quantum scale and then form into more complex and stable systems – verifying a 70-year-old prediction.
Australian Quantum Vortex Team research papers
- Giant vortex clusters in a two-dimensional quantum fluid
- Evolution of large-scale flow from turbulence in a two-dimensional superfluid
- Coherent vortex dynamics in a strongly-interacting superfluid on a silicon chip
Collectively referred to as the Australian Quantum Vortex Team, they comprise another research group at UQ plus a team at Monash University, in Melbourne. Their work earned them a nomination for the 2020 Eureka Prize – the “Oscars” of Australian science.
Despite this, Bowen’s lab didn’t actually set out to study turbulence – it found them.
Part of the UQ Precision Sensing Initiative, the lab focused on using the properties of superfluid helium to build quantum technologies, such as extremely precise inertial sensors and ultrafast quantum computing networks.
“We have a very, very cold fridge that gets us down to about a fiftieth of a degree away from absolute zero – about twenty millikelvin,” Bowen explains – and in this fridge they keep a box of this bizarre quantum fluid.
So what exactly is superfluid helium?
“We don’t 100% understand ourselves,” Bowen admits.
Here’s the gist: if you cool any material to a low enough temperature, it will become a solid – except for helium. A quirk of quantum mechanics means that materials always have a miniscule amount of “quantum zero-point energy”, even when they’re at absolute zero.
“For helium, that energy is enough to melt the solid,” Bowen explains. “In some sense, quantum mechanics is melting the helium and causing it to be a very different type of fluid.”
Superfluids have a range of delightfully peculiar properties, including the fact they have no way to dissipate energy flow. If a physicist set up a flow in a tub full of superfluid helium then went away on a year-long sabbatical, it would still be flowing on their return.
Superfluid helium is also an example of a “matter wave”, another quantum property in which its atoms act more like a wave than a particle, giving rise to the strange flows. It’s this property that Bowen’s lab wants to exploit in quantum technologies.
But to do so, they must observe, control and understand the turbulent flows within this superfluid – and thus grapple with one of the most stubborn mysteries in physics.
“Turbulence is a problem for us, really – we don’t want it!” Bowen says, chuckling. “In our particular case we’d like to understand it just so we can remove it.”
Turns out, superfluid helium is an excellent medium in which to study how turbulent phenomena form and evolve, as evidenced by the lab’s contribution to the landmark demonstrations made in 2019.
They provided the first experimental proof of a prediction made by Nobel-Prize-winning physical chemist Lars Onsager, who in 1949 proposed that turbulence in 2D systems could be understood by observing it on nanoscales. These systems are made up of miniscule working parts called quantum vortices – the quantum equivalent of a tornado or a vortex in water – and Onsager suggested that over time, vortices rotating in the same direction would cluster to form larger ones, making the system become more stable.
By studying the interactions between quantum vortices, he predicted we could understand many characteristics of the system as a whole – such as “why you get these large-scale pattern formations like in the Great Red Spot or in cyclones, and it explains why it persists”, Bowen explains.
Each of the three studies in Science created quantised vortices from a different material and watched as they evolved and stabilised. Bowen’s lab observed this in superfluid helium, using lasers to measure the fluid’s dynamics, while the other two labs used Bose-Einstein condensates, a quantum state that exists at ultra-low temperatures.
What’s counter-intuitive about these vortices, according to Bowen, is that the fluid is only allowed to take particular speeds.
“If I stir a pot, then in a classical system that fluid can take any velocity it likes, but in superfluids it can only take very specific velocities,” he explains. “When I stir it, initially nothing happens – it just ignores the fact that I’m stirring. Then if I increase my speed, at some point it steps up to a specific new velocity, and if I keep stirring it steps up again. But you can only have discrete values.
“It’s a weird behavior. It comes straight out of quantum mechanics and the fact that the atoms are behaving more like a wave than an atom.”
His team observed small clusters of these wacky vortices in superfluid helium by using lasers to “listen” to the vortices, measuring the ripple effects they have on the superfluid’s surface.
“The frequency of that ripple changes when the vortex appears, and we use lasers to hear that,” Bowen explains. “We’re not optically imaging it – we’re acoustically imaging.”
The next goal is to use this technique to see a single quantised vortex – which, remarkably, has never been directly observed, despite the 2016 Nobel Prize in Physics being awarded to a team of physicists who recognised and explained the existence of quantised vortices in 2D films of superfluids.
Bowen’s team didn’t even observe a single vortex in their 2019 study; they only observed ensembles of vortices, then analysed the data assuming the vortices were quantised in order for the results to make sense.
“Of course, they must exist,” he says. “If we discovered that there wasn’t such as a thing as quantised vortices, all kinds of physics would have massive problems.”
To observe a single vortex, Bowen and team will shrink their experiments right down. At the moment, they’re working on scales of hundreds of microns (equivalent to the width of a few human hairs), but they’re aiming for single micron scales (about the size of bacteria).
Pushing vortices into a tiny space will make them interact more strongly, increasing the frequency shift of their “sound” – that is, the ripples they make in the superfluid.
“What I’d like to do,” Bowen says, “is to listen to that sound wave with no vortex at a certain frequency, then add a vortex and see it jump to another frequency.”
This distinct jump would prove the vortex’s quantised nature, directly verifying the assumption underpinning the 2016 Nobel Prize. Bowen’s lab is in the perfect position to achieve this.
“At least in terms of superfluid helium, we’re the only lab in the world able to do what we do,” he says. “We are the field.”
They have the unique capability to combine quantum liquids and silicon-chip technology, by mapping turbulent behaviour onto a thin film of superfluid helium on a chip.
“Normally if you want to understand turbulence, such as the weather, you go to your computer and code in everything you know about the system and then simulate it,” Bowen says.
“This is a completely different way of modelling the turbulence you see in nature, because we can actually build physical objects that display it. There’s no code, no model – we just create the turbulence in miniature and then watch what it does.”
Further experiments with microscopic turbulence will hopefully lead to better models of turbulent phenomena in the world around us.
“The interesting question is, how much can you scale it up to become a useful tool to learn about classical turbulence?” Bowen muses. “I think it’s fair to say we don’t know the answer to that question – yet.”
In the meantime, understanding turbulence will pave the way for Bowen’s lab to create new quantum technologies. They hope to revolutionise inertial sensors, which continually calculate position and velocity to aid in the navigation of aircraft, submarines, ships, spacecraft and even smartphones. Cutting-edge sensors are currently based on lasers – but Bowen reckons that atoms could do the job better, since they interact much more strongly with gravity.
Replacing light waves with matter waves – such as superfluid helium – could improve the sensitivity of inertial sensors by a factor of ten billion.
“In practice we’re a long way from achieving that,” Bowen notes. But in principle, their research could lead to much smaller navigation devices with phenomenal sensitivity.
Their work could also use superfluid flow to solve fundamental challenges in creating a quantum internet, as well as to understand exotic natural phenomena – like the mysterious “chirps” heard from pulsars, which contain a neutron superfluid at their core.
They could even probe the nature of quantum mechanics itself.
Currently we don’t know where the interface between the classical and quantum world is, if there’s an interface at all, or whether there’s a unifying theory to tie everything together.
Theoretically, Bowen’s team could compress their quantum fluid until it starts to mimic the behaviour of a single atom, emitting characteristic frequencies of sound instead of light. By pushing quantum behaviours up to larger and larger scales, we can begin to answer fundamental questions.
The trickiest thing, Bowen says, is choosing what to do. While there are many clear goals being chased by quantum physicists around the world, his lab possesses a completely different technological platform.
“We’re in a unique position,” he says. “My feeling is we should be asking different questions to everyone else and doing something new, something really out there.”
The most exciting thing about being in this field right now, he says, is the unknown.
“We’ve really pushed the frontiers of what you can measure in superfluid helium and how you can control it, far beyond what has been possible before, and that’s opened up this frontier that we can explore and make genuine and important fundamental discoveries.
“The challenge is not ‘What should I do?’ but rather ‘Which of the many things I’d like to do should I do first?’”