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Turbulent science: grappling with physics' oldest unsolved problem

A team of scientists from University of Oxford and the Central Laser Facility (CLF), has used high-power lasers to recreate the sort of supersonic turbulence found in star-forming nebulae. The research, which aims to better our understanding of how the shockwaves created by turbulence triggers and affects star formation, is also tackling one of physics’ oldest mysteries: turbulence itself.

Whether it be water eddying chaotically in a stream as it flows around a rock; smoke swirling, branching and twisting from a cigarette; or air currents in the atmosphere adding a moment of discomfort to an otherwise smooth flight – we are all familiar with turbulence. Yet, despite its ubiquity, turbulence is actually one of the least understood phenomena in nature.

Turbulence’s mysteries are so profound that it led the Nobel Prize-winning physicist (and one of the founding fathers of quantum physics), Werner Heisenberg, to say that, should he be presented with the opportunity to ask God two questions, he would ask: ‘Why quantum mechanics? And why turbulence?’. Supposedly, he was only confident in God’s ability to answer the first of those questions. Almost 90 years later, the effort to understand and predict turbulent behaviours remains one of physics’ great endeavours.

So why has turbulence proven to be such an intractable mystery? Turbulence happens when a smooth flow of fluid (or gas) splits into smaller eddies and vortices. These swirls then break into smaller swirls, which themselves then break into yet smaller swirls in an unpredictable cascade. To make matters worse, those manifold swirls, eddies and vortices interact with each other – creating a chaotic chain of interactions that is impossible to predict. Physicists call this the ‘turbulent cascade’. Despite the complexities, computers are quite good at simulating these interactions – at least in subsonic (below the speed of sound) turbulence.

Supersonic turbulence of the kind found in star-forming nebula is even more complicated. When you add motions beyond the speed of sound, you have to add shockwaves that further complicate the interactions to the equation, which makes them extremely difficult to simulate for the computer. And you can’t just sit back with a telescope and just watch the effects of turbulence in a nebula unfold because they happen over timescales of millions of years, which would test the patience of even the most dedicated physicist.

Luckily, any physical law that scientists discover on Earth, as rule, can be applied anywhere – even millions of light-years away in some distant cloud of swirling cosmic gas. In an emerging field known as ‘laboratory astrophysics’, scientists can use laboratory experiments to improve their understanding of physical processes that occur throughout the Universe.

It is this sort of ‘that which works small will work large’ approach that the team from University of Oxford has taken advantage of. In this case, ‘the small’ is using high-energy lasers at the Central Laser Facility (CLF) to study the turbulence generated from the collision of two supersonic plasma jets and then seeing how that can be applied to ‘the large’: turbulent plasmas in star-forming regions of the Universe.

To achieve this, the team, whose work has been reported in Nature Communications, used the Vulcan laser facility. Vulcan has eight laser beams available with a variety of energies, which gave the scientists a high degree of flexibility in terms of the types of experiment that could be carried out.

Two clusters of three laser beams were used to zap a gas and generate supersonic plasma jets (plasma is a super-hot ionised gas) travelling at speeds of up to Mach 6, which is more than 11,000 kilometres per hour. By passing the jets through a pair of grids and colliding them they were able to produce a region of supersonic turbulence and observe the resulting shockwaves. They were able to watch the turbulence transition from low-velocity turbulence to the sort of high-velocity turbulence seen in star-forming nebulae – the first time this has been studied in the laboratory.

The team hope that the work will open a new avenue in the study of supersonic turbulence that could be used alongside existing computational and observational techniques. They plan to extend their work to include a magnetic field component to their experiments. This is necessary because powerful magnetic fields are known to be present in star-forming clouds, which scientists believe also affects the behaviour of turbulence.

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Last updated: 22 May 2019


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