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In March 2002, the scientific world was rocked by some astonishing news: a distinguished US government scientist claimed he had made nuclear fusion out of sound waves in his laboratory.
Rusi Taleyarkhan's breakthrough was such important news because nuclear fusion is one of the most difficult scientific processes, and also one of the most coveted. It could solve all of our energy problems for ever. In principle, sufficient fuel exists on earth to provide clean, pollution-free energy for billions of people for millions of years.
To make it happen, individual atoms must be slammed into each other with enough energy to make them fuse together, something that requires temperatures found only in the core of stars like our Sun – over 10 million Kelvin. The idea that these temperatures had been reached in a small scale laboratory using only soundwaves took many scientists by surprise. To them, fusion projects were huge multibillion-pound, intergovernmental schemes with the far off goal of producing energy in several decades time.
Taleyarkhan's fusion breakthrough was based on a little-understood process called sonoluminescence. It's a process that magically transforms sound waves into flashes of light, focusing the sound energy into a tiny flickering hot spot inside a bubble. It's been called the star in a jar.
The star in a jar effortlessly reaches temperatures of tens of thousands of degrees, hotter than the surface of the sun. Many scientists had wondered if the core of the bubble was even hotter – maybe even as hot as the core of the sun. If so, fusion would happen there. But until Taleyarkhan, no one had been able to either prove it or disprove it.
The breakthrough and the paper in Science attracted great scepticism. When fusion takes place, particles called neutrons are given off. These are considered by scientists to be the key signature of nuclear fusion – but measuring neutrons on a small, laboratory scale had proven notoriously difficult in the past – and had even killed off an infamous fusion claim in 1989.
Many scientists didn't believe that Rusi Taleyarkhan' neutron detection was absolutely right. So to get to the bottom of the issue, the experiment was re-run by Mike Saltmarsh and Dan Shapiro, colleagues at the Oak Ridge National Laboratory. They couldn't find any evidence of fusion. But the controversy escalated as Taleyarkhan's team stood their ground and then, two years later, brought out a new paper showing even more fusion and more neutrons. This paper was thoroughly reviewed and published in another respected journal.
But the the controversy wouldn't die down. Nuclear fusion from soundwaves would be a huge scientific breakthrough – and to be convinced of it, many scientists wanted to see better evidence, evidence that was absolutely incontrovertible. They wanted to look very precisely at the timing of the neutrons to see just how closely they were related to the flashes of light.
If they occurred at the exact same time, they would finally be convinced that fusion was taking place. But they wanted timing with incredible accuracy, that of a nanosecond, or a billionth of a second. This was one measurement that, though possible, still hadn't been carried out by Taleyarkhan and his team.
So Horizon decided to try to sort out the issue once and for all. And we commissioned an independent team of leading scientists to conduct the experiment. Working from the instructions set out in Taleyarkhan's paper, we assembled the same key scientific conditions to create nuclear fusion from sonoluminescence. To see if we could find fusion, we measured the neutrons and the flashes of light simultaneously with nanosecond accuracy, something that had never been done before.
The experiment was carried out by Seth Putterman, one of the world's leading practitioners of sonoluminescence. His data was analysed by a panel in the UK that included experts in sonoluminescence and neutron detection. They agreed that Putterman had achieved the vital scientific conditions set out in Taleyarkhan's paper and that his experiment was a good attempt at getting the same results.
But then it came down to the crucial question: did Putterman find fusion? The result was negative. Recording data nanosecond by nanosecond, Putterman did not find a single neutron close enough to a flash of light for it to be considered the result of nuclear fusion.
We put our conclusion to Taleyarkhan. He said that several differences in our equipment could have affected our results. It is very possible that other laboratories around the world will reproduce Rusi Taleyarkhan's fusion results but until then, the claim will continue to attract great scepticism from the wider scientific community.
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