麻豆传媒

NUCLEAR fusion can be achieved by popping bubbles in nail polish remover, claims a team led by Rusi Taleyarkhan at Oak Ridge National Laboratory in Tennessee.

鈥淚f it is true, it is truly amazing,鈥� says Andrea Prosperetti, who studies bubbles at Johns Hopkins University in Baltimore. For starters, it would allow physicists to study fusion on their bench tops, although this is already possible with low-power accelerators. More tantalising, but even more controversial, is the prospect of harnessing this form of fusion to produce clean energy.

Bubble fusion is already provoking comparisons with the cold fusion fiasco in 1989, when Martin Fleischmann of Southampton University and Stanley Pons of the University of Utah announced that they had triggered nuclear fusion at room temperature by electrolysing heavy water. Others failed to replicate their results.

The first attempt to duplicate Taleyarkhan鈥檚 results has also failed. 鈥淥ur experiment saw no evidence for nuclear fusion,鈥� says Mike Saltmarsh, a member of another team at Oak Ridge that tried the experiment again with a more sophisticated detection system. 鈥淭his does not prove that no nuclear fusion is going on,鈥� Saltmarsh points out. But if any fusion is occurring, it must be at a very low level.

Nevertheless, Taleyarkhan鈥檚 results are being taken more seriously than Fleischmann and Pons鈥檚. For one thing, they appear in a prestigious journal. More importantly, there鈥檚 nothing cold about this fusion. Collapsing bubbles really might approach the temperatures needed for fusion.

Getting energy from nuclear fusion has been the goal of many scientists. One way is to fuse two deuterium atoms鈥攈ydrogen with a neutron. This reaction yields lots of energy and creates either inert helium and free neutrons, or hydrogen and radioactive tritium鈥攈ydrogen with two neutrons. Tritium has a half-life of just 12 years and can be turned into helium by fusing it with more deuterium. So unlike existing fission reactors, fusion wouldn鈥檛 produce mountains of radioactive waste.

The catch is that to overcome the repulsion between deuterium nuclei, they have to be heated to around 10 million degrees Celsius, the temperature in the heart of the Sun. Containing matter at such extraordinary temperatures is extremely difficult. No one has developed a fusion reactor that generates more energy than it uses up.

It鈥檚 been speculated that collapsing bubbles could be an alternative to giant reactors. For decades it has been known that sound waves in water can generate tiny bubbles that heat to thousands of degrees and emit light as they collapse鈥攁 phenomenon called sonoluminescence. If these bubbles get near the magic 10 million degrees, nuclear fusion might occur.

Taleyarkhan鈥檚 team thought it might be possible to achieve such temperatures in acetone, or nail varnish remover. To achieve fusion, they replaced the hydrogen atoms in the acetone with deuterium and chilled the liquid to 0 掳C. Blasting it with a neutron beam generated tiny bubbles. Sound waves then expanded the bubbles to about 2 millimetres before they imploded.

The team reports detecting emissions of high-energy neutrons that coincided with light and shock waves from the imploding bubbles. The neutrons had energies of 2.5 million electronvolts, which is what you鈥檇 expect for neutrons released as deuterium fuses to form helium. They also detected raised levels of tritium. No evidence of fusion was detected when normal acetone was used instead.

But many scientists doubt that fusion took place. In theory, collapsing bubbles could reach astronomical temperatures if they remain perfectly spherical as they implode. But real-life bubbles don鈥檛 usually behave this way. The actual temperatures in the collapsing bubbles are probably more like 10,000 or 20,000 degrees, Prosperetti says. 鈥淔rom this to the millions you need for fusion is quite a stretch.鈥�

All the experts are now keen to see more attempts to repeat the experiment. But even if these show that fusion does occur, there鈥檚 no evidence that the process could generate more energy than it devours. Of course, it is possible that some clever technology might eventually squeeze enough energy out of the bubbles. 鈥淚 wouldn鈥檛 rule it out,鈥� Prosperetti says. 鈥淏ut this field has seen some weird claims in the past. Lots of people have ended up with mud on their faces.鈥�