UK design 'starts race' to turn pure light into matter

gamma ray burst It would replicate a process that also occurs in gamma-ray bursts - the Universe's biggest explosions

Physicists have uncovered a surprisingly straightforward strategy for turning light into matter.

Published in Nature Photonics, the design adapts existing technology and could be implemented in the UK.

Several locations could now enter a race to convert photons into positrons and electrons, in the absence of other particles, for the very first time.

This would prove an 80-year-old theory by Breit and Wheeler, who themselves thought physical proof was impossible.

Now, according to researchers from Imperial College London, that proof is within reach.

Prof Steven Rose and his PhD student, Oliver Pike, told the BBC it could happen within a year.

"With a good experimental team, it should be quite doable," said Mr Pike.

If the experiment comes to fruition, it will be the final piece in a puzzle that began in 1905, when Einstein accounted for the photoelectric effect with his model of light as a particle.

Several other basic interactions between matter and light have been described and subsequently proved by experiment, including Dirac's 1930 proposal that an electron and its antimatter counterpart, a positron, could be annihilated upon collision to produce two photons.

Breit and Wheeler's theoretical prediction of the reverse - that two photons could crash together and produce matter (a positron and an electron) - has been difficult to observe.

"The reason this is very hard to see in the lab is that you need to throw an awful lot of photons together - because the probability of any two of them interconverting is very low," Prof Rose explained.

In 1997, an experiment at the Stanford Linear Accelerator Centre managed, for the first time, to collide photons with enough energy to produce particles of matter. That team used very high-energy electrons to bounce the photons in a trillion-watt laser beam back into the oncoming stream. The bounced photons had such high energy that some of them collided with several laser photons simultaneously, producing multiple electron-positron pairs.

The new design, according to Prof Rose, should produce "pure two-photon annihilation", without the need for additional particles.

His team proposes gathering a vast number of very high-energy photons by firing an intense beam of gamma-rays into a further cloud of photons, created within a tiny, gold-lined cylinder.

That cylinder is called a "hohlraum", German for "hollow space", because it contains a vacuum, and it is usually used in nuclear fusion research. The cloud of photons inside it is made from extraordinarily intense X-rays and is about as hot as the Sun.

Hohlraum Gamma-rays (white) would be fired into a cloud of X-ray photons inside a "hohlraum" to produce positrons and electrons (red and blue)

Hitting this very dense cloud of photons with the powerful gamma-ray beam raises the probability of collisions that will make matter - and history.

"It's pretty amazing really," said Mr Pike. He says it took some time to realise the value of the scheme, which he and two colleagues initially jotted down on scrap paper over several cups of coffee.

"For the first 12 hours or so, we didn't quite appreciate its magnitude."

But their subsequent calculations showed that the design, theoretically at least, has more than enough power to crack the challenge set by Breit and Wheeler in the 1930s.

"All the ingredients are there," agrees Sir Peter Knight, an emeritus professor at Imperial College who was not involved in the research but describes it as a "really clever idea".

"I think people will seriously start to have a crack at this," Prof Knight told BBC News, though he cautioned that there were a lot of things to get right when putting the design into practice.

"If it's done in a year, then they've done bloody well! I think it might take a bit longer."

Some healthy scientific competition may speed up the process.

There are at least three facilities with the necessary equipment to test out the new proposal, including the Atomic Weapons Establishment in Aldermaston.

"The race to carry out and complete the experiment is on," said Mr Pike.

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