Proving the impossible: experiment devised to convert light into matter

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A practical experiment to prove Breit and Wheelers theory that it would be possible to convert light into matter has been devised by Imperial College London physicists after an 80-year wait. While the theory is accepted to be true, it was initially said to be impossible to prove under laboratory conditions. However, in just one day three physicists have managed to calculate a way of physically proving the hypothesis, which has been published in the journal, Nature Photonics.

By colliding two photons together, the theory states, an electron and a positron can be created. The method suggests a photon-photon collider and would open up an entire new type of experimental high-energy physics.

The scientists suggest colliding high-energy photons within a small gold container called a hohlraum. A high-energy beam is produced by accelerating electrons to close to the speed of light and firing them into a slab of gold. This would create a beam of photons which, the university said, is a billion times more energetic than visible light. A high-energy laser is used to create a thermal radiation field, which contains light similar to that emitted by stars, by directing the beam at the inside of the hohlraum.

The beam from the gold slab is then directed through the centre of the can where the photons then collide with the photons from the thermal radiation field. The electrons and positrons which are created would then be detectable as they exited the hohlraum.

Lead researcher, Oliver Pike, said: ‘Although the theory is conceptually simple, it has been very difficult to verify experimentally. We were able to develop the idea for the collider very quickly, but the experimental design we propose can be carried out with relative ease and with existing technology.’

The scientists had been investigating problems with fusion energy when they realised what they were working on could be applied to the Breit-Wheeler theory. The breakthrough was achieved in collaboration with a fellow theoretical physicist from the Max Planck Institute for Nuclear Physics, who happened to be visiting Imperial.

Pike continued: ‘Within a few hours of looking for applications of hohlraums outside their traditional role in fusion energy research, we were astonished to find they provided the perfect conditions for creating a photon collider. The race to carry out and complete the experiment is on!’

Demonstrating the Breit-Wheeler theory would provide the final jigsaw piece of a physics puzzle which describes the simplest ways in which light and matter interact. The six other pieces in that puzzle, including Dirac’s 1930 theory on the annihilation of electrons and positrons and Einstein’s 1905 theory on the photoelectric effect, are all associated with Nobel Prize-winning research.

Professor Steve Rose from the Department of Physics at Imperial College London said: ‘Despite all physicists accepting the theory to be true, when Breit and Wheeler first proposed the theory, they said that they never expected it be shown in the laboratory. Today, nearly 80 years later, we prove them wrong. What was so surprising to us was the discovery of how we can create matter directly from light using the technology that we have today in the UK. As we are theorists we are now talking to others who can use our ideas to undertake this landmark experiment.’

The research was funded by the Engineering and Physical Sciences Research Council (EPSRC), the John Adams Institute for Accelerator Science, and the Atomic Weapons Establishment (AWE), and was carried out in collaboration with Max-Planck-Institut für Kernphysik.