In what could be a landmark moment in the history of science, physicists working at the Blackett Physics Laboratory in Imperial College London have designed an experiment to validate one of the most tantalizing hypotheses in quantum electrodynamics: the theory that matter could be created using nothing more than pure light.

Premised on a discussion that they had over one day and a few cups of coffee, the three physicists – two from Imperial College and one visiting from the Max Planck Institute in Heidelberg, Germany – recognized that their work on fusion energy also offered possibilities in the theory of light to matter creation, suggested in a theory 80 years ago by two American physicists, Breit and Wheeler. These two physicists had premised the idea that because annihilating electron-positron pairs produce two or more photons, then colliding photons should, in turn, produce electron-positron (or “Breit-Wheeler”) pairs.


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In devising an experiment aimed at attempting to produce these Breit-Wheeler pairs, the physicists working at Imperial College propose a two-step process. Firstly, a high-energy electron beam accelerating electrons in a vacuum close to the speed of light would be fired into a target of pure gold several millimeters thick. Via a process called “Bremsstrahlung” (German for “Braking radiation”) the high-energy electrons bombarding the target would lose kinetic energy but, in so doing, release gamma-ray photons.

Schematic of main elements of photon-photon collider

Secondly, a magnetic field within the apparatus would collimate and direct this gamma-ray photon beam into a hohlraum (German for “cavity”). Simultaneously, to ensure that any subsidiary electron-positron pairs created at this point would be separated by a further magnet containment field, the hohlraum would be bombarded with a high-energy laser beam effectively rendering it as a black-body thermal radiation chamber. As the beam of high-energy photons entered the cavity they would rise to a super-excited state where they would collide en masse with the photons generated by the laser aimed at the hohlraum and, all going to plan, hundreds of thousands of Breit-Wheeler pairs would be generated to form a continuous stream from the cavity. In fact, it is anticipated that, in using a 2-GeV electron beam and a 400-eV hohlraum laser, the yield would be in excess of 100,000 electron-positron pairs.

The seven simple ways that light and matter interact (Photo: Oliver Pike, Imperial College London)

If this experiment comes to fruition it would represent not only the first realization of a pure photon–photon collider, but a method of achieving light to matter transformation at power levels orders of magnitude lower than previously thought possible. And, without the requirement for a massive particle accelerator, it could be easily achieved in a modestly-equipped laboratory.

Given the potential to open up a relatively low-energy, simple way to investigate a cornerstone of quantum electrodynamics, this proposal should allow many more researchers access to this field. As a result, this could help add to our knowledge of the processes that took place in the first 100 seconds of the universe and possibly shed more light on those mysterious denizens of deep-space: gamma-ray bursts emanating from exploding massive stars.

Lastly, validating the Breit-Wheeler theory would also provide the seventh and final in the line theories describing the simplest ways in which light and matter interact. These include Dirac's 1930 theory on the annihilation of electrons and positrons, Einstein's 1905 theory on the photoelectric effect, and Blackett and Occhialini’s single-photon annihilation. Those theories are all associated with Nobel Prize-winning research.

Details of the research were published this week in the journal Nature Photonics.

Source: Imperial College London

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