Theoretical physicists have proposed a way they think will allow us to create matter from light. The experiment has yet to be done, but has passed peer review as practical and the inventors are in discussions with experimentalists with the equipment to carry it out. The proposal has created excitement because, while it has been accepted for 80 years that two photons of light could theoretically create matter, for that time the demonstrational proof was regarded as beyond the reach of lab equipment.
As Einstein's much quoted, but seldom understood, equation e=mc2 tells us, matter and energy are connected. One can turn into another. Light is one of the forms of energy produced from matter in atomic bombs. However, going the other way is more of a challenge.
In 1934 Gregory Breit and John Wheeler proposed that, under the right circumstances, two photons of light would convert into an electron and its antimatter equivalent the positron. Breit-Wheeler pair production is classified as one of seven basic types of light and matter interactions (see chart below). The others all resulted in Nobel Prizes, either for the experimentalist who observed it or the theoretician who explained it. (In some cases, observations came before theory, whereas in others it was the reverse).
"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,” says Professor Steve Rose of Imperial College London. However, the options open to experimentalists have expanded a lot since then.
Earlier this year two physicists from the University of Warsaw, Katarzyna Krajewska and Jerzy Kaminski modeled the distributions of electron-positron pairs created when laser and nonlaser photons collide, noting that the “rapid development of high-power laser technology has led to a renaissance of theoretical interest in strong-field quantum electrodynamics.”
Now Rose and his Phd student Oliver Pike have come up with a way to put this sort of modeling to the test. In Nature Photonics they propose a two step process. High intensity lasers would be used to push electrons until they are traveling close to the speed of light, directing them towards a slab of gold. The electrons' impact would release high energy gamma rays. Ordinary gamma rays, such as those produced by many nuclear decays, will not do it. The photons produced here are 1000 times as energetic as those at at the division between X-Rays and gamma rays, or a billion times that of visible light.
Then it would be necessary to send the output of a high energy laser inside an otherwise hollow gold tube and fire the light at the tube's inner surface to create a spread of wavelengths. If the high energy gamma rays from the first stage are directed into the ultraheated center of the tube Rose and Pike believe the collisions between the two sorts of photons would not only produce electron-positron pairs, but do so in such numbers “of the order of 100,000 pairs” that they would be detectable.
"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,” says Pike.
The idea came not from a systematic search for a Breit-Wheeler demonstration, but Pike's contribution to the quest for nuclear fusion. The hollow gold can, known as a hohlraum is a staple of fusion research. “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,” Pike says.
There is, of course, many a slip between the can and the lip. Although Pike and Rose are collaborating with labs with the equipment to try the experiment out it has yet to be done. The vast expense of energy to create a few electrons would make this a dead loss as a commercial process, but the potential to shed light on the quantum world matters greatly.
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