Proposed Experiment to Convert Light into Matter in Simplest Way Known

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Light and Matter Interactions Over the Years: Image courtesy of Oliver Pike, Imperial College London

Physicists at Imperial College London announced a breakthrough experiment to turn light into matter in the most direct way possible.

Should the experiment work, it would verify an eighty year old theory, and replicate conditions thought to exist when the universe was less than one second old.

Metamorphosis in Physics

In 1928, brilliant British physicist Paul Dirac predicted that every particle has a twin — a doppelganger with the same mass but opposite electrical charge. This particle was dubbed an antiparticle.

An electron, for example, has an electric charge of -1.  The electron’s corresponding antiparticle is called the positron, which has an electric charge of +1.

Dirac’s theory predicted when a matter particle and its antiparticle twin collide, they annihilate each other and produce photons (light particles).

The opposite is also true. When two photons with enough energy collide, they disappear and produce a matter particle and its corresponding antiparticle.

 

A number of laboratory experiments have demonstrated the conversion of matter into light — for example, the collision of an electron and positron to produce two photons. But light to matter conversion has proved more complicated.  These experiments have required the presence of atomic nuclei, electrons, or massive high-energy particles.

The simplest and most direct way to create matter from light was theorized in 1934 by physicists Gregory Breitand John Archibald Wheeler. This involved the collision of two photons of light to produce an electron and positron. There were great doubts at the time as to the practicality of such an experiment.

Despite all physicists accepting the theory to be true, when Breit and Wheeler first proposed the theory, they said they never expected it be shown in the laboratory,” Professor Steve Rose at the Department of Physics at Imperial College London said in a  London press release.

Rose and his colleagues at Imperial’s Blackett Physics Laboratory have published a paper in Nature Photonics which “shows for the first time how Breit and Wheeler’s theory could be proven in practice,” per the press release.  “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.”

The Envisioned Photon Collider

Example of a hohlraum. Mock up to be used on the NIF laser. Image courtesy of the Lawrence Livermore National Laboratory

There are two steps in the proposed photon collider experiment.

In the first step, a very high power laser speeds up electrons to nearly the speed of light. The electrons are then shot into a gold slab. This produces “a beam of photons a billion times more energetic than visible light,” per the press release.

In the second step, a high energy laser is fired at the inner surface of a tiny gold can called a “hohlraum”to produce a thermal radiation field of light. The photon beam from step one is then directed through the center of the gold can. These photons collide with the thermal radiation photons inside the can to produce electron-positron pairs. The plan is to then detect these electrons and positrons as they exit the can.

Where did this idea for a new experiment come from? Lead researcher Oliver Pike and colleagues at Imperial sat down one day to search for hohlraum applications outside their traditional role in fusion energy research.

Within a few hours . . .  we were astonished to find they provided the perfect conditions for creating a photon collider,”  said Pike in the press release. “Although the theory is conceptually simple, it has been very difficult to verify experimentally (in the past),” he added. “The (new photon collider) experimental design we propose can be carried out with relative ease and with existing technology. 

To work, the envisioned photon collider will require high energy photons. Why? Because per E = mc2, the energy of the photons must be at least equal to the mass of the matter particles they are trying to produce times the speed of light squared.  This is called the mass threshold.

Big Bang Redux

Light to Matter Conversion: Given enough energy, two colliding photons (light particles) can produce a matter particle and antiparticle, such as the electron and positron shown here. Image by Mark Egdall, all rights reserved.

Upon implementation, the photon collider experiment will emulate a critical process found in the very early universe. Moments after the big bang, high energy photons collided to produce matter particle-antiparticle pairs. These in turn collided to produce photon pairs.

As the universe expanded and cooled, it stretched the frequencies of the photons and reduced their energies. About 1 second after the big bang, photon energies fell below the mass threshold temperature required for the production of electrons and positrons. Photons continued to collide, but no longer had the energy to transform into electron-positron pairs — so they remained as photons.

Nonetheless, electrons and positrons (and heavier matter particles) continued to collide to produce more and more photons. As a result, the universe filled with a fireball of primordial light. This radiation has since cooled dramatically — to form the Cosmic Microwave Background we see today in the heavens.

Science Past Meets Science Future

The meeting between past theories and present capabilities is always exciting, and when it relates to the origins of the universe, even more so. Decoded Science eagerly awaits the results of this experiment.

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