Physicists Pave the Way to Turn Light into Matter

This artist's conception shows two photons (in green) colliding. Image Credit: ATLAS / LHC

E = mc². It’s one of the most basic and fundamental equations throughout astrophysics. But it does more than suggest that mass and energy are interconnected, it implies that light can be physically transformed into matter.

But can it really — physically — be done? Scientists proposed the theory more than 80 years ago, but only today have they paved the way to make this transformation routinely on Earth.

The concept calls for a new kind of photon-photon collider. It sounds like science fiction, but it could be turned into reality with existing technology.

“Although the theory is conceptually simple, it has been very difficult to verify experimentally,” said lead researcher Oliver Pike from London’s Imperial College in a press release. “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.”

In 1934, two physicists Gregory Breit and John Wheeler proposed that it should be possible to turn light into matter by smashing together only two photons, the fundamental particles of light, to create an electron and a positron. It was the simplest method of turning light into matter ever predicted, but it has never been observed in the laboratory.

Past experiments have required the addition of massive high-energy particles. We’ve seen from the development of nuclear weapons and fission reactors that a tiny amount of matter can yield a tremendous amount of energy. So it seems Breit and Wheeler’s theory would require the opposite effect: tremendous amounts of energy from photons to yield a tiny amount of matter.

This experiment will be a first in that it doesn’t require the addition of massive high-energy particles. It will be performed purely from photons.

The concept calls for using a high-intensity laser to speed up electrons to just below the speed of light, and then smash them into a slab of gold to create a beam of photons a billion times more energetic than visible light. At the same time, another laser beam would be blasted onto a hohlraum — a small gold container meaning “empty can” in German — that would create a radiation field with photons buzzing inside.

The initial photon beam would be directed into the center of the hohlraum. When the photons from the two sources collide, some would be converted into pairs of electrons and positrons. A detector would then pick up the signatures form the matter and antimatter as they flew out of the container.

Theories describing light and matter interactions. Image Credit: Oliver Pike, Imperial College London
Theories describing light and matter interactions. Image Credit: Oliver Pike, Imperial College London

“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 said. “The race to carry out and complete the experiment is on!”

The demonstration, if carried out successfully, would be a new type of high-energy physics experiment. It would complete physicists’ list of the fundamental ways in which light and matter interact, and both recreate a process that was important 100 seconds after the Big Bang and a process visible in gamma ray bursts, the most powerful explosions in the cosmos.

The paper has been published in Nature Photonics.

Surprise Gamma-Ray Burst Behaves Differently Than Expected

Artist's impression of a gamma-ray burst, showing the two intense beams of relativistic matter emitted by the black hole. To be visible from Earth, the beams must be pointing directly towards us. Credit : NASA/Swift/Mary Pat Hrybyk-Keith and John Jones

Roughly once a day the sky is lit up by a mysterious torrent of energy. These events — known as gamma-ray bursts — represent the most powerful explosions in the cosmos, sending out as much energy in a fraction of a second as our Sun will give off during its entire lifespan.

Yet no one has ever witnessed a gamma-ray burst directly. Instead astronomers are left to study their fading light.

New research from an international team of astronomers has discovered a puzzling feature within one Gamma-ray burst, suggesting that these objects may behave differently than previously thought.

These powerful explosions are thought to be triggered when dying stars collapse into jet-spewing black holes. While this stage only lasts a few minutes, its afterglow — slowly fading emission that can be seen at all wavelengths (including visible light) — will last for a few days to weeks. It is from this afterglow that astronomers meticulously try to understand these enigmatic explosions.

The afterglow emission is formed when the jets collide with the material surrounding the dying star. They cause a shockwave, moving at high velocities, in which electrons are being accelerated to tremendous energies. However, this acceleration process is still poorly understood. The key is in detecting the afterglow’s polarization — the fraction of light waves that move with a preferred plane of vibration.

“Different theories for electron acceleration and light emission within the afterglow all predict different levels of linear polarization, but theories all agreed that there should be no circular polarization in visible light,” said lead author Klaas Wiersema in a press release.

“This is where we came in: we decided to test this by carefully measuring both the linear and circular polarization of one afterglow, of GRB 121024A, detected by the Swift satellite.”

Gamma-ray burst 121024A, as seen on the day of burst by ESO’s Very Large Telescope (VLT) in Chile. Only a week later the source had faded completely. Credit: Dr Klaas Wiersema, University of Leicester, UK and Dr Peter Curran, ICRAR.
Gamma-ray burst 121024A, as seen on the day of the burst by ESO’s Very Large Telescope in Chile. Only a week later the source had faded completely. Image Credit: Dr Klaas Wiersema, University of Leicester, UK and Dr Peter Curran, ICRAR.

And to their surprise, the team detected circular polarization, meaning that the light waves are moving together in a uniform, spiral motion as they travel. The gamma-ray burst was 1000 times more polarized than expected. “It is a very nice example of observations ruling out most of the existing theoretical predictions,” said Wiersema.

The detection shows that current theories need to be re-examined. Scientists expected any circular polarization to be washed out. The radiation of so many electrons travelings billions of light-years would erase any signal. But the new discovery suggests that there could be some sort of order in the way these electrons travel.

Of course the possibility remains that this particular afterglow was simply an oddball and not all afterglows behave like this.

Nonetheless “extreme shocks like the ones in GRB afterglows are great natural laboratories to push our understanding of physics beyond the ranges that can be explored in laboratories,” said Wiersema.

The paper has been published in Nature.