A New Study Suggests Light Can Form Without Any Matter at All

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In quantum field theory, a vacuum is never completely empty. Electromagnetic particles like electrons and positrons are always passing through in particle or wave form.
Simulations of what would happen in a vacuum if we fired extremely powerful lasers through it showed that these rogue particles could be rearranged into another beam of light.
Future facilities operating petawatt lasers are looking to attempt carrying out further experiments along these lines.

Empty spaces in the dark only appear to be complete voids. Even intergalactic space (in the absence of dust particles) has a few hydrogen atoms per cubic meter. Now, simulations that sound like they belong in a sci-fi movie have shown that light can—at least, in the quantum realm—be created out of nothing. Welcome to the quantum vacuum.

How can a beam of light materialize out of thin air without even the tiniest motes of dust swirling around? Well, quantum field theory answers this by suggesting that even in regions of space where there is absolutely nothing physical, there are fleeting electromagnetic particles and waves that come and go. Through a series of 3D digital simulations (using software known as OSIRIS), a team of physicists at the University of Oxford—led by Zixin Zhang—found that extreme laser beams can disturb other particles introduced into that field and rearrange them into something else.

Zhang and her team were able to virtually create a phenomenon predicted by quantum physics known as vacuum four-wave mixing. It is thought that when this occurs, two or three interacting wavelengths can create one or two more wavelengths without anything extra being added. Previous studies hypothesized that processes which were intense enough could polarize or possibly even break down the vacuum itself. When three powerful laser pulses joined forces in the simulations, they polarized pairs of electrons and positrons in the vacuum, which act both like particles and waves (in quantum terms). Polarization causes a wave to vibrate in a particular direction relative to where it is coming from.

“The quantum vacuum is filled with energy fluctuations from which virtual electron-positron pairs arise,” Zhang said in a study recently published in Physics Communications. “The presence of these virtual particles creates non-linearities in vacuum that interact with high power laser pulses propagating through, altering their properties.”

Birefringence was another exotic phenomenon produced by the experiment. When light passes through electromagnetic fields with considerable strength, it can experience a shift in polarization because the light bent in two different ways. In the physical world, birefringent materials are often used to make polarizing prisms, which have applications ranging from microscopy to photography. In the recent simulation, vacuum birefringence occurred because the polarization of electrons and positrons changed when they passed through through the laser beams.

These weren’t just any lasers, either—they were petawatt lasers. Multi-petawatt lasers such as Romania’s Extreme Light Infrastructure (ELI) can blast double laser beams at 10 petawatts each. A petawatt is 10¹⁵ watts, or about 10,000,000,000,000 standard lightbulbs of power).

While the electrons and positrons in the simulations would normally be impossible to observe, this recent simulation has shown that zapping them with lasers could change that fact. This opens a portal into testing out some extremely weird physics beyond a computer screen, and it might someday be possible to explore exotic shapes and pulses in laser beams.

Especially once we bring the next wave of petawatt lasers online. The upcoming Central Laser Facility in the UK will have a main beam that can operate on the order of 20 petawatts and get an assist from eight additional, less energetic beams. And the University of Rochester in Rochester, New York, plans to take it even further—its EP-OPAL project will generate two 25-petawatt beams, and one of its first experiments will deal with four-wave mixing that scatters photons.

“Looking ahead, the solver could be applied to studying interactions of novel pulse shapes […] and tightly focused beams,” the Oxford team said. “The simulation results can complement existing theoretical work and provide benchmarks for future high-power laser experiments.”

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