We’ve seen matter become light. What about the other way?

Matter can become energy. You might have seen this in the film Oppenheimer or in videos of nuclear explosions. Just as the nuclei fuse, a blinding light fills the air and sky, so bright that simply closing your eyelids does not help.

Physics allows physicists to do the opposite as well: to create matter from light. The problem is the scale at which the required technologies need to work. Imagine playing that video in reverse. Scientists need to pack a great quantity of light into a really small volume of space, and hold it there for long enough for the light to ‘condense’ into matter.

Specifically, scientists need to create extremely intense electromagnetic fields (photons are packets of electromagnetic energy).

Recently, researchers from Germany, Ireland, the U.K., and the U.S. reported clearing a key technical hurdle on the path to doing so. Their findings were published in Nature.

The theory that describes how light and matter interact is called quantum electrodynamics (QED). Normally, empty space appears empty. But at extremely high energy, QED predicts that a sufficiently powerful electromagnetic field can stir up a pair of subatomic particles from the vacuum. The problem? The strength of the electromagnetic field required far outpaces what current technologies can achieve.

In the new study, the researchers used a different technique. They fired an extremely intense laser pulse at a polished target, immediately parting the atoms in its surface of all their electrons. The electric field in the laser light set the electrons oscillating back and forth at nearly the speed of light. This caused the target’s surface to reflect the laser pulse in multiple copies at higher frequencies, called harmonics.

Physicists have hoped to further compress these harmonics and focus them into small spots. Such coherent harmonic focusing (CHF) would make the harmonics extremely intense, approaching the levels required to create matter. But while the idea has been around for two decades, physicists have struggled to realise it efficiently. Previous experiments produced harmonics but with much less energy than in the original pulse.

A pulse’s intensity increases from the leading point to its middle, roughly. The time taken for the pulse’s peak intensity to enter the target is called its rise time. The authors of the new study reduced the rise time to well under one-trillionth of a second. At peak performance, then, the experiment generated enough energy within the 12th to 47th harmonics to show for the first time that the efficiency required by theory may be within reach.

The authors have stressed that they have not yet demonstrated the final focused state, only that the essential technical ingredients are now in place. At the next generation laser facilities that the U.K. and China, among others, are currently building, the technique is projected to reach an intensity of 10²⁹ W per sq. cm — the threshold where QED effects become apparent.

“The reward would be access to a new regime of physics, where light becomes strong enough to tear down the vacuum and create particles from nothing,” Alexander Pukhov, of the University of Dusseldorf and who helped lay the foundations of CHF, told Physics.