Fast radio bursts are one of the biggest cosmic mysteries of our time. They're extremely powerful but extremely brief explosions of electromagnetic radiation in radio wavelengths, discharging in milliseconds as much energy as 500 million Suns.
For years, scientists puzzled over what could be causing these brief outbursts, detected in galaxies millions to billions of light-years away. Then, in April 2020, we got a really strong lead: a brief, powerful flash of radio waves from something inside the Milky Way – a magnetar.
This suggests that at least some fast radio bursts are produced by these extremely magnetized dead stars. Now, physicists have devised a way to replicate in a lab what we think happens in the first stages of these insane explosions, according to the theory of quantum electrodynamics (QED).
"Our laboratory simulation is a small-scale analog of a magnetar environment," says physicist Kenan Qu of Princeton University. "This allows us to analyze QED pair plasmas."
A magnetar is a type of dead star called a neutron star. When a massive star reaches the end of its lifespan, it blows off its outer material, and the core, no longer supported by the outward pressure of nuclear fusion, collapses under its own gravity to form an ultra-dense object with a powerful magnetic field. That's the neutron star.
Some neutron stars have an even more powerful magnetic field. That's a magnetar. We don't know how they get this way, but their magnetic fields are somewhere around 1,000 times more powerful than those of a normal neutron star, and a quadrillion times more powerful than Earth's.
Scientists think that fast radio bursts are a result of the tension between the magnetic field, so powerful it distorts the magnetar's shape, and the inward pressure of gravity.
The magnetic field is also thought to be responsible for transforming the matter in space around the magnetar into a plasma consisting of matter- antimatter pairs. These pairs consist of a negatively charged electron and positively charged positron, and they are thought to play a role in the emission of the rare fast radio bursts that repeat.
This plasma is called a pair plasma, and it's very different to most of the plasma in the Universe. Normal plasma consists of electrons and heavier ions. The matter-antimatter pairs in pair plasma have equal masses and spontaneously form and annihilate each other. Pair plasmas' collective behavior is very different from that of normal plasmas.
Because the strength of the magnetic fields involved is so extreme, Qu and his colleagues devised a way to create pair plasmas in a lab via other means.
"Rather than simulating a strong magnetic field, we use a strong laser," Qu explains.
"It converts energy into pair plasma through what are called QED cascades. The pair plasma then shifts the laser pulse to a higher frequency. The exciting result demonstrates the prospects for creating and observing QED pair plasma in laboratories and enabling experiments to verify theories about fast radio bursts."
The technique involves generating a high-speed electron beam, traveling at close to the velocity of light. A moderately powerful laser is fired at this beam, and the resulting collision creates a pair plasma.
Moreover, it slows the resulting plasma. This could solve one of the problems found with previous experiments to create pair plasmas – observing their collective behavior.
"We think we know what laws govern their collective behavior. But until we actually produce a pair plasma in the laboratory that exhibits collective phenomena that we can probe, we cannot be absolutely sure of that," says physicist Nat Fisch of Princeton University.
"The problem is that collective behavior in pair plasmas is notoriously hard to observe. Thus, a major step for us was to think of this as a joint production-observation problem, recognizing that a great method of observation relaxes the conditions on what must be produced and in turn leads us to a more practicable user facility."
The observation experiment is yet to be conducted, but it offers a way to conduct these probes that hasn't been possible before. It reduces the need for extremely powerful equipment that may be beyond our technical capabilities and budgets.
The team is currently preparing to test their ideas with a series of experiments at the SLAC National Accelerator Laboratory. This, they hope, will help them learn how magnetars generate pair plasmas, how those pair plasmas might produce fast radio bursts, and to identify whatever previously unknown physics might be involved.
"In a sense what we are doing here is the starting point of the cascade that produces radio bursts," says physicist Sebastian Meuren of Stanford University and SLAC.
"If we could observe something like a radio burst in the laboratory that would be extremely exciting. But the first part is just to observe the scattering of the electron beams, and once we do that we'll improve the laser intensity to get to higher densities to actually see the electron-positron pairs. The idea is that our experiment will evolve over the next two years or so."
So it might be a little bit longer until we get our answers on fast radio bursts. But if we've learned anything over the years, it's that unraveling this fascinating mystery is definitely worth the wait.
The team's paper has been published in Physics of Plasmas.