One of the greatest cosmological mysteries of our time is fast radio bursts. They're massive but transient blasts of electromagnetic radiation at radio frequencies, releasing as much energy as 500 million Suns in milliseconds.
Scientists have been confused for years as to what may be triggering these short outbursts, which have been observed in galaxies millions to billions of light-years away. Then, in April 2020, we got a huge lead: a short, bright burst of radio waves emanating from something deep within the Milky Way — a magnetar.
This implies that these highly magnetic dead stars are responsible for at least some rapid radio bursts. According to quantum electrodynamics theory, scientists have now developed a means to mimic in a lab what we believe happens in the early phases of these wild explosions (QED).
"Our laboratory simulation is a small-scale analog of a magnetar environment," explains Princeton University physicist Kenan Qu. "This allows us to analyze QED pair plasmas."
A magnetar is a sort of neutron star that has died. When a giant star reaches the end of its life cycle, the outer material is blown off, and the core, no longer sustained by nuclear fusion, collapses under its own gravity to produce an ultra-dense object with a strong magnetic field. The neutron star is that.
The magnetic fields of some neutron stars are significantly stronger. That's a magnetic field. We don't know how they got this way, but their magnetic fields are 1,000 times stronger than that of a regular neutron star and a quadrillion times stronger than that of Earth.
Fast radio bursts are thought to be caused by a tension between the magnetic field, which is so strong that it alters the magnetar's form, and gravity's inward push.
The magnetic field is also assumed to be responsible for converting matter in space around the magnetar into plasma made up of matter-antimatter couples. The emission of the rare rapid radio bursts that reoccur is considered to be aided by these pairs, which consist of a negatively charged electron and a positively charged positron.
This plasma is known as a pair plasma, and it's unlike anything else in the universe. Electrons and heavier ions comprise normal plasma. In pair plasma, the matter-antimatter pairs have equal masses and create and destroy each other spontaneously. Pair plasmas behave differently from typical plasmas in terms of collective behavior.
Because the magnetic fields required are so strong, Qu and his colleagues found a method to make pair plasmas in the lab using other methods. "Rather than simulating a strong magnetic field, we use a strong laser," Qu says.
"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 method includes creating a high-speed electron beam that travels at near to light speed. A fairly intense laser is directed at this beam, resulting in a pair plasma collision.
Furthermore, the resultant plasma is slowed. This might overcome one of the issues encountered in prior pair plasma experiments: watching their aggregate 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 Princeton University physicist Nat Fisch.
"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."
Although the observation experiment has yet to be carried out, it provides a new technique to perform these investigations. It eliminates the need for incredibly powerful equipment that may be beyond our technological ability and financial means.
The team is presently preparing a set of tests at the SLAC National Accelerator Laboratory to verify its theories. Scientists expect that by doing so, they will be able to understand more about how magnetars form pair plasmas, how those pair plasmas could produce rapid radio bursts, and what previously undiscovered physics might be involved.
"In a sense what we are doing here is the starting point of the cascade that produces radio bursts," explains Stanford University and SLAC scientist Sebastian Meuren.
"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."
As a result, we may have to wait a little longer for replies on quick radio bursts. But if there's one thing we've learned over the years, it's that solving this enthralling puzzle is well worth the wait.
