Physicists Have Found a Way to Simulate The Beginnings of Fast Radio Bursts

 

Artist's impression of a magnetar. (Sophia Dagnello, NRAO/AUI/NSF)

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 could be triggering these brief outbursts, which have been observed in galaxies millions to billions of light-years away. Then, in April 2020, we got a huge lead: a brief, bright burst of radio waves emanating from something deep within the Milky Way — a magnetar.

 

This shows that these highly magnetic dead stars produce at least some rapid radio bursts. According to quantum electrodynamics theory, physicists have now developed a mechanism to mimic in a lab what we assume happens in the earliest phases of these wild explosions (QED).

 

"Our lab simulation is a small-scale counterpart of a magnetar environment," explains Princeton University physicist Kenan Qu. "With this, we can examine 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 shape, 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 fast radio bursts that reoccur is thought 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 make up normal plasma. In pair plasma, the matter-antimatter pairs have equal masses and spontaneously create and annihilate one another. The collective behaviour of pair plasmas is substantially different from that of normal plasmas.

 

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.

 

Quexplains, "Rather than imitating a strong magnetic field, we employ a strong laser."

 

"Through what are known as QED cascades, it turns energy into pair plasma. The laser pulse is subsequently shifted to a higher frequency by the pair plasma. The interesting finding shows the potential for producing and studying QED pair plasma in laboratories, as well as studies to test hypotheses about rapid radio bursts."

 

The method includes creating a high-speed electron beam that travels at close to light speed. A moderately intense laser is directed at this beam, resulting in a pair plasma collision.

 

Furthermore, the resultant plasma is slowed. This could overcome one of the issues encountered in prior pair plasma experiments: watching their aggregate behaviour.

 

"We believe we understand the laws that govern their aggregate behaviour. However, we won't know for sure until we create a pair plasma in the lab that shows collective phenomena that we can investigate "Princeton University physicist Nat Fisch says".

 

"The issue is that pair plasma collective behaviour is notoriously difficult to monitor. Thus, realising 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, we took a big stride forward by thinking of this as a joint production-observation problem."

 

Although the observation experiment has yet to be carried out, it provides a new technique to conduct these investigations. It eliminates the need for incredibly powerful equipment that may be beyond our technological ability and financial means.

 

The team is currently preparing a set of tests at the SLAC National Accelerator Laboratory to test their theories. Scientists expect that by doing so, they will be able to understand more about how magnetars form pair plasmas, how those pair plasmas might produce fast radio bursts, and what previously undiscovered physics might be involved.

 

"In a sense, what we're doing here is the beginning of the cascade that causes radio bursts," explains Stanford University and SLAC scientist Sebastian Meuren.

 

"It would be incredibly thrilling if we could detect something like a radio explosion in the laboratory. However, the initial step is to see the scattering of electron beams, after which we'll increase the laser power to reach higher densities and view the electron-positron pairs. Our experiment is expected to evolve over the next two years or so."

 

As a result, we may have to wait a little longer for answers on quick radio bursts. But if there's one thing we've learned over the years, it's that solving this intriguing puzzle is well worth the wait.

Reference: Princeton University