Fast Radio Bursts: The Millisecond Signals From Deep Space We Still Can't Fully Explain
Fast radio bursts release as much energy in a millisecond as the Sun does in three days. Discovered in 2007, their origins involve magnetars, but the full picture is still emerging.
In 2007, astrophysicist Duncan Lorimer and his student David Narkevic were sifting through archival data from the Parkes Observatory in Australia when they found something nobody had seen before. Buried in pulsar survey data recorded on July 24, 2001, there was a single radio pulse. It lasted about five milliseconds. It came from the direction of the Small Magellanic Cloud, a dwarf galaxy roughly 200,000 light-years away.
Five milliseconds. That's less time than it takes to blink. And in those five milliseconds, whatever produced the signal released as much energy as the Sun puts out in three days.
They called it the Lorimer Burst. It was the first recognized fast radio burst (FRB), and it opened up one of the most active and confusing areas in modern astrophysics. Since 2007, thousands of FRBs have been detected. Some repeat. Most don't. One pulses every 16.35 days like clockwork. We've traced some to magnetars (neutron stars with unimaginably powerful magnetic fields), but we still don't fully understand the mechanism that produces them, and some FRBs don't fit the magnetar model at all.
What You'll Learn
- •What Exactly Is a Fast Radio Burst?
- •How Was the First FRB Discovered?
- •How Many Fast Radio Bursts Have Been Detected?
- •Why Do Some FRBs Repeat While Others Don't?
- •The Magnetar Breakthrough of 2020
- •What Is FRB 121102?
- •Could Fast Radio Bursts Be Alien Signals?
- •Why Are FRBs So Important to Astronomy?
- •Frequently Asked Questions
What Exactly Is a Fast Radio Burst?
A fast radio burst is a transient pulse of radio waves from deep space, lasting anywhere from a fraction of a millisecond to about three seconds. Despite their extremely short duration, they're phenomenally energetic at their source. The average FRB releases as much energy in a millisecond as the Sun produces in roughly three days.
But here's the catch: by the time these signals reach Earth, they're incredibly faint. The signal strength has been described as about 1,000 times weaker than a mobile phone on the Moon. It takes sensitive radio telescopes and careful data analysis to detect them at all.
FRBs come from all directions in the sky, and most originate from other galaxies, billions of light-years away. We know this because of a property called "dispersion measure," which measures how much the signal has been spread out by passing through the ionized gas between galaxies. The higher the dispersion measure, the more intergalactic material the signal has traveled through, meaning the farther away it originated.
What makes FRBs so puzzling is the combination of their enormous energy output and their tiny duration. Whatever produces them must be compact (to produce such a short burst) and extraordinarily powerful (to release so much energy so quickly). Only a few types of astrophysical objects meet both criteria.
How Was the First FRB Discovered?
The Lorimer Burst wasn't discovered in real time. Duncan Lorimer and David Narkevic found it in 2007 while reviewing archival data from the Parkes radio telescope in New South Wales, Australia. The data had been recorded on July 24, 2001, during a survey looking for pulsars.
The burst stood out immediately. It was a single, bright radio pulse with a dispersion measure far too high to have come from within the Milky Way. Whatever produced it was extragalactic, likely hundreds of millions of light-years away.
The discovery was met with some skepticism. A single detection in archival data could be a glitch, an equipment malfunction, or interference from terrestrial sources. In 2010, the skeptics seemed vindicated when similar signals were detected at Parkes and traced to microwave ovens in the observatory's kitchen. These "perytons," as they were called, mimicked some properties of FRBs and raised questions about the Lorimer Burst's validity.
But subsequent discoveries vindicated Lorimer. More FRBs were found in other datasets, from other telescopes, with properties that couldn't be explained by terrestrial interference. The first FRB observed in real time came on January 19, 2015, when the Parkes Observatory detected one live. By that point, the existence of fast radio bursts as a genuine astrophysical phenomenon was no longer in doubt.
How Many Fast Radio Bursts Have Been Detected?
The floodgates opened with the Canadian Hydrogen Intensity Mapping Experiment (CHIME), a radio telescope in British Columbia that began operating in 2018. CHIME was designed specifically with the sensitivity and sky coverage to detect FRBs at scale.
The numbers are staggering. In June 2021, astronomers reported that CHIME had detected over 500 FRBs from outer space in a single year. By now, thousands have been catalogued. CHIME detects roughly one per day on average, and that rate is limited by the telescope's field of view, not by the actual frequency of FRBs in the universe.
Current estimates suggest that roughly 10,000 FRBs occur somewhere in the observable universe every single day. The sky is constantly being lit up by these millisecond flashes. We just didn't have the technology to notice until recently.
Why Do Some FRBs Repeat While Others Don't?
This is one of the biggest open questions in FRB science. Most detected FRBs appear to be one-time events: a single burst from a specific location, never repeated. But a significant minority do repeat, producing multiple bursts from the same source over days, weeks, or months.
The first repeater discovered was FRB 121102, detected in 2012 at the Arecibo Observatory in Puerto Rico. Its repetition was confirmed in 2016, which was a game-changer. A single burst could theoretically be caused by a cataclysmic event (like two neutron stars colliding), but a repeating source rules out any one-time destructive event. The source has to survive the burst to produce another one.
Even stranger, FRB 180916 was found to repeat on a regular cycle: roughly every 16.35 days, it enters an active phase lasting about four days, then goes quiet for roughly 12 days before repeating. This periodicity could indicate an orbital cycle (the source orbiting a companion object) or a rotational cycle of the source itself.
The key question: are repeating and non-repeating FRBs produced by the same type of source in different conditions, or are they fundamentally different phenomena that happen to produce similar signals? The answer could reshape our understanding of these events.
In October 2021, astronomers reported detecting 1,863 bursts from a single source (FRB 121102) over a 47-day period using China's FAST telescope. That's an average of nearly 40 bursts per day from one object, a rate that constrains what kind of mechanism can produce them.
The Magnetar Breakthrough of 2020
On April 28, 2020, the mystery took a giant leap toward resolution.
Two telescopes, CHIME and the Survey for Transient Astronomical Radio Emission 2 (STARE2), simultaneously detected a pair of millisecond radio bursts from within our own galaxy. The signal was designated FRB 200428, and it came from the same region of sky as SGR 1935+2154, a known magnetar.
A magnetar is a neutron star with an extraordinarily powerful magnetic field, roughly a trillion times stronger than Earth's. Neutron stars are already extreme objects: the collapsed cores of massive stars, packing more mass than the Sun into a sphere about 20 kilometers across. Magnetars are the most extreme version, prone to sudden, violent "starquakes" and magnetic field rearrangements that release enormous amounts of energy.
The April 2020 burst from SGR 1935+2154 was detected simultaneously in radio waves and X-rays, providing the first direct link between FRBs and magnetars. The burst was thousands of times less energetic than typical extragalactic FRBs, but it was also thousands of times closer. Scaled to cosmological distances, it was consistent with the weaker end of the FRB energy spectrum.
This established that magnetars can produce fast radio bursts. But it didn't prove that all FRBs come from magnetars. The mechanism by which magnetar activity generates coherent radio pulses at such extreme power levels remains an active area of theoretical research.
What Is FRB 121102?
FRB 121102 is the most studied fast radio burst source and, in many ways, the Rosetta Stone of the field. Discovered in 2012 at the Arecibo Observatory and confirmed as a repeater in 2016, it was the first FRB to be precisely localized to a host galaxy.
That galaxy turned out to be a small, low-metallicity dwarf galaxy about three billion light-years from Earth. FRB 121102 sits within a compact, persistent radio source embedded in an extreme magneto-ionic environment, suggesting the bursting object is surrounded by (or embedded in) highly magnetized plasma.
The two leading models for FRB 121102 are:
A young magnetar in a supernova remnant. If a magnetar formed recently (in astronomical terms) from a supernova explosion, it would be surrounded by the expanding shell of debris from the explosion. The magnetar's bursts would have to travel through this material, which would explain the extreme Faraday rotation (a measure of magnetic field effects on polarized light) observed in FRB 121102's signals.
A neutron star near an active galactic nucleus. Some researchers have proposed that FRB 121102's source sits near the supermassive black hole at the center of its host galaxy. The intense magnetic environment near such a black hole could explain the persistent radio emission and extreme polarization properties.
In 2024, an international team studying another persistent repeating FRB (FRB 20201124A) found evidence supporting a binary system model, where the bursting object is embedded in a plasma bubble blown by high accretion rates, within a star-forming region. This model could apply to FRB 121102 as well.
Could Fast Radio Bursts Be Alien Signals?
The extraterrestrial intelligence hypothesis has been proposed, and it's worth addressing directly.
In 2017, Harvard physicists Avi Loeb and Manasvi Lingam published a paper exploring whether FRBs could be produced by artificial transmitters powering interstellar light sails. The idea was speculative but mathematically explored: an enormous transmitter (planet-sized) powered by sunlight could, in principle, produce beamed radio emissions consistent with FRB properties.
The argument against is straightforward: we now have a confirmed natural source (the magnetar SGR 1935+2154) that produces FRB-like signals. The properties of most FRBs are consistent with magnetar physics. The distribution of FRBs across the sky matches what you'd expect from a cosmological population of natural objects, not from directed beams aimed at Earth.
That said, the alien hypothesis can't be completely eliminated for every individual FRB. The sheer diversity of FRB properties (some repeat, some don't, some are periodic, some come from unusual environments) means it's theoretically possible that more than one type of source contributes to the observed population. But Occam's razor strongly favors natural explanations, especially now that we've caught a magnetar in the act.
Why Are FRBs So Important to Astronomy?
Fast radio bursts have turned out to be far more than just a curiosity. They're becoming powerful tools for understanding the universe.
Mapping the invisible universe. As FRB signals travel through intergalactic space, they pass through clouds of ionized gas that are otherwise nearly impossible to detect. By measuring how much each FRB signal is dispersed, astronomers can map the distribution of this "missing" baryonic matter between galaxies. In 2020, a study using FRBs confirmed predictions about the density of matter in the intergalactic medium, helping solve the "missing baryon problem" that had puzzled cosmologists for decades.
Probing magnetic fields. The polarization properties of FRBs carry information about the magnetic fields they've passed through. This makes them natural probes of the magnetic structure of the intergalactic medium and of host galaxy environments.
Testing fundamental physics. The precise timing of FRBs over cosmological distances can be used to test the equivalence principle (whether photons of different energies travel at the same speed through gravitational fields) and to constrain models of dark energy.
Understanding extreme physics. The mechanism that produces coherent radio emission at FRB energy levels pushes the boundaries of plasma physics and electrodynamics. Understanding it will likely reveal new physics about how radiation is produced in extreme magnetic environments.
Fast radio bursts went from a single mysterious detection in archival data to one of the most dynamic fields in astrophysics in less than two decades. We know magnetars are involved. We know FRBs come from diverse environments. We know they can be used as cosmic tools. But the full picture, the complete explanation of how a collapsed star the size of a city can produce a flash of radio waves with the energy output of a billion suns, is still being written.
For more signals from the cosmos that defy easy explanation, explore the Wow! Signal, a 72-second radio transmission detected in 1977 that's never repeated. 'Oumuamua brought a different kind of deep-space mystery when the first interstellar object passed through our solar system with unexplained acceleration. And Tabby's Star showed how even a star's dimming pattern can spark debates about alien megastructures.
Frequently Asked Questions
What causes fast radio bursts?
Magnetars (highly magnetized neutron stars) are confirmed as at least one source. In April 2020, a magnetar in our own galaxy produced an FRB-like burst detected simultaneously in radio waves and X-rays. However, the exact mechanism that converts magnetar energy into coherent radio pulses isn't fully understood, and it's possible that multiple types of sources contribute to the FRB population.
How long does a fast radio burst last?
Most FRBs last between a fraction of a millisecond and a few milliseconds. The longest recorded FRB lasted about three seconds. Despite their tiny duration, they release enormous energy, roughly equivalent to what the Sun produces in three days.
Do all fast radio bursts repeat?
No. Most detected FRBs appear to be one-time events. A significant minority repeat, producing multiple bursts from the same source. At least one repeater (FRB 180916) follows a regular 16.35-day cycle. Whether non-repeating FRBs come from fundamentally different sources or are simply repeaters we haven't observed long enough is still debated.
How many fast radio bursts happen per day?
Current estimates suggest roughly 10,000 FRBs occur somewhere in the observable universe every day. The CHIME telescope detects roughly one per day, limited by its field of view. Thousands have been catalogued since the first discovery in 2007.
Could FRBs be signals from aliens?
It's been proposed but is considered extremely unlikely. The confirmed detection of an FRB from a known magnetar in 2020 provided a natural explanation. The distribution and properties of FRBs are consistent with a population of natural astrophysical sources. No FRB has shown characteristics that would require an artificial origin.
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