An intense jet of energy in space appears to be traveling seven times faster than the speed of light—a feat that is considered physically impossible in our universe. Though this rapid pace is only an optical illusion, according to a new study, it still represents a blast of energy shooting towards us at very nearly the speed of light.
The Hubble Space Telescope (HST) has captured incredible views of the jet—which was ignited by an unprecedented collision between two hyperdense objects, called neutron stars—that led to one of the most important breakthroughs in astronomical history at the time it was discovered in 2017.
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While the jet did not actually break the cosmic speed limit, it raced right up to the edge of this impassable threshold, reaching at least 99.97 percent of the speed of light, which translates to about 670 million miles per hour. Scientists led by Kunal Mooley, an astrophysicist at the California Institute of Technology, used Hubble and other telescopes to clock the jet’s “superluminal motion,” meaning the trippy illusion of faster-than-light speed, in a study published on Wednesday in Nature.
“We have demonstrated in this work that precision astrometry with space-based optical and infrared telescopes is an excellent means of measuring the proper motions of jets in neutron-star mergers,” Mooley and his colleagues said in the study. “The James Webb Space Telescope (JWST) should be able to perform astrometry much better than that with the HST, owing to the larger collecting area and smaller pixel size.”
The crash between these neutron stars was so explosive that it created ripples in the very fabric of spacetime, known as gravitational waves. Even though the merger happened a whopping 140 million light years away, scientists were still able to detect these subtle waves when they passed through Earth in August 2017.
The event, named gravitational wave (GW) 170817 after the date it was discovered, quickly earned a momentous place in space history. For starters, it was the first time that scientists had ever identified waves from a merger between two neutron stars. A handful of gravitational waves formed by mergers between black holes had been discovered at that point, but collisions between neutron stars had remained elusive.
The nature of the objects is important because black hole mergers do not produce visible light, and can only be spotted through the novel process of gravitational wave astronomy. In contrast, collisions between neutron stars, which are compact roiling objects formed by the explosive deaths of large stars, do produce luminous blasts of radiation.
The possibility of capturing two different signals of the same event—in this case with gravitational waves and a light signal—can produce a wealth of insights that are impossible to discern from only one observational technique.
For this reason, scientists hustled to get as many telescopes as possible pointed at the place in the sky where GW170817 originated to look for the radiant explosion from the mergers, including the jets that these events shoot out into space. Sure enough, the brilliant aftermath of the collision was spotted by dozens of telescopes, which followed the eruption as it faded. The achievement marked a major advance in the field of multi-messenger astronomy, which describes the observation of multiple types of signals from the same event.
Now, five years later, Mooley and his colleagues have added more detail to this astronomical mosaic with observations from Hubble, as well as from the European Space Agency’s Gaia observatory and several radio arrays on Earth involved with the field of very-long-baseline interferometry (VLBI). The team was able to see the jet slamming through a blob of material that had been blasted into space from the merger, which accelerated the mass to high speeds.
By measuring the motion of the blob, the researchers were able to show that the jet appears to be outpacing the speed of light sevenfold. As far as we know, nothing can travel faster than the speed of light, except for the expansion of the universe itself. The illusory effect of the superluminal motion stems from the ultra-relativistic speed of the jet, which is traveling just slightly slower than the light it emits.
The matter in the jet is just barely trailing its luminous light particles, known as photons, from our perspective on Earth. Because of this effect, photons that the jet emits in the early phases of its eruption can end up arriving at Earth around the same time as photons emitted at later stages, because the jet is more or less keeping pace with its own light output. This trippy phenomenon makes it seem as if the jet is moving faster than light-speed, a result that would shatter our understanding of physics, when in fact the jet is merely moving near light-speed, a result that is still pretty dang mind-boggling.
With this new study, Mooley and colleagues have presented a roadmap for discovering similar features in future unions of neutron stars. These efforts might unravel some of the mysteries of these explosive events, such as the potential link between neutron star mergers and highly luminous flashes known as short gamma-ray bursts.
“Our study represents, to our knowledge, the first proper motion constraint on the Lorentz factor”—which is a special measurement of moving objects—“of a gamma-ray-burst jet indicating ultra-relativistic motion,” the researchers said in the study.
“The combination of optical astrometry and radio VLBI measurements (with current observing facilities) may be even more powerful, and could deliver strong constraints on the viewing angles of neutron-star mergers located as far away as 150 megaparsecs,” equivalent to nearly 500 million light years, “as long as they have favorable inclination angles and occur in relatively dense environments compared with GW170817,” the team concluded.
