How faster-than-light explosions could reveal the universe’s secrets

Let’s engage in a thought experiment: picture directing a powerful laser beam towards the moon, the laser light piercing through the darkness of space until it touches the moon’s surface. Now, imagine flicking the laser, causing the point of light to sweep across the lunar landscape. It would seem to dart from one end of the moon to the other in an instant, traversing thousands of kilometers in mere moments, seemingly outpacing the speed of light itself. How can this perception exist?

The simple answer is that no laser pointer is capable of such a feat. So even if one were created, there’s no reason for alarm; it wouldn’t defy the well-established laws of physics that prohibit faster-than-light travel. What we have here is merely an optical illusion. Yet, there are real, cosmic phenomena that create similar optical illusions. According to astrophysicist Robert Nemiroff from Michigan Technological University, “Nothing with mass can accelerate from below the speed of light to above,” but illuminating fronts like shadows and laser spots can, and do, exceed light speed visually around us.

For many years, astronomers have recorded numerous instances of superluminal motion, previously regarded as mere curiosities. However, recent realizations reveal that some of these occasions can unveil surprising and hidden facts about the universe that other methods cannot. Nemiroff even proposes a novel kind of illusion—light echoes that appear to travel backward in time—which could provide insight into some of our universe’s most enigmatic phenomena. With the advent of the Vera C. Rubin Observatory, we stand on the brink of frequently witnessing these ephemeral illusions.

Light Echoes

Astronomy has long noted multiple occurrences of superluminal motion, the first documented case dating back to 1901. Thomas Anderson, an amateur astronomer and clergyman from Edinburgh, UK, discovered a new star-like point of light in the Perseus constellation that outshone its stellar counterparts. Astronomers flocked to observe it, including those from the Greenwich Observatory in London, and were astonished by their findings—an explosion whose luminous outer layers appeared to expand up to five times the speed of light.

This phenomenon, dubbed Nova Persei 1901, originated from a thermonuclear explosion on the surface of a white dwarf—a dead star. Initially, its strikingly superluminal behavior perplexed astronomers, but it was later elucidated in 1939 by French astronomer Paul Couderc, who pinpointed it as a light echo.

As light from the explosion radiated outward, it struck various clouds of dust at differing angles. Some regions illuminated more quickly than others, not due to the light traveling faster but because of their orientation concerning Earth and the nova. Therefore, the result was an optical illusion of bright arcs of light seemingly outpacing the light that created them.

This isn’t the singular illusion that has emerged. Apparent superluminal motion assists in studying potent cosmic jets—streams of charged particles ejected from active black holes, merging neutron stars, and other exotic systems. Often, these jets travel at velocities approaching the speed of light, and as they illuminate surrounding dust, they can create faster-than-light effects based on our perception, especially when these jets are oriented towards us.

One can understand this through a child playing with shadows created by fingers in front of a flashlight. By moving your hand slightly closer to the light, the shadows dance across the wall. The same principle applies in the cosmos, where distant light sources and clouds of dust function as the lamp and the screen.

Just last year, astronomers examined a superluminal jet from a galaxy named Centaurus A, located approximately 12 million light-years from Earth. Astrophysicist David Bogensberger at the University of Michigan tracked a shining knot within the jet that exhibited unusual behavior; in radio wavelengths, it appeared to move at 80 percent the speed of light, while in X-rays, it seemed to zip ahead at 2.7 times the speed of light.

“This indicates that the radio and X-ray data portray entirely different phenomena, which is a relatively new discovery,” Bogensberger remarks. “A growing consensus is recognizing that we observe two separate populations of plasma within jets, each exhibiting distinct behaviors and properties.”

This differentiation, according to him, could enrich our understanding of jet formation, composition, and evolution as they traverse through the cosmos.

Furthermore, superluminal motion can also assist in determining the angles at which these narrow beams strike Earth—an analysis not easily achievable through other methodologies. According to astrophysicist Matt Nicholl from Queen’s University Belfast, UK, accurate measurements of the energy emitted by a jet observed directly towards us can differ significantly from that of a jet angled slightly away. “This information can provide crucial insights into the jet’s energy, revealing fundamental facets about the stability of neutron stars and pressure within their centers,” Nicholl states. “This aspect of nuclear physics cannot be replicated in terrestrial laboratories.”

However, detecting these swiftly-moving signals presents challenges. Bogensberger mentions that while astronomers routinely observe objects appearing to travel approximately ten times the speed of light, there are rare instances of them suggesting speeds beyond fifty times. Most telescopes rarely re-examine the same segment of sky frequently enough to capture these fleeting effects, but with the proliferation of high-cadence astronomy—characterized by frequent sky scans—this is shifting.

Image Doubling

Researchers such as Nemiroff and Jon Hakkila from the University of Alabama in Huntsville are optimistic that emerging data will bolster a theory they proposed in 2019, addressing one of the most enigmatic phenomena in astrophysics: gamma-ray bursts (GRBs). These are sudden bursts of high-energy light associated with cataclysmic stellar deaths or merging neutron stars. Yet, specific details about their formation and behavior remain elusive.

An enduring mystery involves their light curves—graphs that chart a burst’s brightness over time. Instead of a smooth rise and fall corresponding to the explosion and subsequent dimming, GRB light curves often exhibit a rippled structure, featuring multiple distinct peaks during both the rise and decline phases. Some light pulses showcase numerous such peaks. Strangely, the initial and final peaks often appear to mirror one another, suggesting a sequential unfolding and then rewinding of the event.

“GRB light curves were perplexing,” Hakkila comments.

Previous theorists had offered explanations for the phenomenon—from GRBs reflecting off cosmic barriers to intricate interactions within blobs of plasma, radiation, and magnetic fields. Nevertheless, many of these interpretations felt contrived, and few successfully resolved the commonality of these echo-like structures.

Hakkila derived inspiration from earlier work by Nemiroff, which demonstrated that under specific conditions, a wave or particle moving faster than light through a medium (while remaining slower than light in a vacuum) could trigger a phenomenon he termed “relativistic image doubling.” To an observer, this would seem like two instances of the same event: one progressing normally, the other in reverse.

Hakkila expanded on this notion, suggesting that within a GRB jet, a wave or “impactor” could accelerate from subluminal to superluminal speeds. As it transitions, the wave interacts with plasma, generating a burst of radiation. Because it briefly exceeds the speed of light in that medium, the emitted light arrives in an unusual temporal order: first as a conventional signal and then again, appearing to reverse. The outcome, courtesy of relativistic image doubling, is a light curve manifesting as an echo: flaring, dimming, and then flaring once more.

Hakkila likens it to someone entering a room, illuminating all lights, then exiting while turning them off in the precise reverse order. He hypothesized that this effect might explain around 85 percent of GRBs, as revealed in a paper published in 2021. Recently, in 2023, researchers Dong-Jie Liu and Yuan-Chuan Zou at Huazhong University of Science and Technology in Wuhan, China, replicated this analysis using updated data. Not only did their findings support Hakkila’s idea, but they revealed that these mirrored signals could provide insights into the speed of shockwaves traversing through plasma or their interactions with dense clumps along the way.

Consequently, what was once considered observational noise could potentially serve as subtle indicators of GRB structures and explosive mechanisms.

Nemiroff remains optimistic that the concept could extend even broader. Beyond GRBs, he anticipates that relativistic image doubling events may manifest throughout the universe.

One plausible source for these unusual signals could be rapidly rotating neutron stars, known as pulsars. These celestial objects often possess supremely hot spots on their surfaces, associated with their magnetic poles. If a pulsar is encircled by a disk of dust, one of these spots might scatter off the disk as it rotates toward us, potentially allowing the scattered beam to arrive before the light directly from the spot. This could create the illusion of two identical spots moving in opposite directions.

Nemiroff captures the essence of this phenomenon succinctly: “What you observe is a pair of spots seemingly moving apart from each other; one keeps time as you’d expect, while the other appears to run in reverse.”

As data from the Vera C. Rubin Observatory begins to pile up, scanning the entire southern sky every few nights, he believes there’s a solid chance of directly observing these superluminal image pairs.

While Nemiroff is excited about the potential discoveries, he acknowledges the need for patience. The Rubin Observatory’s initial data won’t focus immediately on esoteric physics pursuits; early observations will prioritize standard scientific endeavors, such as detecting galaxies, asteroids, and supernovae. Nevertheless, combined with detectors like LIGO, Rubin could also be pivotal in uncovering another superluminal phenomenon: gravitational waves that surpass light speed, as noted by cosmologist Tessa Baker from the University of Portsmouth, UK.

This would be a transformative discovery, as Einstein’s theory of relativity posits that gravitational waves and light share a speed limit. Nonetheless, certain theoretical frameworks proposed to elucidate dark energy might feature additional fields that could alter the propagation of gravitational waves and light. Thus, light would remain bounded by its current speed limit, yet gravitational waves could experience different interactive dynamics.

This scenario suggests that the ripples in space-time generated by a gravitational wave event might reach us prior to the light signals, which Rubin could detect, providing support for alternative gravitational theories.

Experimental Light-Speed Illusions

<pWhen superluminal illusions become more commonly observed, researchers aim to optimize their potential scientific value. As part of this aim, some scientists are experimenting with these effects in controlled settings. Theoretical physicist Simon Horsley from the University of Exeter, UK, and his colleagues have explored materials like indium tin oxide with adjustable properties, employing lasers across these materials in setups simulating the moon-laser experiment. According to theory, no information should cross faster than light across the material’s surface. The question remains: Is that true?

Findings suggest that the refractive index—a measure of how much light is bent by the material—appears to shift concurrently with the laser’s movement, seemingly exceeding light speed. “We are looking at a patch that rushes across the material,” Horsley explains. The results create a reflection that seems to emerge from something traveling faster than light—not a real object but an optical illusion.

This phenomenon relies on the Doppler shift—the same principle that results in an ambulance siren lowering its pitch as it passes by. “This can be stretched to encompass movements exceeding the speed of light,” says Horsley, indicating that the angle of reflected light can visibly shift as it crosses the light-speed boundary.

This is pivotal since Doppler shifts are fundamental tools in astronomy, utilized to deduce motion—measuring how quickly galaxies recede, how jets are angled, or how stars orbit one another. If superluminal illusions can unpredictably distort these shifts, laboratory studies like Horsley’s could enhance our interpretation of celestial observations—effectively recalibrating our anticipations against established optical phenomena.

The materials Horsley studies may also facilitate investigations into other undiscovered superluminal effects. “We are in pursuit of any exotic physics we can explore with this,” he remarks, “as we’re not truly moving anything past light speed, yet optical behaviors mimic such movement.” Other experimentation has examined the implications of superluminal motion, too. Earlier this year, Dominik Hornof, a physicist at Vienna University of Technology, and his collaborators deployed laser pulses to simulate a fast-moving object close to light speed. They discovered that as the object crossed our view, it appeared to rotate, with light from the front arriving more rapidly than light from the back.

Hornof’s team also demonstrated that if the object were angled to travel toward or away from us—similar to how jets emanating from black holes or neutron star mergers can occasionally be oriented—it wouldn’t merely appear to rotate; it could suddenly convey the impression of violating the light-speed threshold. “By adjusting the line of sight by just 2.5 degrees, we’d perceive a superluminal motion of 22 times the speed of light,” points out Hornof. “This kind of situation seems absurd.”

These Earth-based simulations could serve as testbeds for understanding the physics behind superluminal illusions, enhancing how astronomers interpret signals they are likely to soon encounter within Rubin’s data.

Many more illusions await discovery, including Nemiroff’s anticipated double images, if they manifest as expected. “This concept is inherently fascinating,” says Nemiroff. “Countless remarkable surprises continue to emerge.” He suspects the floodgates of discovery may open similar to the initial stages of gravitational lensing—an idea where light is bent and warped by massive celestial objects, which initially strived to gain acceptance before being widely observed in abundance.

He reflects, “Today, numerous lensing events are under investigation. Perhaps a similar trend awaits us regarding superluminal events, including double image phenomena or straightforward cases such as a jet traversing a cosmic dust cloud, akin to a laser pointer gliding over the moon.”