We can explain.
Time, as far as we know, moves in only one direction. But in 2018, researchers found events in some of the gamma-ray burst pulses that repeated as if they were returning in time.
Today, new research provides an answer as to what might cause this time reversibility effect. If the waves in relativistic jets that produce gamma-ray bursts travel faster than light – at 'superluminal' speeds – one effect could be time reversibility.
Such accelerating waves may indeed be possible. We know that when light passes through a medium (like gas or plasma), its phase velocity is slightly slower than the speed of light in a vacuum and, as far as we know, the speed limit of the universe.
Consequently, the wave can travel through the jet burst of gamma rays at superluminal speeds without violating relativity. But to understand this, we need to look at the source of these flares.
Gamma-ray bursts are the most energetic explosions in the universe. They can last from a few milliseconds to several hours, they are unusually bright, and we do not yet have an exhaustive list of their causes.
We know from observations of colliding neutron stars in 2017 that these collisions can create gamma-ray bursts. Astronomers also believe that such bursts occur when a massive, rapidly spinning star falls into a black hole, violently ejecting material into the surrounding space in a colossal hypernova.
The black hole is surrounded by a cloud of accretion material around the equator; if it rotates fast enough, the recoil of the initially exploded material will result in the firing of relativistic jets from the polar regions, exploding through the outer shell of the progenitor star to create gamma-ray bursts.
Now let's go back to those waves that travel faster than light.
We know that when moving in a medium, particles can move faster than light. This phenomenon is responsible for the famous Cherenkov radiation, often perceived as a characteristic blue glow. This glow – a 'light boom' – occurs when charged particles, such as electrons, move faster than the phase velocity of light.
Astrophysicists John Hakkila of the College of Charleston and Robert Nemiroff of Michigan Technological University believe that the same effect can be observed in jets of gamma-ray bursts, and have performed mathematical simulations to demonstrate how this happens.
“In this model, a shock wave in an expanding gamma-ray jet is accelerated from light to superluminal speeds, or slows down from superluminal to light,” they write in their paper.
'The shock wave interacts with the environment, creating Cherenkov and / or other radiation when moving faster than the speed of light in that environment, and other mechanisms (such as thermally Compton or synchrotron shock radiation) when moving slower than the speed of light.
'These transitions create a backward time gamma-ray burst light curve in the process of doubling the relativistic image.'
This doubling of the relativistic image is believed to occur in Cherenkov detectors. When a charged particle moving at a speed close to the speed of light hits water, it moves faster than the Cherenkov radiation it generates, and therefore could hypothetically end up in two places at the same time: one image appears to move forward in time, and the other moves in the opposite direction.
Keep in mind this doubling has not yet been observed experimentally. But if this happens, it will create reversibility in time, observed on the light curves of gamma radiation, arising in the case when a shock wave passing through a reactive medium accelerates to speeds exceeding the speed of light, and slows down to light speeds.
The researchers hypothesized that the impactor responsible for creating the gamma-ray burst would be a large-scale wave caused by, say, a change in density or magnetic field. This will require further analysis.
“Standard GRB models neglect time-reversible light curve properties,” Hakkila said. “Superluminal jet motion explains these properties while retaining many of the model's standard characteristics.”
The study was published in the Astrophysical Journal.
