A photon released by a distant explosion appears to have crossed the Universe, even though conventional physics predicts that it should have disappeared along the way.
The story begins in October 2022 with GRB 221009A, an exceptionally bright gamma-ray burst. This type of event involves a sudden release of energy into space. This one occurred more than 2 billion light-years away. Several observatories then recorded gamma rays, that is, photons far more energetic than visible light.
Position of GRB 221009A as seen from Earth and line of sight crossing the Milky Way.
Credit: NASAâs Goddard Space Flight Center
Among them, the Carpet observatory detected a particularly remarkable event. An initial analysis estimated its energy at around 251 teraelectronvolts. The complete analysis of the data now puts it at around 300 TeV. It would be the most energetic photon associated with this gamma-ray burst in this observation.
The problem lies in its immense journey. Space is not perfectly empty: it contains, among other things, the cosmic background radiation, very ancient light present throughout the Universe. A photon this energetic can encounter one of these photons and disappear in the interaction. Over billions of light-years, its chances of arriving intact therefore become extremely smallâan understatement for saying that this is almost certainly impossible.
Giorgio Galanti and Marco Roncadelli studied several explanations. In particular, they examined the idea that photons could temporarily transform into hypothetical particles called axion-like particles. In this form, some of the radiation could travel more easily. According to their calculations, this mechanism alone has difficulty explaining the photon observed by Carpet.
The researchers then tested a more radical possibility: at extreme energies, a fundamental rule of relativity might be slightly different. This rule, called Lorentz invariance, implies in particular that the laws of physics remain the same for observers moving at a constant speed. Modifying it would change the way very energetic photons interact during their journey.
In some of the models studied, such a modification makes the journey of the 300 TeV photon compatible with the observations. The authors also examine scenarios combining this modification with axion-like particles. These calculations can also take into account other highly energetic photons detected after the same gamma-ray burst.
Beware of shortcuts: this does not mean that Einsteinâs relativity has just been disproved. The study tests models capable of explaining a highly unusual observation, but a single detection is not enough to establish a new law of physics. It is also necessary to precisely confirm the photonâs origin and compare this explanation with future similar events.
The next very bright gamma-ray bursts may provide this test. If other photons reach Earth with energies and from distances normally incompatible with their survival, researchers will have several independent cases. They will then be able to check whether the same model explains their energies, paths, and arrival times.