Einstein’s general relativity theory (GR) states that the speed of light in space is an constant, independent from the photon energy (light particles). In a study now published in the journal Physical Review Letters, an international team of researchers –among them are researchers from the Institute of Cosmos Sciences of the UB (ICCUB) – tried to test this theory by using observations of a gamma-ray burst (GFB), detected in January 2019 by the MAGIC telescopes in La Palma.
Einstein’s theory describes gravity as a result from the interaction between mass and space-time and its predictions have been tested in many experiments. However, physicists think a more unknown but more fundamental theory, of quantum nature, could explain this phenomenon. Some of the quantum gravity theories that have been presented suggest that the speed at which the photons travel in space can be energy dependent. This hypothetical phenomenon is called Lorenz invariance violation (LIV). If this exists, the difference in speed would be too tiny to be measured, unless its effects are accumulated during long periods of time or over large distances, such as in the GRBs emission detected on Earth.
The photons that are produced by the GRBs, travel during billions of years before reaching the Earth, which could make the effect of the hypothetical differences in speed measurable. Moreover, quantum gravity theories predict that the higher the energy of protons, the greater the difference would be. Therefore, researchers expect the high-energy gamma ray telescopes such as MAGIC to be highly competitive in the search of LIV effects.
GRBs take place at unpredictable times and places. There are satellite borne detectors around the Earth. On January 14, 2019, after receiving an alert from the GRBs detector in the Swift satellite, MAGIC culminated a search that lasted more than fifteen years, with its first identification of a GRB in the domain of teraelectronvolt energies (TeV). The GRB190114C could be detected thanks to MAGIC, which began its observation only fifty seconds before it occurred. “One of the most positive aspects revealed by the detailed study of the GRB190114 is that it is a more or less common GRB” notes Marc Ribó, researcher at ICCUB and deputy coordinator of physics in the MAGIC Collaboration. “This is good news –he continues– because it means that we will probably detect more. Our detection opens a new phase in the search for LIV effects on cosmic gamma ray source observations”
Scientists tried to use this unique observation to look for effects of quantum gravity. Researchers used theoretic models to describe the temporary evolution of the TeV emission in the gap between the beginning of GRB and observations with MAGIC. Limits in quantum gravity that researchers obtained in this study are compatible with the existing ones to date, and are also the first ones obtained using the observation of the highest energy GRB emission that can occur. With this pioneering study, the MAGIC team established a starting point for future studies in the search of measurable effects of the space-time quantum nature.
Col·laboració MAGIC. Bounds on Lorentz invariance violation from MAGIC observation of GRB 190114C. Physical Review Letters, 9 July 2020. Doi: 10.1103/PhysRevLett.125.021301