Einstein’s General Relativity (GR) is a beautiful theory that explains how mass and energy interact with space-time and creates a phenomenon that is often called gravity. GR has been tested and re-tested in different physical situations and across many different scales, and with the postulation that the speed of light is constant, it always proved to be outstanding to predict the experimental results. Nevertheless, physicists suspect that GR is not the most basic theory and that there may be an underlying quantum mechanical description of gravity, called quantum gravity (QG).
Some QG theories consider that the speed of light can be energy dependent. This hypothetical phenomenon is called Lorentz invariance violation (LIV). The effects are considered too small to be measured unless they accumulate for a very long time. So how can you achieve that? One solution is to use signals from astronomical sources to gamma rays. Gamma-ray bursts (GRBs) are powerful and far-away cosmic explosions, emitting very varied, extremely energetic signals. They are thus excellent laboratories for experimental tests of QG. The higher-energy photons are expected to be more affected by the QG effects, and there should be many of them; these travel billions of years before they reach the earth, which improves the effect.
GRB is detected daily with satellite-borne detectors, which observe large parts of the sky, but with lower energies than the ground-based telescopes such as MAGIC. On January 14, 2019, the MAGIC telescope system discovered the first GRB area in thermal electron voltaic energy (TeV, 1,000 billion times more energetic than the visible light), thus recording by far the most energetic photons ever observed from such an object. Several analyzes were carried out to study the nature of this object and the very high energy radiation.
Tomislav Terzić, a researcher from the University of Rijeka, says: “No LIV study has ever been conducted on GRB data in the TeV energy field, simply because there was no such data to date. For over twenty years, we expected such an observation to increase sensitivity to the LIV effects, but we couldn’t say how much until we saw the final results of our analysis. It was a very exciting period. “
Of course, MAGIC scientists wanted to use this unique observation to hunt for effects of QG. Initially, however, they encountered an obstacle: the signal recorded with the MAGIC telescope was monotonically decayed with time. Although this was an interesting finding for astrophysicists studying GRB, it was not favorable for LIV testing. Daniel Kerszberg, a researcher at IFAE in Barcelona, said: “When comparing the comparative times of two gamma rays with different energies, one assumes that they were emitted directly from the source. But our knowledge of processes in astronomical objects is still not accurate enough to determine the emission time for a particular photo. “
Traditionally, astrophysicists rely on recognizable variations of the signal to limit the photon release time. A monotonically variable signal lacks these features. So the researchers used a theoretical model that describes the expected gamma ray emission before the MAGIC telescope began to observe. The model includes a rapid increase in flow, peak emission and a monotonous decay as observed by MAGIC. This provided the researchers with a handle to actually chase after LIFE.
A thorough analysis then revealed no energy-dependent time delay in the arrival times for gamma rays. Einstein still seems to be holding the line. “However, this does not mean that the MAGIC team was left empty-handed,” said Giacomo D’Amico, a researcher at the Max Planck Institute for Physics in Munich; “We were able to place strong restrictions on the QG energy scale.” The limits established in this study are comparable to the best available limits obtained using GRB observations with satellite detectors or with ground-based observations of active galactic nuclei.
Cedric Perennes, a postdoctoral researcher at the University of Padova, added: “We were all very happy and feel privileged to be able to carry out the first study of Lorentz’s invariance violation ever on GRB data in TeV’s energy field and to crack open the door for future studies! “
Unlike previous works, this was the first such test ever performed on a GRB signal at TeV energies. With this seminar study, the MAGIC team thus established a foothold for future research and even stricter tests of Einstein’s theory during the 2000s. Oscar Blanch, spokesman for the MAGIC collaboration, concluded: “This time we saw a relatively close GRB. We hope to catch brighter and more distant events soon, which would allow even more sensitive testing. “
Reference: “Limits to Violation of Lorentz Invariance from MAGIC Observation of GRB 190114C” by VA Acciari et al. (MAGIC Collaboration), July 9, 2020, Physical review letters.
DOI: 10.1103 / PhysRevLett.125.021301