Einstein’s General Theory of Relativity (GR) is a nice theory that explains how mass and energy interact with space-time and create a phenomenon commonly known as gravity. GR has been tested and retested in various physical situations and on many different scales. Assuming that the speed of light is constant, it turned out that the experimental results were always predicted excellently. Nevertheless, physicists suspect that GR is not the most basic theory and that there may be a quantum mechanical description of gravity called quantum gravity (QG).
Some QG theories assume that the speed of light could be energy-dependent. This hypothetical phenomenon is called Lorentz invariance violation (LIV). Its effects are believed to be too small to be measured unless they accumulate over a very long period of time. How can you do that? One solution is to use signals from astronomical gamma-ray sources. Gamma ray bursts (GRBs) are powerful and distant cosmic explosions that send out very variable, extremely energetic signals. They are therefore excellent laboratories for experimental QG tests. 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 Earth, which increases the effect.
GRBs are captured daily with satellite-based detectors that observe large parts of the sky, but with lower energies than ground-based telescopes like MAGIC. On January 14, 2019, the MAGIC telescope system discovered the first GRB in the field of teraelectronvolt energies (TeV, 1000 billion times more energetic than visible light), thereby recording by far the most energetic photons ever observed by such an object. Several analyzes were carried out to examine the nature of this object and the very high energy radiation.
Tomislav Terzić, a researcher at the University of Rijeka, says: “There has never been a LIV study on GRB data in the TeV energy range simply because there has been no such data. For over twenty years, we had expected such an observation to increase sensitivity to the LIV effects, but we couldn’t tell how much until we saw the final results of our analysis. It was a very exciting time. “
Of course, the MAGIC scientists wanted to use this unique observation to search for QG effects. At the very beginning, however, they faced an obstacle: the signal recorded with the MAGIC telescopes fell monotonically over time. While this was an interesting finding for astrophysicists studying GRBs, it was not favorable for LIV tests. Daniel Kerszberg, a researcher at IFAE in Barcelona, said: “If you compare the arrival times of two gamma rays of different energy, you assume that they were emitted immediately by the source. However, our knowledge of processes in astronomical objects is still not precise enough to determine the emission time of a particular photon. “
Traditionally, astrophysicists rely on recognizable variations in the signal to limit the emission time of photons. A monotonically changing signal lacks these features. Therefore, the researchers used a theoretical model that describes the expected gamma-ray emission before the MAGIC telescopes started observing. The model includes a rapid increase in flow, peak emissions and a monotonous drop as observed by MAGIC. This gave the scientists the opportunity to actually search for LIV.
A careful analysis then showed no energy-dependent time delay in the arrival times of gamma rays. Einstein still seems to keep 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 severely limit the QG energy scale.” The limits set in this study are comparable to the best available limits obtained with GRB observations with satellite detectors or with ground-based observations of active galactic nuclei.
Cedric Perennes, postdoctoral fellow at the University of Padua, added: “We were all very happy and felt privileged to be able to do the first study of Lorentz invariance violation with GRB data in the TeV energy domain and that Breaking open the door for future studies! “
In contrast to previous work, this was the first such test ever to be carried out on a GRB signal at TeV energies. With this groundbreaking study, the MAGIC team set foot in future research and even more rigorous testing of Einstein’s theory in the 21st century. Oscar Blanch, spokesman for the MAGIC collaboration, concluded: “This time we observed a relatively close GRB. We hope to see brighter and more distant events soon, which would enable more sensitive testing. “
Reference: “Limits of Violation of Lorentz Invariance by MAGIC Observation of GRB 190114C” by VA Acciari et al. (MAGIC Collaboration), July 9, 2020, Letters for physical examination.
DOI: 10.1103 / PhysRevLett.125.021301