Cosmic Cataclysm allows precise test of General Relativity
Published on 09.07.2020
The detection of a Gamma ray burst by MAGIC telescopes allows us to study whether the speed of light in vacuum is a constant in nature.
The results, published in the journal Physical Review Letters, show that photons of different energies emitted about 4.5 billion years ago reach Earth with a time difference of less than a minute, thus putting a limit on the hypothesis that the speed of photons depend on their energy.
In 2019, the MAGIC telescopes detected the first Gamma Ray Burst at very high energies [1, 2]. This was the most intense gamma-radiation ever obtained from such a cosmic object. But the GRB data have more to offer: with further analyses, the MAGIC scientists could now confirm that the speed of light is constant in vacuum – and not dependent on energy. So, like many other tests, GRB data also corroborate Einstein’s theory of General Relativity. The study has now been published in Physical Review Letters.
Einstein's general relativity (GR) is a beautiful theory which explains how mass and energy interact with space-time, creating a phenomenon commonly known as gravity. GR has been tested and retested in various physical situations and over many different scales, and, postulating that the speed of light is constant, it always turned out to outstandingly predict the experimental results. Nevertheless, physicists suspect that GR is not the most fundamental theory, and that there might exist an underlying quantum mechanical description of gravity, referred to as quantum gravity (QG). Some QG theories consider that the speed of light might be energy dependent. This hypothetical phenomenon is called Lorentz invariance violation (LIV). Its effects are thought to be too tiny to be measured, unless they are accumulated over a very long time. So how to achieve that? One solution is using signals from astronomical sources of gamma rays.
Gamma Ray Bursts, the most violent explosions in the universe
Gamma-ray bursts (GRBs) are powerful and far away cosmic explosions, which emit highly variable, extremely energetic signals. They are thus excellent laboratories for experimental tests of QG. The higher energy photons are expected to be more influenced by the QG effects, and there should be plenty of those; these travel billions of years before reaching Earth, which enhances the effect.
GRBs are detected on a daily basis with satellite borne detectors, which observe large portions of the sky, which allows them to detect and locate GRBs almost instantaneously when they occur, and send alerts to telescopes around the world, including MAGIC telescopes, to participate in their observation and study. On January 14, 2019, after receiving an alert from the GRBs detector on the Swift satellite, the MAGIC telescope system detected the first GRB in the domain of teraelectronvolt energies (TeV, 1000 billion times more energetic than the visible light), hence recording by far the most energetic photons ever observed from such an object. Multiple analyses were performed to study the nature of this object and the very high energy radiation.
Marc Ribó, Tenure track lecturer from the Institute of Cosmos Sciences (ICCUB) and Deputy Coordinator of Physics of the MAGIC Collaboration, tells us: "One of the most positive aspects revealed by the detailed study of the GRB190114 is that it is a more or less common GRB. This is good news 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 sources observations.”
Naturally, the MAGIC scientists wanted to use this unique observation to hunt for effects of QG. At the very beginning, they however faced an obstacle: the signal that was recorded with the MAGIC telescopes decayed monotonically with time. While this was an interesting finding for astrophysicists studying how GRBs are produced, it was not favorable for LIV testing. Daniel Kerszberg, a researcher at IFAE in Barcelona said: “when comparing the arrival times of two gamma-rays of different energies, one assumes they were emitted instantaneously from the source. However, our knowledge of processes in astronomical objects is still not precise enough to pinpoint the emission time of any given photon”. Traditionally the astrophysicists rely on recognizable variations of the signal for constraining the emission time of photons. A monotonically changing signal lacks those features. So, the researchers used a theoretical model, which describes the expected gamma-ray emission before the MAGIC telescopes started observing. The model includes a fast rise of the flux, the peak emission and a monotonic decay like that observed by MAGIC. This provided the scientists with a handle to actually hunt for LIV.
Testing the quantum nature of space-time
A careful analysis then revealed no energy-dependent time delay in arrival times of gamma rays. Einstein still seems to hold the line. “This however does not mean that the MAGIC team was left empty handed”, said Giacomo D’Amico, a researcher at Max Planck Institute for Physics in Munich; “we were able to set strong constraints on the QG energy scale”. The limits set in this study are comparable to the best available limits obtained using GRB observations with satellite detectors or using ground-based observations of active galactic nuclei.
The limits on quantum gravity that have been obtained in this work are compatible with those already existing to date, and are the first to be obtained by observing the most energy GRB emission that can occur. With this pioneering study, the MAGIC team has established a starting point for future research in the search for measurable effects of the quantum nature of space-time.
In contrast to previous works, this was the first such test ever performed on a GRB signal at TeV energies. With this seminal study, the MAGIC team thus set a foothold for future research and even more stringent tests of Einstein’s theory in the 21st century. Oscar Blanch, spokesperson of the MAGIC collaboration, concluded: "This time, we observed a relatively nearby GRB. We hope to soon catch brighter and more distant events, which would enable even more sensitive tests."
MAGIC and the MAGIC collaboration
MAGIC (Major Atmospheric Gamma Imaging Cherenkov) is a system of two 17 meter diameter telescopes located at 2200 meters above sea level at the Observatorio El Roque de los Muchachos (ORM), in the Canary island of La Palma, Spain. The pioneering telescopes are designed to detect very high energy gamma-rays in the energy range from 30 GeV to more than 50 TeV, using the imaging atmospheric Cherenkov technique. The MAGIC telescopes are run by an international collaboration of around 280 people from 12 countries, including scientists, engineers, technicians and other staff.