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A cosmic cataclysm allows a test of the quantum structure of space-time
09/07/2020
 
The detection of a gamma-ray burst by the MAGIC telescopes allows researchers to study whether the speed of light in vacuum is a constant of nature.

The results, published in the journal Physical Review Letters, show that photons with different energies emitted 4500 million years ago reach the Earth with a difference in time smaller than one minute, setting limits to the hypothesis that the speed of photons depends on their energy.

Researchers from the Institut d’Estudis Espacials de Catalunya (IEEC), the Universitat Autònoma de Barcelona (UAB), the Institute of Cosmos Space of the Universitat de Barcelona (ICCUB), the Institute for High Energy Physics (IFAE), the Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), the Universidad Complutense de Madrid (UCM) and the Instituto de Astrofísica de Canarias (IAC) participate in the research. 

Einstein's theory of Relativity postulates that the speed of light in vacuum is a constant independent of the energy of photons (light particles). In a study published in the journal Physical Review Letters, an international team of researchers tested this postulate using observations of a gamma-ray burst (GRB) detected in January 2019 by the two MAGIC telescopes from La Palma.

The attempt was based on the fact that Einstein's theory describes gravity as a result of the interaction of mass with space-time. His predictions have been confirmed by many experiments. Despite this, physicists suspect that there is a more fundamental theory, of a quantum nature, still unknown. Some of the quantum gravity theories consider that the speed at which photons travel in vacuum depends on their energy. This hypothetical phenomenon is called Lorentz invariance violation (LIV). It is believed that, if it existed, this speed difference would be too tiny to be measured, unless its effect accumulates over long periods of time or over long distances, such as it is the case of the emissions that occur in GRBs and are detected on Earth.

Gamma-ray bursts, the most violent explosions in the Universe

GRBs are brief flashes of very high-energy photons (or gamma rays) emitted by distant cosmic explosions. The photons produced by GRBs travel for billions of years before reaching Earth, which could make the effect of the hypothetical differences in their speed measurable. Furthermore, quantum gravity theories predict that the higher the energy of the photons, the greater the difference. Therefore, very high-energy gamma-ray telescopes, such as MAGIC, are expected to be especially efficient in the search for LIV effects.

GRBs occur at unpredictable times and places in the sky. There are GRB detectors on the board of satellites in Earth’s orbit that have a very wide field of view, allowing them to detect and locate GRBs almost instantaneously as they occur, and send alarms to telescopes around the world, including MAGIC. On January 14, 2019, after receiving an alert from the Swift satellite's GRB detector, MAGIC made the first detection of a GRB in the very high-energy gamma-ray band, thus completing a search that has lasted more than 15 years. The so-called GRB190114C could be detected due to the fact that MAGIC began its observation just 50 seconds after it occurred. Marc Ribó, researcher from the Institut d’Estudis Espacials de Catalunya (IEEC) at the Institute of Cosmos Sciences of the Universitat de Barcelona (ICCUB) and Assistant Physics Coordinator of the MAGIC Collaboration, said: “One of the most positive aspects that the detailed study of the GRB190114 has revealed is that it is a more or less common GRB. This is good news because it means we will likely detect other similar ones. Our detection inaugurates a new phase in the search for the effects of LIV on observations of cosmic gamma-ray sources.”

Scientists wanted to use this unique observation to look for quantum gravity effects. However, they faced an obstacle at the very beginning: the gamma-ray signal recorded by MAGIC decreased monotonously with time. Although this fact is interesting to understand how GRBs occur, this is not favorable for looking for LIV effects. Daniel Kerszberg, a postdoctoral scientist at the Institute for High Energy Physics (IFAE) in Barcelona and one of the main authors of the study, explains: "To know if gamma rays travel at different speeds we should be able to compare the arrival times of gamma rays that were emitted by the GRB at the same time. Because it is not possible to know the exact time of the emission of individual photons, we usually use sudden temporal variations of the signal to recognize photons that were probably emitted at the same time." However, a decreasing signal does not show these variations. So the researchers used theoretical models to describe the temporal evolution of the very high-energy band emission in the interval between the onset of GRB and the observations with MAGIC. Kerszberg adds: "To look for LIV signals in our data we used two different methods to model their evolution in time. We wanted to be sure not to make mistakes in drawing conclusions from this exceptional signal, the first one of a GRB in the very high-energy band".

Testing the quantum nature of space-time

The careful analysis of the data found no significant difference in the speed of gamma rays of different energy. This does not mean that the effort was useless, since the MAGIC scientists managed to set limits on possible theories of quantum gravity. Javier Rico, researcher at IFAE ??and Coordinator of Analysis and Publications of the MAGIC Collaboration, says: “GRB190114C occurred when the Earth was still forming, 4.5 billion years ago. Since then, the gamma rays emitted have been traveling through the Universe until, just over a year ago, we detected hundreds of them with MAGIC telescopes. By analyzing them, we were able to determine that the time it took for the different photons to travel differed by a maximum of approximately one minute.”

The limits to quantum gravity that have been obtained from this work agree with those that already existed, and they are the first ones to be obtained by observing the highest energy emission that occurs in a GRB. 

With this groundbreaking study, the MAGIC team has established a starting point for future research in search of measurable effects of the quantum nature of space-time. Oscar Blanch, IFAE researcher and Spokesperson for the MAGIC Collaboration, says: “We are confident that future detections of GRBs in the very high-energy band will include early emission, prior to the monotonous decrease, which is expected to be rich in temporal structure, thus significantly increasing the sensitivity to LIV effects."

MAGIC telescopes

MAGIC (Major Atmospheric Gamma Imaging Cherenkov) is a system of two 17-meter diameter telescopes located 2200 meters above the sea level at El Roque de los Muchachos Observatory (ORM), on the Canary Island of La Palma, Spain. Telescopes are designed to detect very high-energy gamma rays, using the Cherenkov atmospheric imaging technique. MAGIC telescopes are run by an international collaboration of around 280 people from 12 countries, including scientists, engineers, technicians and other personnel.

The Spanish community has participated in MAGIC since its beginnings. Currently the members of MAGIC are the Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), the Instituto de Astrofísica de Canarias (IAC), the IFAE, the Autonomous University of Barcelona (UAB), the Institute of Cosmos Space of the Universitat de Barcelona (ICCUB) and the Universidad Complutense de Madrid (UCM). The IEEC participates in this project through researchers from the units ICCUB and the Center of Space Studies and Research (CERES, UAB). In addition, the MAGIC's data center is the Port d'Informació Científica (PIC), a collaboration of IFAE and CIEMAT.

References

Bounds on Lorentz invariance violation from MAGIC observation of GRB 190114C, MAGIC Collaboration, Phys. Rev. Lett. 125 (2020), doi: 10.1103/PhysRevLett.125.021301.

Acciari, V.A., Ansoldi, S., Antonelli, L.A. et al. Teraelectronvolt emission from the γ-ray burst GRB 190114C. Nature 575, 455–458 (2019).

Acciari, V.A., Ansoldi, S., Antonelli, L.A. et al. Observation of inverse Compton emission from a long γ-ray burst. Nature 575, 459–463 (2019).

Links

- IEEC
- Magic Collaboration
- Research paper

More information

This research is presented in the paper “Bounds on Lorentz invariance violation from MAGIC observation of GRB 190114C”, by MAGIC Collaboration, to be published in the journal Physical Review Letters on 9th July 2020.

The Institute of Space Studies of Catalonia (IEEC  — Institut d’Estudis Espacials de Catalunya) promotes and coordinates space research and technology development in Catalonia for the benefit of society. IEEC fosters collaborations both locally and worldwide and is an efficient agent of knowledge, innovation and technology transfer. As a result of over 20 years of high-quality research, done in collaboration with major international organisations, IEEC ranks among the best international research centers, focusing on areas such as: astrophysics, cosmology, planetary science, and Earth Observation. IEEC’s engineering division develops instrumentation for ground- and space-based projects, and has extensive experience in working with private or public organisations from the aerospace and other innovation sectors.  

IEEC is a private non-profit foundation, governed by a Board of Trustees composed of Generalitat de Catalunya and four other institutions that each have a research unit, which together constitute the core of IEEC R&D activity: the University of Barcelona (UB) with the research unit ICCUB — Institute of Cosmos Sciences; the Autonomous University of Barcelona (UAB) with the research unit CERES — Center of Space Studies and Research; the Polytechnic University of Catalonia (UPC) with the research unit CTE — Research Group in Space Sciences and Technologies; the Spanish Research Council (CSIC) with the research unit ICE — Institute of Space Sciences. IEEC is integrated in the CERCA network (Centres de Recerca de Catalunya).

Images

PR_Image1 
Caption: The MAGIC telescopes at El Roque de los Muchachos Observatory, La Palma, Canary Islands, Spain.
Credit:  Giovanni Ceribella (MAGIC Collaboration).

PR_Image2 
Caption: Artistic vision of a GRB observed by the MAGIC telescopes.
Credit: Superbossa.com and Alice Donini

Contacts

Oscar Blanch Bigas
Spokesperson of MAGIC Collaboration

Institut de Física d’Altes Energies, Barcelona 
blanch@ifae.es

IEEC Communication Office
Barcelona, Spain

Ana Montaner Pizà
E-mail: comunicacio@ieec.cat 
 
Attached Documents
Generalitat de CatalunyaUniversitat de BarcelonaUniversitat Autònoma de BarcelonaUniversitat Politècnica de CatalunyaConsejo Superior de Investigaciones CientíficasCentres de Recerca de Catalunya