Physicists demonstrate with unprecedented accuracy the constancy of the speed of light

One of the challenges of modern physics is to successfully combine the theories of general relativity and quantum mechanics. As a result of these efforts, several leading quantum gravity theories predict that the speed of light depends on the energy of its particles, the photons. 

The Institute of Space Studies of Catalonia (IEEC — Institut d’Estudis Espacials de Catalunya) and the Universitat Autònoma de Barcelona (UAB), with researchers at UAB’s Centre for Space Studies and Research (CERES), have collaborated with the University of Aveiro and the University of Algarve (both in Portugal) to look for this dependence based on the analysis of astrophysical observations of very energetic light from gamma-ray emissions from very distant sources. With unprecedented accuracy, the research shows that the speed of light remains a universal constant.

Determining the speed of light accurately is a problem with a long history. In 1887 one of the most important experiments in this regard took place. American scientists Michelson and Morley failed to measure the speed of the Earth by comparing the speed of light in the direction of the Earth’s motion with that perpendicular to it. The speed was exactly the same in every direction, regardless of our planet’s motion. 

That arguably most important ‘zero measurement’ in the history of science led Einstein to postulate that the speed of light is constant and consequently to formulate his theory of special relativity. This theory implies that all laws of physics are the same, independent of the relative motion between observers—a concept known as Lorentz invariance.

Meanwhile, quantum theory was developed, with Lorentz invariance at the heart of all its theoretical frameworks, in particular quantum field theory and the Standard Model of Particle Physics. The latter is the most precisely tested theory ever developed and has been verified to incredible precision.

So why doubt Lorentz invariance after 115 years of uninterrupted success? The answer once again starts with Albert Einstein—this time with his theory of general relativity, which describes gravity as a deformation of geometry of spacetime. This theory has also proven to be extremely successful, having been tested to great precision in many circumstances ranging from weak to very strong gravity.

The problem lies in the fundamental incompatibility between the probability wave functions of quantum field theory—the mathematical entities that describe quantum systems—with their movement through curved geometry and at the same time their modifications of spacetime curvature. Most attempts to reconcile the two theories—general relativity and quantum mechanics—into a common framework of quantum gravity have resulted in the need to break Lorentz invariance and consider that the speed of light is not constant, albeit only slightly. Thus, Michelson and Morley’s quest continues today.

A team of researchers led by Mercè Guerrero, former UAB student currently at University of Aveiro, and Anna Campoy-Ordaz, IEEC PhD student at the Department of Physics of the UAB, with the participation of Robertus Potting from the University of Algarve and Markus Gaug, associate professor at the Department of Physics of the UAB and also member of the IEEC, has now tested Lorentz invariance, that is to say, the constancy of the speed of light, to unprecedented precision with the help of astrophysics. The results are published in the journal Physical Review D.

One prediction of several Lorentz-Invariance-Violating quantum gravity theories is a dependence of the speed of light on photon energy. Any deviation from a constant speed of light must be extremely small to remain compatible with current constraints, but may become detectable at the very highest photon energies, known as very-high-energy gamma rays.

“This is possible because tiny differences in the group velocity of photons may accumulate into measurable arrival-time delays on Earth if the photons were emitted simultaneously from a source located at a very large distance,” explains Gaug, also affiliated to CERES.

“We combined a collection of existing bounds from astrophysical measurements of very-high-energy gamma rays using a new statistical method to test a series of Lorentz-invariance-violating parametres, currently favoured by theoreticians, of the Standard Model Extension (SME),” details the CERES researcher Campoy Ordaz.

The researchers hoped to prove Einstein wrong but, like so many others before them, did not succeed. Nevertheless, the research has increased tenfold the precision limit in measuring the constancy of the speed of light.

In the meantime, the quest to experimentally test the predictions of quantum gravity theories continues, with next-generation instruments just around the corner, such as the Cherenkov Telescope Array Observatory—a collaboration involving several research units of the IEEC—, designed to greatly improve performance on the detection of very-high-energy gamma rays from distant sources.

Press release prepared in collaboration with the Universitat Autònoma de Barcelona (UAB).

Lead Researcher at the IEEC

Anna Campoy Ordaz

Institute of Space Studies of Catalonia (IEEC)
Universitat Autònoma de Barcelona (UAB)
E-mail: campoy@ieec.cat, anna.campoy@uab.cat

Markus Gaug

Institute of Space Studies of Catalonia (IEEC)
Universitat Autònoma de Barcelona (UAB)
E-mail: gaug@ieec.cat, markus.gaug@uab.cat