Gamma-ray burst tests quantum gravity

Gamma ray burst

Artist’s impression of a gamma-ray burst (Image:, Alice Donini / MAGIC Collaboration)

The speed of light is constant, according to Einstein’s theory of relativity. But if this is to be compatible with modern quantum mechanics, this constancy of light would have to fall at extreme energy. At least that is what some theories of quantum gravity say. A cosmic catastrophe has now given astronomers the chance to test this assumption. Using the light of an extremely strong gamma ray burst, they were able to determine whether the speed of light for the most energetic parts of the rays differs from those with less energy. The result: If there is this energy-dependent deviation, it must be less than 1.7 billiardths.

Einstein’s general theory of relativity forms the basis of classical physics. It explains almost all phenomena that can be observed on earth and in space – but not the processes in the quantum world. The phenomenon of gravity, which Einstein describes as the curvature of space-time, is also incompatible with the laws of quantum physics. That is why physicists have been trying for decades to resolve these contradictions using theories such as quantum gravity. According to these models, space-time is not a continuous matrix like Einstein’s, but is divided into the smallest units. With some variants of quantum gravity, however, this quantized space-time would mean that light could no longer pass through the vacuum unhindered and at constant speed. Instead, radiation with particularly high energy would have to “feel” this quantization of the matrix, so to speak, and thereby slow down a tiny bit. Physicists also refer to such subtle deviations from the constancy of the speed of light as Lorentz invariance violation (LIV).

Superlative gamma ray burst

The problem, however, is that because these tiny deviations – if they exist – only occur at extremely high energies and because they are so small that they only get within the measurable range over enormous distances, this Lorentz invariance violation was so far neither provable nor refutable. But now a cosmic catastrophe has come to the aid of the physicists. This is a gamma ray burst – a flash of extremely short-wave and high-energy radiation, such as can be released when some stars explode or when neutron stars collide. In January 2019, several observatories, including the two MAGIC telescopes on La Palma, detected a gamma-ray flash that was unusually intense: For about 30 seconds, its afterglow was more than 100 times as strong as the Crab Nebula, the brightest known gamma source in our Milky Way . With an energy of around two teraelectron volts, GRB190114 c was the most energetic gamma-ray flash ever detected.

And this is exactly what made this burst of gamma rays a perfect measuring instrument for the violation of Lorentz invariance required by some quantum gravity theories. Because the radiation of this event is highly variable and is distributed over many different frequency and energy ranges. At the same time, the source of this gamma-ray burst was 4.5 billion light years from Earth. It was closer to us than some other outbursts of this kind, but still far enough away to at least contain possible violations of the constant speed of light. To do this, the researchers from the MAGIC collaboration checked whether there were energy-dependent differences in the arrival times of the various radiation components.

It is constant – at least up to a certain value

The analysis of the observation data showed that the arrival time of the gamma rays did not depend on their energy. At least in the range that can be read from this gamma ray burst, the speed of light seems to be constant. “But that doesn’t mean that the MAGIC team is left empty-handed,” explains Giacomo D’Amico from the Max Planck Institute for Physics. “We were able to further narrow the possible energy range for the occurrence of quantum gravity effects.” According to this, the energy-related deviation of the photons from the speed of light cannot be greater than 1.7 times 10 to the power of minus 15 – i.e. 1.7 quadrillionths. This result makes a decisive contribution to narrowing down the validity of some variants of quantum gravity.

At the same time, this measurement is just the beginning: “The observed gamma-ray flash was relatively close to Earth,” says Oscar Blanch, spokesman for the MAGIC collaboration. “We hope to be able to observe brighter and more distant objects in the future, with which even more sensitive tests of quantum gravity will be possible.”

Source: MAGIC Collaboration (Physical Review Letters, doi: 10.1103 / PhysRevLett.125.0213

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