Statistical evidence of spacetime quantization collected from space

The quantumization of spacetime can be indirectly revealed by its imprint on particle propagation. Recently, a set of data combining the IceCube Neutrino Observatory and the Fermi Gamma Ray Space Telescope showed preliminary statistical evidence for such quantum spacetime effects.

The research results were published in Nature Astronomy on June 12 under the title "

If the principles of quantum mechanics and general relativity are to be combined into a quantum theory of gravity, then it seems inevitable that spacetime itself has to be quantized. This quantization can be shown indirectly through its effect on particle propagation: in the most studied cases, this would lead to a small correction to the particle velocity, and this correction increases with energy.

Neutrinos may be ideal for testing this possibility. They can reach the Earth from very distant astrophysical sources, so that the small corrections to the velocity act long enough to have a significant effect: for example, telescopes like the IceCube Neutrino Observatory in Antarctica can observe them at energies up to 1015 eV.

The source of neutrinos should be a very short burst. For example, a giant explosion like a gamma-ray burst (GRB) can occur after a stellar collapse or merger and also produce high-energy neutrinos. This investigation uses gamma-ray bursts observed by the Fermi telescope and ultra-high energy neutrinos detected by the IceCube Neutrino Observatory to test the hypothesis that some neutrinos and some gamma-ray bursts may have a common origin, but are observed at different times due to energy-dependent velocity reduction.

Specifically, the detected neutrino must be linked to its GRB source, which can be achieved using the directional and temporal coincidence of the electromagnetic signals of the neutrino and the GRB: if there is an effect on the propagation of the quantized spacetime (QTS), there is a clear temporal misalignment.

Then, in order to utilize neutrino data in these investigations, the requirement for temporal coincidence with the GRB source must be lenient enough to detect the mismatch. However, this also opens up the possibility of incorrectly associating neutrino and GRB sources. In a set of associations, we cannot assume that they are all "signals" and that some of them will be "noise"; however, using statistical techniques, it is possible to determine with a high degree of confidence that some neutrino-source associations must be correct (although it is not known exactly which associations are correct).

In this experiment, the team said, "We have found nine associations between IceCube neutrinos and GRBs (from data recorded by NASA's Fermi Gamma-ray Telescope)."

Statistical analysis suggests that at least two of these correlations may be just noise (the neutrino-GRB pairing is fortuitous), but it is unlikely that all nine are noise; moreover, for the nine correlations identified it can be concluded that there is a strong correlation between the energy of the neutrinos and the observed time difference between the neutrinos and the GRB - -this correlation is exactly as expected if the quantized spacetime causes an energy-dependent correction to the velocity of the particle.

Correlation of quantum spacetime? For the nine identified GRB-neutrino correlations (black dots are the initial seven correlations; red triangles are the subsequent high-energy correlations), the difference Δt between the GRB observation time and the neutrino observation time is shown as a relation to к, which combines the energy of the neutrino with the redshift value of the GRB. The correlation between the high value of к and the high value of Δt is consistent with some quantum spacetime models: the blue line shows their the predicted linear dependence. This correlation is noteworthy because it can be inferred that at least two of the data points are noisy and that the redshift is roughly estimated for most of the data points.

This marks an important milestone in the field of quantum gravity research, as it is the first time that such a level of statistical evidence in support of quantum gravity has been found.

"This result shows that tests based on the time-of-flight of high-energy GRB neutrinos can be a reliable tool for probing Planck-scale physics." Commenting on this scientific result, the peers said, "It thus suggests that the GRB neutrino signal can be used in future studies to explore the quantum properties of spacetime, especially when better quality neutrino direction data and a richer sample of GRB neutrinos are available."

Whether or not this preliminary statistical evidence stands up to the test of future data, neutrino analyses such as this one mark the beginning of a more mature stage of quantum-gravity phenomenology. For decades, it has been thought that the focus in studying the interactions of quantum mechanics and general relativity should be entirely on the development of new mathematics, since the minuteness of the observable effects would prevent them from entering laboratory experiments: however, in the past 20 years, studies of the universe as a "laboratory" have finally begun to gain the necessary observations.

"Although these findings are preliminary, they provide a solid basis for further detailed investigations as we continue to collect data from gamma-ray and neutrino telescopes." The team added: "Even if future data do not confirm this effect, our findings will still provide tight constraints on the parameters of relevant models, which already represent a rare and noteworthy step in the study of quantum gravity."

Even that may eventually lead to firm constraints on model parameters: quantum-spacetime studies will finally enter a phase where observations can guide model building - a goal that may be achieved more quickly if the entire IceCube data set can be utilized.