The highest energy particles are so rare and special that they receive a name. In 1991, the most energetic particle ever detected, with an energy of 3.2×10^{20} electron Volt or equivalently 320 billion times the rest-mass energy of a proton. This particle was named the “Oh-My-God particle.” In 2023, another ultra-high energy cosmic-ray with 75% of this record energy had been reported.
A cosmic-ray is a high-energy elementary particle — often a proton — that moves near the speed of light. Ultrahigh-energy cosmic rays have energy levels that are more than a million times larger than those reached by the most powerful human-made particle accelerators. Cosmic-rays with energies of hundreds of billion times the proton rest-mass energy, are rarely found — fewer than one particle per square kilometer per century. They might flag new physics if they are produced by some unknown physical processes that might enable them to travel over much vaster distances than previously thought.
Protons with energies above fifty billion times the proton rest-mass energy lose energy over cosmological distances as a result of pion production through their scattering on the cosmic-microwave background — the relic radiation from the Big-Bang. This so-called Greisen–Zatsepin–Kuzmin effect, is named after the three physicists who first pointed it out in 1966.
What are the highest energy particles produced in the cosmos? We do not know because our detectors can only spot the cosmic-rays if their impact rate on Earth is less than once per ten square kilometers per century. The highest limit imaginable, without collapsing by its own gravity to a black hole, is the Planck energy — 19 orders of magnitude above the proton rest-mass energy. Planck-energy particles may exist, but we need detectors much bigger than Earth to detect them over a reasonable timescale. It is also possible that extremely-high energy particles are being produced in the cosmos but they lose their energy on their way towards Earth. Weakly-interacting particles, like neutrinos, could survive their journey through the cosmos better than protons.
On February 13, 2023, a neutrino with an energy that is 120 million times the proton rest-mass energy was discovered by the Cubic Kilometer Neutrino Telescope (KM3NeT), a collection of glass spheres which detect flashes of light sourced by muons on the floor of the Mediterranean Sea. Muons are continuously raining down on Earth’s surface, because they are produced when cosmic-rays hit air molecules. But occasionally, a cosmic neutrino that smashes into the planet’s surface also produces a muon. The KM3NeT particle’s path was close to horizontal relative to Earth’s surface and travelled eastwards, towards Greece.
Where did this neutrino come from? The possibilities include a collision of a more energetic cosmic-ray with the cosmic microwave background. It could have also resulted from high-energy particles accelerated in relativistic flows near supermassive black holes at the centers of galaxies, or from the births of stellar-mass black hole which are known to generate a relativistic jet and flashes of energetic photons in the form of Gamma-Ray Bursts. In collaboration with Eli Waxman, I published a paper suggesting that neutrinos with the energies achieved in the best current terrestrial accelerator, the Large Hadron Collider at CERN, might be produced in a supernova explosion, when the blast wave breaks out from the surface of the exploding star. In a follow-up paper, we placed constraints on higher energy neutrino factories in the Universe, irrespective of their identity.
KM3NeT observed the record-setting neutrino with 21 strings of detectors. Since then, 12 more strings were added, increasing the rate of detectable events and the precision of the related measurements. The project’s total cost is about 364 million dollars. This cost is directed towards detecting the products of cosmic accelerators. The expense would have been astronomically larger if we were to produce the detected particles in a human-made accelerator.
A speculative possibility is that we are detecting high-energy neutrinos produced by an accelerator developed by a more advanced scientific civilization, possibly in a nearby galaxy. The required accelerator power and cost would scale up with increasing distance. Here on planet Earth, CERN recently announced its plan to develop the biggest accelerator ever, the so-called Future Circular Collider. The accelerator will reside in a tunnel with a circumference of 90.7 kilometers, and its development will require about 800,000 person-years of employment. With proper funding, it might begin operations in the 2070s at an estimated cost of 17 billion dollars.
Aliens might be more ambitious in their scientific plans than humans, because of their enhanced curiosity and recognition of the practical benefits from science. They may have realized that scientific progress helps their cosmic survival against predators. If so, they may have developed stealth spacecraft and electromagnetically-invisible communication technologies based on their highest-energy accelerators. Examples for invisible communication signals may involve dark matter particles or waves of dark energy. In the process of reaching the high energies needed to produce these signals, these civilizations may generate residual waste of other particles that might appear for our telescopes as high-energy neutrinos or cosmic rays.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
