One of the most limiting constants of modern physics is the speed of light. It implies that we cannot transfer information to astronauts visiting the nearest star, Proxima Centauri, in less than 4.25 years.
Despite wishful thinking, quantum entanglement cannot transfer messages faster than light. Indeed, a measurement of an entangled quantum system at one location would affect its measurement at another location at a speed that is faster than light, but there is no transfer of an intentional message through these two measurements.
No known particles move faster than light. However, some physicists have conjectured as recently as July 2024 in a paper published in Physical Review D, that a hypothetical brand of elementary particles could do that. These particles are called tachyons.
If tachyons exist, they could be used to send signals into the past. This would violate causality according to Einstein’s Special Relativity, leading to the so-called grandfather paradox by which you could travel to the past and interact with your grandfather to prevent your own birth. The speed of tachyons increases with decreasing energy. No experimental evidence for the existence of tachyons exists.
In September 2011, the OPERA experiment of CERN reported that neutrinos travel faster than light, but later updates indicated that the conclusion resulted from a faulty fiber-optic cable in the experimental timing system.
Of course, laboratory constraints depend on the types of interactions that tachyons have. If a tachyon has significant interactions with Standard Model particles, we may expect it to be especially easy to find signals that are not being seen from our future. Furthermore, we may expect to have already found evidence for tachyons through precision laboratory tests at CERN or particle detectors of various kinds. On the other hand, it is possible that tachyons interact with ordinary matter weaker than neutrinos. In the limit of extremely weak interactions, this would mean that only gravitational tests could constrain tachyons, as gravity is universal for all particles and is the one interaction that cannot be avoided. This is the limiting case, where the possible theoretical problems and direct observational constraints are minimized.
Nevertheless, even if the interaction is only gravitational, one could find indirect ways to constrain the existence of tachyons.
A range of astrophysical observations from X-ray binaries of a star feeding a black hole to LIGO sources of gravitational waves, show that stellar-mass black holes exist, and are long-lived for billions of years. Tachyons must be compatible with this fact.
A new paper that I just wrote with the brilliant Mark Hertzberg and Aidan Morehouse shows that the long-term existence of black holes can be used to rule out massive tachyons. The idea is simple. Black holes can be regarded as the ultimate prison, but only for particles that do not exceed the speed of light. Tachyons could escape quantum-mechanically from the vicinity black holes and cause them to evaporate more vigorously than Stephen Hawking calculated in his famous 1974 paper.
Based on Classical (non-Quantum) Physics, we can ask whether tachyons can escape a black hole. This seems plausible given that they travel faster than light. However, our paper shows that from the point of view of a distant observer, even tachyons do not escape. However, tachyons can escape quantum-mechanically from black holes. Our paper computed the Hawking radiation of tachyons, finding that for heavy tachyons – the emergent flux of energy is dramatically enhanced compared to standard Hawking radiation of photons. This leads to black holes evaporating quickly if the tachyons are sufficiently massive. We use this insight to place a direct lower bound on any tachyon’s mass.
Our paper derives direct observational constraints on tachyons. We compute the Hawking radiation of tachyons from black holes, finding it to be significantly enhanced in the presence of heavy tachyons. For a black hole of mass M and tachyons of mass m, the black hole lifetime is proportional to M/m2. This implies that the observation of astrophysical black holes of a few solar masses with a lifetime of several billion years, rules out tachyons of mass m larger than a billion times the mass of the proton.
In other words, there cannot be any tachyons associated with Grand Unification scales or Quantum Gravity scales within a tenth of a billionth of the Planck mass, which is 10^{19} times the proton mass. As a result, while there already are theoretical reasons to be skeptical of the existence of tachyons based on causality, our paper provides a new observational constraint based on the long lifespan of astrophysical black holes.
The future discovery of primordial black holes in the asteroid mass range of 10^{17}-10^{21} grams, which could potentially constitute dark matter, would rule out tachyon masses down to 7-700 times the proton mass.
Of course, if we ever encounter events from a future generation of CERN’s particle colliders, or from what happened before the Big Bang, we may have to reconsider the possible existence of tachyons.
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.