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.
To read more, click here.