Physicists suggest that tachyons can be reconciled with special relativity

Tachyons are hypothetical particles that travel at speeds greater than the speed of light. These superluminal particles are the “enfant terrible” of modern physics. Until recently, they were generally considered as entities that did not fit into special relativity.

However, an article recently published in Physical assessment D by physicists from the University of Warsaw and the University of Oxford has shown that many of these prejudices were unfounded. Tachyons are not only not ruled out by the theory, but also allow us to better understand their causal structure.

Motion at speeds exceeding the speed of light is one of the most controversial issues in physics. Hypothetical particles that could move at superluminal speeds, called tachyons (from the Greek tachýs—fast, swift), are the “enfant terrible” of modern physics. Until recently, they were generally regarded as creations that did not fit into special relativity.

At least three reasons for the non-existence of tachyons in quantum theory have been known so far. The first: the ground state of the tachyon field would be unstable, which would mean that such superluminal particles would form “avalanches”. The second: a change in the inertial observer should lead to a change in the number of particles observed in his reference system, but the existence of, say, seven particles cannot depend on who is looking at them. The third reason: the energy of the superluminal particles could take on negative values.

Meanwhile, a group of authors: Jerzy Paczos, who is pursuing his PhD at Stockholm University, Kacper Dębski, who is completing his PhD at the Faculty of Physics, Szymon Cedrowski, a final year Physics student (Studies in English), and four more senior researchers: Szymon Charzyński, Krzysztof Turzyński, Andrzej Dragan (all from the Faculty of Physics, University of Warsaw) and Artur Ekert from the University of Oxford, have just pointed out that the difficulties with tachyons so far had a common cause. It turned out that the ‘boundary conditions’ that determine the course of physical processes include not only the initial state but also the final state of the system.

Past and future merge

Simply put, to calculate the probability of a quantum process involving tachyons, it is necessary to know not only the initial state in the past, but also the future final state. Once this fact was included in the theory, all the previously mentioned difficulties disappeared completely, and the tachyon theory became mathematically consistent.

“It’s a bit like internet advertising: one simple trick can solve your problems,” says Andrzej Dragan, the main inspiration behind the entire research project.

“The idea that the future can influence the present, rather than the present determining the future, is not new in physics. However, until now this kind of view has been at best an unorthodox interpretation of certain quantum phenomena, and this time we were forced to this conclusion by the theory itself. In order to ‘make room’ for tachyons, we had to expand the state space,” Dragan concludes.

The authors also predict that the extension of the boundary conditions has consequences: a new kind of quantum entanglement appears in the theory, mixing past and future, which is not present in conventional particle theory. The paper also raises the question of whether tachyons described in this way are purely a ‘mathematical possibility’ or whether such particles are likely to be observed someday.

According to the authors, tachyons are not just a possibility, but are in fact an indispensable part of the spontaneous breaking process responsible for the formation of matter. This hypothesis would mean that Higgs field excitations, before the spontaneous symmetry breaking, could travel at superluminal speeds in the vacuum.