A team of physicists at the University of Bristol found that the line between time moving forward and backward can blur in quantum mechanics.

The authors of the new work have shown that time in quantum systems can develop in two opposite directions, both forward and backward.

For centuries, philosophers and physicists have speculated about the existence of time. In the classical sense, we have no question whether time exists and how it moves. In physics, this concept is described using “entropy” – this is a physical quantity that determines the degree of disorder in the system. The growth of entropy is a necessary condition for any movement. On the basis of this, you can build an arrow of time.

If a phenomenon produces a large amount of entropy, it is almost impossible to observe its change over time. But when the entropy produced is small enough, then the change in time will occur naturally.

Julia Rubino, Ph.D., Quantum Engineering Technology Laboratory, University of Bristol

Researchers have applied this idea to the quantum realm. One of its features is the principle of quantum superposition, according to which, if two states of a quantum system are possible, then it can be in them simultaneously.

If we extend this idea to the usual course of time, then it turns out that quantum systems developing in one direction or another can go in the opposite direction.

Dr. Gonzalo Manzano of the University of the Balearic Islands said that in the work they quantified the entropy that a system evolves in a quantum superposition creates.

We have found that this most often results in the system being projected onto a well-defined direction of time that corresponds to the most likely process of the two. And yet, when an action produced a small amount of entropy, then one can physically observe the consequences of the system developing simultaneously in forward and backward temporal directions.

Gonzalo Manzano, Ph.D. and research co-author from the University of the Balearic Islands

This work is also of practical importance in quantum thermodynamics. If we take into account the alternative scenario of the development of events, then the quality of the equipment can be improved.