In quantum reality, even time doesn’t flow the way it’s supposed to

A team of physicists from the Universities of Bristol, Vienna, and the Balearic Islands and the Institute for Quantum Optics and Quantum Information (IQOQI-Vienna) has demonstrated how quantum systems can simultaneously evolve in two opposite temporal directions, both forward and backward in time.

The study, published in the latest issue of Communications Physics, calls for a rethinking of how the flow of time is understood and presented in contexts where quantum laws play a crucial role.

For centuries philosophers and physicists have speculated about the existence of time. Nevertheless, in the classical world, our experience seems to remove any doubt that time exists and goes on. Indeed, in nature, processes tend to evolve spontaneously from states of less disorder to states of greater disorder, and this tendency can be used to determine the direction of time. In physics, this is described in terms of “entropy,” which is a physical quantity that determines the amount of disorder in a system.

Dr. Julia Rubino of the Quantum Engineering Technologies Laboratory at the University of Bristol (QET labs) and lead author of the publication says: “If a phenomenon produces a large amount of entropy, observing its reversal in time is so unlikely that it becomes almost impossible. However, when the entropy produced is small enough, there is a negligible probability that the time reversal of the phenomenon will occur naturally.”

“As an example, consider the sequence of actions we perform during our daily morning routine. If we were shown our toothpaste moving from the toothbrush back into the tube, we would have no doubt that it was a rewinding of our day. However, if we gently squeezed the tube so that only a small portion of the toothpaste came out, it would not be so unlikely to see it go back into the tube, being sucked up when we unclenched it.”

The authors of the study, led by Professor Kaslav Bruckner of the University of Vienna and IQOQI-Vienna, applied this idea to the quantum domain, one of whose features is the principle of quantum superposition, according to which if two states of a quantum system are possible, then that system can be in both states simultaneously.

Dr. Rubino said: “If we extend this principle to the arrows of time, it turns out that quantum systems that evolve in one or the other temporal direction (toothpaste coming out of the tube or going back into it) may also find themselves simultaneously evolving in both temporal directions.”

“Although this idea seems rather nonsensical when applied to our everyday experience, at the most fundamental level the laws of the universe are based on quantum mechanical principles. This raises the question of why we never encounter such superpositions of temporal fluxes in nature.”

Dr. Gonzalo Manzano, co-author of the paper from the University of the Balearic Islands, emphasized: “In our work, we quantified the entropy produced by a system evolving in a quantum superposition of processes with opposite time directions. We found that more often than not it leads to projection of the system to a well-defined time direction corresponding to the most probable process of the two. And yet, when a small amount of entropy is involved (for example, when so little toothpaste has been spilled that one can observe it being re-absorbed into the tube), one can physically observe the consequences of the system evolving simultaneously along the forward and backward directions of time.”

In addition to the fundamental feature that time itself may not be well-defined, the work also has practical implications for quantum thermodynamics. Placing a quantum system in a superposition of alternate time directions can provide advantages in the operation of heat machines and refrigerators.

Dr. Rubino added: “Although time is often viewed as a continuously increasing parameter, our study shows that the laws governing its flow in quantum mechanical contexts are much more complex. This may mean that we need to rethink the way this quantity is represented in all those contexts where quantum laws play a crucial role.”

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