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Physicists show that a quantum system can evolve, at the same time, towards the past and the future

An international team of physicists from several universities, including those of Bristol, Vienna, and the Balearic Islands, has managed to show that quantum systems can evolve simultaneously along two opposite time arrows, into the future and into the past.


The study, published in the latest issue of 'Nature Communications Physics', forces us to reconsider the way in which the flow of time is understood and represented in contexts where quantum laws play a fundamental role.


For centuries, philosophers and physicists have pondered the nature of time . However, in the world around us, our own experience indicates that time flows in only one direction, from the present to the future, and never the other way around.


In fact, in nature, all processes tend to spontaneously evolve towards more disordered states, and that propensity can be used to identify a particular arrow of time, which is what we all experience. In physics this is described in terms of 'entropy', which defines the amount of disorder present in a system.


"If a phenomenon produces a large amount of entropy: - explains Giulia Rubino , from the University of Bristol and the first author of the study -, "observing its time reversal is so unlikely that it becomes essentially impossible. However, when the entropy produced is small enough, there is a non-negligible probability that the time reversal of a phenomenon occurs naturally."


According to the researcher, "we can take as an example the sequence of things we do in our morning routine . If we were shown our toothpaste moving from the brush to its tube, we would have no doubt that we are watching a rewound recording of our day. However, if we squeeze the tube gently so that only a small part of the toothpaste comes out, it would not be so unlikely that we would see it go back into the tube, sucked in by the decompression of the tube itself."


Led by Caslav Brukner of the University of Vienna, the study's authors applied this idea to the quantum realm, one of whose peculiarities is the principle of quantum superposition, according to which if two states of a quantum system are possible, then that The system can be in both states at the same time. Extending this principle to the arrows of time, it turns out that quantum systems evolving in one or another time direction (toothpaste going out or going back into the tube) can also be simultaneously evolving along the opposite time direction.


Basis of the laws of the universe

"Although this idea seems quite absurd when applied to our daily experience," Rubino continues, "at its most fundamental level, the laws of the universe are based on principles of quantum mechanics . And that makes us wonder why we never find these superpositions of time streams in nature."


In the words of Gonzalo Manzano, from the University of the Balearic Islands and co-author of the research, «in our work, we quantify the entropy produced by a system that evolves in quantum superposition of processes with opposite time arrows. We find that this often results in projecting the system in a well-defined time direction , corresponding to the more likely process of the two. And yet, when small amounts of entropy are involved (for example, when so little toothpaste is spilled that it can be seen to be reabsorbed into the tube), then one can physically observe the consequences of the system having evolved towards forwards and backwards, in both temporal directions and simultaneously.


Aside from the fundamental feature that time itself might not be well defined, the work also has practical implications in quantum thermodynamics. Placing a quantum system in a superposition of alternative time arrows could offer, for example, performance advantages for heat engines and refrigerators.


“Although time is often treated as a continually increasing parameter - Rubino concludes - our study shows that the laws governing its flow in quantum mechanical contexts are much more complex. This may suggest that we need to rethink the way we represent this quantity in all those contexts where quantum laws play a crucial role."


References: Nature Communications 

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