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Quantum systems break the linearity of time

A new study suggests that systems governed by quantum mechanics do not show an exclusively linear evolution in time: in this way, they are sometimes able to unfold into the past and into the future simultaneously.

An international group of physicists concludes in a recent research published in the journal Communications Physics that quantum systems that evolve in one direction or another in time can also be found evolving in unison along both directions. This property shown by quantum systems in certain contexts breaks with the classical temporal conception, in which it is only possible to move forward or backward in time.

The work, carried out by scientists from the universities of Bristol (United Kingdom), Vienna (Austria), the Balearic Islands (Spain) and the Institute of Quantum Optics and Quantum Information (IQOQI-Vienna), shows that the limit between the time that going back and forth can be blurred in quantum mechanics. According to a press release from the University of Bristol, the new study forces us to rethink how the flow of time manifests itself in contexts in which quantum laws play a fundamental role.

Always in one direction?

In 1927, the British astronomer Arthur Eddington used the expression "arrow of time" to indicate its unidirectional property within the framework of the second law of thermodynamics: for the scientist, the arrow of time is an exclusive condition of entropy and does not register equivalence in space. Entropy measures the degree of organization of a system in equilibrium and marks its irreversibility: entropy or disorder always tends to increase over time.

However, both this new study and previous research have shown that the idea of ​​the arrow of time or classical temporal evolution are not absolute concepts, but rather relative. Now, researchers have shown that, under certain circumstances, quantum systems can move along two opposite time arrows , both forward and backward in time.

Entropy and its role

The central element of this proof would be entropy . If a phenomenon produces a large amount of entropy, its reversal in time is essentially impossible. However, if the amount of entropy produced is less, there is a concrete probability of observing the time reversal of the phenomenon.

While we usually think that in nature time always tends to move in a single direction , that is, from the past to the present and from the present to the future, in quantum systems with low entropy this concept can be broken. Physicists show it clearly with an example from everyday life: when we put the toothpaste on the brush every morning, we don't think it can go back into the tube. However, if a small amount of paste does not get onto the toothbrush, it is very likely that we will see it return to the tube, reabsorbed by decompression.

A similar phenomenon would happen at a quantum level, when the entropy produced is small enough. Since quantum physics also admits overlaps between the processes of time forward and backward, the arrow of time would become indefinite in the realm of quantum mechanics.

The Observer and the Quantum Superposition

Even previous studies have analyzed the role that the observer would play in these phenomena, which could partly determine the evolution of the arrow of time. This is not so strange or absurd if we consider that elementary particles can be in several places at the same time and simultaneously possess two or more states ( quantum superposition ) or travel through space without following the classical timeline.

Although time is usually considered a parameter that tends to increase permanently, the new study shows that the laws that govern its flow in quantum mechanical contexts are clearly more complex. In addition to the impact on philosophical and scientific conceptions on the subject, the researchers also point to a practical application of the discovery: in thermodynamics, using a quantum system with a superposition of time arrows could offer comparative advantages in the performance of thermal machines and refrigerators.


Quantum superposition of thermodynamic evolutions with opposing time's arrows. Rubino, G., Manzano, G. and Brukner, Č. Communications Physics (2021). DOI: https: //

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