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November 26, 2021
In the quantum realm, not even time flows as you might expect
by University of Bristol
In the quantum realm, not even time flows as you might expect
Artistic illustration of a gondolier trapped in a quantum
superposition of time flows. Credit: (c) Aloop Visual & Science,
University of Vienna, Institute for Quantum Optics and Quantum
Information of the Austrian Academy of Sciences
A team of physicists at the Universities of Bristol, Vienna, the
Balearic Islands and the Institute for Quantum Optics and Quantum
Information (IQOQI-Vienna) has shown how quantum systems can
simultaneously evolve along two opposite time arrows--both forward and
backward in time.
The study, published in the latest issue of Communications Physics,
necessitates a rethink of how the flow of time is understood and
represented in contexts where quantum laws play a crucial role.
For centuries, philosophers and physicists have been pondering the
existence of time. Yet, in the classical world, our experience seems
to extinguish any doubt that time exists and goes on. Indeed, in
nature, processes tend to evolve spontaneously from states with less
disorder to states with more disorder, and this propensity can be
used to identify an arrow of time. In physics, this is described in
terms of 'entropy', which is the physical quantity defining the
amount of disorder in a system.
Dr. Giulia Rubino from the University of Bristol's Quantum
Engineering Technology Labs (QET labs) and lead-author of the
publication, said:
"If a phenomenon produces a large amount of entropy, observing its
time-reversal is so improbable as to become essentially impossible.
However, when the entropy produced is small enough, there is a
non-negligible probability of seeing the time-reversal of a
phenomenon occur naturally.
"We can take the sequence of things we do in our morning routine as
an example. If we were shown our toothpaste moving from the
toothbrush back into its tube, we would be in no doubt it was a
rewinded recording of our day. However, if we squeezed the tube
gently so only a small part of the toothpaste came out, it would not
be so unlikely to observe it re-entering the tube, sucked in by the
tube's decompression."
The authors of the study, under the lead of Professor Caslav Brukner
of the University of Vienna and the IQOQI-Vienna, 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 both possible, then that system can also be in both states
at the same time.
"Extending this principle to time's arrows, it results that quantum
systems evolving in one or the other temporal direction (the
toothpaste coming out of or going back into the tube), can also find
themselves evolving simultaneously along both temporal directions.
"Although this idea seems rather nonsensical when applied to our
day-to-day experience, at its most fundamental level, the laws of the
universe are based on quantum-mechanical principles. This begs the
question of why we never encounter these superpositions of time flows
in nature," said Dr. Rubino.
Dr. Gonzalo Manzano, co-author from the University of the Balearic
Islands, said: "In our work, we quantified the entropy produced by a
system evolving in quantum superposition of processes with opposite
time arrows. We found this most often results in projecting the
system onto a well-defined time's direction, corresponding to the
most likely process of the two. And yet, when small amounts of
entropy are involved (for instance, when there is so little
toothpaste spilled that one could see it being reabsorbed into the
tube), then one can physically observe the consequences of the system
having evolved along the forward and backward temporal directions at
the same time."
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's arrows could offer advantages in the performance
of thermal machines and refrigerators.
Dr. Rubino said: "Although time is often treated as a continuously
increasing parameter, our study shows 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."
"Quantum superposition of thermodynamic evolutions with opposing time
's arrows" is published in Communications Physics.
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More information: Quantum superposition of thermodynamic evolutions
with opposing time's arrows, Communications Physics (2021). DOI:
10.1038/s42005-021-00759-1
Journal information: Communications Physics
Provided by University of Bristol
Citation: In the quantum realm, not even time flows as you might
expect (2021, November 26) retrieved 27 November 2021 from https://
phys.org/news/2021-11-quantum-realm.html
This document is subject to copyright. Apart from any fair dealing
for the purpose of private study or research, no part may be
reproduced without the written permission. The content is provided
for information purposes only.
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