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.
Reference:
Quantum superposition of thermodynamic evolutions with opposing time's
arrows, Communications Physics (2021).
DOI: 10.1038/s42005-021-00759-1
Tags:
Physics