spaceSpace and Physics

Time Might Not Flow In Just One Direction In The Quantum World


Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

clockNov 26 2021, 10:43 UTC
Artistic illustration of a gondolier trapped in a quantum superposition of time flows

Artistic illustration of a gondolier trapped in a quantum superposition of time flows. Image credit: © Aloop Visual & Science, University of Vienna, Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences

New research suggests that the flow of time at a quantum level doesn’t have to go in just one direction. Quantum systems, in certain conditions, might move both forward and backward in time. And this is a very exciting discovery.

We are not talking about time machines or the quantum realm of the Marvel Universe here. But just how we experience time at our level might not exactly be the true nature of time. Time is one of the great mysteries of the universe. We experience it and we know that it works in a certain way, going in the same direction from the past into the future.


This particular direction – the arrow of time as it is called – has been connected with the concept of entropy, the measure of the disorder of a system. In an isolated system, such as our universe, the entropy will always increase and the direction of the increase matches the direction of time passing.

As reported in the journal Communications Physics, the researchers looked at phenomena where entropy changes are small. By the fundamental properties of quantum mechanics, it is possible that the entropy could spontaneously decrease (like going backward in time). They also employed another important quantum mechanical property: superposition. In that state, your physical system can exist in multiple configurations.   

“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,” lead author Dr Giulia Rubino from the University of Bristol, said in a statement.


“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 international team looked at a quantum superposition with a state that evolves both backward and forward in time. They found that more often than not, this results in the system ending up forward in time. But for small entropy changes, the system can continue to evolve both forward and backward in time.

This work is pointing at the big unknown that is Time at its most fundamental level. And this work hints at we might understand it even less than we previously thought.


“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,” Dr Rubino concluded.

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