What if time is not as fixed as we thought? Imagine that instead of flowing in one direction – from past to future – time could flow forward or backwards due to processes taking place at the quantum level. This is the thought-provoking discovery made by researchers at the University of Surrey, as a new study reveals that opposing arrows of time can theoretically emerge from certain quantum systems.

For centuries, scientists have puzzled over the arrow of time – the idea that time flows irreversibly from past to future. While this seems obvious in our experienced reality, the underlying laws of physics do not inherently favour a single direction. Whether time moves forward or backwards, the equations remain the same.

Dr Andrea Rocco, Associate Professor in Physics and Mathematical Biology at the University of Surrey and lead author of the study, said:

“One way to explain this is when you look at a process like spilt milk spreading across a table, it's clear that time is moving forward. But if you were to play that in reverse, like a movie, you'd immediately know something was wrong – it would be hard to believe milk could just gather back into a glass.

“However, there are processes, such as the motion of a pendulum, that look just as believable in reverse. The puzzle is that, at the most fundamental level, the laws of physics resemble the pendulum; they do not account for irreversible processes. Our findings suggest that while our common experience tells us that time only moves one way, we are just unaware that the opposite direction would have been equally possible."

The study, published in Scientific Reports, explored how a quantum system – the world of the sub-atomic – interacts with its environment, known as an ‘open quantum system’. Researchers investigated why we perceive time as moving in one direction, and whether this perception emerges from open quantum mechanics.

To simplify the problem, the team made two key assumptions. First, they treated the vast environment surrounding the system in such a way that they could focus only on the quantum system itself. Second, they assumed that the environment – like the entire universe – is so large that energy and information dissipate into it, never returning. This approach enabled them to examine how time emerges as a one-way phenomenon, even though, at the microscopic level, time could theoretically move in both directions.

Even after applying these assumptions, the system behaved the same way whether time moved forward or backwards. This discovery provided a mathematical foundation for the idea that time-reversal symmetry still holds in open quantum systems – suggesting that time’s arrow may not be as fixed as we experience it.

Thomas Guff, postdoctoral researcher who led the calculations, said:

“The surprising part of this project was that even after making the standard simplifying assumption to our equations describing open quantum systems, the equations still behaved the same way whether the system was moving forwards or backwards in time. When we carefully worked through the maths, we found that this behaviour had to be the case because a key part of the equation, the "memory kernel," is symmetrical in time.

“We also found a small but important detail which is usually overlooked – a time discontinuous factor emerged that keeps the time-symmetry property intact. It’s unusual to see such a mathematical mechanism in a physics equation because it's not continuous, and it was very surprising to see it pop up so naturally.”

The research offers a fresh perspective on one of the biggest mysteries in physics. Understanding the true nature of time could have profound implications for quantum mechanics, cosmology and beyond.

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