What if time is not as set in stone it seems? Imagine that time could move forward or backward due to quantum-level processes rather than in a single direction. According to a recent study published in Scientific Reports, researchers at the University of Surrey have uncovered the intriguing discovery that some quantum systems have the potential to produce competing arrows of time.
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The arrow of time—the notion that time moves irrevocably from the past to the future—has baffled scholars for ages. The fundamental principles of physics do not favor one path over another, even though this appears to be evident in the reality humans experience. The equations are the same whether time goes forward or backward.
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.
Dr Andrea Rocco, Associate Professor (Reader), Physics and Mathematical Biology, University of Surrey
He added, “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 investigated the interactions between an “open quantum system,” or quantum system, which is the subatomic world, and its surroundings. Researchers looked into whether open quantum mechanics explains why we believe time is moving in a single direction.
The team established two important assumptions to simplify the challenge. Initially, they handled the extensive surroundings of the system in a way that allowed them to concentrate just on the quantum system. Second, they believed that because the environment, like the universe as a whole, is so vast, information and energy vanish into it and never come back.
Despite the fact that time might theoretically move in both directions at the microscopic level, this method allowed them to investigate how time manifests as a one-way phenomenon.
The system acted in the same manner whether time went forward or backwards even after these presumptions were applied. The assumption that time-reversal symmetry still persists in open quantum systems was given a mathematical basis by this discovery, indicating that the arrow of time may not be as fixed as one perceive it to be.
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.
Thomas Guff, Research Fellow, Quantum Thermodynamics, University of Surrey
She further added, “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 study provides a new insight into one of physics’ greatest puzzles. Determining the actual nature of time may have significant ramifications for cosmology, quantum mechanics, and other fields.
Journal Reference:
Guff, T. et. al. (2025) Emergence of opposing arrows of time in open quantum systems. Scientific Reports. doi.org/10.1038/s41598-025-87323-x