The concept of time's arrow-the idea that time moves irreversibly from past to future-has been a longstanding scientific puzzle. While human experience suggests a clear forward direction, the fundamental equations of physics remain unchanged regardless of whether time progresses forward or in reverse.
Dr. Andrea Rocco, Associate Professor in Physics and Mathematical Biology at the University of Surrey and lead author of the study, explained:
"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."
Published in Scientific Reports, the study examined how a quantum system-interacting with its environment as an 'open quantum system'-may influence our perception of time's direction. Researchers sought to determine whether the forward flow of time arises naturally from open quantum mechanics.
To approach this question, the team made two simplifying assumptions. First, they focused solely on the quantum system, treating its vast environment in a way that removed unnecessary complexity. Second, they assumed the environment-akin to the entire universe-is so large that energy and information dissipate without returning. This allowed them to explore how time's one-way progression emerges, despite the microscopic laws of physics permitting time to move in both directions.
Surprisingly, even after applying these constraints, the system's equations remained unchanged regardless of whether time moved forward or backward. This mathematical symmetry supports the idea that time reversal remains a valid possibility in open quantum systems, challenging the traditional notion of a fixed arrow of time.
Thomas Guff, a postdoctoral researcher who led the calculations, commented:
"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."
This research offers new insights into one of the most profound mysteries in physics. A deeper understanding of time's nature could have significant implications for quantum mechanics, cosmology, and other fields of fundamental science.
Research Report:Emergence of opposing arrows of time in open quantum systems
Related Links
University of Surrey
Understanding Time and Space
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