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A theoretical study from researchers at the University of Surrey suggested that the direction of time may not be fundamentally fixed in certain quantum systems. The work, published in Scientific Reports, examined how the “arrow of time” could emerge from microscopic physics and found that time-reversal symmetry can remain intact even in models used to describe processes such as energy loss and thermalisation.
The arrow of time refers to the observed one-way direction from past to future in everyday life. In macroscopic processes, this is easy to see. Spilled milk spreads across a table and does not gather back into a glass, and heat flows from hotter objects to colder ones. These processes shape the common sense idea that time moves in a single direction.
However, at the level of fundamental physics, many equations do not prefer a direction of time. Time-reversal symmetry means that the same physical laws can describe a system whether time moves forward or backward. This has made it difficult to explain why irreversible behaviour appears in the large-scale world even when the underlying rules do not require it.
Dr Andrea Rocco, Associate Professor in Physics and Mathematical Biology at the University of Surrey, described this contrast: "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.
