Evrim Yazgin
Cosmos science journalist
New theoretical research shows that time can flow forwards or backwards in quantum systems, challenging our everyday perception of how time progresses.
Over the centuries, scientists have tried to understand the arrow of time – the idea that time flows inevitably in one direction. How can we make sense of the seemingly unrelenting march of time from past to future?
Answers in fundamental physics haven’t been forthcoming. The laws of nature don’t inherently favour a single direction. Whether time moves forward or backward, the equations remain the same.
The explanation of our everyday experience of time is the second law of thermodynamics derived by Austrian physicist Ludwig Boltzmann.
This law states that entropy – the inherent disorder in a system – cannot decrease with the passage of time. This law explains very well the macroscopic world of our everyday experiences.
“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,” says Andrea Rocco, an associate professor in physics and mathematical biology at the University of Surrey in the UK. “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,” Rocco continues. “The puzzle is that, at the most fundamental level, the laws of physics resemble the pendulum; they do not account for irreversible processes.”
Rocco is the lead author of a study which examines the arrow of time in quantum systems. The research is published in Scientific Reports.
“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.”
Physicists have spent a great deal of time showing that quantum mechanics allows for the opposite flow of time in microscopic systems. This has been shown theoretically, and experimentally in quantum computers.
The new study follows on this research area and explores how theoretical “open” quantum systems interact with the environment. They tried to find out if it’s this interaction which is why we perceive time as moving in one direction.
To simplify the problem, the team made 2 key assumptions.
First, they treated the environment as so vast that they could focus on the quantum system in isolation. Second, they assumed that the environment – e.g. the entire universe – is so large that energy and information dissipate into it and not return into the quantum system.
Even with these assumptions, the theoretical system behaved the same way whether time moved forward or backwards, giving a mathematical foundation for time reversal symmetry in open quantum systems.
“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,” says first author Thomas Guff.
In quantum mechanics, the arrow of time may not be as fixed as our experience may have us believe.
