Everyone knows that time flows in one direction, with events occurring in a sequence. Time’s unidirectionality is linked to entropy, the unstoppable urge of any system to become increasingly disordered. A castle made of sand will eventually collapse, left to its own devices in the desert, but it won’t magically form again in this ‘ordered’ structure. Once you crack an egg, it won’t “uncrack” itself. A broken glass shatters and does not reassemble itself. Entropy guides what we call the arrow of time.
But in the quantum realm, the rules are far stranger. A new study says that down to the tiniest things possible, the arrow of time might point in two directions at once.
The physicists at the University of Surrey in the UK claim that the physical equations that describe the workings of the universe don’t distinguish between forward and backward time. This means that, in theory, time could flow in reverse, and the math would still hold up.
“We are stuck in this universe in which time actually goes in one direction,” Andrea Rocco, a theoretical physicist at the University of Surrey and a co-author of the study, told Scientific American. “But the equations of motion that we are considering would have allowed the universe to go in the other direction.”
Is time’s arrow an illusion?
Ludwig Boltzmann, a 19th-century physicist, proposed that the arrow of time arises from the statistical behavior of particles. His famous H-theorem suggested that entropy — a measure of disorder — tends to increase over time. This, of course, leads to the irreversible processes we observe in nature. His ideas became the foundation for the thermodynamic arrow of time, which explains why we remember the past but not the future.
But critics like Joseph Loschmidt and Ernst Zermelo pushed back. They pointed out that if the fundamental laws of physics are time-symmetric (meaning they work the same whether time runs forward or backward), then for every process that increases entropy, there should be a corresponding reverse process that decreases it. This led to Loschmidt’s paradox, which shows that entropy-decreasing processes must exist in principle.
Indeed, Newton’s laws of motion, Schrödinger’s equation in quantum mechanics, and even Einstein’s theory of relativity treat time as reversible. Imagine an animation of a planet orbiting a star. If you played the video backward, the physics would still make sense.
The new study revisits this old tension among physicists in the context of quantum systems. They show that even in quantum systems, where the equations of motion are time-symmetric, irreversibility can emerge through interactions with the environment. Their equations predict that a quantum system can evolve both forward and backward in time, implying that time’s arrow is not an inherent feature of quantum mechanics.
Our perception of linear time
So, why does time feel so linear to us? The answer, according to Rocco and his colleagues, lies in the concept of “open quantum systems.” These are systems that interact with their environment, like a cup of water spilling onto the floor. In such systems, time’s arrow emerges spontaneously, not because the laws of physics demand it, but because of the way energy and information flow.
If time’s arrow isn’t baked into the fabric of the universe, is it just a construct of the human mind?
“Sometimes people call this a ‘psychological’ arrow of time,” says Nicole Yunger Halpern, a physicist at the National Institute of Standards and Technology who was not involved in the study. She explains that our perception of time as linear might be an “emergent phenomenon,” a byproduct of the way we observe and interpret the world.
James Cresser, a professor of physics, takes this idea even further. He suggests that time might be nothing more than “scaffolding” for the human brain, a way to make sense of the events around us. “The events themselves don’t rely on the scaffolding,” he told Scientific American. “And what we’re doing with these equations is applying one kind of temporal scaffolding as compared to another possible temporal scaffolding.”
This doesn’t mean linear time is meaningless. On the contrary, it’s deeply tied to our existence. “We are unavoidably buried in time — that is a profound part of our existence that we can never, ever escape,” Cresser says.
Recent experiments have proposed observing quantum interference between forward and backward processes. If successful, such experiments could provide direct evidence of the dual arrow of time in quantum systems.
A Quantum Dance with Time
These conclusions came when researchers applied the Markov approximation, a simplifying assumption that the open quantum system’s future depends only on its present state. The resulting equations of motion remained time-symmetric. This means that the system could evolve irreversibly in both the forward and backward directions of time.
“The Markov approximation does not imply a violation of time-reversal symmetry,” the authors write. Instead, it allows for two opposing arrows of time. The system can thermalize (reach a state of equilibrium) in both directions.
In response, the physicists propose new definition of Markovianity. Traditionally, Markovian dynamics are defined for positive times only, introducing an implicit arrow of time. The researchers argue that Markovianity should be extended to include both past and future directions.
“Markovian dynamics are typically only valid after a particular reference time, commonly chosen to be t = 0,” they explain. By extending the definition to include negative times, the researchers show that the Markov approximation preserves time-reversal symmetry.
The researchers emphasize that their results do not contradict the second law of thermodynamics, which states that entropy tends to increase over time. Entropy increases in both directions — it’s the choice of the arrow of time that determines which direction we perceive as the future.
A Universe with Two Timelines?
One of the biggest mysteries in physics is why the universe started in a state of extremely low entropy. Some cosmologists argue that this initial condition is what set the arrow of time in motion.
But if the new study is correct, the arrow of time could have emerged in both directions from the Big Bang. This idea echoes recent work in cosmology suggesting that time might flow in two directions, creating two universes that evolve away from a common origin. In one, entropy increases in the direction we experience; in the other, time moves in the opposite direction, and what we call the past is their future.
So, in the quantum world, time’s arrow might not be a single path but a branching road, pointing in two directions at once. Yes, my head is running in circles, too.
The findings appeared in the journal Scientific Reports.
