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Illustration: Henry Wong

At subatomic level, the past can be the future: quantum researchers

  • Conventional theory that time can only move forward challenged by study, but the conditions for a ‘backward arrow’ are limited
  • The question of whether time can be reversed ‘one of the fundamental challenges’ of quantum physics
The flow of time can go either way in the world of quantum physics, and this rare phenomenon can be observed and measured in a well-designed experiment, according to a new study.

One of the most puzzling things about quantum mechanics is that a subatomic particle can have different physical properties at the same time – such as spinning in opposite directions, known as superposition.

For almost a century, quantum physicists have been debating whether the law of superposition can be applied to time, so that a subatomic particle can experience the past and future simultaneously.

The debate is far from settled, in part because it remains unclear under what circumstances time could be reversed, and how to detect such an event.

Giulia Rubino, quantum physicist with Britain’s University of Bristol, and her colleagues found the time arrow points backwards in a simple quantum system containing just a few subatomic particles, when the chaos in the system decreases. Their paper was published on Friday in the journal Communications Physics.

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The phenomenon is observable only when the system is extremely stable, with just a small amount of disorder, or “entropy”. “We don’t know if a qubit (quantum particle) is evolving ‘forward in time’ or ‘backward in time’ until it is measured,” Rubino told the South China Morning Post.

Whether the particles are time-travelling to the past or future remains unknown until they are measured.

“Although this idea seems rather nonsensical when applied to our day-to-day experience, at its most fundamental level, the laws of the universe are based on quantum-mechanical principles,” Rubino said.

Time is often treated as a continuously increasing parameter, but “our study shows the laws governing its flow in quantum mechanical contexts are much more complex. This may suggest we need to rethink the way we represent this quantity in all those contexts where quantum laws play a crucial role”.

In complex systems, time always moves forward, but scientists say in a small, stable system they can detect a backward motion in time. Image: Shutterstock

Quantum physics researcher Wang Yingdan, a professor with the Institute of Theoretical Physics, Chinese Academy of Sciences in Beijing, said most scientists – herself included – still took time to be a one-way journey.

In quantum physical experiments involving a small number of atoms or light particles, “almost every operation can be reserved. We have treated it as a physical process rather than time-travelling”, said Wang, who was not involved in the British study.

“We don’t consider the effect of time very often because the movement of particles in quantum experiments is usually much slower than the speed of light,” she added.

The matter of time has been considered by great thinkers like Isaac Newton, who believed in the existence of a “universal clock” governing the life and death of everything in the universe.

Albert Einstein challenged Newton’s theory of absolute time (and space) with his theory of relativity, which argued that, to a person travelling at the speed of light, the clock would stop ticking.

Someone travelling faster than light would, in theory, see time moving backwards, according to Einstein. But he went on to say that nothing goes faster than light, because everything was determined from the moment the universe was born.

In other words, the arrow of time always points forward.


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Rubino and her colleagues said the common definition of time applied to a large system with lots of chaos. In these systems, it was indeed impossible to catch a backward motion in time. But in a small, stable system the phenomenon was easier to detect, she said.

“We can take the sequence of things we do in our morning routine as an example. If we were shown our toothpaste moving from the toothbrush back into its tube, we would be in no doubt it was a rewound recording of our day,” she said.

“However, if we squeezed the tube gently so only a small part of the toothpaste came out, it would not be so unlikely to observe it re-entering the tube – sucked in by the tube’s decompression.”

Rubino and her team proposed two types of quantum machines, one working like an engine that can convert heat to mechanical movements; and the other as a refrigerator, to test their theory – that by measuring the disorder of quantum particles in these devices, they could identify the direction of time arrows.

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Quan Haitao, professor of physics with Peking University, said the question of whether time can be reversed remains one of the most fundamental challenges to quantum physics.

“Take an empty box separated by a wall in the middle, for example. If you fill one side of the box with randomly bouncing ping-pong balls and remove the wall, the balls will spread out to the entire box. The balls will not all go back to the side because time cannot be reversed,” Quan said.

“But if we reduce the number of ping-pong balls in the box, the chance of this happening will increase. This is known as time-symmetry in quantum physics, which means time can go either forward or backward,” he said.

“Why the symmetry disappears in a large, complex system remains a puzzle.”