Scientists discover a photon traveling back in time

The quantum world is indeed a strange world, where nothing is impossible.

In one of the most iconic scenes in Christopher Nolan's Tenet (2020) , the main character (The Protagonist) holds an unloaded gun and aims it at a concrete target.

Scientist Barbara then gave him a hint: " You don't shoot the bullet. You catch it ." At the moment the protagonist pulled the trigger, the bullet suddenly flew back from the target, back into the barrel and into the previously empty magazine.

The scene contains Christopher Nolan's entire idea in Tenet, that we can turn back the arrow of time, sending some objects, or even people back to the past. (Illustration).

This mechanism, called "Time inversion," could solve a series of paradoxes of traditional time travel , most notably the grandfather paradox, in which a person travels back in time to kill his grandfather and thus negates his own existence.

But can time really be reversed?

In a new experiment conducted at the University of Toronto, Canada, scientists observed a phenomenon similar to that in Tenet, when they shot a particle of light through a cloud of supercooled rubidium atoms, the light particle appeared to fly out of the cloud before entering it.

"Our experiments have shown that photons can make atoms appear to spend a negative amount of time in an excited state ," said physicist Professor Aephraim Steinberg, a founding member of the Centre for Quantum Information and Quantum Control at the University of Toronto.

In theory, Steinberg and his colleagues demonstrated that time could be a negative quantity.

In a paper published on the pre-print platform arXiv.org, Steinberg said he had been working on the experiment since 2017. At the time, Steinberg and Josiah Sinclair, a PhD student in the same lab, were interested in the interaction between light and matter, specifically a phenomenon called atomic excitation.

This phenomenon occurs when photons pass through a medium and are absorbed, causing electrons orbiting atoms in that medium to jump to higher energy levels. When these excited electrons return to their original state, they emit the absorbed energy as re-emitted photons, creating the observed time delay as the light passes through the medium.

Sinclair's team wanted to measure that delay and find out whether it depended on the fate of that photon: Would it be scattered and absorbed in the atom cloud, or would it pass through without encountering any interactions?

'At the time, we weren't sure what the answer was, and we felt that such a basic question about such a basic thing should have an easy answer ,' Sinclair said. 'But the more we talked to other scientists, the more we realized there was no expert consensus, everyone was just making judgments based on their own intuition.'

Shoot a photon into a cloud of atoms and, intuitively, the photon should go in and out of it.

So Sinclair and Professor Steinberg spent three years planning and developing a device to test this question in the lab. Their experiments involved firing photons through a cloud of ultracold rubidium atoms and measuring the degree of atomic excitation that occurred.

Two surprising things emerged from the experiment: Sometimes the photons passed through without creating any interactions, but the rubidium atoms remained excited—and for as long as if they had absorbed the photons.

Even more strangely, when the photons are absorbed, they appear to be re-emitted almost immediately, before the rubidium atoms return to their ground state—as if the photons, on average, are escaping the atomic cloud faster than expected.

The team then teamed up with Howard Wiseman, a theoretical and quantum physicist at Griffith University in Australia, to come up with an explanation. The trio came up with a theoretical framework that suggests the phenomenon can only be explained by a negative time-valued quantity.

To understand this, you have to think of the photon as a quantum, not a particle. As the Heisenberg uncertainty principle shows, you cannot simultaneously measure both the position and momentum of a particle with absolute precision.

This means that the absorption and re-emission of any photon through a cloud of atoms is not guaranteed to occur in a fixed time interval. Instead, it occurs over a range of probabilistic times.

Experiments show that sometimes photons leave the cloud before entering it.

So when you shoot a photon into a cloud of atoms, sometimes the probability of the photon leaving the cloud at a given moment is higher than the probability of it entering the cloud at that moment. This leads to negative time.

'We were completely surprised by this prediction ,' Sinclair said. 'A negative time delay might seem paradoxical, but it means that if you build a 'quantum' clock to measure the amount of time atoms spend in an excited state, the clock hands will in some cases move backwards instead of forwards.'

The discovery by a team at the University of Toronto comes two months after a team at the US National Institute of Standards and Technology and the University of Colorado Boulder built the most advanced atomic clock ever.

Its mechanism of action is based on the same principle that Sinclair and Professor Steinberg are testing: They use ultraviolet light to excite the nuclei of a thorium-229 atom in a solid crystal.

They then measured the frequency of the energy pulses hitting the nucleus—the equivalent of a pendulum in a conventional clock—and counted the wavelengths in the UV light using a tool called an optical frequency comb.

Based on this principle, the atomic clock can achieve outstanding accuracy, only losing 1 second after 30 billion years - twice the age of the universe and 6 times the age of the Earth.

Professor Aephraim Steinberg, physicist, founding member of the Centre for Quantum Information and Quantum Control at the University of Toronto.

But with the new discovery, the accuracy of this clock will become a question mark. Could it be that at any moment, it will turn back? And how might this affect the arrow of time that we have assumed to point in one direction?

More research will be needed to answer those questions. For now, we are like photons in a cloud, and as uncertain as Heisenberg's principle. All of which suggests that the quantum world is indeed a strange place, and in it, it seems, anything is possible.