Scientists discover “negative time” in bizarre quantum experiment where photons exit atoms before entering

Credit: Unsplash/Sonja Langford.

In the world of quantum physics, where the rules of everyday reality seem to bend and twist, researchers have made a discovery so puzzling that it seems to warp time itself. Photons, those fundamental particles of light, are exiting materials before they’ve even entered — at least according to new experiments from a team of quantum physicists at the University of Toronto.

A Mind-Boggling Observation

“It sounds crazy, I know,” admitted Aephraim Steinberg, one of the lead researchers on the project, in a post on X. And yet, the team’s data reveals something deeply weird: under the right conditions, photons appear to spend a negative amount of time interacting with atoms. As if the field wasn’t confusing enough, their findings add yet another layer of mystery to the already mind-bending world of quantum mechanics.

Light, as we know it, doesn’t usually behave this way. When photons enter a material, they typically interact with the atoms inside. The atoms, in turn, absorb the energy from the photons, causing their electrons to jump to higher energy levels — a process called atomic excitation. After a delay, the photons are re-emitted, and the material’s atoms return to their normal state. It’s a process familiar to physicists: light goes in, atoms absorb, light comes out.

But in the world of the very small, the ordinary rules often break down. Steinberg and his team were shooting photons through a cloud of ultracold rubidium atoms when they noticed something strange. The photons didn’t just pass through — they seemed to exit the cloud of atoms before they should have. Almost like they’d gotten a head start. In fact, it seemed as though the atoms were excited before the photons had even arrived.

“We were completely surprised,” Josiah Sinclair, a quantum physicist who helped design the experiments, told the German popular science magazine Spektrum der Wissenschaft. “A negative time delay may seem paradoxical, but what it means is that if you built a ‘quantum clock’ to measure how long an atom spends in the excited state, the clock hand would move backward rather than forward.”

The Puzzle of Negative Time

To be clear, this isn’t the kind of time travel that would let us change the past or glimpse the future. Instead, it’s a phenomenon that arises from the strange way quantum particles behave when they interact with matter. In this case, the photons appeared to leave the atomic cloud before the atoms had even fully absorbed them. It was as if they were bending time just enough to defy normal expectations.

In an X thread that aimed to clear up some misunderstandings, Sinclair used a vivid analogy: a chess clock. Picture a chess match, he suggests. At the end of the game, you can tell how long each player spent on their moves by looking at their clocks. Now imagine, he continues, that one player’s clock showed not just zero but a negative value — indicating they somehow took less than zero time to make their moves. This, in essence, is what the team saw with their photons.

In his thread, he clarified that this “negative time” doesn’t mean reversing the past or seeing the future. In the quantum world, particles can exist in “superposition,” where multiple outcomes happen simultaneously. Sometimes, this means events appear to unfold out of order or even backward from our everyday perspective.

Now, imagine a game of chess. At the end of the game, you find that one hour has elapsed and that each player made 30 moves. You would conclude that each turn took on average 1 minute, but what can you infer about the average turn time of each player? It might be reasonable to… pic.twitter.com/agtWof7czW

— Josiah Sinclair (@JosiahSinclair) October 4, 2024

Rewriting the Rules of Light

The experiment was designed to study how long photons spend exciting atoms — a measurement that, in classical physics, always yields a positive value. But in the quantum world, things don’t always follow the rules. Using a technique called the cross-Kerr effect, Steinberg and his team measured the phase shifts in a secondary beam of light, which allowed them to determine how long the rubidium atoms stayed excited.

The results were baffling: under certain conditions, the photons’ transit time through the atomic cloud was negative. The team ran the experiments repeatedly, using carefully synchronized pulses of light to ensure accuracy. After gathering data for hours, they were sure of what they had found. Negative time was real. Well, at least in the strange, probabilistic way that governs the quantum realm.

The explanation lies in the weirdness of quantum mechanics itself. In the everyday world, time always moves forward, but at the quantum level, when a photon interacts with an atom, the interaction doesn’t occur in a straightforward way. Instead, the process is fuzzy, smeared across a range of possibilities. And that range includes events seeming to happen out of order.

“When you see a transmitted photon, you can’t know exactly how it behaved,” Steinberg explained told science journalist Manon Bischoff. “In some cases, the photon might ignore the atom entirely. In other cases, it interacts, excites the atom, and then leaves. But in quantum mechanics, both outcomes can happen at the same time.”

What does this mean for quantum physics?

Though this discovery won’t let us build a time machine, it has intriguing implications for the field of quantum computing. Quantum computers rely on particles like photons to carry and process information. If scientists can better understand how photons interact with atoms — even in cases where the timing seems reversed — it could lead to more efficient quantum systems.

By controlling these delicate interactions, researchers might develop more stable quantum circuits, the fundamental building blocks of quantum computers.

But for now, the team is focused on understanding the basic physics of what they’ve discovered. The results raise profound questions about how light and matter interact in the quantum realm, and whether this phenomenon could appear in other systems.

Still, some scientists remain cautious. Andrew Jordan, a quantum physicist at Chapman University, praised the experiment but urged further tests. “Time is what is measured by a clock,” he said, quoting Einstein, adding that this is an indirect way of measuring the passage of time which requires further experiments to validate.

For now, negative time remains a puzzle — and a thrilling one at that. Sometimes, in the quantum universe, what happens next might already be in the past.

The findings were reported in the preprint server arXiv.

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