December 21, 2024 by Ben Simon, Issam Ahmed in Washington
Collected at: https://phys.org/news/2024-12-scientists-negative-quantum.html
Scientists have long known that light can sometimes appear to exit a material before entering it—an effect dismissed as an illusion caused by how waves are distorted by matter.
Now, researchers at the University of Toronto, through innovative quantum experiments, say they have demonstrated that “negative time” isn’t just a theoretical idea—it exists in a tangible, physical sense, deserving closer scrutiny.
The findings, posted on the preprint server arXiv but not yet published in a peer-reviewed journal, have attracted both global attention and skepticism.
The researchers emphasize that these perplexing results highlight a peculiar quirk of quantum mechanics rather than a radical shift in our understanding of time.
“This is tough stuff, even for us to talk about with other physicists. We get misunderstood all the time,” said Aephraim Steinberg, a University of Toronto professor specializing in experimental quantum physics.
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While the term “negative time” might sound like a concept lifted from science fiction, Steinberg defends its use, hoping it will spark deeper discussions about the mysteries of quantum physics.
Laser experiments
Years ago, the team began exploring interactions between light and matter.
When light particles, or photons, pass through atoms, some are absorbed by the atoms and later re-emitted. This interaction changes the atoms, temporarily putting them in a higher-energy or “excited” state before they return to normal.
In research led by Daniela Angulo, the team set out to measure how long these atoms stayed in their excited state. “That time turned out to be negative,” Steinberg explained—meaning a duration less than zero.
To visualize this concept, imagine cars entering a tunnel: before the experiment, physicists recognized that while the average entry time for a thousand cars might be, for example, noon, the first cars could exit a little sooner, say 11:59 am. This result was previously dismissed as meaningless.
What Angulo and colleagues demonstrated was akin to measuring carbon monoxide levels in the tunnel after the first few cars emerged and finding that the readings had a minus sign in front of them.
Relativity intact
The experiments, conducted in a cluttered basement laboratory bristling with wires and aluminum-wrapped devices, took over two years to optimize. The lasers used had to be carefully calibrated to avoid distorting the results.
Still, Steinberg and Angulo are quick to clarify: no one is claiming time travel is a possibility. “We don’t want to say anything traveled backward in time,” Steinberg said. “That’s a misinterpretation.”
The explanation lies in quantum mechanics, where particles like photons behave in fuzzy, probabilistic ways rather than following strict rules.
Instead of adhering to a fixed timeline for absorption and re-emission, these interactions occur across a spectrum of possible durations—some of which defy everyday intuition.
Critically, the researchers say, this doesn’t violate Einstein’s theory of special relativity, which dictates that nothing can travel faster than light. These photons carried no information, sidestepping any cosmic speed limits.
A divisive discovery
The concept of “negative time” has drawn both fascination and skepticism, particularly from prominent voices in the scientific community.
German theoretical physicist Sabine Hossenfelder, for one, criticized the work in a YouTube video viewed by over 250,000 people, noting, “The negative time in this experiment has nothing to do with the passage of time—it’s just a way to describe how photons travel through a medium and how their phases shift.”
Angulo and Steinberg pushed back, arguing that their research addresses crucial gaps in understanding why light doesn’t always travel at a constant speed.
Steinberg acknowledged the controversy surrounding their paper’s provocative headline but pointed out that no serious scientist has challenged the experimental results.
“We’ve made our choice about what we think is a fruitful way to describe the results,” he said, adding that while practical applications remain elusive, the findings open new avenues for exploring quantum phenomena.
“I’ll be honest, I don’t currently have a path from what we’ve been looking at toward applications,” he admitted. “We’re going to keep thinking about it, but I don’t want to get people’s hopes up.”
More information: Daniela Angulo et al, Experimental evidence that a photon can spend a negative amount of time in an atom cloud, arXiv (2024). DOI: 10.48550/arxiv.2409.03680 , arxiv.org/abs/2409.03680
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