Researchers discover “negative time” in quantum physics, challenging traditional concepts of time

In a new experiment, the researchers from the University of Toronto discovered “negative time.” 

“It took a positive amount of time, but our experiment observing that photons can make atoms seem to spend a *negative* amount of time in the excited state is up!” wrote Aephraim Steinberg in a post on X. He was a physicist at the University of Toronto and collaborated in this study.

They set up an apparatus to explore the interaction between photons and ultracold rubidium atoms. Their experiment involved shooting photons through a cloud of these atoms and measuring the level of atomic excitation that occurred. 

Surprisingly, they found two unexpected results. First, even when photons passed through the atomic cloud without being absorbed, the rubidium atoms still became excited, as though they had interacted with the light. 

Second, in cases where the atoms absorbed photons, the reemission of the photons occurred almost instantaneously, much faster than expected, implying that the photons were exiting the atoms before the usual atomic excitation process was completed.

To explain these strange results, the team collaborated with Howard Wiseman, a quantum physicist from Griffith University, Australia. They developed a theoretical framework that showed the time the transmitted photons spent in the atomic excitation state corresponded perfectly with the expected group delay of the light. 

Group delay refers to the time shift in the peak of a light pulse as it moves through a medium. 

What was truly bizarre in this case was that even when it appeared as though the photons were being reemitted before the atomic excitation had finished, the timing still matched the theoretical predictions, suggesting that the photons were somehow moving faster than expected, or even traveling “backward” in time.

This phenomenon arises from quantum mechanics, in which particles such as photons do not follow classical physics principles. Events in the quantum realm can appear to occur out of order or simultaneously as particles exist in probabilistic states. This means that, in certain situations, photons can behave in ways that contradict our traditional concept of time.

Although it may appear contradictory, negative time delay does not break any known physical principles, including Einstein’s theory of relativity. 

The findings do not imply that information travels faster than light but rather that how photons interact with atoms at the quantum level can occasionally yield unexpected, paradoxical effects.

This breakthrough occurred in a cutting-edge quantum physics laboratory at the University of Toronto in Canada. 

The finding came after years of planning and months of experimentation. The final vital observations were made in late 2023, and the results were released as a preprint on arXiv.org on September 5, 2024.

The discovery was made while studying atomic excitation.

“I can promise you that we were completely surprised by this prediction,” Sinclair, a University of Toronto doctoral fellow at that time, referred to the matchup between the group delay and the time that the transmitted photons spent as atomic excitations. “And as soon as we were confident we hadn’t made a mistake, Steinberg and the rest of the team—I had moved on to do a postdoc at [the Massachusetts Institute of Technology] by this point—began planning to do a follow-up experiment to test this crazy prediction of negative dwell time and see if the theory would hold up.”

The discovery of “negative time” began with a team led by Josiah Sinclair, who, as a doctoral student, collaborated with physicist Aephraim Steinberg at the University of Toronto. 

Sinclair’s team initially focused on the interaction between light and matter, specifically how photons influence atomic excitation. They developed an experimental apparatus to measure the time delay in light as it passed through a cloud of ultracold rubidium atoms, leading to the strange finding of harmful time delays. 

Daniela Angulo, also from the University of Toronto, led follow-up experiments that further explored and confirmed the strange behavior of photons in this context, cementing the discovery of negative time in quantum systems.

“[Angulo] and the rest of the team have accomplished something really impressive and produced a beautiful set of measurements. Their results raise interesting questions about the history of photons traveling through absorptive media and necessitate a reinterpretation of the physical meaning of the group delay in optics,” Sinclair said.

References:

¹ Evidence of ‘Negative Time’ Found in Quantum Physics Experiment – Manon Bischoff, Jeanna Bryner – SciAm – September 30, 2024

² Experimental evidence that a photon can spend a negative amount of time in an atom cloud – Daniela Angulo et al. – Arxiv.org – September 5, 2024

Harsha Borah

Harsha Borah is an experienced content writer with a proven track record in the industry. Harsha has worked with LitSpark Solutions and Whateveryourdose, honing skills in creating engaging content across various platforms. A gold medalist in a state-level writing competition organized by Assam Tourism, Harsha’s travelogue on Tezpur was widely appreciated. Harsha’s article, "The Dark Tale of the Only Judge in India to Be Hanged," ranks second on Google and has garnered over 11 000 views and 8 900 reads on Medium. Outside of writing, Harsha enjoys reading books and solving jigsaw puzzles.

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