Scientists find evidence of ‘negative time’

Quantum physicists say ‘crazy’ result would make a quantum clock appear to move backward rather than forward

Physicists from Canada observed photons exiting a material before they entered it, experiencing what they call ‘negative time’ (iStock/ Getty Images)

Scientists claim to have found evidence of “negative time” after observing photons exiting a material before entering it.

A team of quantum physicists from the University of Toronto in Canada made the discovery after studying the behaviour of photons – the wave particles of light – for seven years in order to better understand a phenomenon known as atomic excitation.

This is where photons absorbed by a material experience a time delay – also known as a group delay – before exiting the material due to their interactions with atoms within it.

To better understand this process, the researchers came up with an experiment to shoot photons through a cloud of ultracold atoms and observe the atomic excitation.

To their surprise, the transit time of some of the photons through the atoms concluded before the atomic excitation had finished, giving it a negative value and making it appear as though the photons exited the material before entering it.

“These results suggest that negative values taken by times such as the group delay have more physical significance than has generally been appreciated,” the researchers noted.

The results were detailed in a study, titled ‘Experimental evidence that a photon can spend a negative amount of time in an atom cloud’, which is currently awaiting peer review.

“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!” Aephraim Steinberg, an experimental quantum physicist at the University of Toronto, wrote on X. “It sounds crazy, I know.”

Speaking to Scientific American, Josiah Sinclair from the University of Toronto said that his team was “completely surprised” by the results.

“A negative time delay may seem paradoxical, but what it means is that if you built a ‘quantum’ clock to measure how much time atoms are spending in the excited state, the clock hand would, under certain circumstances, move backward rather than forward,” he said.

The findings have no practical impact on our understanding of time, however it does raise questions about previous studies relating to photons and optics, while also demonstrating the mysterious nature of the quantum realm.