Physicists have just uncovered fresh evidence for a bizarre quantum phenomenon called negative time, where photons appear to leave a cloud of atoms even before fully entering it.

The odd quantum effect was revealed in an experiment conducted by scientists at the University of Toronto in Canada and Griffith University in Australia. The team monitored the atoms directly as light passed through them.

Howard Wiseman, PhD, a theoretical quantum physicist at Griffith University and study co-author, said the results should not be mistaken as evidence for practical time travel. “It can all be understood with standard physics, but it’s yet one more weird property of quantum physics that people hadn’t suspected,” he added.

The negative time effect

According to the scientists, the phenomenon occurs when a beam of light travels through a cloud of atoms. Photons, the smallest possible particles of light, briefly disappear by being absorbed into atoms. They temporarily turn into stored energy known as atomic excitation before being emitted again.

A 1993 study suggested that some photons moving through these atomic clouds seemed to arrive at detectors earlier than expected. In some cases, the light pulse appeared to emerge before the center of the pulse had fully entered the cloud. It implied a negative transit time.

Scientists at the time said there might be a simpler reason behind the effect. They claimed that photons at the front of the pulse may simply have been more likely to pass through than those at the back, which created the illusion of faster travel.

“People were convincing themselves that this is not actually as crazy as it sounds,” Wiseman told Live Science. Now, to finally test whether the effect was real, he and his colleagues changed their approach and watched the atoms interacting with the light.

Testing the mystery

To estimate how long the photons effectively remained inside the cloud, the team measured how long the atoms stayed in an excited state. A second beam of light helped them detect tiny phase shifts linked to the atoms’ excitation levels.

This allowed the atoms to reveal what was happening during the experiment in real time. The result aligned with earlier findings, as the atoms appeared to reveal a negative interaction time.

Quantum systems are very sensitive to observation, which means that measuring them can, in certain cases, disrupt the phenomenon entirely. To prevent this, the team relied on ‘weak measurements’. However, while this method introduces far less disturbance, it produces enormous amounts of noise.

To get a clear signal, the team repeated the experiment roughly one million times. Data collection across multiple experimental setups took approximately 70 hours. Wiseman said simple photon-atom interactions have been studied for almost 100 years. “Just the fact that it can still show surprises after all this time is interesting,” he concluded.

The researchers now aim to investigate photons that scatter away from the cloud, instead of passing through it. Some theories predict that the scattered photons could offset the negative time effect as they carry extra positive excitation time.

The study has been published in the journal Physical Review Letters.