Quantum physics experiment leads to bizarre "negative time" phenomenon

No, it has nothing to do with time travel

Serving tech enthusiasts for over 25 years.
TechSpot means tech analysis and advice you can trust.

A real mind-melter: Quantum mechanics is strange, but even for a field of science that regularly defies our conventional understanding of reality, the latest discovery is particularly baffling. Researchers claim they have observed photons exhibiting a peculiar behavior, which they have described as "negative time."

The bizarre discovery, detailed in a study that has yet to be peer-reviewed, stems from an experiment in which photons were fired into a cloud of atoms cooled to just above absolute zero. In cases where the photons passed through without interacting, researchers found that the atoms were still briefly excited, as though the photons had been absorbed and re-emitted. Meanwhile, when the photons were absorbed, they seemed to reappear before the atoms could even become excited.

"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," Josiah Sinclair of the University of Toronto explained to Scientific American. Sinclair's earlier work helped lay the foundation for the study.

So, what's really happening? When photons travel through a medium like the atom cloud, they can be absorbed, causing the atoms' electrons to jump to a higher energy level (excitation). The atoms then de-excite, re-emitting the photons' energy, which observers see as the light being delayed as it passes through.

// Related Stories

• US agency develops a new standard for timekeeping on the Moon
• Researchers propose way to "see" gravity for the first time

The researchers were surprised to find no consensus on precisely what happens to individual photons during the delay process, so they conducted experiments to investigate further.

Based on those tests, they believe the phenomenon can be explained by the strange quantum effect known as "superposition" – the ability of quantum particles to exist in multiple states simultaneously.

From the perspective of the detector measuring the photons' journey, this quantum uncertainty allows the photons to register both positive and negative time values as they pass through the atom cloud. In this context, "negative time" appears to mean that photons seem to travel faster when the atoms are excited, compared to when they remain inactive.

While the findings don't change our broader understanding of time, they serve as a reminder that reality at the quantum level often defies our everyday intuitions.

In other recent quantum news, physicists have proposed an ingenious detector that could potentially allow us to observe graviton particles, thought to carry gravity's quantum forces. If successful, this breakthrough could unlock some of the universe's deepest mysteries.