Quantum mechanics just can’t keep from getting freaky. The latest thing is negative time, and how light going through a cloud of atoms might appear to come out before it goes in. Unfortunately, it is not a time machine – your best bet there is still a DeLorean – but a curious phenomenon with intriguing implications for optical applications.

Imagine that you are sending a pulse of light across a cloud of atoms. The atoms are at a temperature close to absolute zero, just tens of micro-degrees above it. Light passing through them would normally interact with them. The photons would be absorbed (creating an atomic excitation) and then reemitted. Overall, the light would gain a group delay.

What’s fun is that this group delay can theoretically be in the negative. The team used light whose frequency is close to the atomic resonance frequency of the atoms in the cloud – that means that excited atoms take a long time to release their photon. But in this experiment, the group delay can end up being negative: something weird is going on.

Obviously, the photons – particles of light – are not time traveling. The experimental setup is indicating the quantum weirdness of the interaction of a specific light with a specific set of atoms. The concept of "now" in quantum terms is a little less fixed, making this interaction seem impossible for our standard view of time as a linear progression from the past to the future. The atoms spend a negative time in an excited state; or simply, the photons do not accumulate any delay passing through – actually, they come out before they got in.

“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!” senior author Aephraim Steinberg, from the University of Toronto, wrote on X.

The work awaits peer-review but raises a good point about the concept of negative time. The team argue that “[these] results suggest that negative values taken by times such as the group delay have more physical significance than has generally been appreciated.”

The preprint, which is yet to undergo peer-review, is available on the arXiv.

[H/T: Scientific American]