Within our comfortable world of causality we expect that reactions always follow an action and not vice versa. This why the recent chatter in the media about researchers having discovered ‘negative time’ with photons being emitted before the sample being hit by source photons created such a stir. Did these researchers truly just crack our fundamental concepts of (quantum) physics wide open? As it turns out, not really.

Much of the confusion stems from the fact that photons aren’t little marbles that bounce around the place, but are an expression of (electromagnetic) energy. This means that their resulting interaction with matter (i.e. groupings of atoms) is significantly more complicated, often resulting in the photonic energy getting absorbed by an atom, boosting the energy state of its electron(s) before possibly being re-emitted as the excited electrons decay into a lower orbit.

This dwell time before re-emission is what is confusing to many, as in our classical understanding we’d expect this to be a very deterministic process, while in a quantum world it most decidedly is not.

This is highlighted in the Scientific American article on the subject as well, specifically quantum probability. Within this system, it’s possible that there can be re-emissions before the atomic excitation has fully ceased. It was this original 2022 finding that was recently retested, with the findings confirmed.

As confusing as this all may sound, the authors of the recent paper stress that the core of the issue here is the so-called ‘group delay’ of the original pulse as it excites the cloud of rubidium atoms. If one were to think of this pulse as discrete quanta of photon particles, it’d seem to break causality, but as a wave function within quantum physics this is perfectly acceptable. Observations such as the rubidium atoms becoming excited despite photons passing through the cloud, and emitting a photon before the electrons returned to their ground state do not seem to make sense, but here we also have to consider how and what we are measuring.

The short version is that causality remains unbroken, and the world of quantum physics is intuitive in its own, strange ways. Research like this also gives us a much better fundamental understanding of photonics and related fields, none of which involve time travel.

Experimental setup and measured optical depth. (Credit: Josiah Sinclair et al., PRX Quantum, 2022)