Physicists have shown that photons passing through a cloud of atoms can appear to spend a negative amount of time inside, and surprisingly, the atoms themselves confirm the strange result.
The experiment, published in Physical Review Letters, used photons, the quantum particles of light, and a cloud of rubidium atoms to test one of quantum mechanicsโ most puzzling effects.
When a photon passes through atoms, it can temporarily transfer its energy to them. This creates an atomic excitation, meaning the photonโs energy briefly โdwellsโ in the atoms before being released again as light.
For this interaction to happen, the photon must have the right energy to match the atomsโ resonance. However, because of the uncertainty principle, a photon with well-defined energy has uncertain timing. This means researchers cannot say exactly when the photon enters the atomic cloud, but they can calculate its average entry time.
For decades, experiments have shown that some photons emerge from such atomic clouds earlier than expected. In fact, when scientists compare the average entry and exit times, the photon appears to have spent less than zero time inside the cloud.
This โnegative timeโ had often been dismissed as a misleading effect caused by the shape of the light pulse. The idea was that only the earliest part of the photonโs long pulse passed through, while the rest was scattered away, making the photon seem to arrive too early.
But researchers wanted to know whether the atoms would tell the same story.
To test this, they used an extremely weak measurement method. A separate weak laser beam was passed through the atomic cloud while the photon moved through it. By measuring tiny changes in the laser beam, the team could gently probe whether the atoms were excited without strongly disturbing the system.
This was important because precise quantum measurements can interfere with what is being measured. A strong measurement would have disrupted the photon-atom interaction entirely, through what is known as the quantum Zeno effect.
Instead, the researchers made very weak measurements across millions of experimental runs. Each individual measurement was imprecise, but when averaged together, the results revealed how long the atoms had been excited.
The surprising result was that the atoms showed the same negative dwell time suggested by the photonโs early arrival.
This means the negative time was not merely an illusion caused by the front of the light pulse passing through. It had a measurable effect on the atomic cloud itself.
The finding does not violate the laws of physics, nor does it mean time travel is possible. The result is fully explained by standard quantum mechanics.
However, it shows that โnegative dwell timeโ is a real measurable quantum effect, even if it seems paradoxical from an everyday point of view.
The experiment deepens scientistsโ understanding of how light and matter interact at the quantum level. It also shows that quantum particles can behave in ways that challenge ordinary ideas about time, motion, and cause.
While it does not open the door to time machines, the discovery reveals that quantum mechanics still holds strange and measurable surprises.
Journal Reference:
Angulo, D., Thompson, K., Nixon, V.-M., Jiao, A., Wiseman, H. M., & Steinberg, A. M. (2026). Experimental Observation of Negative Weak Values for the Time Atoms Spend in the Excited State as a Photon Is Transmitted. Physical Review Letters, 136(15). https://doi.org/10.1103/gjfq-k9dv