The strange case of “negative time” observed in the laboratory

In classical physics, the notion of time is associated with a direction known as the arrow of time, which points from the past to the future. This behaviour is linked to the increase of entropy and, for this reason, time intervals are expected to always be positive. However, this is not the same in quantum mechanics, where the concept of “time” is somewhat different from that in classical systems. Because of this, the idea of “negative time” arises, even though it is counterintuitive.

In quantum mechanics, time is a parameter that describes the evolution of a system, and there are definitions of time associated with processes, such as delay times, dwell times or traversal times. In certain contexts, these quantities can take negative values, especially in interference and scattering phenomena. The effect of “negative time” in quantum mechanics is consistent with physics when correctly interpreted.

A recent experiment, published in Physical Review Letters, investigated a phenomenon known since the 1990s involving the propagation of photons. In certain systems, measurements indicate that photons can exhibit negative delay times when passing through a region, suggesting that they leave before they “enter”. The current experiment showed consistency between different measurements, reinforcing the validity of the result. Although it may seem paradoxical, the phenomenon agrees with what is expected within the theory.

Photons going back in time

Experiments with photons in certain media show temporal behaviours that appear counterintuitive. In a typical system, a pulse of light passes through a cloud of rubidium atoms, which have energy levels resonant with the energy of the photon. When the levels are resonant, it means that the photon’s energy can be temporarily absorbed by the atoms and then re-emitted. This process suggests that the photon “remains” in the medium for a certain amount of time. For resonance to occur, the photon must have a well-defined energy.

When these photons interact with the cloud, most are scattered after transferring energy to the atoms, being re-emitted in random directions. A small fraction manages to pass through the medium without being scattered. When analysing the average arrival time of these photons, they appear to arrive earlier than the expected time. On average, this corresponds to a “negative dwell time” within the cloud. This interpretation suggests, apparently, that the photon would have exited before entering. This behaviour had already been reported in experiments since the 1990s.

Heisenberg Principle

This entire experiment is based on something called the Heisenberg Uncertainty Principle, which states that there is a natural limit to how much we can know certain properties of a particle at the same time. This limit does not arise because of instruments or measurements; it is intrinsic to nature itself. In the quantum world, particles do not have completely defined values for everything at the same time. In other words, these values simply do not exist exactly and simultaneously, only as probabilities.

More intuitively, this means that the more you try to “fix” one property, the more the other becomes undefined. No matter how advanced the technology or method used, this limit will always be present. This happens because, at the quantum level, the very idea of a particle with well-defined properties at all times does not apply. Instead, particles behave probabilistically until they are measured.

New study

A recent experiment investigated this “negative time” in quantum systems using photons interacting with a cloud of atoms. To avoid disturbing the system, the researchers used a weak measurement technique, which allows information to be extracted without fully collapsing the quantum state. Instead of directly measuring the photon, an independent weak laser beam was used to probe the state of the atoms. Small variations in this beam indicated whether the atoms had been excited by the passage of the photon.

The results showed that the dwell time measured by this technique exactly matches the “negative time” inferred from the average arrival time of the photons. This equivalence between two independent definitions of time was unexpected. The fact that both approaches converge on the same negative value suggests that the effect has a robust physical basis. Even so, it does not imply a violation of causality or time travel.

Why is it impossible to measure?

Another issue that arises in experiments with quantum systems is measurement. In quantum mechanics, directly measuring the position of photons during their interaction with atoms disturbs the system. This happens because any measurement process alters the quantum state of both the photon and the atoms. By trying to locate the photon within the medium, the measurement modifies its propagation dynamics. As a result, the phenomenon itself ceases to exist in its original form.

For this reason, it is not possible to continuously track the trajectory of a photon without altering the outcome of the experiment. Therefore, the group specialised in a technique that uses a process that perturbs the system very weakly and does not cause a collapse as other techniques might. Even so, these approaches only provide average values and not well-defined trajectories.

References of the news

Angulo et al. 2026 Experimental Observation of Negative Weak Values for the Time Atoms Spend in the Excited State as a Photon Is Transmitted Physical Review Letters