A team of Canadian and Australian researchers has just put an end to a decades-long debate by providing experimental proof of a concept that challenges our intuition : the negative residence time.
The experience, published in the prestigious journal Physical Review Lettersconsisted of passing particles of light through a cloud of rubidium atoms to measure a delay which, mathematically, turns out to be less than zero.
By directly measuring the state of the atoms, they proved that this “negative residence time” is not an illusion but a real and measurable quantum effect, without allowing time travel.
How can time become negative in quantum physics?
The phenomenon occurs when particles of light pass through a specific medium, such as a cloud of atomswhich enters into resonance (a specific energy frequency where the atom can absorb the photon) with them.
In this case, the photons are absorbed and then re-emitted. If we compare the average entry time to the average exit time, some photons appear to arrive earlier than expectedas if they had spent negative time inside the cloud.
This apparent paradox is an oddity authorized by the laws of the strange world of quantum physics. Heisenberg’s uncertainty principle states that a photon with a very precise energy has a very uncertain temporal position.
It is therefore described as a long pulse. The result is that the particle appears to have left the cloud of atoms even before its “center” entered it, giving birth to this famous negative length of stay.
Why were the old experiments inconclusive?
The effect had already been observed since 1993 but the scientific community remained skeptical. A simple explanation had been put forward to rule out what seemed to be too strange a result.
According to this theory, only the photons located in front of the light pulse managed to pass through the cloud without being scattered, while the rest of the pulse was blocked.
This created a pure statistical illusion : looking only at the “survivors”, we had the impression of traveling faster than light. This interpretation, although reassuring, did not satisfy everyone, notably Aephraim Steinberg, one of the authors of the 1993 study.
The doubt persisted: was this negative time a simple artifact of measurement or a real physical phenomenon ? To find out, we no longer had to just look at the photons upon arrival, but directly interrogate the atoms during the interaction. This is the whole challenge of new experiment.
What is the new game-changing method?
To solve this quantum puzzle, the scientists decided to “ask the atoms” how long the photon had stayed with them. The problem is that any precise measurement disrupts a quantum system, a phenomenon known asquantum Zeno effect (observing a quantum system continuously can freeze its state).
The solution was to use “weak measurements”, that is to say a very gentle measurement technique which minimally disturbs the observed system. By very delicately probing the atoms with a second laser, they were able to detect the tiny changes indicating whether an atom was in a state of “atomic excitation.”
After repeating the experiment millions of times to filter out the noise, the result was clear. Residence time measured directly on atoms was negative and corresponded exactly to the value deduced from the early arrival of photons.
Should we expect time machines?
The answer is a resounding no. As Howard Wiseman, co-author of the study, explains, “ This doesn’t mean we’re about to build a time machine ».
This phenomenon, although counterintuitive, is perfectly explained with standard physics and does not violate the principle of causality. It is impossible to send information into the past with this process. The negative time measured is a statistical average over a large number of events.
This experiment proves that concepts which seemed purely mathematical or illusory to us can have a measurable physical reality.
