In the world of magic, turning the clock has proven itself: thanks to a time turner, the ambitious magic student Hermione Granger was able to attend several school lessons at the same time in the Harry Potter novels. This doesn’t work in the non-magical world. But now a study is making people sit up and take notice: negative time has manifested itself in a quantum physics experiment.
For the first time, researchers have observed a physical process that has a negative duration. This process, how could it be otherwise, takes place in the quantum world, which often eludes intuitive understanding. Quantum particles such as photons – i.e. light particles – can take two paths at the same time or penetrate barriers, they can be entangled with one another over large distances and lose their properties when measured. Now researchers from Aephraim Steinberg’s team at the University of Toronto have experimentally proven that photons leave an atom cloud before they have even entered it. You spend a negative time in it.
Steinberg and his colleague Josiah Sinclar had suspected that something like this could exist since 2020. At that time, they had set up an experiment that was intended to investigate how individual photons moved through clouds of rubidium atoms. Photons can be absorbed by atoms, which absorb their energy and thus enter an excited state. The photon is first swallowed and then spit out again when the atom returns to its ground state – that’s what the textbooks say.
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Two strange things happened in the experiment at that time: rubidium atoms were excited, even if the photons emerged unmolested at the other end of the cloud. The atoms could, in a sense, have the cake and eat it at the same time, as a counter-evidence to the English proverb. And when an atom swallowed a photon, it spit it out almost immediately, much faster than it would normally return to its ground state. Sinclair writes on the Platform X: “Ultimately, this raised more questions than answers.”
Steinberg’s team designed a new experimental setup with which they wanted to investigate the length of time over which the photons are absorbed. Mathematically they had proventhat the duration always corresponds to the time by which the wave packet of photons is delayed as it passes through the atom cloud. The movement of the wave packet describes the so-called group speed.
But the crux of the matter is that sometimes the group speed of the photons in rubidium is not slower, but actually faster than light in a vacuum. Instead of a delay, there is a gain in time.
Something takes minus three seconds? This seems outrageous, but it does not violate the theory of relativity
This was known, but it was generally not considered to be physically significant, but rather an artifact of complicated turbulences in the wave packet that describes the movement of the photon. The best way to imagine this is as a wave in the ocean moving towards the beach on the surface, while a single water molecule within that wave flows in the other direction.
But if you think about the negative delay further in physics, it follows that the time period in which photons are absorbed by the atoms can also be negative. The team has now confirmed this with their experiments. The negative time therefore has measurable physical consequences. As The preprint of the study is already publicthis document has not yet been peer-reviewed.
But even if it seems unheard of for something to take a negative time, this does not violate the special theory of relativity. This prohibits information from being transported faster than light. However, at group speeds faster than light, no information is transmitted, which is why this behavior – unlike Hermione’s time turner – is compatible with the laws of physics.
And anyone who thinks of the 2020 film Tenet, which twists the brain in knots, will also be disappointed: the direction in which time runs is not reversed in the experiments of the Canadian research group. So projectiles cannot be sucked back into weapons like in the film. Only the length of time an event lasts can be negative. This is extremely counterintuitive, but it is the quantum world after all.