Particles of light can spend "negative time" passing through a cloud of extremely cold atoms – without breaking the laws of physics.
In the quantum realm, seemingly impossible things are happening every day. Thanks to quantum effects, a particle of light can exit a cloud of extremely cold atoms before it even enters.
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| Light can get away with an impossibly fast journey due to quantum fuzziness Михаил Руденко/Getty Images/iStockphoto |
When light goes into a material, its speed changes as particles of light, or photons, interact with the atoms around them. Atoms inside a material absorb and then release any photons that enter, causing a delay in their travel time. But in some cases, a photon can exit early rather than late – so early in fact, that it spends a negative amount of time absorbed by the material.
Daniela Angulo at the University of Toronto in Canada and her colleagues sent a photon through a collection of tens of thousands of rubidium atoms almost as cold as absolute zero. Such ultracold atoms are susceptible to quantum effects, and their quantum states can be precisely controlled with lasers and electromagnetic fields. The researchers sent two laser beams through the atoms, each moving towards a different detector.
One beam carried the photons that were either reflected off the atoms or absorbed and then released by them, while the other served as a probe. This probe beam did not get absorbed, but revealed the change in quantum state that happened when any atom in the cloud absorbed a photon from the first beam – this let the researchers determine whether an atom had changed its state while the photon was inside the cloud, and for how long.
They were surprised to find that there was a combination of laser beam frequency and atoms’ quantum states for which this number was negative. After detecting one such instance, they spent almost two years perfecting their setup, and then took data non-stop for weeks to find more, says Angulo. For quantum objects like photons, there is a fundamental trade-off between how much information about their properties one can gather without changing their behaviour. The team opted to make only very weak measurements while photons were in the atom cloud for this reason, but did so repeatedly for up to 15 hours for a single data point, she says.
Aephraim Steinberg, also at the University of Toronto, who worked on the experiment, says although the idea of negative time may seem paradoxical, it doesn’t break any rules of causality or Albert Einstein’s special relativity because the photons are not being used to communicate any information. The experiment simply adds to our understanding of all the ways light and matter can interact in the quantum world, he says.
“These experiments explore what has previously been mainly a theoretical curiosity,” says Peter Milonni at the University of Rochester. “This is an important contribution to a part of physics that continues to fascinate.” He says that it is possible to interpret the odd negative time measurement as a consequence of the inherent fuzziness of quantum mechanics, where several options for how a photon travels through an atom cloud are possible at once. This includes the possibility that it spends no time at all – as well as negative time – among the atoms. When all the possibilities combine just right, sometimes one of these counterintuitive results wins out.
Andrew Jordan at Chapman University in California says the experiment is fantastic, but still suggests the results should be taken with caution. “To recall the words of Einstein: time is what is measured by a clock,” he says, whereas this experiment obtains the time a photon spent within these atoms more indirectly. He says future experiments should attempt even more direct measurements.
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