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Physics

Light has been seen leaving an atom cloud before it entered

Particles of light can spend "negative time" passing through a cloud of extremely cold atoms – without breaking the laws of physics

By Karmela Padavic-Callaghan

18 September 2024

Light can get away with an impossibly fast journey due to quantum fuzziness

Михаил Руденко/Getty Images/iStockphoto

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.

When light goes into a material, its speed changes as its particles, 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…

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