A paper published in arXiv a few weeks ago is making the rounds after having being highlighted by Scientific American on September 30.

The Scientific American headline reads “Evidence of ‘Negative Time’ Found in Quantum Physics Experiment.” According to a group of experimental physicists, photons have been found to exit a material before entering it.

The research results, interesting to say the least, had previously been highlighted by New Scientist. The New Scientist story is paywalled, but open copies are easy to find online, for example here.

The arXiv paper is titled “Experimental evidence that a photon can spend a negative amount of time in an atom cloud.” So far, the paper has been published only in arXiv. That is, the paper has not been peer-reviewed yet. However, the paper is authored by a team of scientists that includes well-known physicists such as Howard Wiseman and Aephraim Steinberg. “Woo-hoo!,” the latter posted to X. Atoms “seem to spend a negative amount of time” in an excited state.

When photons of light pass through a material substrate, in this case a cloud of rubidium atoms at a very cold temperature, prepared in a magneto-optical trap formed using two beams of light and a magnetic field gradient, the photons are absorbed and then re-emitted by the excited atoms of the substrate. This process influences the group delay, an interval of time that can be intuitively thought of as the time that the light spends in the substrate. In certain experimental conditions, the observed group delay is negative. Or in other words, the photons seem to exit the cloud of cold atoms before they have entered it.

What does it mean?

The proposed explanations of the experimental results involve weird quantum phenomena. Before beginning to speculate about time travel and all that, one should bear in mind that quantum mechanics, as it is understood today, rules out the possibility of leveraging weird quantum effects to send signals faster than light and backward in time.

In this case, it can be argued that the group delay is not a physically meaningful quantity. But the sober suggestion of the paper is that it is.

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