Physicists have discovered the phenomenon of “negative time,” but it won’t help you travel back in time.

Scientists from the University of Toronto have discovered further evidence of a counterintuitive perception of the quantum world. In a series of experiments, it was proven that under certain conditions, atoms and photons can behave as if time were going backwards. The scientists’ work has not yet been peer-reviewed and has been on the arxiv.org website since September.

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During the experiments, the researchers passed light (photons) through a cloud of atoms cooled to almost absolute zero. Light passes through such a medium with a certain delay, called group delay. This is due to the fact that some photons are absorbed by atoms and excite them. This occurs due to the absorption of photon energy by the electrons of atoms and their transition to a higher level. The electrons then emit photons and return to their previous energy level, and the atoms exit the excited state. The light comes out as if nothing had happened, but after some detectable time.

Things get interesting when the frequency of photons approaches the resonant frequency of atoms. In such situations, the group delay becomes negative. In the experiment, scientists determined this by the phase shift between the reference beam and the probing beam – this is the so-called Kerr effect. According to the observations and calculations made, the negative group delay of light is not a measurement error, but a given. The atoms of the substance seemed to be excited in advance even before the passage of photons, which, apparently, can be explained by their superposition in the quantum world. Schrödinger explained how this can happen in our world using the example of a cat.

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At least for the group delay of light passing through a substance, the “negative time” has a tangible physical significance, which will need to be taken into account in future studies. For those who dream of going to the past or future, this will not help fulfill their cherished desire, but it once again makes sure that real miracles happen in the quantum world.