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arXiv:physics/0510278

Motion backwards in time? A second example

Evangelos Chaliasos

Published Oct 31, 2005• 2 citations
arXivHTMLPDF

Abstract

It is a striking result the one found by Wang, Kuzmich & Dogariu 1 (see also 2) experimentally last year, that a pulse of light, entered into an atomic caesium (Cs) vapour cell, appeared at the exit before its entrance (!) by 62 nanoseconds. It has to be mentioned that the shape of the pulse was maintained (cf. fig. 4 of 1), so that its peak was not shifted forward inside the pulse. Thus the explanation that the peak of the pulse was advanced at the exit, with the result of emerging there before the entrance of the peak of the incident pulse, does not apply.

DOI
10.48550/arxiv.physics/0510278

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It is a striking result the one found by Wang, Kuzmich & Dogariu 1 (see also 2) experimentally last year, that a pulse of light, entered into an atomic caesium (Cs) vapour cell, appeared at the exit before its entrance (!) by 62 nanoseconds. It has to be mentioned that the shape of the pulse was maintained (cf. fig. 4 of 1), so that its peak was not shifted forward inside the pulse. Thus the explanation that the peak of the pulse was advanced at the exit, with the result of emerging there before the entrance of the peak of the incident pulse, does not apply.
Nothing can travel faster than light. Despite a recent raft of reports in the media, this statement is as true now as it ever was. Nonetheless, experiments over the past 20 years have been forcing us to re-examine what we mean by the word "nothing". In the latest experiment, a group of researchers at the NEC Research Institute in Princeton, US, observed the peak of a laser pulse leave a small cell filled with caesium gas before it had even entered the cell (L J Wang, A Kuzmich and A Dogariu 2000 Nature 406 277). Apparently, the peak of this pulse is simply not the kind of "thing" to which Einstein's famous law applies.
Analysis of an infinitely long train of sine-squared pulses—as a toy model—propagating in a medium exhibiting anomalous dispersion shows that although the shape of the light pulses seems to be almost preserved on a small scale, the pulses are severely distorted on a large scale. The frequency of the sine-squared pulses is doubled before they propagate a tenth of the width (FWHM) of a pulse in the medium. The analysis and a numerical calculation show that the front of a wave with a step-function envelope propagates at the speed of light, c , through a medium with two gain lines. Finally, the propagation of a Gaussian pulse—a more realistic pulse—in a medium exhibiting anomalous dispersion is studied. The distortion of the pulse in a Wang–Kuzmich–Dogariu (WKD) experiment (Wang L J, Kuzmich A and Dogariu A 2000 Nature 406 277) should be observable at a precision of ±1 ns. If a Gaussian pulse travels in the medium for 1/50 of a width (FWHM), it will collapse. 'Snapshots' of a Gaussian pulse show that the shape of the pulse in space will have more than one peak at certain times. A possible picture of light propagation in WKD-like experiments is proposed on the basis on these findings.