The claim seems to be that the researchers can make light travel faster than the speed of light.

Is this true? How does it even make sense?

It’s in Nature so presumably there’s something in it, but the claim seems nonsensical to me. What’s going on?

“Researchers develop way to control speed of light, send it backward”, 4 April 2019, University of Central Florida. https://phys.org/news/2019-04-researchers-develop-way-to-control.html

Abouraddy and study co-author Esat Kondakci demonstrated they could speed a pulse of light up to 30 times the speed of light, slow it down to half the speed of light, and also make the pulse travel backward.

Referring to: H. Esat Kondakci et al. Optical space-time wave packets having arbitrary group velocities in free space, Nature Communications (2019). www.nature.com/articles/s41467-019-08735-8

By jointly modulating the spatial and temporal degrees of freedom, arbitrary group velocities are unambiguously observed in free space above or below the speed of light in vacuum, whether in the forward direction propagating away from the source or even traveling backwards towards it.

DOI https://doi.org/10.1038/s41467-019-08735-8

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    The keyword here is group velocity. You can achieve an arbitrary speed by sweeping a laser pointer across the Moon's surface, but you can't carry any information on the resulting wave. Commented Apr 6, 2019 at 21:14
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    @AE Check this link: gregegan.net/APPLETS/20/20.html Commented Apr 6, 2019 at 21:29
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    Just a quick heads up that there is a very famous and extremely prestigious journal called Nature. They belong to a very large publishing house that also publishes a journal called Nature Communications which is much younger and less prestigious. Not that it means the scientific results in there are untrue but confusing the two journals is a common mistake for non scientists (and arguably exactly what is intended by the publishers: extending the glamour and prestige of the Nature brand name to make more money). Commented Apr 8, 2019 at 12:55
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    Unlike your title, the quote does not say "light" travels that fast. It says a "pulse" travels that fast.
    – GEdgar
    Commented Apr 9, 2019 at 1:00
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    This is not a question for skeptics.SE, but for physics.SE: There is no claim to be checked, only a lack of understanding to be fixed.
    – DonFusili
    Commented Apr 9, 2019 at 7:57

1 Answer 1


An analogy: suppose there's a long line of swings. You run along the line, pushing each swing as you go. This will result in each swing being slightly out of phase with the one before it. There will be appear to be a "wave" traveling down the line of swings at the same speed that you ran down it. For instance, if you look for a swing that at its maximum displacement (i.e. the farthest forward part the cycle of it swinging back and forth), which swing is displaying that property will change, and this will appear to be a "thing" that's moving through the line at the speed you ran past.

For instance, suppose the swings are 1m apart, and each swing is 1s behind the previous one. Then it will look like there's a wave moving at 1m/s. But what if you set one swing moving, then walk up to the next swing, wait for the previous swing to get through one cycle, and then time your push so that this swing is .01s behind. That is, you push the current swing .01s after the previous swing gets back to its original position. Now the wave will be moving at 100m/s, much faster than you can run.

Similarly, when light travels through substances, it sets up electromagnetic vibrations. If those phase of those vibrations varies by position, then there will be appear to be a wave moving through the substance, and this wave can travel faster than the speed of light.

However, this wave can't be controlled faster than the speed of light. For instance, if you want to set up it up so this wave has a length of one light-second, then it will take at least a second (and the way it works out, it actually takes more) to create the wave, and if you try to send a signal by varying the strength of the wave, it will take at least one second for those changes to propagate to the other end.

  • Great answer! I've deleted mine. Reached upvote limit today :( Will upvote tomorrow. Commented Apr 8, 2019 at 23:26
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    – Sklivvz
    Commented Apr 12, 2019 at 9:12

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