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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. – John Dvorak Apr 06 '19 at 21:14
  • @john Thanks for the reply but I don’t understand. Isn’t the presence/absence of light in itself information, like a signal beacon? Is there a words-of-one-syllable explanation you could link to? – A E Apr 06 '19 at 21:17
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    @AE Check this link: https://www.gregegan.net/APPLETS/20/20.html – Barry Harrison Apr 06 '19 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). – user2705196 Apr 08 '19 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 Apr 09 '19 at 01:00
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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 Apr 09 '19 at 07:57
  • Fair point from @DonFusili there. If it's suitable for physics.SE and mods want to move it across then please go ahead. – A E Apr 10 '19 at 08:55
  • @GEdgar You're right. The difference between 'light' and 'a pulse of light' is part of what has been escaping me. That and the idea that anything can travel faster than light which I thought experimental results post Einstein had proven to be the speed limit for the universe. (Also "way to control speed of light" in the headline, but I guess that's just brevity). – A E Apr 10 '19 at 08:58
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    The criticism of this question seems apt: that it's not really pseudo-science so much as a conceptual misunderstanding. Still, feels like a good question to me. If anything, that the claim about light can be read so misleadingly from reputable sources makes it _more_ hazardous as potential misinformation, such that an SE.Skeptics question debunking the misunderstanding seems constructive. – Nat Apr 10 '19 at 17:24
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    @user2705196 I haven't really seen anything from Nature Communications; but Phys.org is basically a science news aggregator. They aren't a journal like Nature, so they don't peer review their content; even if their content is _based on_ peer reviewed articles. They seem to misword things in a pop-sci way on a regular basis, which is unfortunate. – JMac Apr 10 '19 at 18:11

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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.

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