# Evanescent waves and faster-than-c
**URL:** https://boards.straightdope.com/t/evanescent-waves-and-faster-than-c/20898
**Category:** Factual Questions
**Created:** [May 30, 2000, 4:40pm UTC](https://boards.straightdope.com/t/evanescent-waves-and-faster-than-c/20898 "2000-05-30T16:40:22Z")
**Posts on this page:** 9
**Page:** 1
### Author:  [@JonF](https://boards.straightdope.com/u/JonF)
#### Post date: [May 30, 2000, 4:40pm UTC](https://boards.straightdope.com/t/evanescent-waves-and-faster-than-c/20898/1 "2000-05-30T16:40:22Z")
I hesitate to post this, because of the things we see in some other threads, but …
Any of the real physicists here care to comment on faster-than-c propagation of evanescent waves?
[New York Times article](http://partners.nytimes.com/library/national/science/053000sci-physics-light.html)
[_Nature_ : physics : Faster than light](http://helix.nature.com/nsu/000601/000601-5.html)
[An abstract from Physics Review Letters](http://ojps.aip.org/journal_cgi/getabs?KEY=PRLTAO&cvips=PRLTAO000084000021004830000001&gifs=Yes)
[Some comments on Slashdot](http://slashdot.org/comments.pl?sid=00/05/30/1223244&threshold=1&commentsort=0&mode=thread&pid=53#87)
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### Author:  [@Scylla](https://boards.straightdope.com/u/Scylla)
#### Post date: [May 30, 2000, 5:32pm UTC](https://boards.straightdope.com/t/evanescent-waves-and-faster-than-c/20898/2 "2000-05-30T17:32:04Z")
I’m no physicist, but as a garden variety idiot, I found the articles intriguing.
In the Cesium experiment, I’m not sure that anything is travelling faster than light.
The light pulse that enters the cesium chamber actually gets cancelled by the backlash of the light pulse that exits. It’s not the same pulse.
I don’t know if this is an important distinction, or not, since something clearly travelled faster than light.
As for the focused microwaves, I wish they would have explained what potential mechanism might have bee at work. All that I can come up with is a watermelon seed analogy. If they focus the beam tightly enough, microwaves may collide with each other at odd angles, and you get an effect like squeezing a watermelon seed between your fingers. Still, Einstein’s speed limit should still apply, shouldn’t it?
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### Author:  [@bibliophage](https://boards.straightdope.com/u/bibliophage)
#### Post date: [May 30, 2000, 6:17pm UTC](https://boards.straightdope.com/t/evanescent-waves-and-faster-than-c/20898/3 "2000-05-30T18:17:15Z")
I’m not up-to-date on this topic but there are a few things I remember:[list]1. In QED (Quandtum ElectroDynamics), it is possible for a small particle like an electron to travel faster than light over a very short distance. The heavier the particle is and the longer the distance, the less likely this is.
2. In ordinary space, the speed of light is slowed down by electromagnetic fields. In an environment that screens out these fields, light travels slightly (but measurably) faster.
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### Author:  [@Silo](https://boards.straightdope.com/u/Silo)
#### Post date: [May 30, 2000, 8:03pm UTC](https://boards.straightdope.com/t/evanescent-waves-and-faster-than-c/20898/4 "2000-05-30T20:03:29Z")
Well I looked in my QED book then came over to the computer to find that bibliophage beat me to it.
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### Author:  [@JonF](https://boards.straightdope.com/u/JonF)
#### Post date: [May 31, 2000, 2:38pm UTC](https://boards.straightdope.com/t/evanescent-waves-and-faster-than-c/20898/5 "2000-05-31T14:38:19Z")
> [@](#):
>
> The light pulse that enters the cesium chamber actually gets cancelled by the backlash of the light pulse that exits. It’s not the same pulse.
True, but that’s true for any light travelling through a material medium, and maybe even travelling through vacuum. All photons with the same energy and quantum states are indistinguishable.
> [@](#):
>
> In QED (Quandtum ElectroDynamics), it is possible for a small particle like an electron to travel faster than light over a very short distance. The heavier the particle is and the longer the distance, the less likely this is.
Also true, but I’m not aware of any situation in which the faster-than-light travel probability isn’t cancelled as soon as the distance gets to something comparable to atomic size.
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [May 31, 2000, 11:42pm UTC](https://boards.straightdope.com/t/evanescent-waves-and-faster-than-c/20898/6 "2000-05-31T23:42:24Z")
One more thing: Never base your science information on the _New York Times_, or any other general-readership periodical (except for Cecil’s column). You at least want a publication dedicated to science, like _Scientific American_, or _Science News_– They may be oversimplified at times, but they’re usually accurate. The popular press has a tendancy to latch onto whichever preliminary results they consider most sensational, and then don’t follow up. I’ve heard about the experiments with evanescent waves, and the results are very preliminary, and most physicists in the field seem to doubt them. Others will try to replicate the effect, and if they repeatedly fail, or show some specific reason that the first experiment should have gotten the wrong result, then the theory will be discarded. When/if this happens, though, you won’t see it in the _Times_– This is also why folks get the idea that certain practices are healthy/unhealthy, that there’s miracle cures for AIDS and cancer that the Government is hiding, etc.: A preliminary result looks promising, the popular press publishes it early, but the method turns out not to work.
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### Author:  [@Wood\_Thrush](https://boards.straightdope.com/u/Wood_Thrush)
#### Post date: [June 1, 2000, 1:05am UTC](https://boards.straightdope.com/t/evanescent-waves-and-faster-than-c/20898/7 "2000-06-01T01:05:19Z")
Almighty Cecil wrote a column discussing (albeit breifly) faster than light stuff [here](http://www.straightdope.com/columns/990430.html).
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### Author:  [@coffeecat](https://boards.straightdope.com/u/coffeecat)
#### Post date: [June 4, 2000, 8:48pm UTC](https://boards.straightdope.com/t/evanescent-waves-and-faster-than-c/20898/8 "2000-06-04T20:48:46Z")
Is this anything like phase velocities in a wave guide? Or am I totally off base?
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [June 4, 2000, 9:41pm UTC](https://boards.straightdope.com/t/evanescent-waves-and-faster-than-c/20898/9 "2000-06-04T21:41:05Z")
Quantum mechanical phase velocities are always greater than c (except for photons and other light-speed particles), but it’s impossible to transmit information at the phase velocity. Specifically, v[sub]p[/sub] = c[sup]2[/sup]/v[sub]g[/sub] , where v[sub]p[/sub] is the phase velocity, and v[sub]g[/sub] is the group velocity, the velocity at which you can transmit information.