Slashdot Mirror
'Superluminal' Laser Questioned
mreece writes: "Recently on Slashdot there was a discussion about researchers who claimed to have observed superluminal propagation of a laser pulse. Three researchers from the Naval Research Laboratory, Plasma Physics Division, in Washington D.C., claim that this is not a valid interpretation, and that the propagation was not superluminal. The preprint is available in PDF format from http://arxiv.org/abs/physics/0010033. IANAP (yet), but it looks valid."
BTW, third post.
IIRC, information does indeed have some energy, and therefore effective mass. This isn't the point, though. Relativity prohibits any reaction to an action occuring farther away from the action than light would communicate the information.
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Peter Cordes ; e-mail: X(peter@cordes ,
What is the basis for the concern about information travelling faster than light?
Because one of the things that Relativity claims is relative is the idea of simultaneity. "Travels faster than light" in one reference frame always equates to "travels backwards in time" in another reference frame.
If you can send information faster than light in any reference frame (and remember, they're supposed to be indistinguishable), then you can send information back to its starting point at an earlier time than it is sent, and then causality goes right out the window.
> Even though the peak of the light pulse travels faster than c, no information actually travels faster than c.
What is the basis for the concern about information travelling faster than light? I thought the problem with FTL was acceleration to or beyond light speed, because of the relativistic increase in mass. Does information have intrinsic mass? Or is there some other problem that should prevent FTL transfer of information?
Sheesh, evil *and* a jerk. -- Jade
hubie wrote:
>The effect of apparent superluminal speeds, which
>has to do with propagation of waves through a
>medium with a large changing index of
>refraction, was well known for almost a century.
I'm aware of this, but this article shouldn't be dismissed as "old news." The researchers who published the paper in Nature recently claimed to have not only had apparent superluminal speeds, but to actually have a negative group velocity, and apparently there was some speculation that this process could actually have applications. The paper that was submitted to arXiv.org recently argues that their group velocity calculation was incorrect and the pulse did not have a superluminal group velocity.
So I don't think this article should be dismissed. I was hoping someone with more physics knowledge than I have could read through the paper; I read it and it seems correct but I don't have enough knowledge to be entirely sure their approximations are valid. The basic question is not whether it is possible to have apparent superluminal speeds - it certainly is - but whether the researchers actually achieved this.
Matt Reece
The effect of apparent superluminal speeds, which has to do with propagation of waves through a medium with a large changing index of refraction, was well known for almost a century. Arnold Sommerfeld described the mathematics for it around 1917 as well as a nice discussion on the speed of information propagation (which was a hot topic at the time). A reprint of this paper is nicely given in Leon Brillouin's book Wave Propagation and Group Velocity, Academic Press, 1960.
As far as I can tell, it seems to me that it appears that a wave is travelling faster than c, where this is not the case. In reality, each wave is travelling at something less than c, and when they happen be in phase the sum of the waves has a pulse which propogates faster than light.
Of course, I'm in the process of high school physics right now, so I'm not an expert.
Even though the peak of the light pulse travels faster than c, no information actually travels faster than c. This animation nicely demonstrates the difference: http://www.netspace.net. au/ ~gregegan/APPLETS/20/20.html