'Superluminal' Laser Questioned 10
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."
Old news? (Score:1)
BTW, third post.
mass-energy of information (Score:1)
#define X(x,y) x##y
Re:Nice animation (Score:2)
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.
Re:Nice animation (Score:2)
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?
Re:Old news? (Score:2)
>Hasn't this already been discussed? I seem to
>remember reading about some scientists supposedly
>speeding up light, then later saying that the
>media had completely misinterpreted the
>experiment and they didn't speed up light. Or am
>I remembering a completely different incident?
Yes, the experiment was discussed; no, this paper was not. Here is what happened: the original researchers claimed to have created a laser pulse with superluminal group velocity. Still, no claim was made that information was transferred faster than light. Some of the media misinterpreted, thinking information had been transferred and this conflicted with relativity. THe original researchers made no claim that their results conflicted with relativity. The first Slashdot article came at this point, and there was a lot of discussion about how the media misinterpreted the findings.
The new paper posted to arXiv.org by the researchers from the Plasma Physics lab, however, disputes the claim made by the researchers that the pulse had superluminal group velocity. It is not disputing any media interpretation, but the actual paper in Nature that first presented the results.
So, in short, it may be that both the original research and the media interpretations were incorrect.
Re:This is almost 100 year old news! (Score:2)
>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.
This is almost 100 year old news! (Score:2)
Re:This is almost 100 year old news! (Score:2)
I read through the paper and would have to agree with you. The paper seems correct after a quick reading and all of there approximations are fairly standard (e.g. I would have to agree that the shouldn't be need for any thing above gain-velocity dispersion) Also, they said they did do a numerical simulation and got result similar to seen in the experiment. But I could just be biased because I do computational physics:-)
More interesting is that there were experiment earlier showing superluminal propagation of information on microwaves I haven't seen the original paper (nor do I have links) but supposedly they were able to hear music at 1.2c. The basic affect was caused by the microwaves tunneling though a barrier.
Re:Nice animation (Score:2)
Of course, I'm in the process of high school physics right now, so I'm not an expert.
Nice animation (Score:3)