"Faster than light" results published today

Marcus, Matthew (Matthew.Marcus@No-Spam-tencor.com)
Fri, 21 Jul 1999 00:27:40 -0700

I looked at the paper, too. The novel thing is that the group velocity is
negative, which is rather wierd. However, if you look at the before and
after pulses, you find that the advancement of the pulse is much less than
its length, and that the pulse has smooth edges. I suspect that if you go
through the math, you'll find that this continues to hold true even if the
medium is very long. The result is that the time it takes to set up the
pulse is
greater than that which you 'save' by doing your signaling through this
strange medium. If you go through the math of anomalous dispersion, you
find that causality is one of the necessary conditions. There has been lots
of theoretical work on this, and the result is that if you stick any kind
of pulse into any kind of medium, absolutely nothing comes out the other
side in a time less than L/c after the first part of the input pulse hit the
medium. With gently-tapered pulses like the ones used in this experiment,
there's no real "first part". If you tried to sharpen the pulse, you'd
introduce frequency components which were outside the wierd band where the
action happens, so the sharp bits would still take L/c to get there.
The same holds for all those other attempts involving dispersive
transmission lines, tunnel barriers, etc.

The last paragraph of the paper is instructive. The authors discuss the
causality question and come to the conclusion that they haven't invented
subspace radio (my phrase).
mam

> ----------
> From: Leonard Tramiel[SMTP:leonard@No-Spam]
> Reply To: sf-bay-tac@No-Spam
> Sent: Friday, July 21, 1999 12:16 AM
> To: sf-bay-tac@No-Spam
> Subject: Re: "Faster than light" results published today
>
> A "wee bit technical" is quite an understatement. This result is a
> consequence of the characteristics of group velocities in materials with
> an
> unusual index of refraction. The experiment is very clever, looks like it
> was carefully done and included safeguards against experimental error. I
> have little doubt that it can (and shortly will) be replicated.
>
> There are other cases where strange dispersion effects cause velocities
> that
> exceed c. The one I have seen most often is used in X-ray mirrors. The
> index
> of refraction is less than 1, That means that the speed of light in the
> material is greater than c. This results in total EXTERNAL reflection as
> opposed to the total INTERNAL reflection we get in prism diagonals. When I
> began studying X-ray astronomy this was quite a shock. A careful analysis
> showed everything still made sense but I was never able to find an
> explanation that wasn't really hard to understand.
>
> The most interesting thing about this paper (to me at least) is that the
> result is not in violation of any well established theory. The apparent
> violations are the result of the oversimplifications that is an
> inescapable
> consequence of physics without the math.
>
> -Leonard
>
>
>
>

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