# Re: poly: faster than light in a super-vacuum

From: <hal@finney.org>
Date: Tue Jul 11 2000 - 21:00:29 PDT

Wil McCarthy writes:
>the FTL experiments of Gunter Nimtz and Lijun Wang?

Damien responds:
> I understood that these are group velocity effects. My sense of this is
> that if you squeeze a horse through a narrow gate and infer from the
> arrival of the tip of its nose that it's on its way, and even that it's a
> horse, this knowledge can't be used either to accelerate or abrogate the
> arrival of its tail.

WIL again:
>
> Well, yes and no. The tip of the nose itself isn't detectable, although
> it's generally not necessary to receive the tail for detection either. If
> the horse's mass-energy is front-loaded, just the head may be enough to
> trip your detector. And if the only possible messages are "one horse" and
> "zero horses," then the arrival of the head does indeed carry a 1-bit
> superluminal message. And a modulated stream of horses can carry, for
> example, a Mozart concerto at 4c, which is exactly what Gunter Nimtz
> demonstrated.

As I understood these experiments, a pulse was sent into a medium which
was "primed" and ready to be stimulated into emission. The leading edge
of the pulse is enough to stimulate the medium into firing. The result
is that the peak of the pulse moves forward in the wave packet. If you
consider the location of the wave packet to be the location of the peak,
then yes, the peak travels faster than light.

However this is an artifact of the change in shape of the pulse. In fact
all you have done is to amplify the leading edge. Schematically, the
pulse changes from the first shape below to the second (pulses travelling
to the left):

++
+ +
+ +
+ +
+ +
+ +
+ +
++ ++
+++ +++
++++++ ++++++
++++++++++ +++++++++++
+++++++ ++++++++

+++++++
++ ++++++
+ ++++
+ +++
+ ++
+ ++
+ ++
+ +++
+ ++++
+ +++++
+ ++++++
++ +++++++
+++++++ +++++

As soon as the leading edge of the pulse reaches a critical intensity
level, the medium fires, causing a rapid increase of intensity at the
front of the pulse. The pulse then decays as the energy in the medium
is exhausted.

It is the shift forward of the peak, in a pulse which is otherwise moving
at the speed of light, which gives the illusion of FTL transmission.
Actually all that is happening is that the medium has a lower threshold
of sensitivity than the detection apparatus. If the detection apparatus
were set to trigger at the leading edge of the pulse, no illusion of
FTL would be generated.

Hal
Received on Tue Jul 11 21:03:02 2000

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