Empirically Confirmed Superluminal Velocities?

wrote in message news: ...
I'll add here as a comment that the issue of group velocity is
generally misunderstood, perhaps due to the fact that lower level
textbooks don't explain it well. Group velocity *is not* signal
velocity. Under some circumstances, when the dependence of phase
velocity on frequency over the bandwidth of the signal is weak, group
velocity is a good approximation to signal velocity over distances
short enough so that the pulse shape does not change appreciably in
propagation. That's all. The conditions listed above are reasonably
well satisfied in most practical situations, but they totally fail
under anomalous dispersion situation.

Mati Meron | "When you argue with a fool,
| chances are he is doing just the same"

You're aware of the discussions on sci.physics.relativity that to
determine if a signal travelled superluminally what would be required
is a round-trip measurement. This is because of the uncertainty of
synchronizing clocks in two different locations. The standard SR
method of using light-signals requires the assumption of the constancy
of the one-way speed of light. [1]
I was interested to see that in some descriptions of the experiment
of Wang et.al. using lasers in cesium gas, that the light pulse was
said to exit the chamber before it entered:

"Can c, the speed limit of the universe, the speed of light in vacuum,
be exceeded? In July, 2000, the science-oriented news media were full
of reports that pulses of laser light had broken the speed-of-light
barrier. Physicists L. J. Wang, A. Kuzmich, and A. Dogarliu of the
NEC Research Institute in Princeton, NJ, had a paper about to be
published in the prestigious journal Nature describing an experiment
in which not only had laser pulses traveled faster than light, but had
actually emerged from the apparatus before they had entered it." [2]

It occurs to me that this is what would appear to happen if the
entrance and exit were synchronized using light signals but the actual
signal to be measured was traveling superluminally or if light signals
provided an inaccurate means of synchronizing clocks. What would be
needed to see if the signal pulse was being propagated superluminally
would be to have the pulse reflected back to the source and seeing if
the total travel time was less than the time for light to make the
round trip.

Another experiment might have made such round trip measurements.
These were experiments by Ranfagni et.al. of microwaves propagating in
a wave guide:

"In the new experiments, led by Anedio Ranfagni of the Italian
National Research Council in Firenze, the setup looks innocent enough:
The team sent microwaves (3.5 cm wavelength) through a narrow,
ring-shaped opening onto a large and nearby focusing mirror, which
collimated the waves into a beam propagating back from the mirror,
beyond and behind the source. They "modulated" the microwaves with
rectangular pulses (sharp "amplify" and "attenuate" commands, in rapid
succession) and detected the pulses at positions between 30 and 140 cm
from the source, along the beam axis. The slope of their plot of
arrival times vs. distance led to an apparent propagation speed of 5
to 7% above c, although beyond about 1 m, the speed approached c, all
of which agreed with previous predictions." [3]

The experimenters claim some speeds exceeding c in the reflected
waves but it is difficult to tell here if any of these speeds are for
a round trip signal back to the source.


Bob Clark



1.)Conventionality of Simultaneity
http://plato.stanford.edu/entries/sp...e-convensimul/

2.)Faster-than-Light Laser Pulses?
by John G. Cramer
http://www.npl.washington.edu/AV/altvw105.html

3.)Faster than a Speeding Light
http://focus.aps.org/story/v5/st23

In article , (Robert Clark) writes:
wrote in message news: ...
I'll add here as a comment that the issue of group velocity is
generally misunderstood, perhaps due to the fact that lower level
textbooks don't explain it well. Group velocity *is not* signal
velocity. Under some circumstances, when the dependence of phase
velocity on frequency over the bandwidth of the signal is weak, group
velocity is a good approximation to signal velocity over distances
short enough so that the pulse shape does not change appreciably in
propagation. That's all. The conditions listed above are reasonably
well satisfied in most practical situations, but they totally fail
under anomalous dispersion situation.

Mati Meron | "When you argue with a fool,
| chances are he is doing just the same"

You're aware of the discussions on sci.physics.relativity

I'm certainly not aware of any discussions on sci.physics.relativity,
nor do I have any interest in these.

that to
determine if a signal travelled superluminally what would be required
is a round-trip measurement. This is because of the uncertainty of
synchronizing clocks in two different locations. The standard SR
method of using light-signals requires the assumption of the constancy
of the one-way speed of light. [1]
I was interested to see that in some descriptions of the experiment
of Wang et.al. using lasers in cesium gas, that the light pulse was
said to exit the chamber before it entered:

"Can c, the speed limit of the universe, the speed of light in vacuum,
be exceeded? In July, 2000, the science-oriented news media were full
of reports that pulses of laser light had broken the speed-of-light
barrier. Physicists L. J. Wang, A. Kuzmich, and A. Dogarliu of the
NEC Research Institute in Princeton, NJ, had a paper about to be
published in the prestigious journal Nature describing an experiment
in which not only had laser pulses traveled faster than light, but had
actually emerged from the apparatus before they had entered it." [2]

It occurs to me that this is what would appear to happen if the
entrance and exit were synchronized using light signals but the actual
signal to be measured was traveling superluminally or if light signals
provided an inaccurate means of synchronizing clocks. What would be
needed to see if the signal pulse was being propagated superluminally
would be to have the pulse reflected back to the source and seeing if
the total travel time was less than the time for light to make the
round trip.

Another experiment might have made such round trip measurements.
These were experiments by Ranfagni et.al. of microwaves propagating in
a wave guide:

"In the new experiments, led by Anedio Ranfagni of the Italian
National Research Council in Firenze, the setup looks innocent enough:
The team sent microwaves (3.5 cm wavelength) through a narrow,
ring-shaped opening onto a large and nearby focusing mirror, which
collimated the waves into a beam propagating back from the mirror,
beyond and behind the source. They "modulated" the microwaves with
rectangular pulses (sharp "amplify" and "attenuate" commands, in rapid
succession) and detected the pulses at positions between 30 and 140 cm
from the source, along the beam axis. The slope of their plot of
arrival times vs. distance led to an apparent propagation speed of 5
to 7% above c, although beyond about 1 m, the speed approached c, all
of which agreed with previous predictions." [3]

The experimenters claim some speeds exceeding c in the reflected
waves but it is difficult to tell here if any of these speeds are for
a round trip signal back to the source.

Actually all the above experiments yield results fully consistent with
the existing theoretical predictions, based on classical
electrodynamics (even QM is not really required) and fully consisten
with relativity. You should read the scientific publications, not
writeups in popular media. There are all sorts of funny htings you
can do with pulses which change shape across small distnaces.

Mati Meron | "When you argue with a fool,
| chances are he is doing just the same"

Robert Clark:
wrote in message news: ...
I'll add here as a comment that the issue of group velocity is
generally misunderstood, perhaps due to the fact that lower level
textbooks don't explain it well. Group velocity *is not* signal
velocity. Under some circumstances, when the dependence of phase
velocity on frequency over the bandwidth of the signal is weak, group
velocity is a good approximation to signal velocity over distances
short enough so that the pulse shape does not change appreciably in
propagation. That's all. The conditions listed above are reasonably
well satisfied in most practical situations, but they totally fail
under anomalous dispersion situation.

Mati Meron | "When you argue with a fool,
| chances are he is doing just the same"

You're aware of the discussions on sci.physics.relativity that to
determine if a signal travelled superluminally what would be required
is a round-trip measurement.

That is not the case.

This is because of the uncertainty of synchronizing clocks in
two different locations.

It's completely unnecessary to synchronize anything.

(Bilge) wrote in message ...
Robert Clark:
wrote in message news: ...
I'll add here as a comment that the issue of group velocity is
generally misunderstood, perhaps due to the fact that lower level
textbooks don't explain it well. Group velocity *is not* signal
velocity. Under some circumstances, when the dependence of phase
velocity on frequency over the bandwidth of the signal is weak, group
velocity is a good approximation to signal velocity over distances
short enough so that the pulse shape does not change appreciably in
propagation. That's all. The conditions listed above are reasonably
well satisfied in most practical situations, but they totally fail
under anomalous dispersion situation.

Mati Meron | "When you argue with a fool,
| chances are he is doing just the same"

You're aware of the discussions on sci.physics.relativity that to
determine if a signal travelled superluminally what would be required
is a round-trip measurement.

That is not the case.

This is because of the uncertainty of synchronizing clocks in
two different locations.

It's completely unnecessary to synchronize anything.

It is well known among researchers in the foundations of relativity
the need to synchronize clocks at two locations for comparing times at
those locations.
See this post by Stephen Speicher:

From: Stephen Speicher )
Subject: Speed of light
Newsgroups: sci.physics.research, sci.physics.relativity
Date: 2003-06-24 20:06:03 PST
http://groups.google.com/groups?selm...st.localdomain


Bob Clark

Robert Clark:
(Bilge) wrote in message
e-al.net...
Robert Clark:
You're aware of the discussions on sci.physics.relativity that to
determine if a signal travelled superluminally what would be required
is a round-trip measurement.

That is not the case.

This is because of the uncertainty of synchronizing clocks in
two different locations.

It's completely unnecessary to synchronize anything.

It is well known among researchers in the foundations of relativity
the need to synchronize clocks at two locations for comparing times at
those locations.

Why is it necessary to compare times at two locations? That would
seem to be the hard way to determine whether or not a signal is
superluminal and it would be less accurate in making the determination.
It would seem to me that the simplest way to make this determination
is for the source to arrange that the pulse be split, with one part
of te pulse propagating in vacuum and the other through the apparatus.
You then compare the two pulses and see which leads which.

(Bilge) wrote in message ...
Robert Clark:
(Bilge) wrote in message
e-al.net...
Robert Clark:
You're aware of the discussions on sci.physics.relativity that to
determine if a signal travelled superluminally what would be required
is a round-trip measurement.

That is not the case.

This is because of the uncertainty of synchronizing clocks in
two different locations.

It's completely unnecessary to synchronize anything.

It is well known among researchers in the foundations of relativity
the need to synchronize clocks at two locations for comparing times at
those locations.

Why is it necessary to compare times at two locations? That would
seem to be the hard way to determine whether or not a signal is
superluminal and it would be less accurate in making the determination.
It would seem to me that the simplest way to make this determination
is for the source to arrange that the pulse be split, with one part
of te pulse propagating in vacuum and the other through the apparatus.
You then compare the two pulses and see which leads which.

Your suggestion may indeed work. What the experimenters appear to
have done in the Wang experiment was to note the time of departure at
the starting point and compare that to the time of the signals arrival
at the endpoint. To make that comparison you need some means of
synchronizing the starting and end points. The standard SR way is to
send a light signal (or some electromagnetic signal) from a point
midway between the two points both ways and the clocks will be started
when they each receive the light signal. This assumes that the light
signal will travel the same speed each way.
Suppose though that light speed is actually slower in the one
direction that the other. Then if there is a signal that travels at a
faster than light speed then it may *appear* to have reached the end
point before it left the start point.
The objection might be made that light speed has been measured and
has been found not to vary. But as discussed in the post by Speicher,
in reality what has been measured is the round trip light speed since
the starting and endpoints have to be synchronized and since this is
done by EM signals this results in a circularity when you deduce the
one way light speed by dividing the round trip travel time by 2.


Bob Clark

(Robert Clark) wrote in message . com...

[Bilge]
Why is it necessary to compare times at two locations? That would
seem to be the hard way to determine whether or not a signal is
superluminal and it would be less accurate in making the determination.
It would seem to me that the simplest way to make this determination
is for the source to arrange that the pulse be split, with one part
of te pulse propagating in vacuum and the other through the apparatus.
You then compare the two pulses and see which leads which.

Your suggestion may indeed work. What the experimenters appear to
have done in the Wang experiment was to note the time of departure at
the starting point and compare that to the time of the signals arrival
at the endpoint.

snip
Bob Clark

[EL]
Not true, and I have the Adobe file of Wang Superluminal thing and
here is the label under the fugure.

{{{
Fig. 2. Pulse propagation through a medium of a length L at a group
velocity
vg = c/(n + ν dn/dν). and through vacuum for the same
length.
}}}

As you can see, they planned the equation to force a negative quantity
for (ν dn/dν) such that it exceeds the fraction above unity
in the refractive index of caesium.

If you care to inspect carefully the physical meaning of (ν
dn/dν) you shall realise that relativistic issues are hidden
within this innocent classic form of simple calculus.

The resulting relative velocity assumes [c] to be the moving
observer's speed and then multiply the modulation slipping velocity by
it. So no wonder they came up with a velocity that is 315 times faster
than light.

The experimental setup is almost perfect if not literally perfect and
it is exactly as Bilge said.
However, they do not have any physical electronic devices that can
respond to times as small as less than 0.2 ns delays.
That is why they have to record complete traces of waves with equally
compensated delays.
The consequence of the mathematical manipulations is that relative
frequency shifts is taken as a direct consequence of relative velocity
with respect to that of the LASER beam in vacuum.
Naturally, the wave is dispersed due to the different way the medium
responds to different frequencies such that it is quite possible that
the fastest component's propagation speed could be 315 times as fast
as the slowest while all speeds stay below c.
If the slowest wave was assumed to be propagating at c then they must
conclude accordingly that they have achieved superluminal speeds, but
did they?

EL

(Robert Clark) wrote in message

. com...



[Bilge]
Why is it necessary to compare times at two locations? That would
seem to be the hard way to determine whether or not a signal is
superluminal and it would be less accurate in making the determination.
It would seem to me that the simplest way to make this determination
is for the source to arrange that the pulse be split, with one part
of te pulse propagating in vacuum and the other through the apparatus.
You then compare the two pulses and see which leads which.

[Clark]
Your suggestion may indeed work. What the experimenters appear to
have done in the Wang experiment was to note the time of departure at
the starting point and compare that to the time of the signals arrival
at the endpoint.

snip
Bob Clark

[EL]
Not true, and I have the Adobe file of Wang Superluminal thing and
here is the label under the fugure.

{{{
Fig. 2. Pulse propagation through a medium of a length L at a group
velocity
vg = c/(n + ν dn/dν). and through vacuum for the same
length.
}}}

As you can see, they planned the equation to force a negative quantity
for (ν dn/dν) such that it exceeds the fraction above unity
in the refractive index of caesium.

If you care to inspect carefully the physical meaning of (ν
dn/dν) you shall realise that relativistic issues are hidden
within this innocent classic form of simple calculus.

The resulting relative velocity assumes [c] to be the moving
observer's speed and then multiply the modulation slipping velocity by
it. So no wonder they came up with a velocity that is 315 times faster
than light.

The experimental setup is almost perfect if not literally perfect and
it is exactly as Bilge said.
However, they do not have any physical electronic devices that can
respond to times as small as less than 0.2 ns delays.
That is why they have to record complete traces of waves with equally
compensated delays.
The consequence of the mathematical manipulations is that relative
frequency shifts is taken as a direct consequence of relative velocity
with respect to that of the LASER beam in vacuum.
Naturally, the wave is dispersed due to the different way the medium
responds to different frequencies such that it is quite possible that
the fastest component's propagation speed could be 315 times as fast
as the slowest while all speeds stay below c.
If the slowest wave was assumed to be propagating at c then they must
conclude accordingly that they have achieved superluminal speeds, but
did they?

EL

(Robert Clark) wrote in message . com...


Your suggestion may indeed work. What the experimenters appear to
have done in the Wang experiment was to note the time of departure at
the starting point and compare that to the time of the signals arrival
at the endpoint. To make that comparison you need some means of
synchronizing the starting and end points. The standard SR way is to
send a light signal (or some electromagnetic signal) from a point
midway between the two points both ways and the clocks will be started
when they each receive the light signal. This assumes that the light
signal will travel the same speed each way.
Suppose though that light speed is actually slower in the one
direction that the other. Then if there is a signal that travels at a
faster than light speed then it may *appear* to have reached the end
point before it left the start point.
The objection might be made that light speed has been measured and
has been found not to vary. But as discussed in the post by Speicher,
in reality what has been measured is the round trip light speed since
the starting and endpoints have to be synchronized and since this is
done by EM signals this results in a circularity when you deduce the
one way light speed by dividing the round trip travel time by 2.


Bob Clark

[EL]
I was under the impression that in all dispersive media the phase
velocity was a function of the wavelength, which also means that the
refractive index is a function of frequency.

If my information was correct then that is why a dispersion prism can
separate a compound wave into its components by separating each
frequency along a different angle of refraction.

If that was true then dispersive prisms should have no wave packets or
a group velocity at all.
Why in theory, the mathematical formulism does not state that the
angle of incidence must be normal to the plane of the dispersive
medium surface to avoid separating the packet into its components?

Also if FTL experiments are not fallacious then why is there no mixing
of blue and violet to obtain ultraviolet ahead of the group, which
should be separable by a prism?

Indeed I know why, because those waves are ink on paper only and
fabricated hoopla that is founded on hypnotized audience clapping for
every clown under the light spot in the modern physics circus.

Kind regards.

EL

(Bilge) wrote in message ...

It's completely unnecessary to synchronize anything.

[EL]
Hi Bilge.

Why is it so difficult for people to imagine group velocities as wave
envelopes!

All decent empirical wave-group-velocity measurements show that they
are much lower than phase velocity in the same dispersive medium.

I wish to make an analogy to establish a law.

When many people walk in a corridor while their legs are not equal in
length and their steps are not synchronized the group velocity of the
crowd is obviously slower than the fastest person in that crowd.

When six sportsmen are rowing a boat the speed of that boat is the
same as the collective speed of that synchronized group.

The envelope of the rowing mechanics may not exceed the speed of the
boat and the boat tip-front must lead at all times and never be
exceeded.

EL