wrote in message ...

[Mati Meron]
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.


[EL]
You sound like you do know what is a group velocity so why do you also
sound as if you are endorsing the superluminal lie?

I am not going to write the equations or derivations but to give you a
hinting example:

v_g = v_p - [Lambda (div-v_p/div-Lambda)]

also

v_g = dw/dk

Without going into meticulous details, a pure sign wave is not a
natural phenomenon but we usually have interference or a packet of
interacting waves and this is not a light in vacuum or gases exclusive
thing.

For example in deep water v_p = c.k^(-1/2), where c is a constant.
Thus w= c.k^(1/2) and v_g =dw/dk = 0.5 v_p.
These waves are strongly dispersive, and the individual wave crests
slide through the group at twice the group speed.

Those of you who still do not get it and find it hard to imagine why
it is a very funny lie or simply ignorance from those who claim
superluminal velocities, please recall Lord Kelvin's smoke rings.

Air is a dispersive medium and smoke is a visual indicator of the flow
of a wave.

The air compression along the axis of symmetry of the wave is at a
maximum and the velocity too is at its maximum along that axis.
However, the ring demonstrates velocities reduction as we step away
from that axis until they turn negative, which means that smoke
particles move backwards relative to the (Ring) group even though the
whole group is advancing forward, and this particular phenomenon is
observed on the outer shell topology of that smoke ring group.

The contrary may be observed on the smoke particles at the core of the
ring, where those particles would be moving at twice the group
velocity being accelerated on there way in and decelerated on there
way out at the group-wave-front.

Can any of you claim that a smoke ring may move in air faster than
sound can move in the same medium?

No, certainly not, because you are all wise and sane.


LASSER light pumping is neither better nor worse.

Caesium atoms undergo similar dispersion anomalies and a magnetic
field of 1Gauss is maintained parallel to the direction of light
packet propagation to define an axis.

Obviously, caesium atoms are huge particles that attain velocities far
slower than c.

While light propagates at c exactly along the empty axis that caesium
atoms do not enter due to preparing them by preparatory light pulses,
the group velocity is expected to be at half c but very roughly.

If the atoms absorb all the energy or the medium become saturated we
have no anomalies.

The anomaly is a replication of Lord Kelvin's smoke ring in a wave of
air gist but this time using a caesium ring-group hoping to accelerate
LASSER light packets at the core axis of the wave group.

So yes, indeed the light speed at the core axis is at a maximum and it
is at c but the group velocity is much lower.

The terrible mistake is to assume that the light packet is a group
that is propagating at c and then one discovers that relative to that
velocity a much faster speed is detected.

The equations are supposed to find the group velocity not to assume
it.

The group velocity by definition must be a fraction of the maximum
velocity that the medium allows.

Vacuum allows a maximum of c.

Caesium gas at 30 degrees Celsius should slow down light to a maximum
less than c.

The new refractive index may not be a fraction less than unity or it
would be a calculation mistake.

So what they really have is a set of data that they fail to interpret
properly and an oscilloscope that they have no freaking idea about
interpreting its screen or recorded tapes.

Those clowns need to go back to school and they disgrace their
credentials and smear our credentials too by being so irresponsible
and reckless.



EL

In article , (EL) writes:
wrote in message ...

[Mati Meron]
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.


[EL]
You sound like you do know what is a group velocity so why do you also
sound as if you are endorsing the superluminal lie?

Frankly, I've no idea what you're talking about and, since you're a
nice guy, I won't even check. If you've any issues with group
velocity, feel free to consult Jackson's "Electrodynamics", chapt. 7.

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

"Richard" wrote in message ...


Dirk Van de moortel wrote:

"Richard" wrote in message ...


[snip]


Where in the lorentz transform is it implicit that v corresponds only to
tangible particles? You are only repeating what you've heard over and
over. This was an outright lie imposed in order to save SR from very
observable superluminal speeds. It was a rationalization, it was a
fabrication, it was pulled out of their asses. Tell me, If a spot
(generated by a rotating laser) is moving at 2c along a fence wrt me,
then what speed will that spot be moving wrt an observer moving at x m/s
wrt the fence, as measured by that observer?

Primed frame moving with velocity vc w.r.t. unprimed frame.

Spot_event 1: (x',t') = ( 0, 0 )
Spot_event 2: (x',t') = ( wt', t' )
velocity of spot = wt' / t' = w
for instance 2c

Spot_event 1: (x,t) = ( 0, 0 )
Spot_event 2: (x,t) = ( g(wt'+vt'), g(t'+vwt'/c^2 )
velocity of spot = (w+v)/(1+vw/c^2)

w c == (w+v)/(1+vw/c^2) c
w = c == (w+v)/(1+vw/c^2) = c
w c == (w+v)/(1+vw/c^2) c

Subluminal in one frame == subluminal in all frames
Luminal in one frame == Luminal in all frames
Superluminal in one frame == superluminal in all frames

Dirk Vdm

Which I have already stated Dirk.

No. You did not state. You just *asked*:
| "Tell me, If a spot (generated by a rotating laser) is moving
| at 2c along a fence wrt me, then what speed will that spot
| be moving wrt an observer moving at x m/s wrt the fence,
| as measured by that observer?"

You asked so you got an answer.

Dirk Vdm

wrote in message ...
In article , (EL) writes:
wrote in message ...

[Mati Meron]
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.


[EL]
You sound like you do know what is a group velocity so why do you also
sound as if you are endorsing the superluminal lie?

Frankly, I've no idea what you're talking about and, since you're a
nice guy, I won't even check. If you've any issues with group
velocity, feel free to consult Jackson's "Electrodynamics", chapt. 7.

[EL]
Frankly, you still sound like you do know what is a group velocity so
why do you also sound as if you are endorsing the superluminal lie?

Let us discuss Jackson's "Electrodynamics", chapt. 7 another time.
First we need to know how a compound wave is compounded from a group
of waves.
The topology is critical to the interpretation.

Here is a reminder of what was snipped.

I am not going to write the equations or derivations but to give you a
hinting example:

v_g = v_p - [Lambda (div-v_p/div-Lambda)]

also

v_g = dw/dk

Without going into meticulous details, a pure sign wave is not a
natural phenomenon but we usually have interference or a packet of
interacting waves and this is not a light in vacuum or gases exclusive
thing.

For example in deep water v_p = c.k^(-1/2), where c is a constant.
Thus w= c.k^(1/2) and v_g =dw/dk = 0.5 v_p.
These waves are strongly dispersive, and the individual wave crests
slide through the group at twice the group speed.

Those of you who still do not get it and find it hard to imagine why
it is a very funny lie or simply ignorance from those who claim
superluminal velocities, please recall Lord Kelvin's smoke rings.

Air is a dispersive medium and smoke is a visual indicator of the flow
of a wave.

The air compression along the axis of symmetry of the wave is at a
maximum and the velocity too is at its maximum along that axis.
However, the ring demonstrates velocities reduction as we step away
from that axis until they turn negative, which means that smoke
particles move backwards relative to the (Ring) group even though the
whole group is advancing forward, and this particular phenomenon is
observed on the outer shell topology of that smoke ring group.

The contrary may be observed on the smoke particles at the core of the
ring, where those particles would be moving at twice the group
velocity being accelerated on there way in and decelerated on there
way out at the group-wave-front.

Can any of you claim that a smoke ring may move in air faster than
sound can move in the same medium?

No, certainly not, because you are all wise and sane.


LASSER light pumping is neither better nor worse.

Caesium atoms undergo similar dispersion anomalies and a magnetic
field of 1Gauss is maintained parallel to the direction of light
packet propagation to define an axis.

Obviously, caesium atoms are huge particles that attain velocities far
slower than c.

While light propagates at c exactly along the empty axis that caesium
atoms do not enter due to preparing them by preparatory light pulses,
the group velocity is expected to be at half c but very roughly.

If the atoms absorb all the energy or the medium become saturated we
have no anomalies.

The anomaly is a replication of Lord Kelvin's smoke ring in a wave of
air gist but this time using a caesium ring-group hoping to accelerate
LASSER light packets at the core axis of the wave group.

So yes, indeed the light speed at the core axis is at a maximum and it
is at c but the group velocity is much lower.

The terrible mistake is to assume that the light packet is a group
that is propagating at c and then one discovers that relative to that
velocity a much faster speed is detected.

The equations are supposed to find the group velocity not to assume
it.

The group velocity by definition must be a fraction of the maximum
velocity that the medium allows.

Vacuum allows a maximum of c.

Caesium gas at 30 degrees Celsius should slow down light to a maximum
less than c.

The new refractive index may not be a fraction less than unity or it
would be a calculation mistake.

So what they really have is a set of data that they fail to interpret
properly and an oscilloscope that they have no freaking idea about
interpreting its screen or recorded tapes.

Those clowns need to go back to school and they disgrace their
credentials and smear our credentials too by being so irresponsible
and reckless.



EL

(Starblade Darksquall) wrote in message . com...
"Robert J. Kolker" wrote in message ...
Starblade Darksquall wrote:
I know what goes faster than the speed of light.

The stars really really far from me all go faster than the speed of
light when I spin around in circles because my spinning reference
frame is just as good as anybody else's.

You are not talking about the motion of the stars. You are talking about
changing coordinate systems.

Bob Kolker

Motion is relative. They are not moving in their own reference frame,
but they are in mine.


Wrong. However, possibly the majority of everthing in the universe
*is* moving away from us at superluminal velocities.

Octafish wrote:


Wrong. However, possibly the majority of everthing in the universe
*is* moving away from us at superluminal velocities.

Wrong. You are making the Galilean error of assuming that velocities add.

Bob Kolker

"Robert J. Kolker" wrote:

Octafish wrote:


Wrong. However, possibly the majority of everthing in the universe
*is* moving away from us at superluminal velocities.

Wrong. You are making the Galilean error of assuming that velocities add.

You are making the special relativistic error (i.e., the universe is not
modeled by special relativity) that there is a unique definition of
velocity. Some (reasonable) definitions of cosmological velocity give
superluminal speeds. This doesn't violate the spirit of relativity since
these superluminal speeds cannot be used for information transfer.

Another possible stance is that there is no reasonable general
definition of distance and speed in general relativity, and therefore
this whole discussion is meaningless.

Regards,
George

Richard,

[...]
No, it hasn´t. SR forbids *transport of energy-momentum at
superluminal speeds*, and in the case of anomalous dispersion, as in
this experiment, energy-momentum is not transported with the group
velocity of the wave package. So superluminal group velocities don´t
violate SR.

Where in the lorentz transform is it implicit that v corresponds only to
tangible particles?

Where did I say this?

You are only repeating what you've heard over and
over. This was an outright lie imposed in order to save SR from very
observable superluminal speeds.

You will have to prove this.

It was a rationalization, it was a
fabrication, it was pulled out of their asses.

You should avoid such vocabulary if you want further discussion.

Tell me, If a spot
(generated by a rotating laser) is moving at 2c along a fence wrt me,
then what speed will that spot be moving wrt an observer moving at x m/s
wrt the fence, as measured by that observer?

Tell me why do you think that your light spot is transporting
energy-momentum along the fence at superluminal speed. Energy-momentum
in this example is only transported from the light source to the fence
with speed c.

regards,
Jürgen

EL,

...chicken poop.
...****...chicken
poop

Is this your usual way of discussing? You should not only learn some
physics, but also some manners.

regards,
Jürgen

"J?rgen Clade" wrote in message om...
EL,

...chicken poop.
...****...chicken
poop

Is this your usual way of discussing? You should not only learn some
physics, but also some manners.

Impossible.
http://users.pandora.be/vdmoortel/di...rsenLogic.html
Look at the "interesting follow-ups" at the bottom ;-)
Enjoy...

Dirk Vdm