[EL]
Thank you dear Ulrich, I know myself that I am an old ranting fart,
but I still appreciate your testimony.


ueb wrote in message ...
J\"urgen Clade wrote:
EL,

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

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

Don't let irritate, if this man leaves some farts. He is only a chemist,
but he sees more than lots of paid physicists. See also
http://home.t-online.de/home/Ulrich....z/article2.txt
(Reading that, you may better understand the subject of this thread.)

Regards
Ulrich Bruchholz

Randy Poe wrote in message . ..
[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.

Except in negatively-dispersive media, where both theory and
experiment show the opposite.

- Randy

[EL]
Bring back the grudge and let us grind.
What is the merit in being a beautifully coloured parrot?
The merit is to admire the colourful feathers (mostly in a mirror).
However, a parrot is a sound mimicking bird with a bird's brain.

Dispersion is another word for scattering.
Scattering if you did not know is like what happened to the Jews all
over history.
Scattering is like holding a handful of seeds and then tossing them
for random distribution during planting processes.
A dispersive medium is a medium as media are defined in being either
homogenous or not and isotropic or not.
The quality of a medium indicates if it was able to induce scattering
or not.

Now try to imagine pinball the game and look for Pachinko.
The many steal marbles are supposed to be scattered randomly by design
to induce a factor of luck.

The dispersion of light and this means its scattering among air
molecules or any transparent or semitransparent medium should make you
understand that the waves are being physically scattered by deflection
and reflection on the particles of that medium.

Of course by now you should have realised that negative-dispersion is
an expression coined by an idiot.

The phenomenon being the heart of this debate is definition dependant.
In optics dispersion is also defined as the Rate of Change of the
Refractive Index over wavelength scale at a specific wavelength.

Therefore, that definition implies that a wavelength scale must be
constructed by arbitrating a periodical interval indicating wavelength
increments against which we plot the refractive index to extract a
rate of change of that RI with respect to the change in wavelength
about the wavelength in question to illustrate a dispersion figure.

If my sentence was too complex for you to understand, here it is again
in different words.

We have a specific frequency of a wave of light.
We put a point on a graph's x-axis to represent that frequency and we
extend our scale to the left and to the right.
The graph's y-axis then represents the refractive index.
This means that in that specific material the refractive index is
frequency dependant.
By plotting all the different refractive index values we illustrate
the scattering of the refractive index about (before and after) the
frequency at question.

What does the idiotic negative-dispersion supposed to mean!

Pick up any respectable reference that tabulates the refractive index
of materials and try to find any negative value.
The overwhelming majority of indexes have a value between 1 and 2 and
they are POSITIVE VALUES.

Now the rate of change of positive values over positive intervals is
quite unlikely to make sense being negative.
A faster rate and a slower rate are simply seen in the aggregation/
dispersion of the plotted points of the refractive index against the
wavelengths.

So please educate yourself before defending idiots because it only
makes an idiot out of you too.

EL

On 25 Oct 2003 14:01:38 -0700, (EL) wrote:

Randy Poe wrote in message . ..
[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.

Except in negatively-dispersive media, where both theory and
experiment show the opposite.

[EL]
Bring back the grudge and let us grind.
What is the merit in being a beautifully coloured parrot?
The merit is to admire the colourful feathers (mostly in a mirror).
However, a parrot is a sound mimicking bird with a bird's brain.

Nice poetry, if unfathomable. Shall we discuss physics?


Dispersion is another word for scattering.

Incorrect.

Dispersion in wave physics means wavelength-dependence of the speed of
propagation. A medium can be dispersive but, in theory, lossless.
Scattering is inherently lossy. It refers to energy being sent in many
other directions away from the line of propagation.

Normal (positive) dispersion does have the tendency to spread signals
out in time, but this is a different phenomenon than dispersion.

Scattering if you did not know is like what happened to the Jews all
over history.
Scattering is like holding a handful of seeds and then tossing them
for random distribution during planting processes.

Again, nice poetry, but not very closely related to either dispersion
or scattering as the terms are used in physics.

A dispersive medium is a medium as media are defined in being either
homogenous or not and isotropic or not.

A dispersive medium is one in which speed depends on frequency. As we
all know from prisms, glass is a dispersive medium. Most real media
area, though the slope depends on the details of the interaction
between the energy and the molecules of the medium. Most of the time,
the slope of a speed vs. wavelength curve is positive, i.e., the speed
increases with wavelength, or decreases with frequency. Low frequency
waves propagate faster. You can see this in water with storm systems,
with low frequency components traveling reaching shore far ahead of
the rest of the system.

But there are materials which exhibit an opposite slope over some
wavelength regimes, with higher frequencies having higher speeds. This
is not magic, but it leads to a different effect on the shape of a
multiple-frequency pulse as it propagates.

What does the idiotic negative-dispersion supposed to mean!

Indicated above.


Pick up any respectable reference that tabulates the refractive index
of materials and try to find any negative value.
The overwhelming majority of indexes have a value between 1 and 2 and
they are POSITIVE VALUES.

Dispersion refers to slope. How low frequencies move compared to high
frequencies is what leads to the effect.


Now the rate of change of positive values over positive intervals is
quite unlikely to make sense being negative.

Huh? You're saying it doesn't make sense for a positive value to
decrease? Why the hell not?

http://en.wikipedia.org/wiki/Dispersion_(optics)
http://www.corning.com/opticalfiber/...ture_page2.asp
http://adsabs.harvard.edu/cgi-bin/np...ptCo.179..107W

- Randy

Randy Poe wrote in message . ..

Huh? You're saying it doesn't make sense for a positive value to
decrease? Why the hell not?

[EL]
Because less positive and more positive is still positive and I am
talking about refractive indexes.
The refractive index may increase or decrease away from the tested
frequency (wavelength) about which dispersion is being measured but in
all those cases dispersion is positive, absolute or simply unsigned.
What you are talking about is not negative dispersion at all but we
may describe it better as the rate of dispersion.
Since dispersion itself is the rate of change in the refractive index
then what you are talking about is the rate of the rate of change,
which is irrelevant to group velocity.



http://en.wikipedia.org/wiki/Dispersion_(optics)

[EL
Just as an example to demonstrate how silly you can be Randy I copied
the contents of that link and here it is.
{{{
Dispersion (optics
From Wikipedia, the free encyclopedia.
Find out how you can help support Wikipedia's phenomenal growth.

(There is currently no text in this page)
}}}

So you are referring me to a page that has no text in it which means
that you did not even read the content of the links you supplied.

Stop fabricating responses and get serious please.
I do not even know why you are being so enthusiastic defending those
idiots while you are much better a parrot than that.

Please invest your precious time in mathematics where you know better.

Regards.

EL

"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.

Besides, as long as two clocks are synchronized from the frame in which the
experiment is being carried out, the measurement they give for the speed of
light should be C.

On 25 Oct 2003 23:07:46 -0700, (EL) wrote:

Randy Poe wrote in message . ..

Huh? You're saying it doesn't make sense for a positive value to
decrease? Why the hell not?

[EL]
Because less positive and more positive is still positive and I am
talking about refractive indexes.

Yes. But why does that mean a positive function can't decrease?

Consider exponential decay: a function which is positive everywhere,
but always decreasing.

The refractive index may increase or decrease away from the tested
frequency (wavelength) about which dispersion is being measured but in
all those cases dispersion is positive, absolute or simply unsigned.

What? Dispersion is the slope of the curve. If the slope is negative,
it's not "positive, absolute, or unsigned", it's negative.

What you are talking about is not negative dispersion at all but we
may describe it better as the rate of dispersion.

Instead of making up definitions, why don't you do a search for
"negative dispersion" and read a definition? Speed is positive. Index
of refraction is generally greater than 1. We can all agree on that.
Now imagine a curve which is always above 1. Can you picture such a
curve sometimes increasing and sometimes decreasing? Where it
decreases, the grownups say "this has a negative slope".

As the name for this slope is "dispersion", then where the slope is
negative we call that "negative dispersion". It has nothing to do with
negative speeds, with speeds greater than c, or with any other
crackpot strawman you seem to have latched onto.

Since dispersion itself is the rate of change in the refractive index

Very good. The slope of the curve vs. frequency.

then what you are talking about is the rate of the rate of change,
which is irrelevant to group velocity.

No, what I'm talking about is the rate of change. In a negatively
dispersive regime, shorter waves move faster than longer waves. They
have a higher speed. A lower index of refraction.

Draw a line representing index of refraction vs. frequency. Draw the
line so it slopes downward. Can we agree we're talking about the slope
of the index of refraction curve? I hope so. Can we agree that a
downward slope on this plot means higher frequencies have a lower
index, i.e. a higher speed? I hope so.

Negative dispersion: higher frequencies have higher speed. Is that
really so difficult?

And you're incorrect, the dn/d(lambda) value has everything to do with
group velocity.

- Randy

In article ,
I am really sorry to have offended you, and I never meant to offend
you in retaliation to being offended by your post.
Most probably you did not intentionally mean to offend me except that
there is nothing you can do about yourself being offensive.

Try posting science and I shall show you elite manners.

Post the same chicken **** you posted and I shall identify it on the
spot.

EL


bwaahaha. this is excellent. I can't tell from the post if EL is the
kook or if EL is responding to the kook, but either way, excellent
rant.

--- edt

Richard wrote in message ...


[snip]


Here's something interesting to play with:

Suppose an observer is moving at 0.5c wrt lab frame. A superluminal beam
spot is moving wrt him at 2c.

Plugging these into the SR velocity equation:

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

gives

W ~ 2.5c


Go back to 4th grade and study arithmetic.

Paul Cardinale

Randy Poe wrote in message . ..
On 25 Oct 2003 23:07:46 -0700, (EL) wrote:

Randy Poe wrote in message . ..

Huh? You're saying it doesn't make sense for a positive value to
decrease? Why the hell not?

[EL]
Because less positive and more positive is still positive and I am
talking about refractive indexes.

Yes. But why does that mean a positive function can't decrease?

Consider exponential decay: a function which is positive everywhere,
but always decreasing.

The refractive index may increase or decrease away from the tested
frequency (wavelength) about which dispersion is being measured but in
all those cases dispersion is positive, absolute or simply unsigned.

What? Dispersion is the slope of the curve. If the slope is negative,
it's not "positive, absolute, or unsigned", it's negative.

What you are talking about is not negative dispersion at all but we
may describe it better as the rate of dispersion.

Instead of making up definitions, why don't you do a search for
"negative dispersion" and read a definition? Speed is positive. Index
of refraction is generally greater than 1. We can all agree on that.
Now imagine a curve which is always above 1. Can you picture such a
curve sometimes increasing and sometimes decreasing? Where it
decreases, the grownups say "this has a negative slope".

As the name for this slope is "dispersion", then where the slope is
negative we call that "negative dispersion". It has nothing to do with
negative speeds, with speeds greater than c, or with any other
crackpot strawman you seem to have latched onto.

Since dispersion itself is the rate of change in the refractive index

Very good. The slope of the curve vs. frequency.

then what you are talking about is the rate of the rate of change,
which is irrelevant to group velocity.

No, what I'm talking about is the rate of change. In a negatively
dispersive regime, shorter waves move faster than longer waves. They
have a higher speed. A lower index of refraction.

Draw a line representing index of refraction vs. frequency. Draw the
line so it slopes downward. Can we agree we're talking about the slope
of the index of refraction curve? I hope so. Can we agree that a
downward slope on this plot means higher frequencies have a lower
index, i.e. a higher speed? I hope so.

Negative dispersion: higher frequencies have higher speed. Is that
really so difficult?

And you're incorrect, the dn/d(lambda) value has everything to do with
group velocity.

- Randy

[EL]
I think that you are only trying to make me angry by telling lies and
putting words in my mouth.
I know very well that dn/d(lambda) is the rate of change in the
refractive index with respect to the change in frequency.
I did say up here and there that that is what dispersion is.
This is analogous to speed being a rate of change in distance with
respect to change in time (mathematically speaking) and we know that
such a scalar may be represented as a slope and that speed may
increase or decrease while stile being a positive mathematical entity.

To decrease speed you need NEGATIVE acceleration (deceleration), which
is the rate of change in the rate of change in distance with respect
to time.

The same applies here and that is precisely what I said.
We do not care about the rate of dispersion here because dispersion
itself is the rate we need for group velocity not group acceleration.

A negative velocity is an absolute-quantity-velocity in the opposite
direction of a positive velocity reference direction.
However, the speed scalar is an absolute quantity that cannot be less
than zero.

That is why I insist that the terminology of {negative-dispersion} is
Oxymoronic because an increasing index with an increasing frequency or
a decreasing index with an increasing frequency does not imply that
rate of change itself is negative but the relation between the
direction of frequency magnitudes and the index magnitudes are zusamen
or entgegen.

You certainly do not call counter clockwise to be a negative circle.

However, I noticed now that delta-refractive-index could be
mathematically negative when we subtract a higher index from a lower
index. So your terminology is purely mathematical and not physical.

So I do agree of course that all positive values of refractive indexes
measured to coincide with specific frequencies may be increasing or
decreasing proportionally to the frequencies. While you keep the sign
as if it was a vector I remove the sign because dispersion is absolute
because it is a scalar.

The error being avoided by taking the absolute value is to assume a
negative refractive index which is nonexistent physically.

A negative refractive index violates the conservation of energy law of
physics, and an inert "negatively dispersive" medium implies that the
medium can accelerate a wave beyond its propagation constant.

In fact I am grateful to you for helping me to pinpoint the source of
the error that the idiots made.

This reminds me of similar silliness regarding the sign of time and
assuming a negative causality chain to be tenable.

Mathematics is a tool for physics and physics is a field of
applications for mathematics but you may not create physics by
applying wanton mathematics that does not reflect physical concepts.

EL

(EL) wrote in message . com...
Randy Poe wrote in message . ..

http://en.wikipedia.org/wiki/Dispersion_(optics)

[EL
Just as an example to demonstrate how silly you can be Randy I copied
the contents of that link and here it is.
{{{
Dispersion (optics
From Wikipedia, the free encyclopedia.
Find out how you can help support Wikipedia's phenomenal growth.

(There is currently no text in this page)

I noticed that when I posted the part after the underscore
was not underlined (optics) but that is indeed part of
the URL.

So you are referring me to a page that has no text in it which means
that you did not even read the content of the links you supplied.

It's a longish article that begins like this:
"In optics, dispersion is a phenomenon that causes the
separation of a wave into components with different
frequency. It is most often observed in light waves,
though it may happen to any kind of wave that interacts
with a medium, such as sound waves."

Had you searched Wikipedia you would have found it. Had
you followed my suggestion to search google for "negative
dispersion" you would have found it. Had you done any rudimentary
amount of searching you would have found it. Try again.

Stop fabricating responses and get serious please.
I do not even know why you are being so enthusiastic defending those
idiots while you are much better a parrot than that.

I'm attacking your silly version of "negative dispersion".
There are people who see relativity under every bed and are
ready to jump on it with their favorite rant. Negative dispersion
is an effect in classical optics.

Other links I posted were from fiber-optic cable manufacturers
who are using negatively-dispersive materials to achieve
better propagation properties (no, not faster than light
communication, but better cohesiveness on pulses).

Here is the next section of the article you think doesn't
exist.

"Another consequence of dispersion manifests itself as
a temporal effect. The formula above, v = c / n
calculates the phase velocity of a wave; this is
the velocity at which the phase of any one frequency
component of the wave will propagate. This is not the
same as the group velocity of the wave, which is the
rate that changes in amplitude (known as the envelope
of the wave) will progagate. The group velocity vg
is given by:

vg = c*[n - dn/d(lambda)]^(-1)

The group velocity vg is often thought of as the velocity
at which energy or information is conveyed along the wave.
In most cases this is true, and the group velocity can be
thought of as the signal velocity of the waveform. In some
unusual circumstances, where the wavelength of the light
is close to an absorption resonance of the medium, it is
possible for the group velocity to exceed the speed of
light (vg c), leading to the conclusion that superluminal
faster than light) communication is possible. In practice,
in such situations the distortion and absorption of the wave
is such that the value of the group velocity essentially
becomes meaningless, and does not represent the true signal
velocity of the wave, which stays less than c."

Immediately below this point you will see a discussion of
negative dispersion, also called anomalous dispersion.

- Randy