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Empirically Confirmed Superluminal Velocities?
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" |
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Empirically Confirmed Superluminal Velocities?
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. |
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Empirically Confirmed Superluminal Velocities?
(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 |
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Empirically Confirmed Superluminal Velocities?
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Empirically Confirmed Superluminal Velocities?
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Empirically Confirmed Superluminal Velocities?
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 |
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Empirically Confirmed Superluminal Velocities?
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 |
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Empirically Confirmed Superluminal Velocities?
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 |
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Empirically Confirmed Superluminal Velocities?
"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. |
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