Talk:Phase velocity

message: I removed this:
:For light, group velocity and phase velocity are related by the formula 
::v_\mathrm{g} v_\mathrm{p} = c^{2}
:where c is the speed of light in a vacuum.
Since I don't think it's right. Cite, please? -- Bob Mellish 20:40, 4 October 2005 (UTC)
replies: text: See "differentiation of the dispersion relation (1155) yields eq.(1160)" in , and also this quote in : "the product of the group velocity and phase velocity of the GPS signals is equal to the speed of light squared". fgnievinski (talk) 15:27, 16 May 2016 (UTC)
replies: text: It's only true in what is known as a cold electron gas, which apparently models the ionosphere at certain frequencies.--Guy vandegrift (talk) 21:52, 16 May 2016 (UTC)
replies: text: I can confirm that vg*vp is quite far off from c^2 for visible light passing through glass. --Steve (talk) 02:54, 17 May 2016 (UTC)
replies: text:   Thanks for your input.  Would you object to this sentence: "For the ionosphere at radio frequencies (and other cold plasmas), the following relation holds: v_\mathrm{g} v_\mathrm{p} = c^{2}." Thanks. fgnievinski (talk) 10:36, 17 May 2016 (UTC)
replies: text: Not my article, but would rather prefer a short section on phase velocity for a few well known waves: glass, cold plasma, deep water waves, and perhaps whistlers.  Adding one equation for the ionosphere is like allowing Pluto to be a planet:  If you let Pluto in there are a lot of others waiting to join the club.  But as I said, I am not involved with that article...just saying. I am not a Wikipedian but a Wikiversarian.  Why don't you write a Wikiversity article?  --Guy vandegrift (talk) 13:33, 17 May 2016 (UTC)
thread_title:  vg*vp = c*c 

message: The reference site says the group velocity can be greater than c, not the phase velocity. Probably someone should edit this.
----
The article currently states that "phase velocity of electromagnetic radiation may under certain circumstances exceed the speed of light in a vacuum."  This sentence should be changed to something like "phase velocity of electromagnetic radiation may under certain circumstances exceed the speed of light in the medium of propagation", since propagation faster than the speed of light in vacuum is unphysical.  I'll try to find references on this topic before I edit the entry extensively. -- Crosswalkcs 05:35, 31 August 2006 (UTC)
thread_title: Older discussions

message: According to many sources including the Feynman lecture on Physics III (Quantum Mechanics) and here, the following formulas should be correct instead of those currently in the article:
:v_p = E/p = \hbar \omega / \hbar k = \omega/k, 
where E is total energy, 
i.e. E = \gamma m_{rest}c^2 = m_{rest}c^2 + E_k = \sqrt{m_{rest}^2 c^4 + c^2 p^2}
and
:v_g = \frac{d\omega}{dk} = v_{particle} = c^2 / v_p ,
which holds for both light wave and matter wave. 
--KasugaHuang 03:11, 4 January 2007 (UTC)
thread_title:  Phase velocity of matter wave 

message: Is there any way someone could do an animated gif or similar to demonstrate what the topic is about? I think it would help its ability to be understood. --El Pollo Diablo (Talk) 09:38, 5 June 2008 (UTC)
thread_title:  Pic? 

message: Man,,, you are all wrong here. Phase velocity is related to the ---velocity of propagation of the wave in the Fourier space of frequencies --- that is why Phase velocity can be greater then c!speed of light! Because it is not related to the real velocity of propagation of mass and energy. 
what is right for sure is that Vf . Vg  = c^2   , where Vg is the group velocity, and that is the velocity related to the propagation of mass , information,,, whatever.. 
that is it ! Trust me!  
I have exam about this Tomorrow!  : ) See you
I think the article have to be change and a analised taking into account the Fourier transformation and Fourier space, that is completely related to the development of the ideas about phase velocity.        —Preceding unsigned comment added by 193.49.162.11 (talk) 22:35, 26 April 2009 (UTC)
thread_title: Fourier transformation and Fourier space

message: I have reverted the velocity -> speed change.  WP is not a tool for language reform, and phase velocity is the usual formulation.
The objection seems to be the usual line about "velocity is a vector and speed is a scalar", which is all very well for high-school physics, because students at that level are apt to confuse the vector and scalar meanings, and this arbitrary word-choice helps them keep it straight. However it does not really reflect scholarly usage.  Velocity is used frequently in contexts where only the magnitude is important.  Etymologically this makes perfect sense; the Latin root (I'm assuming it's velox) presumably means only "fast", not "fast in a direction". --Trovatore (talk) 21:34, 5 June 2010 (UTC)
thread_title:  velocity/speed 

message: This topic would be fastly easier to grasp if you would pertain to using derivatives in the introduction, instead of memorized nomenclature in relation to long dead individual. You aren´t going to resurrect those dead by using their names in physics.
Take any equation of the form y=f(x). y´=f´(x) is the first derivative, which is defacto the tangent line to the curve at that location. That tangent has an angle, and that angle is the phase of the ´particle´ at that location as that particle moves through the trajectory described by that equation.
Defacto, for any equation y=f(x), the integral of that equation, makes y=f(x) the tangent line change (the phase), of that integral.
A good question in relation is, is the constant in any equation of importance, or is it negligible in that it soley pertains to an operating point differenciation (IE: [f(x+Δx)-f(x)]/Δx is the derivative function and in that function, the constant does not add information at all). Given that that is true, must the phase have that constant included, or would that be overloading that equation?  — Preceding unsigned comment added by 201.209.204.34 (talk) 11:48, 13 July 2013 (UTC)
thread_title:  y, y´, y´´ Add introduction in relation to derivatives. 

message: Female 2405:201:C01F:4459:4168:55A7:5EDF:865 (talk) 14:18, 2 January 2023 (UTC)
thread_title:  Wave velocity