Why Do Pendry's Cloaking Equations Differ in My Calculations?

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Discussion Overview

The discussion revolves around discrepancies in the equations related to Pendry's cloaking theory, specifically focusing on the transformation of permittivity in different coordinate systems. Participants are examining the mathematical formulations presented in Pendry's article and comparing them to their own calculations, raising questions about the assumptions and definitions used in the derivations.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents their derived equations for permittivity, which differ from those in Pendry's article, noting the presence of an extra factor of \( r' \) and \( \sin(\theta') \).
  • Another participant suggests that the extra \( \epsilon \) in the derived equations may stem from the assumption in Pendry's paper that the relative permittivity is 1, indicating a vacuum or air context.
  • There is a request for assistance in understanding the differences in the equations and the implications of the coordinate transformations used.
  • A participant acknowledges the effort involved in the calculations but does not provide specific help.
  • Another participant shares a link to a tool that may assist in typesetting mathematical expressions, indicating a supportive community aspect.

Areas of Agreement / Disagreement

Participants have not reached a consensus on the reasons for the discrepancies in the equations. There are multiple viewpoints regarding the assumptions made in the calculations, and the discussion remains unresolved.

Contextual Notes

The discussion highlights potential limitations in the assumptions regarding the relative permittivity and the coordinate transformations, which may affect the derived equations. The exact definitions and contexts of the variables used are not fully clarified, contributing to the ongoing debate.

andresordonez
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Hi, I'm reading this article (you may need to register to view it, the registration is free though).

http://www.sciencemag.org/content/312/5781/1780.full

(can I post a link to this article in Dropbox so that people reading this don't have to register without getting an infraction from the moderators??)

and I'm getting this:

<br /> \epsilon&#039;_{r&#039;} = \epsilon \frac{R_2}{R_2-R_1} (r&#039;-R_1)^2 \sin(\theta&#039;)<br />
<br /> \epsilon&#039;_{\theta&#039;} = \epsilon \frac{R_2}{R_2-R_1} \sin(\theta&#039;)<br />
<br /> \epsilon&#039;_{\phi&#039;} = \epsilon \frac{R_2}{R_2-R_1} \sin(\theta&#039;)<br />

instead of equations (7) in Pendry's article:

<br /> \epsilon&#039;_{r&#039;} = \frac{R_2}{R_2-R_1} \frac{(r&#039;-R_1)^2}{r&#039;}<br />
<br /> \epsilon&#039;_{\theta&#039;} = \frac{R_2}{R_2-R_1}<br />
<br /> \epsilon&#039;_{\phi&#039;} = \frac{R_2}{R_2-R_1}<br />

The difference between these equations and the ones I get is not only the missing r&#039; and the extra sin(\theta&#039;) but also the extra \epsilon

This is what I'm doing. The new coordinates are given by equations (6):

<br /> r^{\prime}=R_{1}+r\frac{\left(R_{2}-R_{1}\right)}{R_{2}}<br />
<br /> \theta^{\prime}=\theta<br />
<br /> \phi^{\prime}=\phi<br />

The permittivity transforms according to:
<br /> \epsilon_{r}^{\prime}=\epsilon\frac{Q_{\theta&#039;}Q_{\phi&#039;}}{Q_{r&#039;}}<br />
<br /> \epsilon_{\theta}^{\prime}=\epsilon\frac{Q_{r&#039;}Q_{\phi&#039;}}{Q_{\theta&#039;}}<br />
<br /> \epsilon_{\phi}^{\prime}=\epsilon\frac{Q_{r&#039;}Q_{\theta&#039;}}{Q_{\phi&#039;}}<br />

where Q_{u} is given by:
<br /> Q_u^2 = \left(\frac{\partial x}{\partial u}\right)^2 + \left(\frac{\partial y}{\partial u}\right)^2 + \left(\frac{\partial z}{\partial u}\right)^2<br />

Then:
<br /> Q_{r^{\prime}}^{2}=\left(\frac{\partial x}{\partial r^{\prime}}\right)^{2}+\left(\frac{\partial y}{\partial r^{\prime}}\right)^{2}+\left(\frac{\partial z}{\partial r^{\prime}}\right)^{2}<br />
<br /> \frac{\partial x}{\partial r^{\prime}}=\frac{\partial x}{\partial r}\frac{\partial r}{\partial r^{\prime}}+\frac{\partial x}{\partial\theta}\frac{\partial\theta}{\partial r^{\prime}}+\frac{\partial x}{\partial\phi}\frac{\partial\phi}{\partial r^{\prime}}=\frac{\partial x}{\partial r}\frac{\partial r}{\partial r^{\prime}}=\sin\theta\cos\phi\frac{R_{2}}{R_{2}-R_{1}}=\sin\theta^{\prime}\cos\phi^{\prime}\frac{R_{2}}{R_{2}-R_{1}}<br />
<br /> \frac{\partial y}{\partial r^{\prime}}=\frac{\partial y}{\partial r}\frac{\partial r}{\partial r^{\prime}}+\frac{\partial y}{\partial\theta}\frac{\partial\theta}{\partial r^{\prime}}+\frac{\partial y}{\partial\phi}\frac{\partial\phi}{\partial r^{\prime}}=\frac{\partial y}{\partial r}\frac{\partial r}{\partial r^{\prime}}=\sin\theta\sin\phi\frac{R_{2}}{R_{2}-R_{1}}=\sin\theta^{\prime}\sin\phi^{\prime}\frac{R_{2}}{R_{2}-R_{1}}<br />
<br /> \frac{\partial z}{\partial r^{\prime}}=\frac{\partial z}{\partial r}\frac{\partial r}{\partial r^{\prime}}+\frac{\partial z}{\partial\theta}\frac{\partial\theta}{\partial r^{\prime}}+\frac{\partial z}{\partial\phi}\frac{\partial\phi}{\partial r^{\prime}}=\frac{\partial z}{\partial r}\frac{\partial r}{\partial r^{\prime}}=\cos\theta\frac{R_{2}}{R_{2}-R_{1}}=\cos\theta^{\prime}\frac{R_{2}}{R_{2}-R_{1}}<br />
<br /> Q_{r^{\prime}}^{2}=\left(\frac{R_{2}}{R_{2}-R_{1}}\right)^{2}<br />

<br /> Q_{\theta^{\prime}}^{2}=\left(\frac{\partial x}{\partial\theta^{\prime}}\right)^{2}+\left(\frac{\partial y}{\partial\theta^{\prime}}\right)^{2}+\left(\frac{\partial z}{\partial\theta^{\prime}}\right)^{2}<br />
<br /> \frac{\partial x}{\partial\theta^{\prime}}=\frac{\partial x}{\partial\theta}=r\cos\theta\cos\phi=\frac{R_{2}}{R_{2}-R_{1}}\left(r^{\prime}-R_{1}\right)\cos\theta^{\prime}\cos\phi^{\prime}<br />
<br /> \frac{\partial y}{\partial\theta^{\prime}}=\frac{\partial y}{\partial\theta}=r\cos\theta\sin\phi=\frac{R_{2}}{R_{2}-R_{1}}\left(r^{\prime}-R_{1}\right)\cos\theta^{\prime}\sin\phi^{\prime}<br />
<br /> \frac{\partial z}{\partial\theta^{\prime}}=\frac{\partial z}{\partial\theta}=-r\sin\theta=-\frac{R_{2}}{R_{2}-R_{1}}\left(r^{\prime}-R_{1}\right)\sin\theta^{\prime}<br />
<br /> Q_{\theta^{\prime}}^{2}=\left[\frac{R_{2}}{R_{2}-R_{1}}\left(r^{\prime}-R_{1}\right)\right]^{2}<br />

<br /> Q_{\phi^{\prime}}^{2}=\left(\frac{\partial x}{\partial\phi^{\prime}}\right)^{2}+\left(\frac{\partial y}{\partial\phi^{\prime}}\right)^{2}+\left(\frac{\partial z}{\partial\phi^{\prime}}\right)^{2}<br />
<br /> \frac{\partial x}{\partial\phi^{\prime}}=\frac{\partial x}{\partial\phi}=-r\sin\theta\sin\phi=-\frac{R_{2}}{R_{2}-R_{1}}\left(r^{\prime}-R_{1}\right)\sin\theta^{\prime}\sin\phi^{\prime}<br />
<br /> \frac{\partial y}{\partial\phi^{\prime}}=\frac{\partial y}{\partial\phi}=r\sin\theta\cos\phi=\frac{R_{2}}{R_{2}-R_{1}}\left(r^{\prime}-R_{1}\right)\sin\theta^{\prime}\cos\phi^{\prime}<br />
<br /> \frac{\partial z}{\partial\phi^{\prime}}=\frac{\partial z}{\partial\phi}=0<br />
<br /> Q_{\phi^{\prime}}^{2}=\left[\frac{R_{2}}{R_{2}-R_{1}}\left(r^{\prime}-R_{1}\right)\right]^{2}\sin^{2}\theta^{\prime}<br />

Finally:
<br /> \epsilon_{r^{\prime}}=\epsilon\frac{\left[\frac{R_{2}}{R_{2}-R_{1}}\left(r^{\prime}-R_{1}\right)\right]^{2}\sin\theta^{\prime}}{\frac{R_{2}}{R_{2}-R_{1}}}=\epsilon\frac{R_{2}}{R_{2}-R_{1}}\left(r^{\prime}-R_{1}\right)^{2}\sin\theta^{\prime}<br />
<br /> \epsilon_{\theta^{\prime}}=\epsilon\frac{\left(\frac{R_{2}}{R_{2}-R_{1}}\right)^{2}\left(r^{\prime}-R_{1}\right)\sin\theta^{\prime}}{\frac{R_{2}}{R_{2}-R_{1}}\left(r^{\prime}-R_{1}\right)}=\epsilon\frac{R_{2}}{R_{2}-R_{1}}\sin\theta^{\prime}<br />
<br /> \epsilon_{\phi^{\prime}}=\epsilon\frac{\left(\frac{R_{2}}{R_{2}-R_{1}}\right)^{2}\left(r^{\prime}-R_{1}\right)}{\frac{R_{2}}{R_{2}-R_{1}}\left(r^{\prime}-R_{1}\right)\sin\theta^{\prime}}=\epsilon\frac{R_{2}}{R_{2}-R_{1}}\csc\theta^{\prime}<br />

Any kind of help is more than welcome!
 
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That must have taken some time to type. sorry no actual help from me here.
 
Well, the extra \epsilon (relative permittivity) is just because in the paper it is assumed to be 1 (vacuum or air)
 

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