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Forum: Processes Involving Optical Photons
Re: Question Faster Than Light Photons? (Joel Long)
Re: More Re: Faster Than Light Photons? (Joel Long)
Re: More Re: Faster Than Light Photons? (Joel Long)
Re: Feedback Re: Faster Than Light Photons? (Gumplinger Peter)
Date: 16 Jul, 2012
From: Joel Long <Joel Long>

Hello Peter,

> Sorry, I meant to answer your post when you first posted it but 'it fell
> under the radar'.

No problem, I completely understand.

> You found the reason why you are getting 'faster than light photons'.
> You are specifying a dielectric with an index of refraction <1. Optical
> photons will not travel inside aluminum; not very far into the volume
> anyway, before they are either absorbed or reflected. You may assign a
> REAL and IMAGINARY index of reflection to your Al surface (see the LXe
> example) which are then used to calculate the reflectivity of the
> surface.

Okay, I will try with the real and imaginary indices, rather than the refraction index and absorption index.

> > My understanding was that the group velocity *could* be greater than the
> > speed of light,
> That's news to me...

To quote: "A widespread misconception is that since, according to the theory of relativity, nothing can travel faster than the speed of light in vacuum, the refractive index cannot be lower than 1. This is erroneous since the refractive index measures the phase velocity of light, which does not carry energy or information, the two things limited in propagation speed. The phase velocity is the speed at which the crests of the wave move and can be faster than the speed of light in vacuum, and thereby give a refractive index below 1."

Maybe that's wrong - it's not my area of physics at all - but that's where I got that understanding from.

> Your source is probably specifying the real and imaginary part of the
> metal's index of refraction. If you use that as explained in LXe
> example, you'll get G4 to calculate the reflectivity of this surface
> (dielectric_metal)

My source specifies both values. For example, at 400 nm, the real part of the index (n) is 0.48787 and the imaginary part (k) is 4.8355. Currently what I have done is put in the n value as is, then convert the k to an absorption coefficient.

> 
> > Should I simply force my refractive indices to 1 if they are
> > actually less than 1?
> 
> Do you think it is physical to track optical photons in a metal?

Well, obviously setting my refractive index to one doesn't make any sense; that was born out of frustration and confusion. As to your question, though, I obviously intent for nearly everything to be reflected, and the rest to be absorbed. I know that I could potentially do that using surfaces, but I have a whole lot of surfaces I would have to define, and so I preferred to get the correct behavior from material properties instead if at all possible.

In any case, I might actually see some photons go through in reality. I'm not actually working with aluminum, I'm working with aluminized mylar. If my understanding is correct, that means a sheet of mylar (which is almost perfectly optically clear) with an atoms-thin layer of aluminum put on one side via vacuum deposition of some kind; the purpose being to get a true aluminum surface, rather than the aluminum oxide that results from any exposure to air. I honestly do not know whether some photons might make it through such a thin layer of aluminum (I suppose if I pulled out some old textbooks I could manage, but it would take me a while), but regardless, if a few make it through it shouldn't materially affect my simulation.

Thanks for your help! Joel

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1 Feedback: Re: Faster Than Light Photons?   (Gumplinger Peter - 16 Jul, 2012)
(_ Idea: Re: Faster Than Light Photons?   (Erik Dietz-Laursonn - 14 Sep, 2012)
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