Re: On permittivity and permeability
- From: Timo Nieminen <timo@xxxxxxxxxxxxxxxxx>
- Date: Mon, 17 Oct 2005 11:01:11 +1000
On Fri, 13 Oct 2005, Ken S. Tucker wrote:
> Let's look at the "piezoelectric effect", where an applied
> mass distorts the relations of a charge latice to produce
> a voltage, that is the basis of electronic weigh scales
> unless Timo likely understands better, but that's what I
> understand.
Luckily, most of the stuff that I deal with falls into the
can-ignore-electrostriction etc category. Landau has a brief coverage of
such things, and notes that stresses due to applied fields are
proportional to E^2 in isotropic dielectrics, but proportional to E in
piezoelectrics. As field -> deformation, so deformation -> field, E
proportional to the deformation. I don't know the microscopic details, but
one can go quite some way without them (as per Landau). [That's Landau vol
8, Electrodynamics of continuous media.]
> In the presence of the Sun's "MASS" , light bends
> (deflection) and slows down, (Shapiro), indicating
> an alteration to "e" and "m" that seems to vary the
> refractive index by the presence of matter, from
> that PoV.
[Caveat: I'm rather non-expert in GR, but here goes ... ]
Local Lorentz invariance, with e0 and m0 being 4-scalars, would mean the
"local observer" would disagree on there being a refractive index other
than 1, while the "distant observer" can happily model the effect as a
change in refractive index. I saw many years ago a nice plastic lens that
modelled gravitational lensing.
> But this effect might also appear as a polarization,
> due to mass, like the "piezo" does, you see Timo, GR
> doesn't affix a certain mechanical reason to P, on
> the contrary I think P indicates the presence of
> matter, _even in a so-called vacuum_!
In non-GR electrodynamics, P is only non-zero in the presence of matter.
I'd guess that in GR, P is only non-zero according to the "observer on the
spot" in the presence of what everybody will agree is matter. To that
extent, I think it's pretty clear that nonzero P indicates the presence of
matter. As for what a distant observer sees, I dunno.
--
Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/
E-prints: http://eprint.uq.edu.au/view/person/Nieminen,_Timo_A..html
Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html
.
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