From: nospam@de-ster.demon.nl
[dear moderators, the moderation software, or something else,
still makes a mess of the reference headers.
This makes it difficult sometimes to reply to the right person,
or to see who has replied to a given posting)
Tom Roberts wrote:
> On 7/7/21 12:20 PM, J. J. Lodder wrote:
> > Tom Roberts wrote:
> >> The constancy of the vacuum speed of light applies only locally, so
> >> everyone who understands the issues will agree for a massless field.
> >> But of course that's the rub -- we don't really know whether the photon
> >> field is truly massless.
> >
> > Why this insistence on photons being or not being 'truly' massless?
>
> Because it is an important aspect of whether the (vacuum) speed of light
> varies. In our current best models of the world, nonzero photon mass and
> a varying vacuum speed of light are equivalent (either both are valid or
> both are invalid).
>
> > All troubles that might arise from a non-zero photon mass
> > are easily killed in advance
> > by adding 'in the limit of infinite frequence'
> > to the definition of the speed of light.
>
> But a) we don't do that, and b) that would make it impossible to
> actually measure the speed of light, one could only measure it
> approximately -- hopeless for such a fundamental aspect of the world we
> inhabit, and one used in so much technology.
Your points are completely beside the subject of this sub-thread,
which was the question if the speed of light can vary with -time-,
and if we could in principle measure this.
> The correct way to deal with a nonzero photon mass is to distinguish
> between the two quite different meanings of c:
> 1) the vacuum speed of light
> 2) the symmetry speed of Lorentzian manifolds
> If (1) is found to vary, no fundamental revolution in physics is
> involved, we just start using a nonzero photon mass [#]. If (2) is found
> to vary, it would refute every theory of physics we have today.
>
> [Historically, in 1905 this distinction was not known
> and Einstein intermixed them inappropriately. His
> second postulate is actually about (2), not (1).
> Today we consider SR to be a theory of geometry,
> not electrodynamics (the subject of his 1905 paper).]
>
> [#] See Proca theory.
You are discussing the by comparison trivial point
whether the speed of light could depend on frequency.
(through a finite photon mass for example)
If so, we would measure the dispersion relation,
and obtain the speed in the limit of infinite frequency.
(like so many other idealisations in physics)
> > Since the photon mass cannot be measured,
> > even the longest radio waves that we can make
> > still have an 'infinite' frequency.
>
> This is just plain wrong:
> a) The photon mass has been measured many times; at present the
> best measurements are consistent with zero and an upper limit
> of 10^-18 eV.
> b) EM waves with frequencies from kilohertz to terahertz have
> been measured -- NONE are "infinite".
You are wrong in that, the photon mass has never been measured.
All we have are upper limits.
So -every- photon we have ever seen has an infinite frequency,
as far as photon mass is concerned.
> > A photon mass corresponding to a wavelength of the size of the universe
> > cannot be measured in principle.
>
> Hmmmm. Mass does not "correspond" to wavelength in any way.
???
\lambda = 2\pi/m, naturally, or 2\pi\hbar/mc,
if you prefer plumber's units.
It really does not matter how you express the characteristic
for where Maxwell's equations end,
as a mass, a frequency, or a wavelength,
Jan
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
|
