[nudephil (OP)]

Listener Victor sent in this:

I recently had a personal eureka moment when I realised that there can be no such thing as "free space" - as in the permittivity, permeability, and impedance of free space and the radar range equation for free space. If there is nothing there, how can it have properties?

So I suddenly thought these properties must refer to dark matter.

This got me thinking further. Why should electromagnetic waves such as light have a fixed speed? Is it possible the 30,000,000 m/s is only because we are not measuring over a sufficiently large distance? If light has to plough through dark matter could it eventually come to standstill? Could the strings making up the photons simply stop vibrating and become dark matter?

Can anyone help untangle the issue?

[alancalverd]

1. Yes, in theory, and pretty close to it in practice.

2. No, though there are some theories that suggest there are bits of dark matter in interstellar space.

3. Strings are an unprovable hypothesis, not bits of vibrating catgut.

[Janus]

Listener Victor sent in this:

I recently had a personal eureka moment when I realised that there can be no such thing as "free space" - as in the permittivity, permeability, and impedance of free space and the radar range equation for free space. If there is nothing there, how can it have properties?

Permittivity, permeability and impedance of free space are not "properties", they are baseline physical constants.
So for example
 F = q1q2/4pi e0 r^2   gives the force acting between two charges (q1,q2) a distance of r apart when there is nothing between them.  It's a constant of proportionality, and it's value depends on the units used to measure q and r. 
It has nothing to do with the "nothing" between them having a "property", It is just the factor you have to use to get the right value for the force using the chosen units.

[Bill S]

Presumably,  the "nothing" between them means nothing that would influence the force in question; rather than "absolutely" nothing.

[alancalverd]

Very confusing semantics, Bill. Anything will alter the force compared with a true "nothing".

As Janus says, we define ε₀ as the number that relates charge (in coulombs) distance (meters) and force (newtons) when there is nothing but space between the charges. Its value depends only on the units we use for charge, distance and force. Any actual material has a dimensionless relative permittivity ε that you multiply by ε₀ to calculate the force between charges separated by a chunk of that material.     

Fortunately dry air has ε very close to 1 so we usually ignore it.

[Bill S]

Very confusing semantics, Bill. Anything will alter the force compared with a true "nothing".

I was anticipating a problem that might never have arisen.  I think that’s sorted, thanks.

[yor_on]

As far as I get it the permittivity of free space is a 'arbitrarily' set number, connecting to other constants. In a way similar to the Plank units, they all have a relation to each other.

  " More fundamentally, it isn't that meaningful to ask whether physical constants with units vary, either in space or time, since they are measured in terms of other things that could potentially vary instead. The only really meaningful question is to ask whether dimensionless combinations of physical constants, such as the fine structure constant α, vary. "

You can take a look at those   https://en.wikipedia.org/wiki/Planck_units 

https://astronomy.stackexchange.com/questions/36614/has-the-permittivity-or-the-permeability-of-space-been-measured-in-a-gravitation

https://physics.stackexchange.com/questions/440007/why-does-the-vacuum-even-have-permeability-and-permittivity
=

and related to the quote I used. https://arstechnica.com/science/2020/04/fine-structure-constant-may-vary-with-space-constant-in-time/