[PmbPhy]

What you are measuring is dielectric and magnetic property of atomic structures but vacuum has no such property...

He's referring to the permittivity and permeability of free space. Those quantities are not measured as a property of atomic structure since it's measured in a vacuum. I.e. the force between two charged particle in vacuum is

F = [1/4pi*e]qq'/r²

where "e" is the permittivity of free space. There is no atomic structure to speak of here. However I myself don't see this as a property of space.

[evan_au]

What you are measuring is dielectric and magnetic property of atomic structures but vacuum has no such property...

Air has a relative permittivity of ε_r=1.00058986, which is almost the same as a vacuum (ε_r=1).

This should not be surprising, because there is a lot of space (vacuum) between each molecule in the air. You could imagine air as >99.8% vacuum (ie if you condensed 1 liter of air to a liquid, it would take up <1.5ml).

If you connect a pump to a container of air, and halve the pressure of the air, ε_r will be around 1.0002, ie the molecules of oxygen, nitrogen & water will have a reduced impact on the permittivity of the vacuum.

As you reduce the air pressure towards zero, the relative permittivity of the gas will asymptotically approach the permittivity of a vacuum.

So the permittivity of a vacuum is not a fiction - and when we measure the permittivity of air, we are (mostly) measuring the permittivity of a vacuum.

[PmbPhy]

What you are measuring is dielectric and magnetic property of atomic structures but vacuum has no such property...

Air has a relative permittivity of ε_r=1.00058986, which is almost the same as a vacuum (ε_r=1).

This should not be surprising, because there is a lot of space (vacuum) between each molecule in the air. You could imagine air as >99.8% vacuum (ie if you condensed 1 liter of air to a liquid, it would take up <1.5ml).

If you connect a pump to a container of air, and halve the pressure of the air, ε_r will be around 1.0002, ie the molecules of oxygen, nitrogen & water will have a reduced impact on the permittivity of the vacuum.

As you reduce the air pressure towards zero, the relative permittivity of the gas will asymptotically approach the permittivity of a vacuum.

So the permittivity of a vacuum is not a fiction - and when we measure the permittivity of air, we are (mostly) measuring the permittivity of a vacuum.

What gives you the impression that the permittivity of free space is a physical property of the vacuum? In free space all it is is a constant of proportionality, nothing more. In fact in Gaussian units Coulombs law is

F = qq'/r²

i.e. the constant isn't even there. See: https://en.wikipedia.org/wiki/Vacuum_permittivity

[lightarrow]

What definition is that?

ε₀ = 10⁷/4πc².

Anyway it's not a real, physical property of vacuum, as PmbPhy wrote.

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[evan_au]

[Permittivity is] not a real, physical property of vacuum

I can charge up 2 metal plates, measure the area of the plates, and the distance between them. I can apply a known electric charge, and measure the voltage between the plates. That represents measurement of real, physical properties. From these real, physical properties I can calculate the permittivity of the dielectric (yet another physical property).

I can repeat the experiment with a glass dielectric, an air dielectric and vacuum as a dielectric. These are all real, physical properties measurable in the lab.

The permittivity I get for a vacuum is just as real as the result I get for glass. It is just as real as the answer I get for air (and extremely close to the answer I get for air).

So I don't understand why the properties of glass and air should be real, and the result of the same experiment in a vacuum should be, in some sense, "unreal"? Please clarify.

[lightarrow]

[Permittivity is] not a real, physical property of vacuum

I can charge up 2 metal plates, measure the area of the plates, and the distance between them. I can apply a known electric charge, and measure the voltage between the plates. That represents measurement of real, physical properties. From these real, physical properties I can calculate the permittivity of the dielectric (yet another physical property).

I can repeat the experiment with a glass dielectric, an air dielectric and vacuum as a dielectric. These are all real, physical properties measurable in the lab.

The permittivity I get for a vacuum is just as real as the result I get for glass. It is just as real as the answer I get for air (and extremely close to the answer I get for air).

So I don't understand why the properties of glass and air should be real, and the result of the same experiment in a vacuum should be, in some sense, "unreal"? Please clarify.

The fact voltage in air (or other material medium) is different from voltage in vacuum is ascribed to electric properties of the first: its molecules polarize in presence of an electric field.
The fact you find relations between electric quantities in the vacuum is ascribed to properties of the field sources (charges, currents).

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[mathew_orman (OP)]

Vacuum capacitor has no dielectric material...
Empty space between the plates has no property of any kind ...
Ones again if there is no physical influence on matter placed in-between the capacitor plates then no dielectric property can be defined, but to satisfy the mathematical model a fudge number is assigned and it is a starting point of influence... Any single atom between plates influence both displacement current and self capacitance  and mutual capacitance of the plates...