Permittivity is the measure determining the electric field produced by charge in a particular medium.
Now electric field, $E$ increases as ε (permittivity) decrease, and E decreases as ε increases, due to inverse proportionality of E to ε.
Talking in material (practical) terms, the permittivity – that is how much E field would be allowed in a medium- is due to the material of medium. For example, water medium has water molecules, so when two charges are placed in water, the Field from the two charges are resisted by water molecules, and so less NET field would be produced by the charges(as compared to when the two charges would have been placed in vacuum), and there would be less force between them.
In vacuum, there is no such mass or material object. So it should have permittivity approaching 0(and in fact 0 itself). But permittivity of free space (free space means- no electromagnetic waves, no particles, no charges, nothing in space, only absolute space) is 8.85×10-¹² F m-¹.
It is though a fact, that if ε of vacuum (free space) be 0, then there would be infinite force between two objects kept in free space, and it is physically not possible. But hypothetically it is possible. (Or is this hypothesis wrong?).
What makes the vacuum not have 0 permittivity?
Best Answer
Both the previous answers (though correct) are somewhat misleading. What $\epsilon_0$ is measuring is the strength of the electric force. The force between two point charges is stated by Coulombs law, which states
$F_e = \dfrac{1}{4\pi\epsilon_0} \dfrac{q_1q_2}{r^2}$, where q represents their charges and r is the distance between them. Electric forces exist everywhere in the universe, and $\epsilon_0$ is just a fundamental constant.
You seemed to have the notion that an interposed material like water decreases this force, somehow blocks the electric field. The actual affect is the opposite: the presence of a material between two charges increases their attraction. Why?
Pretend we have a positive and negative charge separated by a metal conductor. The charges will polarize the material, causing some of the electrons in the material to move closer to the positive charge, like this:
Though the net charge in the dielectric is zero, the charges on the electrodes will feel an attractive force in addition to the attraction that already exists between them, due to the material.
Anyway, materials have a property called permittivity, which quantifies how much they increase the force between two charges ($\epsilon$). I prefer to think in terms of relative permittivity, or $\kappa$, which is a unitless number that gives the ratio between electric forces in a vacuum vs. through a material. By definition, for a vacuum, $\kappa = 1$. Various materials augment the electric forces by various amounts, but in all cases, they have values of $\kappa$ greater or equal to one.
Footnote: even in insulators, where electrons don't move between atoms, this effect is still observed, due to electron orbits being slightly skewed to one side of individual atoms.