View Full Version : epsilon naught (permetivity)

2012-May-07, 01:22 PM
this is the vacuum permetivity constant whose units are farads per metre, found in the equation of the force between charges (such as in a simple capacitor) and also (squared) in the Rydberg constant in analysing an atom. It is a fundamental property of the vacuum at atomic and at macro scales and it seems to link electrostatic and electromagnetic properties. ( via c^2) My question is how do we interpret this physical quantity in vacuum space, how does it work? Are there assumed to be virtual photons mediating the "link" How can the vacuum have this property?

2012-May-07, 03:31 PM
Don't electron positron pairs created from the vacuum have something akin to a frequency dependent epsilon naught, in QED? I've always looked at it as a scaling factor in SI units. Like the gravitational constant. We can ask what G means, and we'll hear over and over again that it represents how weak gravity is. Before we can attempt to answer your question, we must first agree on what polarization of the vacuum means.

Edit: here's a good read. http://ab-initio.mit.edu/~meep/meep.pdf
MIT electromagnetic equation propagation

2012-May-07, 10:05 PM
thanks for the link. As I understand it that paper is about materials and I suppose all materials have to work inside the vacuum and so could be thought of as modifying the vacuum property. The right angle shift between electrostatic and electromagnetic waves which can also be polarised in vacuum so the dielectric effect of materials can always be seen as "just" modifying the vacuum. However at the moment I cannot envisage what the mechanism in vacuum is. When we use epsilon in the charge force equation it does just look like a scale factor for a very simple relationship, charge times charge divided by distance squared but then it is related to the magnetic constant by c^2. Perhaps I askedthe wrong question to understand.

2012-May-08, 10:55 PM
It's a tough question to answer. I'm still learning Maxwell's equations and yet despite their success, I can't help but think "did you just say these equations are based on the absence of other charges around? No currents? No dielectrics?".
Epsilon has the units of Coulombs built into it. What is charge? It's not like trying to imagine the usual quantities of mass, length, and time.