Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: DoctorBeaver on 04/07/2008 06:39:44

We can't produce a true vacuum, so how do we know how fast light travels through 1?

We can't produce a true vacuum, so how do we know how fast light travels through 1?
One reason is this: at low pressures, the electromagnetic properties (electric permittivity ε_{0} and magnetic permeability μ_{0}) of air (for example) vary very little with pressure, so making the extrapolation to complete void gives the same result. You know that light's speed is related to these properties:
c = 1/Sqrt(μ_{0}ε_{0})

So it's done by deduction. OK. Thanks Alberto.