Naked Science Forum

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

Title: How do we know the speed of light in a vacuum?
Post 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?

Title: How do we know the speed of light in a vacuum?
Post by: lightarrow on 04/07/2008 08:30:34

Quote from: 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?

One reason is this: at low pressures, the electromagnetic properties (electric permittivity ε₀ and magnetic permeability μ₀) 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(μ₀ε₀)

Title: How do we know the speed of light in a vacuum?
Post by: DoctorBeaver on 04/07/2008 13:54:19

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