# The Naked Scientists Forum

### Author Topic: How do we know the speed of light in a vacuum?  (Read 2067 times)

#### DoctorBeaver

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##### How do we know the speed of light in a vacuum?
« 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?

#### lightarrow

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##### How do we know the speed of light in a vacuum?
« Reply #1 on: 04/07/2008 08:30:34 »
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)
« Last Edit: 04/07/2008 08:36:38 by lightarrow »

#### DoctorBeaver

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##### How do we know the speed of light in a vacuum?
« Reply #2 on: 04/07/2008 13:54:19 »
So it's done by deduction. OK. Thanks Alberto.

#### The Naked Scientists Forum

##### How do we know the speed of light in a vacuum?
« Reply #2 on: 04/07/2008 13:54:19 »