On 24/12/2017 20:43, Bill Wright wrote:

What are the parameters that set the speed of electromagnetic
transmission in a vacuum? I've googled everywhere but I can't find the
answer. It's easy enough to find the figure but WHY? Why not
29,979,245.8 metres per second or 2,997,924,580 metres per second?

The electro-magnetic behaviour of space is described by Maxwell's
Equations. As newshound has suggested, it is possible to show* that a
wave equation is a possible solution of Maxwell's Equations, and that
the speed of the resulting wave would be equal to ...
1 / sqroot( permittivity of space * permeability of space)
.... and that this value is exactly equal to the measured speed of light
in space. This is how we know that light is an electro-magnetic wave.

* This is science speak:

It is trivial to show that = an undergrad can prove it
It is easy to show that = a postgrad can prove it
It may be proved that = The prof can prove it

In this particular case, I was able to prove it at uni, and did so
during a physics tutorial, which, I discovered much later from a
bus-stop conversation between another student who'd been present with my
then girl-friend, gave me something of a reputation for the subject.
But I wouldn't want to be asked to prove it now!