How do we know the maximum speed of light?

How do we know the speed of light in a vacuum if we've never been able
to measure it? Please correct me if I'm mistaken.

1) All observable space is saturated with CMBR, i.e. electromagnetic
radiation, which is a form of energy.
2) As asserted by the Mass-Energy Equivalence and the Strong
Equivalence Principle energy and mass produce a gravitational field in
the same way.
3) Light must obey the laws of space-time like all other things, as
such it is affected by gravity. Light travailing over locally-
irregular gravitational fields is refracted, e.g. a gravitational
lens, etc.

Thus we cannot observe the behavior of light in a "vacuum" devoid of
both mass and energy, as would be the case on the fringe of an
expanding. Or did I miss something?

JSD


[[Mod. note -- If you work out the likely magnitude of these effects,
they're *very* tiny. Any experiment has some level of experimental
error, and if effects like (1), (2), and (3) above are well below that
level, then it's ok to neglect them. More generally, the "speed of
light in a vacuum" is an *abstraction*; any actual experimental
realisation is going to have experimental limitations and approximations.
What's important is that we understand and can quantify these limitations
and approximations.
-- jt]]
.

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