# How do we know the maximum speed of light?

1. Nov 5, 2008

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]]

2. Nov 5, 2008

### Uncle Al

> 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]]

1) Lightspeed is finite *precisely* because there is stuff in the
vacuum: non-zero permeablity and permitivity of free space; Maxwell's
equations, Lorentz invariance. The stuff that isn't there is
measurable as the Casimir effect, Lamb shift (try U(91+) rather than
H(+)), Rabi vacuum oscillations, electron anomalous g-factor....

1) Do you want a faster lightspeed?

http://www.npl.washington.edu/AV/altvw43.html
Scharnhorst effect
http://arXiv.org/abs/gr-qc/0107091
http://arXiv.org/abs/quant-ph/0010055
Phys. Lett. B236 354 (1990)
Phys. Lett. B250 133 (1990)
J Phys A26 2037 (1993)

2) http://arXiv.org/abs/0706.2031

Pookie pookie.

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/lajos.htm#a2