Re: Simple Sagnac

The problem here is really that the "classical" case is a squishy fiction
without hard rules.

Light is carried by a aether in vacuum and travels at c relative to that
aether, sure, no prob. But then when the light's passing through air, or
glass, or water, how fast does it go, and what's that speed relative to?
If you're talking about a fictitious "classical universe" you can make up
any rule you like, so it's hard to say the "classical" case doesn't
match some experiment involving a medium -- just change the rules until it
does, hey hey.

Sagnac himself didn't address the presence of a medium; he assumed he
could ignore the effect of the air in his apparatus, and just assume the
light traveled at C relative to the "fixed frame" (I think he thought
there was an aether but I'm not sure).

But we know now, from experiments, that light _is_ dragged with the
medium, at least when the medium is something substantially thicker than
air. So, just assuming "no drag" in all cases when talking about the
effect from a classical perspective doesn't make a lot of sense. (And for
a "thick" medium with substantially reduced speed of light, it also
doesn't lead to the correct results, as mentioned previously.)


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