Re: Simple Sagnac


"sal" <pragmatist@xxxxxxxxxx> wrote in message news:pan.2005.07.25.15.11.10.44873@xxxxxxxxxxxxx
> Here's a (relatively) simple view of the Sagnac effect:
>
> http://www.physicsinsights.org/sagnac_1.html
>
> I tried to cut through the fog of algebra that often obscures web pages
> that talk about the Sagnac effect: The math on the page includes nothing
> harder than a Lorentz transform, and no messy integrals. In fact, the
> Sagnac effect is pretty simple if one avoids getting bogged down in the
> details of the rotation.
>
> Unlike Eugene and Wittke I don't claim this page contains earth-shattering
> new physics, nor that it displays such blinding brilliance on my part that
> you should wear sunglasses while reading it, but none the less some might
> find it worthwhile. If you don't understand the Sagnac effect, take a
> look at it; maybe my point of view will help.
>
> And the effect certainly seems to put paid to ballistic theory, as Paul
> Andersen has been saying for ages and ages (though I never quite
> understood what he meant until I worked it out for myself).
>
> Comments welcome, of course!

If I'm not mistaken, working with a moving medium, in
the classical treatment you apparently use a dragged,
(source speed dependent) signal speed u- = k - v,
but in the relativistic treatment you use standard
undragged, source speed independent Lorentz composed
speed u- = (k-v)/(1-k v).
So you get a discrepancy between classical and
relativistic. That doesn't surprise me.

In other texts the effect is the same classically as
relativistically. Light speed is taken to be c (or unity)
and nothing is dragged or composed. No medium is used
and closing speed between signal and detector is used
in both cases. So there is no difference.

See for instance sections 1.1 and 1.2 of
http://www.physics.berkeley.edu/research/packard/Competition/Gyros/LaserRingGyro/Steadman/StedmanReview1997.pdf
| "Sagnac (1913) drew on purely classical ideas to
| predict the effect..."
and even more so:
http://www.mathpages.com/rr/s2-07/2-07.htm
| "This analysis is perfectly valid in both the classical and
| the relativistic contexts."
and
| "It's worth emphasizing that the Sagnac effect is purely a
| classical, not a relativistic phenomenon,..."

These are rather sharply contrasting with your line:
| "However, from the point of view of Newtonian mechanics,
| the effect is simply impossible. If anything, the Sagnac
| effect is a powerful demonstration of the validity of
| relativity."

Other than that, a few typo's/nitpicks:
- "fiber object" ==> "fiber optic" ?
- "vaccuum" ==> "vacuum"
- second part of equation (10)
g . -v .(-L_S) ==> g .( -v .( -L_S ) )
- "circular motion introduces centripetal acceleration"
==>
"circular motion requires centripetal acceleration"
or
"centripetal acceleration introduces circular motion"

Cheers,
Dirk Vdm


.

Relevant Pages

  • Re: Simple Sagnac
    ... >> Here's a simple view of the Sagnac effect: ... > If I'm not mistaken, working with a moving medium, in the classical ... implies the signal is light traveling in a vacuum. ... > | a powerful demonstration of the validity of relativity." ...
    (sci.physics.relativity)
  • Re: On the Similarities and Differences between Lorentz 1904 (LET) and Einstein 1905 (SR)
    ... "greywolf42" wrote in message ... All they needed was a reliable tick rate. ... Relativity explains the Sagnac effect just fine. ...
    (sci.physics.relativity)
  • Re: Simple Sagnac
    ... Ken S. Tucker wrote: ... >> that talk about the Sagnac effect: The math on the page includes nothing ... > Rocket scientists don't use relativity, ... relativistic correction, rocket science wouldn't ...
    (sci.physics.relativity)
  • Re: More M & Ms: so whats the story on this "Sagnac effect" then, anyway?
    ... > that the theory of relativity cannot explain ... > explain the Sagnac effect while maintaining ... But he explained them with the stationary ether concept. ... Also Sagnac works well with the Lorentz transformations. ...
    (sci.physics.relativity)