Re: Limitations of Sagnac generality

From: Bill Hobba (bhobba_at_rubbish.net.au)
Date: 11/16/04 

Date: Tue, 16 Nov 2004 12:25:23 GMT

"Ben Bean" <kavs_delethis_@sysmatrix.net> wrote in message
news:WtadnZYcwdp3ewTcRVn-oA@sysmatrix.net...
>
> "Bill Hobba" <bhobba@rubbish.net.au> wrote in message
> news:5Nhmd.37814$K7.24406@news-server.bigpond.net.au...
> >
> > "Ben Bean" <kavs_delethis_@sysmatrix.net> wrote in message
> > news:leCdnWPT7t2HxgTcRVn-gg@sysmatrix.net...
> > > [...]
> > > Look at the
> > > local football stadium, oriented east-west perchance. You invent a
> > perfectly
> > > valid inertial frame that is momentarily (when the test run starts)
> > tangent
> > > to Earth at the stadium's 50-yard line, and it is moving eastward WRT
> the
> > > ECI frame.. moving at Earth's local tangential velocity.
> > > That's a perfectly
> > > valid FoR and according to it,
> >
> > But not interial.
>
>
> Ah but it is inertial. You expressly define one that is inertial. It
doesn't
> rotate with earth, it is tangential to earth, and only touches the 50-yd
> line "momentarily", as I clearly stated. The frame of which I spoke IS
> inertial (discounting gravity of course and discounting earth's solar
orbit
> et al for the sake of this examination).

That is the point - what you described has gravity present so is not
inertial. Now from memory ECIF stands for earth centered inertial frame,
which, again if memory serves me correctly, describes a satellite in orbit -
it certainly does not describe a frame tangential to the earth. But as a
matter of principle even a satellite in orbit is not a strictly inertial
frame due to tidal forces - even though it would be pretty damn close. But
if you are considering matters of principle - which is what it looks like
you are doing - then we had best be careful.

>
>
> >
> > > EM signals sent to either goal post from the
> > > 50-yd line will definitely arrive simultaneously.
> >
> > Well actually no.
>
>
> No?? Which one hits first then, in that concocted IF? Whoa! don't try to
> answer that until you first see that the tangential frame of which I spoke
> IS an inertial one.

Remember you have gravity so in what you describe - a frame that momentarily
is not participating in the earths movement - then during the time period
the gravity will be 'cutting' across the frame so it is nether inertial nor
symmetrical. However on second thought you may be considering a frame in
free fall in which case we will be left with tidal effects - but those
effects will be symmetrical about the 50 yard line so if that is what you
meant then yes I would say the light beams would meet simultaneously - but
even then the frame is not strictly inertial.

>
>
> >
> > > If you fiddled a bit with
> > > your defined tangential IF, then you could concoct one where light
hits
> > the
> > > east goal post first, and you could concoct one a tad different in
which
> > > light hits the west goal post first. How can one fundamentally presume
> > then
> > > that "in actual truth", it hits the west side first? I'm not certain
> that
> > is
> > > a valid deduction from Sagnac's test results. Just because rotation is
> > > unequivocal doesn't mean that the ECI frame, and detections made from
> the
> > > ECI frame, have a priori validity/truth. Surely zillions of
relativists
> > > would volunteer without hesitation: "Oh, the westbound signal from the
> > > 50-yard line definitely/absolutely hits first". And that, I opine, is
> not
> > > quite telling it like it is.
> > >
> > > If anyone out there with great savvy agrees, then we're done
quibbling,
> > > period.
> >
> > Rather than make sarcastic comments that indicates you have no idea what
> you
> > are talking about why not study SR? BTW SR deals with inertial
reference
> > frames - frames attached to the earth, while for many practical purposes
> are
> > inertial, in fact they are not. Learn to crawl before you can walk.
> >
> > Bill
> >
> >
>
> Kewl. But I wasn't employing sarcasm, and the tangential frame I cited is
> inertial.

You admitted gravity was present - thus it is not inertial. However it
should be easy to unambiguously describe what I think you are trying to get
at (which I think is a frame in instantaneous free fall) that would do what
you suggest. However I fail to see the point.

Bill

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