Re: The Spin Proviso to Relativity

From: Ben Bean (kavs_delethis__at_sysmatrix.net)
Date: 10/21/04 

Date: Thu, 21 Oct 2004 19:25:43 -0400

"Ben Bean" <kavs_delethis_@sysmatrix.net> wrote in message
news:3M2dncxUAuaje-rcRVn-jg@sysmatrix.net...
>
> "Paul B. Andersen" <paul.b.andersen@hia.no> wrote in message
> news:cl899p$25n$1@dolly.uninett.no...
> >
> > "Ben Bean" <kavs_delethis_@sysmatrix.net> skrev i melding
> news:ktKdnS2bLsm8GurcRVn-rg@sysmatrix.net...
> > >
> > > "Paul B. Andersen" <paul.b.andersen@hia.no> wrote in message
> > > news:cl6d07$8jb$1@dolly.uninett.no...
> > > >
> > > > "Ben Bean" <kavs_delethis_@sysmatrix.net> skrev i melding
> > > news:B8SdncZcW_z2v-ncRVn-oA@sysmatrix.net...
> > > > > I am eager to hear wisdoms in answer to the quandary below stated.
> > > > >
> > > > > SCENARIO: You stand on a planet just like Earth, but there's no
> > > atmosphere.
> > > > > You stand on the equator and hold your hands up to the air so that
> they
> > > are
> > > > > a meter apart. [Relax, this is NOT a study in relative
simultaneity
> like
> > > the
> > > > > Barn/Pole thing.] As you stand there a huge spacecraft coasts by
> just
> > > > > overhead, just beyond your reach. The ship seems motionless to
you,
> > > > > hovering, because it is going eastward at a speed to exactly match
> the
> > > > > planet's tangential rotational speed. As you reach up, your
> outstretched
> > > > > fingertips are just about touching the 842-meter mark and the
> 843-meter
> > > mark
> > > > > on the rule graduated on the enormous ship's straight exterior.
> > > > >
> > > > > Here's the quandary: the spaceship occupants can EMPHATICALLY
assert
> > > that an
> > > > > all-way light beacon pulse emitted midway between their ship's 842
&
> > > > > 843-meter marks will hit the two nearby meter marks
simultaneously,
> > > > > according to their native frame's clocks and such. Yet the guy on
> the
> > > planet
> > > > > cannot make the same claim?? When does an arbitrary local span
> become
> > > > > tantamount to an SR scenario. In spite of Sagnac, there must
surely
> be
> > > some
> > > > > carry-over; I mean you're just about TOUCHING that other frame,
> > > comoving.
> > > > >
> > > > > -Ben
> > > >
> > > > Of course the guy on the planet will agree that the light will hit
the
> two
> > > metre
> > > > marks simultaneously. That is, if he had one clock at each side of
> > > himself,
> > > > and he E-synched those clocks, they would show the same when hit
> > > > by the light.
> > > > However, if the two clocks were showing UTC, they would NOT
> > > > show the same when hit by the light.
> > > > Clocks on the surface of the Earth showing UTC are NOT synchronous
> > > > in the Earth fixed frame. They are synchronous in the non rotating
> > > ECI-frame.
> > > >
> > > > Paul
> > > >
> > > >
> > > Great answer! Uh, but, whereas I know what ECI stands for, I am at a
> loss as
> > > to what UTC stands for.
> >
> > UTC means "Coordinated Universal Time", and to say it simple,
> > it is the same as local mean time.
> > See also my response to "sal".
> >
> > > But I think it matters not. You say that Earth's
> > > surface clocks can all be synched to the non-rotating ECI, which
> suffices.
> >
> > Yes, and I say that this IS how we synchronize clocks on the Earth.
> >
> > > I
> > > don't necessarily buy your dismissal of Sagnac (of course I am
> > > misinterpreting perhaps), but your answer suggests that a light signal
> takes
> > > the same time to go from NY to LA as the reverse, as long as you use
the
> > > non-rotating ECI clocks as your basis.
> >
> > UTC clocks on the Earth ARE "rotating" along with the Earth.
> > I said they are synchronised (simultaneously showing the same)
> > in the ECI-frame, NOT that they are stationary in the ECI-frame.
> > And if you measure the time light takes to go from NY to LA with
> > these UTC clocks, you will find that it is different from the time
> > it takes to go in the reverse direction.
> > This IS the "Sagnac effect".
> >
> > > I guess that makes sense alright, but
> > > it's insufficient. The guy reaching up and touching the inertially
> moving
> > > space ship -- he has a wrist watch on each arm and he claims they are
> > > synchronized. His clocks belong to a frame that IS rotating. I'm just
> not
> > > sure. Again, what is UTC, Universal Time something?
> >
> > If this guy claims his writs watches to be synchronized, he will
> > probably mean that they simultaneously show the same in
> > his instantly inertial rest frame. (The inertial frame in which he
> > instantly is at rest.)
> > And his clocks will stay in synch.
> > If he measure the speed of light with these clocks, he will find
> > that it will use the same time in both directions.
> >
> > But if he compare them to two adjacent UTC clocks, he will
> > see that they are different, because his watches are NOT
> > synchronous in the ECI-frame.
> >
> > > The central question is, "How does light behave in the frame of the
man
> > > standing with his arms outstretched (over his head), and ONLY
according
> to
> > > that man's native clocks & measures"? Can the man say that a light
> signal
> > > emitted midway between his hands arrives at each hand at precisely the
> same
> > > time? Probably not.
> >
> > Yes, he can.
> > He will use his local frame.
> >
> > > But light would clearly not move relative to a
> > > theoretical aether fixed at the planet's center either. So there must
be
> > > some give. Light must take less time to go westward than eastward (on
> the
> > > spinning planet), which would be in line with the Sagnac findings, but
> not
> > > so much less time as would be predicted by imagining the light
> travelling
> > > through some fixed aether frame anchored at the planet's center.
> > >
> > > It's confusing alright.
> >
> > Yes, it IS confusing. :-)
> > But remember this:
> > According to SR, the speed of light is c _in an inertial frame_.
> > As long as you remember this, it is quite obvious that if you
> > emit light in both directions from some point on the equator,
> > and guide the light (mirrors) around the Earth, the two light beams
> > will meat each other at the same point _in the ECI frame_
> > as they were emitted from. But then the point on the Earth
> > has moved, so one of the light beams has already passed
> > this point, while the other one has not yet reached it.
> > So _because_ the speed of light is c in the ECI-frame,
> > the two beams will NOT simultaneously reach the point
> > on the Earth from where they were emitted.
> > A clock at that point will thus measure the light to use
> > different times around the Earth in opposite directions.
> >
> > Paul
> >
> >
> >
>
> **********************************
> I REALLY REALLY GOT TO HAND IT TO YOU, PAUL B. ANDERSON...
> That's some mighty fine STRAIGHT SHOOTING and you sure as dang heck got me
> to understand PERFECTLY!
>
> The guy can invent an inertial frame in which his two wristwatches are
> synched (but NOT E-synched, which is WRT the non-rotating ECI frame), so
> long as the times and distances are minute enough. I garner this from your
>
> > > ..Can the man say that a light signal emitted midway between
> > > his hands arrives at each hand at precisely the same
> > > time? Probably not.
> > Yes, he can.
> > He will use his local frame.
> >
>
> It must get messy if you try to concoct an inertial frame (too large) to
> encompass eg. the span NY to LA for the duration it takes light to
transit.
>
> Yeah but nevermind -- I get it -- MANY MANY THANKS!
>
> -BB
>
>

PBA:
I'm here to fix an apparent misunderstanding I had of your terminology. What
E-synched (capital E) implies I'm not 100% certain, but it is NOT
specifically associated with UTC. The standing man can synchronize the
watches on his left and right hands WRT a concocted inertial frame that
closely approximates the actual arc he is moving on, and thus conclude that
any light/radio pulse emitted halfway between his hands arrives at those
hands simultaneously. This works for the suitably small scale, but not the
large?? ie. NY to LA.

HONEST! I understood every word of your explanation (Many thanks) about how
Sagnac truth messes up any hope of surface-resident synchronized clocks,
except by resorting to UTC (which is ECI-based); but under that UTC scheme
you cannot make the claim which the standing man so yearns to make, ie. that
light will expand outward uniformly east/west from a source point, per his
vantage.

Further jabber would be welcome, too; if the above isn't right on.

-Ben

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