"Paul B. Andersen" wrote in message
...

"Ben Bean" skrev i melding
...

"Ben Bean" wrote in message
...

"Paul B. Andersen" wrote in message
...

"Ben Bean" skrev i melding
...

"Paul B. Andersen" wrote in message
...

"Ben Bean" skrev i melding
...
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 wris****ches 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.

More or less, yes.
Your shouting tells me that elaboration would be futile.

You're one sick & slimey limey, boy.


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.

Let me say it like this:
I can claim that the light will expand uniformy from
the point it is emitted _in the inertial frame in which he
is instantly at rest when the light is emitted_.
He is not stationary in said frame, but during the 2 ns the light
takes to reach his hand, he will have moved 10^-33 meters
from the point of emission.
But 12 hours later, the light he emitted westwards will
be 12 light hours east of him.

Paul

"Ben Bean" wrote
...
You're one sick & slimey limey, boy.

Thanks to you to.
It's always a pleasure to explain something to
someone who appreciates it.
I can see that my time was a good investment.

Paul

"Paul B. Andersen" wrote in message
...
Your shouting tells me that elaboration would be futile.

"Ben Bean" wrote
...
You're one sick & slimey limey, boy.

It's always a pleasure to explain something to
someone who appreciates it.

Like the garden variety hypocrite, you can dish it out but you can't take
it.