Re: Motion Through Space Is Meaningful


"2ndPostulateDude" <cadwgan_gedrych@xxxxxxxxx> wrote in message news:1126126018.569567.131290@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Dirk Van de moortel
> Dirk wrote:

[unsnip]
> > On can define the "true physical length" of a rod as its
> > proper length, i.o.w. as its length as measured in its own
> > restframe. But that is a matter of taste.
[/unsnip]
> > Comoving with a rod, from one side of it send a signal
> > at time ts toward other side of it. Receive an echo at
> > time te. Proper length of the rod = 1/2 c ( te - ts ).
>
> Since your entire distance analysis was based on your
> above (i.e., on proper length)

It was not.
This definition was based on something far more elementary.
You seem to have some kind of reading problem.

>, and since proper length
> is based on the baseless assumptions that (a) rod's do
> not physically contract, (b) clocks do not physically
> slow, and (c) Einstein's clocks are properly related
> temporally, you have not disproved my claim that SR
> cannot correctly measure even relative speeds correctly.
>
> To expand on my above:
>
> (a) You have no proof that half the round-trip light travel
> time gives the rod's physical or actual or intrinsic length.
>
> (b) You cannot assume that Einstein's clocks can correctly
> measure the round-trip time because his clocks may be
> intrinsically slowed.
>
> (c) You cannot assume that Einstein's clocks can correctly
> measure the round-trip time because his clocks may not
> be correctly related.
>
> You just ran around in a circle, and bit yourself in the
> arse.

It seems that you are severely obsessed with something.

I have explained what I can choose to define when I assume
something (isotropy of light speed), and that, doing so, I can
measure the speed of an object - without using a rod.
Indeed I have no proof that light speed is isotropic.
I say that, assuming that it is, I can choose to define time and
distance of a remote event the way I implicitly explained in the
part that you snipped:

Send a signal to an object at time ts, receive echo at time te.
I can define time and distance of reflection event on the object
as
t = 1/2 ( te + ts )
d = 1/2 c ( te - ts )
In this setup, there are no rods, no multiple clocks, and
certainly no Einstein's clocks. There is one clock in my hand
and there is a light signal and an echo of the signal, both of
which I assume I will measure to have the value c.

And therefore, without using rods:
Send signal1 to object at time ts1, receive echo at time te1.
Send signal2 to object at time ts2, receive echo at time te2.
Time and distance of reflection event1
t1 = 1/2 ( te1 + ts1 )
d1 = 1/2 c ( te1 - ts1 )
Time and distance of reflection event2
t2 = 1/2 ( te2 + ts2 )
d2 = 1/2 c ( te2 - ts2 )
Average speed of object = abs ( (d2-d1)/(t2-t1) )
I measured a speed without using a rod.

This is what SR can do, whether you like it or not.

[ Since you seem to be obsessed by rods and since you
somehow seem to believe in absolute motion, I will make
it easier for you. Sitting absolutely still in the absolute frame
of your choice, holding a rod, from one side of it send a
signal at time ts toward the other side of it. Receive an
echo at time te. You read those times on the clock in your
hand at the sending side of the rod. If you think that absolute
stillness is possible, then you can (but are not obliged to)
define the Absolute Proper Length of the rod as the value
1/2 c ( te - ts ) - provided you assume light speed
isotropy in that absolute frame of your choice.
But that is a matter of taste. You could just as well call
the expression the "Brian D. Jones of the rod". ]

Dirk Vdm


.

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