Re: length dilation

evabern <brothenstein@xxxxxxxxx> wrote in message
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On Feb 15, 8:28 am, "Dirk Van de moortel"
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evabern <brothenst...@xxxxxxxxx> wrote in message

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On Feb 14, 10:55 pm, "Dirk Van de moortel"
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On Feb 13, 10:19 pm, "Dirk Van de moortel"
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Dirk Van de moortel <dirkvandemoor...@xxxxxxxxxxxxxxxxxx> wrote in message
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evabern <brothenst...@xxxxxxxxx> wrote in message
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Conider a rod of proper length L(0) at rest in I' and located along
the x' axis. Observers of that frame detect simultaneously the spac
coordinates of the two ends associated with events
1'[x'(1),t'] and 2'(x'(2),t').

They don't have to detect them simultaneously since the end
points of the rod are going nowhere in their frame. You can
take any pair of sparks to measure the length its rest frame.
You must make sure the sparks are simultaneous in the frame
in which the rod is *moving*.

Detected from I the same two events are
characterized by the space coordinates
x(1)=g[x'(1)+Vt']
x(2)=g[x'(2)+Vt']
resulting that the measured length of the moving rod is
x(2)-x(1)=gL(0)
i.e. a length dilation takes place. g stands for the Lorentz factor.

If the rod is moving in I, then you must use sparks that are
simultaneous in I.
You would not measure the length of a moving train by measuring
the distance to the front now, and the distacne to the back 10 minutes
later. Your train would be ridiculously long.

Did Einstein take into account that measurement procedure? I mention
that gL(0) is the component of a four vector whereas L(0)/g is not!
Thanks for your point of view.

Here's how you do it the proper way.
If at rest in I, then
L = x(2) - x(1)

Typo. That should be:
If at rest in I', then in I you get:
L = x(2) - x(1)

Then, transformed to I', where the time does not matter:
x'(1) = g ( x(1) - V t )
x'(2) = g ( x(2) - V t )
so
L(0) = x'(2) - x'(1) = g ( x(2) - x(1) ) = g L
and thus
L = L(0) / g
Thanks for your answer. Nobody can interdict the simultaneous
detection of the ends of the rod even if the result of the measurement
is time independent!

The time of DETECTION of the sparks is of no importance.
The time of OCCURANCE of the sparks is important.

If you want to use the Lorentz transformation for this, then
you are going to use only two sparks (i.e. events) to allow
two observers to measure the length of a rod. So you must
make sure that you take two sparks that occur simultaneous
for the observer for which the rod is moving (think of the
train). The time (of occurance) of the sparks is of no imporance
for the observer for which the rod is at rest. You can measure
the distance to the front of a non-moving train now and the
distance to the rear next week. The difference is the lenght,
provided the train does not move - for you.

Photographic detection of the ends of the moving rod by stationary
observers could lead to length dilation, length contraction and no
distorsion depending on the way in which the rod is located in its
rest frame. If the middle of the rod is at the origin O' and
stationary photogtapher is located at O', then length expansion takes
place. My intenstion is to show that length dilation can be achieved
by "simultaneous" detection in the rest frame of the rod. Is there
something wrong in that?

Simultaneous DETECTION halfway in the restframe implies
simultaneous OCCURANCE in the rest frame, which in turn
implies NON-SIMULTANEOUS OCCURANCE in the
moving frame. And that is ridulous. I would NOT measure
the length of a moving train by measuring the distance to the
front now, and the distance to the back 10 minutes later. My
train would be ridiculously long.

Is this really so hard?

Dirk Vdm
Thanks again. Please have a look at Roy Weinstein, "Observation of
length by a single observer,"
Am.J.Phys. 28, 607 (1960). I quote from the abstract
"...It is shown here that the length seen by a single observer is not
the usual contraction and indeed under certain circumstances, one sees
a body lenghtened rather then contracted."
Kind regards

I am looking athttp://www.wbabin.net/physics/rothenstein27.pdf
where Bernhard Rothenstein and Marius Costache carefully and
elaborately measure the length of a moving train by measuring
the distance to the front now, and the distance to the back 10
minutes later. Then they subtract the numbers and call it the
length of the train.
That is not a careful and elaborate discovery of length dilation.
It is a careful and elaborate demonstration of stupidity.

Dirk Vdm

That is my last invitation to the literature of the subject
V.N. Strel'tsov and M.S. Khvastunos, "Invariance of interval and
length in the theory of relativity, Russian Physics Journal, 38, 213
1995. Have you seen Weinstein's paper?

Yes of course I have. It is online at
http://3bl.azazelo.org/3bl/wp-content/uploads/2007/05/observation_length.pdf
It is a carefully worded and well written article. Weinstein knew
what he was talking about. Unlike Rothenstein who clearly is a confused
crackpot with no idea what he talking about or dealing with. Just have a
look at one of hs other painfully pathetic attempts at playing physicist:
http://www.wbabin.net/physics/rothenstein25.pdf
This is just plain horrible.

I hope you use other sources to get introduced to this subject.
Did you ever have a high school introduction to basic physics?

Dirk Vdm


I consider that our discussion should be stoped.



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