Re: Understanding SR - simultaneity


"Nicolaaas Vroom" <nicolaas.vroom@xxxxxxxxxx> wrote in message news:JTifg.457519$cz.12152965@xxxxxxxxxxxxxxxxxxxxxxxx

"Dirk Van de moortel" <dirkvandemoortel@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
schreef in bericht news:6mWeg.455490$Ti3.11978818@xxxxxxxxxxxxxxxxxxxxxxxx

The diagram is for you to express yourself properly, and for
us to understand you properly.

Correct.
The Question is does it corresponds with the physical reality.

That is my problem.
When I perform experiment 3 for any speed
does the observer R see the signal and front simultaneous ?

Depends on what you want. Your drawing says he does.
That implies that the train's proper length is equal to the
distance between L and R multiplied by gamma.

Correct.
That implies that you have to define in advance for which speed v
you want to perform this test because you have to correct the proper
length at v= 0 in order to compensate for length contraction
when v not equal to 0.

Is that person the same as when
"I" would perform experiment 2,
being Observer R and measure no difference ?
I expect "your" answer is Yes.
But how would you convince "me".

I made the same mistake twice. Sorry.
This should have been experiment 3.

The correct text is:

Is that person the same as when "I" would perform experiment 3,
being Observer R and measure no difference ?
I expect "your" answer is Yes.
But how would you convince "me".

Again, I have no idea what you mean. I don't understand that
question.

My problem is that when you perform this experiment
and when you take length contraction into account
should Observer R actual measure no difference.
Accordingly to SR there should be no difference.

The problem is you cannot perform an actual experiment
to demonstrate this because of the dimensions involved.
That means I'am not convinced that SR is correct.

We also can't perform an actual experiment where we verify
the velocity composition when someone fires a bullet from a
moving car.
We never performed an experiment where we dropped you
from a building. I don't think that means that you are not
convinced that you will fall to ground.
If you want to learn about experimental verification of
special relativity, have a look at
http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html
If that is not enough for you, then ...


My problem is what is the correct reasoning to convince me
that SR is correct to describe the outcome of experiment 3.
Is it something described in this document:
http://farside.ph.utexas.edu/teaching/em/lectures/lectures.html
http://farside.ph.utexas.edu/teaching/em/lectures/node114.html

In the following document is written:
http://users.pandora.be/nicvroom/simultaneity.jpg
"From the configuration it is clear that A will judge that the two events
occur simultaneously"
Unfortunately for me it is not clear that when the events occur
simultaneous that A will see the flashes simultaneous.

If the events are simultaneous in the frame of A and A is
halfway between the flashes, then the flashes are simultaneous
*according to A*, and then A will see the flashes simultaneously
because we assume that light speed is the same in both directions.

Accordingly to the drawing it does but IMO that is no reason
to accept that it actual does.

There is a reason why we assume that light speed is independent
of direction.


Of course when A sees them simultaneous than B will not see
them simultaneous.
That is simple.
But the reverse is also possible.
That is my problem, or is it lack of understanding ?

I think you missed something very basic somewhere, but I still
don't know what. If you try to make the single spacetime diagram,
we might find out...


Since the remainder of what you write after this is just a repeat
of what you wrote before, I will snip, but my comments stand.

Most important:
Draw everything on one single spacetime diagram.
Mark the two pairs of signals with different colors.

By the way, do you realize that for the train observer (for instance
at B, but also at the moving mirror places a and b) the events
where the signals hit the mirrors are simultaneous? I.o.w. you can
draw a line between the place where you marked t1 on mirror a,
and t4 on mirror b. This line is parallel to the x'-axis of the train.
You can also look at the a-line (or the B-line, or the b-line, or
any other line parallel with it) as the t'-axis. It helps when you
explicitly draw these lines.

I fully agree with you, but currently this not under discussion..

But this is:
Draw everything on one single spacetime diagram.
Mark the two pairs of signals with different colors.
....

Dirk Vdm


.

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