Re: Understanding SR - simultaneity


xray4abc wrote:
Nicolaaas Vroom wrote:
"xray4abc" <lemhenyil@xxxxxxxx> schreef in bericht
news:1150823905.958529.85710@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Nicolaaas Vroom wrote:
1. Consider an Observer A standing at the centre between two mirrors.
Observer A. sends out two lightsignals
i.e. one towards each mirror.
Each signal will be reflected by its corresponding mirror.
This will define two events.
IMO Observer A will SEE the reflected signals simultaneous.
I think everyone will agree. Sofar no problem.

2. Place this whole experiment on a moving platform
(this becomes a train in the next experiment)
including the mirrors
and lets us perform this same experiment.

Observer A is now called Observer B.
Observer B sends out two lightsignals.
IMO again as before Observer B will also
see the reflected signals simultaneous.
(independent of his speed v relative to Observer A).
I think everyone will agree. Again sofar no problem.

3. Inbetween the mirrors, before and after we build a track.
At the track there is a train, which has the same length
as the distance between the mirrors.
The train starts left from the mirrors.
There is an Observer at rest at each mirror called R and L
We again will perform the same experiment as in #1.
That means Observer A will send two lightsignals.
The Observers at the mirrors have an important task
when they see a light signal (being reflected)
The Observer R at the right mirror should observe the position
of the front of the train.
The Observer L at the left mirror should observe the position
of the back of the train.
We will perform this experiment in such a way that when Observer L
sees a lightsignal he also sees the back of the train.
That means the position of the mirror and the Back of the train
coincides.

Question: will Observer R at the right mirror also see that
there is a signal which coincides with the front of the train ?

Accordingly to SR it should.
See for example the the book "Introducing Einsteins Relativity"
by Ray d'Inverno page 23.
In that case the light signals are generated by Firing Devices
which are at the position of the mirrors and which are observed
simultaneous by Observer A.
(If this is not clear I will explain it in more detail)

4. The same as above but know we will perform experiment 2.
That means Observer B at the centre of the train will send two
lightsignals to the moving mirrors.
The Observers R and L stay at rest near the mirrors at rest

We will perform this experiment in such a way that when Observer L
sees a lightsignal he also sees the back of the train.
That means the position of the moving mirror at the Back of the train
and the observer with a mirror at rest all coincide.

Question: will Observer R at rest at the right mirror also see that
there is a signal which coincides with the front of the train ?
(and which is reflected by the moving mirror)

If the Observer R in experiment 3 sees that the signal and the front
of the train coincide than that should also be the case in experiment 4.

IMO there should be no difference because Observer B
(as established in #2) also sees the two lightsignals simultaneous.

5. The same as above but know we will perform
both experiment 3 and experiment 4 "together."
That means Observer A at the centre of the track will send two
red lightsignals.
That means Observer B at the centre of the train will send two
blue lightsignals.

We will perform this experiment in such a way that when Observer L
also sees the back of the train he also receives (sees) the two
lightsignals
That means the position of the moving mirror at the Back of the train
and the mirror at rest coincide.

Question: will Observer R at the right mirror also see that
the two lightsignals (red and blue) and the front of the train
(moving mirror) coincide?

Accordingly to the results of experiment 3 and 4
(Based on the results of experiment 1 and 2)
it should
however this raises a serious issue.
For Observer L because he receives both signals simultaneous
this becomes one event and one reflected signal
If Observer R also sees/receives the two signals simultaneous
then this becomes also one event and one reflected signal.

The problem is how can this combined signal be both
seen simultaneous by Observer A and by Observer B ?

In experiment 3 a moving observer B will not see the
reflected signals generated by A simultaneous.
(If Observer R sees the signal and the front simultaneous).
In experiment 4 observer A at rest will not see the
reflected signals generated by B simultaneous.
(If Observer R sees the signal and the front simultaneous).

This raises a problem if it is possible to combine the two experiments
such that the two left signals combine in one (and the two right signals)
All of a sudden Observer A should see the two reflected signals
simultaneous and not simultaneous and that is impossible.
The same for Observer B.

I hope that my description is clear.

http://users.pandora.be/nicvroom/

Nicolaas Vroom

I will not show what your mistake is, as it would take
too much of my time.

I do not think I made any mistake.
Except that it is possible to describe the issues involved
in a simpler way.

Instead, I will give you some
clues of where I consider the origin of the difficulties is.
I would suggest you, to take into account the fact, that
the occurrence of 2 simultaneous events and the
seeing (i.e. perception) of them as 2 simultaneous events
are 2 completely different things.

I fully agree.
You can start with two (simultaneous) events.
The question where should the Observer be situated
to SEE them as simultaneous.
The example I want to understand is described at:
http://users.pandora.be/nicvroom/simultaneity.jpg
This example describes a one dimensional situation.
That means the events and the Observer(s) are all on one line.

Accordingly to the text the Observer that sees the two events
simultaneous should be at the center of the two FD's
I do not understand that.
Why can the Observer that SEES them simultaneous
not be at a different place, for Example at the moving train ?

It is a very bad and complicated example !

Have you ever considered why electrodynamics isn't taght in
primary school? That is the simplest example there is because
it IS the fundabental mechanism of the Lorentz force.

You can simulate the delay dragging a roller skate with a
gumband. Yank on the gum band, theier is a delay before
the skate move. The band simulates the Coulomb force
the mass of the skate simulates the electrons mass:

http://physics.nist.gov/cuu/Images/alphaeq.gif
http://physics.nist.gov/cuu/Constants/alpha.html

Sue...

( These things are difficult to be judged even when we consider
only one source of light/signal ! )
By my opinion,
the person who wrote the explanation is surely wrong!
First of all, the firing method uses the assumption that there is
no length contraction/dilation and the length of the moving train is
the
same with the distance of the firing devices on the embankment.
The light sources , according to the drawing, are installed "in"
the train. Then, the Observer from the train is the one who can
have 2 simultaneous events.
(The Observer A will receive light then from 2 moving sources !)

One thing is for sure that if Observer A SEES them simultaneous
than Observer B does not (That is easy to understand).

(And visa versa)
It is considered as such, yet I would say......I am not
so sure. I could give a counter-example, where 2 events are
simultaneous in IRF 1 and the events have different values of
time associated in IRF 2 , yet this thing
as a matter of a principle DOES NOT SAY
ANYTHING about the simultaneity of the events in IRF 2.
To understand what I am saying, think of observer A or
B ( the one who is supposed to see simultaneous events in
the example given by you) like NOT BEING in the middle of
the distance between light sources. Then he does not record
simultaneity ! So, not seeing simultaneity does not mean
anything after all. It's question is still open .

What makes this example more difficult is the question if
Lorentz Length Contraction is involved ?

That is why the discussions should be limited to examples
with only one source of the signal!

That is why one needs to
define a reasonable procedure, to select from the "seen" events
the ones which "are" simultaneous, without leading to a
conflicting notion of time.
LL

Nicolaas Vroom

.

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