Comments about the book "Introducing Einstein's Relativity" by Ray d'Inverno.

• The text in italics is copied from the book
  
• Immediate followed by some comments
  

In the book at page 23 we read:

(1) Imagine a train travelling along a straight track with velocity v relative to an observer A on the bank of the track. (2)In the train, B is an Observer situated at the centre of the train. (3) We assume that there are two electrical devices on the track which are the length of the train apart and equidistant from A. (4) When the train goes over these devices, they fire and activate two light sources (LS) situated at each end of the train. (5) From the configuration it is clear that A will judge that the two events, when the light sources switch on, occur simustaneously. (6) However B is travelling towards the light emanating from light source 2 (LS2) and away from light emanating from light source 1. (7) Since the speed of light is constant, B will see the light from source 2 before seeing the light from source 1 and so will conclude that the light source comes before the other."

                                             t3
                                            ..                 
                                          . .                 
                                        .  .                 
                                      .   .                 
                                    .    .                 
                                  .     .                 
                                .      .                 
                              .       . t2                
                            .        .  .                 
                          .         .     .                 
                        .          .        .  
                      <-----------B----------->
                     FD1                     FD2
---------------------ED1---------------------ED2-------------------
                     LS1          A          LS2              
                        .         .         .
                          .       .       .
                            .     .     .
                              .   .   .
                                . . .
                                  .
                                  t1
                                  

ED1 and ED2 are the two Electrical Devices on the Track.
LS1 and LS2 are the two Light sources on the Track.
FD1 and FD2 are the two Firing Devices on the train.
The bottom part represents the situation for Observer A at the track at equal distance between the Electrical Devices. He will see the two light sources simultaneous at t1.
The top part represents the situation for Observer B, at the center of the train at equal distance between the Firing Devices. He will the light from LS2 at t2 and light from LS1 at t3.

Reflection

There are many problems with this thought experiment.

1. There is no mention that Length Contraction is involved. That means that the length of the moving train l (i.e. the difference between the two firing devices on the train) is less than the length l0 of the train at rest i.e. the difference between the two Electrical Devices at the track. If the length of the train decreases than FD1 will fire before FD2 and A will not see the two lightsignals from LS1 and LS2 simultaneous.
2. The text at line 5 starts with "From the configuration it is clear etc". I agree from the drawing it is clear, but is it also true in reality ?
3. In Wikipedia the same subject is discussed. See Relativity of Simultaneity. It is interesting to study the two experiments. In the book by "Ray" starting point is a physical experiment with two simultaneous events. In Wikipedia starting point is one event (a light source shining in two directions) which results in two events (i.e. the reflections on the mirrors). Those two events are declaired simultaneous. In effect this experiment demonstrates clock synchronisation.clock synchronisation

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