Relativity
The Special and General Theory

UP to now our considerations have been referred to a particular body of reference, which we have styled a "railway embankment." We suppose a very long train traveling along the rails with the constant velocity v and in the direction indicated in Fig 1. People traveling in this train will with a vantage view the train as a rigid reference-body (co-ordinate system); they regard all events

in reference to the train. Then every event which takes place along the line also takes place at a particular point of the train. Also the definition of simultaneity can be given relative to the train in exactly the same way as with respect to the embankment. As a natural consequence, however, the following question arises:

Are two events (e.g. the two strokes of lightning A and B) which are simultaneous with reference to the railway embankment also simultaneous relatively to the train? We shall show directly that the answer must be in the negative.

When we say that the lightning strokes A and B are