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Let me first reformulate the problem of the constant speed of light.  Tom (at the rear of the train) says that when you (your car) are at the the square flag, the photon is at the the triangle flag.  According to Tom and the sketch, you (in your car, arrived at the square flag) say: "When I am at the square flag, the photon is at the triangle flag." The distance traveled by the photon from Tom to the triangle flag is not the same as the distance traveled by the photon from you (the car) to the triangle flag. Relative to you the photon traveled a much shorter distance. This means that according to what I show on the spacetime map the photon's speed is not the same for you as for Tom. But we know from experience that the speed of light is the same for everybody. What's wrong with this Spacetime map?  For Tom the photon is at the at the triangle flag, and he thinks (and he is pretty sure about this because he has never heard of Einstein's Special Relativity) that you (the car) will confirm this. But ... you will NOT! For you, .... 

=> Go to ....
 Introduction
 Relativity for Dummies
 4D Spacetime Block Universe
 Reciprocal time dilation
 Reciprocal length contraction
 Lengths do not 'appear' contracted!
 Measuring the contracted train
 Constant light speed
 Where is the bullet?
 Train experiment
 Slower and shorter  case study
 Minkowski vs Loedel
 Spacetime video
 Further reading