Let’s have a look at a relativistic train robbery

There are many strange twists and turns in this high speed drama

Some readers may not know that in physics the speed of light is absolute and nothing can travel faster than it. The relativistic train robbery story below has been designed to help you better understand what this means in terms of phenomena taking place in different locales in relationship to the speed of light (sometimes called as ‘C’).

You will see that there is a moving train, a train robber, and two train guards depicted in the story. The text describes how each of these for entities (including the moving train) are relativistically connected to each other in quite different ways n relationship to the speed of light. This includes the firing of two different types of guns by the two guards.


Let’s board this mysterious train

In the diagram below there are two variations on the same event. In the first instance the guards in the upper half of the diagram are armed with stun guns that fire bullets. In the lower half of the diagram the guards are armed with stun guns that fire beams of energy at the speed of light (sometimes referred to as ‘C’).

trains master copy

In the case of the upper diagram the speed of the train itself is a factor in the firing of the stun guns. Guard A discovers the train is being robbed. If he fires a warning shot near the robber, the stun bullet would hit the ground near the train at 150 mph. This is because the speed of the train is added to the speed of the stun bullet.

Guard B is on top of the stationary tower next to the moving train. Since the train is traveling at 50 mph, her stun bullet would hit the robber at 50 mph. In such a situation one has to subtract the speed of the stun bullet from that of the speed of the train, (i.e. 100 mph less 50 mph) to arrive at the correct speed. This is because the robber on the train is already traveling at 50 mph.

When guard A fires a stun bullet that hits the robber no calculation for speed adjustment is needed. This is because guard A is on the train, so he and the robber are traveling at the same speed. The effect cancels out as if the guard and robber are both motionless. The stun bullet would hit the robber at 100 mph.

In the lower half of the diagram, both the guards are using stun guns that fire beams of energy (possibly lasers, for this example it is not important). The respective beams from both guns travel at the speed of light. (The speed of light is a constant.) No calculations about changes to speed are needed here. The speed of light does not change, no matter what motion the origin of the light, or motion of the target of the light (in this case laser light). In such a situation the beams of energy from both stun guns hit the robber at the same speed, i.e. the speed of light.


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