The Railroad Analogy


Albert Einstein often used a railroad as an analogy to explain relativity to lay people, specifically, the concept of simultaneous events.  It goes something like this.  Imagine you’re on a train that’s stationary on a track.  Two bolts of lightning hit the track, one in front of the train, and one behind it. Both lightning bolts strike the track at the same distance from the train.  A person standing in the middle of the train would perceive both strikes as simultaneous.  A person standing on the side of the track would also perceive both strikes as simultaneous.

Now imagine that the train is moving.  Two more bolts of lightning strike the track, equidistant to the train.  The person standing next to the track still perceives both bolts as hitting the track simultaneously.  The person on the train has a much different experience, though.  To him, the bolt that struck the track in front of the train happened slightly before the bolt that struck behind the train.  The light from the lightning travels at the speed of light, so the light from one bolt reaches the train before the other because the train is moving towards one bolt, and away from the other.

Since we’re all in a constant state of motion, relativity affects all of our perceptions.  Most of the time, we’re not aware of the effect because we all occupy the same relative state.  We all orbit on the same planet, so we perceive the solar system as if we’re on the moving train.  If it were possible to leave a satellite in a stationary position outside our solar system, we might find that our perception of our own home isn’t what we thought.

2 Responses to “The Railroad Analogy”

  1. ian Says:

    but did the lightning hit it at same time..whether he sees it or not..that’s the real question..

  2. Travis William Hecox Says:

    I bet the person on the train would percieve it as simultaneous also simply because the speed of light is so fast, anything at a distance visible to the naked eye on this planet would perceivably be visible instantly. so any distance difference is imperceivable by timing visually. The sound of the thunders from the lightning strikes, on the other hand, would probably be noticably not simultaneous.

    Einstein sucked at analogies is my conclusion.

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