The relativity of Simultaneity
  • Simultaneity refers to the idea that different things can happen at the same time.
  • Einstein's introduction of Special Relativity complicated this idea.
  • The principle of simultaneity is that: Events that are simultaneous in one frame of reference will not be simultaneous in another frame of reference.
  • This is explained through a thought experiment (Click here for a demonstration):
    • A man at Point C, equidistant from Point A and Point B, observes a railroad
    • The instant the man at Point D passes by Point C in a train moving at a relativistic speed $v$, Lightning strikes at Point A and Point B
    • To the observer at Point C, the strikes will be simultaneous as he is equidistant from A and B
    • Yet, by the time the light waves reach the observer on the train, the train has travelled $vt$ to Point D'.
    • Thus the light waves from B take less time to reach the observer than light waves from A as the distance is shorter.
    • Therefore, the person in the train would perceive that Lightning struck B before it struck A.
  • Although there is a difference in the perception, Einstein states that both the observations are correct.