Third Effect: Simultaneous Events Are Not Simultaneous
Two events, simultaneous for an immobile observer, are not necessarily simultaneous for a second observer moving fast enough.
The third effect is the strangest of all. It states that the chronology of events can change for objects moving at high speed. The general rule is that the events at the rear of a moving object tend to happen earlier than normally expected. For the application of this rule, the front is defined as the direction of the movement.
For example, consider two students raising their hands at the same time in a bus. When these two students look at each other, they agree on the fact that they are synchronized (because they are not moving one relative to the other). However, for an observer looking at the bus moving toward or away from him, the two students will be desynchronized. For a bus moving to the right, away from the observer, he will see the student on the left raise his hand first. For a bus moving to the left, toward the observer, he will see the student on the right raise his hand first.
This third effect is also called the relativity of simultaneity: as you get more speed, your reality desynchronizes relative to others.