Created by: roberto.c.alfredo in physics on
Suppose two events happen far apart.
At event A, a detector clicks. At event B, another detector clicks somewhere else.
One observer says A happened first.
Another observer, moving relative to the first, says B happened first.
Neither observer made a mistake.
That sounds dangerous. If relativity allows people to disagree about which event came first, what stops one observer from seeing an effect occur before its cause?
The answer is that relativity does not treat every pair of events the same way.
For events that could causally influence each other, their order is protected. For events that are spacelike-separated, their order does not have to be.
That is not a loophole in causality. It is part of the structure that preserves it.
1. Begin Outside the Light Cone
Imagine events A and B on a spacetime diagram.
They are separated by enough distance, and little enough time, that even light could not travel from one to the other before both have occurred.
In other words, they are spacelike-separated.
Neither event can have caused the other. A light signal leaving A would reach B's location too late. A light signal leaving B would likewise reach A's location too late.
The light cone already tells us that much.
But now ask a different question:
Which event happened first?
It is tempting to assume that the universe must contain one correct answer, even if the two events cannot affect each other.
Special relativity says otherwise.