Causal order

The iron chains of causality link events together into a definite order: a cause always precedes its effect. If, however, there are distant events that do not influence each other, what decides their order?

As noted above, time dilation implies that different sets of rulers and clocks moving relative to each other will assign different orders to distant events. Consider, for example, three events: event A, which causes event B, and a distant event, X. Since A and B are connected by some causal process, their order is fixed. But if event X is distant enough from both, then different clocks may register any of the three orders

A, B, X or A, X, B or X, A, B

That is, the distant event may follow both A and B, happen between them or precede both.

According to the mainstream interpretation of special relativity, durations and other temporal intervals are not invariant, and are therefore not real. According to this view, there is no fact of the matter about which of the three orders above is real and physical. The events all occur and are all real, but there is no physical fact that makes X later or earlier than the others. Just as there are no unicorns or pink elephants, there is no order between distant events that do not influence each other.

Consider another illustration. Suppose that there are two long queues leading into two doors at a club or music concert.