Chapter 6.0 Time travel is possible – 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. Within each queue the order is clear. The people closer to the door will enter first.

But as the two queues shuffle past each other, sometimes one is faster and sometimes the other. Thus there is no clear order between people in different queues. In the future, when they meet inside the club, they will influence each other, and they may have influenced each other in the past. But while they are separated from each other in different queues there are no influences, and therefore no meaningful order between them. According to the mainstream interpretation, events in our world are like this. Some are linked in chains of causes, but between the parts of chains that do not influence each other there is no definite order.

Which events are chained together into an order by causes, and which are not? Since causes are carried by things with energy or mass, and these cannot travel faster than the speed of light, no cause can propagate faster than light.

Thus if light cannot pass from one event to another, then no causal influence can and the pair of events is not causally ordered. There is no fact of the matter about which is earlier and which is later.

Thus, according to the mainstream interpretation, if light emerges from a distant star and travels this way but cannot reach Earth before the next election, then the emission of the light was neither earlier nor later than the next election. Similarly, it would take light about a billionth of a second to cross an object the size of a human brain.

If two synapses fire in such a way that light could not travel from one firing to the other, then there is no physical order between these events.

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