The traveller contributes to events at the time they visit. In Wells’s The Time Machine, many thousands of years of public time after a dinner party, the time traveller kills a Morlock. In the traveller’s own personal time, the Morlock attacks the traveller earlier than the dinner party. Indeed, the Morlock’s attack on the traveller is part of what causes the traveller to tell the story to his friends (at his origin time). However, to the listeners, in public time, the Morlock attacks later than the party.
There are many different kinds of causation proposed in time travel stories. The most significant are (a) causal loops and (b) changing the past.
The time traveller causes situations to exist or occur at the origin time. Examples of this are the films Bill & Ted's Excellent Adventure (1989) and Twelve Monkeys (1999). Bill and Ted pass their history exam, which causes a future world of peace and prosperity. That future world sends Lucius back in time to help Bill and Ted, causing them to pass their exam. Thus, we have at least two ways of describing this causal loop:
- 1. Through helping Bill and Ted, the future world causes the future world.
- 2. Through accepting the future world’s help. Bill and Ted’s exam success causes the exam success.
We might wonder what starts the loop. Is it the future world sending back Lucius, revealing to the two that they are set to cause a world of peace and prosperity, or is it Bill and Ted’s exam success? One possibility is that the loop starts with the earliest cause. Yet, this raises a further question: what is the earliest cause? Is it the earliest cause in public time or personal time?
In personal time, there seems to be no earliest cause. The younger Bill and Ted may hold their decision to climb into the machine is the earliest cause. For them, nothing happened earlier than that. However, this action is driven by Lucius time-travelling to meet them. For Lucius’s personal time, this is not the earliest cause. Instead, it is the moment when he accepted the mission. Again, in public time, Bill and Ted’s decision is earlier than Lucius’s acceptance, but that is not personal time.
Here is a sample of some stages in the causal chain leading to Bill and Ted passing their history exam:
- 1. Lucius travels to meet them: in Lucius’s personal time, this is earlier than Bill and Ted getting into the machine.
- 2. Still in Lucius’s personal time, Lucius is sent by his peers into the past.
- 3. His peers, knowing that Bill and Ted need help, decide to send Lucius into the past.
- 4. They acquire that knowledge from records of the past.
- 5. Bill and Ted climb into the machine.
All of these are part of Lucius’s personal chain of cause and effect. They are all causes and all effects. They are also exaggerated forms of any causal loop that causes one to participate in events about which one knows something, while at the origin time, before participation.