In the world of time travel fixed time is one of the most popular theories mentioned in the effort to make sense of how time would work if anyone, or anything, could experience it out of sequence.  The last article in this series gave a brief overview of the major theories of time.  This time we are focusing on Fixed Time Theory.

This may be marginally the most popular theory of time among scientists.  According to it, you cannot change the past--not merely because, as the library computer in The Time Machine suggests, you cannot go to the past, but because the past is immutable.  Thus even if it were possible to travel to the past, you could not change anything.

Could not change anything does not mean, as suggested in the aforementioned movie, that Emma who died at the hand of a mugger will die somehow within a few hours of the same time and a few miles of the same place; it means that Emma will die in exactly that way, at the same time and place, by the hand of that same mugger, as she already died.  If you were able to travel to the past, you would find that whatever part you played in events was the part you had already played in those events, leading to exactly the same outcome.  Once you step into your own past, you are fated to do whatever you already did.  Concerning whether you can kill your own grandfather, a fixed time theorist will reply that you have already failed to do so, so your attempt will fail--or more precisely, your attempt has already failed.  This is seen by some as a failure of the theory.

It is also inherent in fixed time that the future is equally determined.  It is, after all, the past for any point in the yet further future.  As mentioned last time, the very concept that you can travel to the past or the future means that all of history already exists, like a movie in which we are characters moving through the frames but someone outside the film can choose what scenes to watch next.  It is thus inherent to the theory that choice is an illusion and future is fated.  Time is not a medium within which the world forms and changes, but a path along which we move to experience what already is.  Many fixed time stories play on this theme.

Fixed time also holds that events can form a causal loop, sometimes called a predestination paradox, in which causes in the future bring about consequences in the past which in turn are the causes of the events in the future which caused them.  This uncaused cause is another significant source of objection to the theory, but those fixed time theorists who allow these (and not all do) assert that these are entirely possible, as long as they are self-supporting.

Although it is not always possible to ascertain the theory behind a film, films which might be built on a fixed time theory include 12 Monkeys (in which James Cole has seen his own death but cannot avoid it), Kate and Leopold (in which Kate was already her boyfriend's ancestor, and was in pictures from the party before she traveled back to the party), Happy Accidents (in which a great deal of the story is about whether it is possible, based on some rule related to a focus of emotion on a specific moment, to change history), and The Final Countdown (in which the designer of the ship is the man who was left behind when it returned from the past).  None of these are perfect examples under the theory, as each has aspects that do not fit it, and most can be explained to some degree under different theories.

Next time we will consider Parallel Dimension Theory.