Time travel is a difficult subject. It also happens to be a very popular subject of many stories in books, television, movies, video games, and so on. But human kind's limited understanding of the workings of time and the possibility of time travel (especially the limited understanding of the casual book reader, movie viewer, or video game player) leads to depictions of this subject being wrought with logical inconsistencies, paradoxes, and plot holes.
Regarding paradoxes
One of the most common problems with time travel stories is the creation of paradoxes. Of these, some of the most common paradoxes are the "predestination paradox", the "bootstrap paradox", and the "grandfather paradox" (or the "reverse grandfather" paradox). You're probably familiar with all of these, but you may not know them by name, so I'll take a moment to define them for you:
- Predestination Paradox = a time traveller goes back in time to participate in historic events (intentionally or not) and creates a causal loop whereby the time traveller creates the very future that he or she remembers. Examples of this paradox include the classical Greek myth of Oedipus and the destruction of the T-800 in The Terminator and the subsequent use of its components to construct Skynet in Terminator 2: Judgement Day. This paradox is similar to a "self-fullfilling prophecy" and is related to the "bootstrap paradox" discussed below.
- Bootstrap Paradox = an object or information is sent through time and used, and then sent back to the same point in time again (either the original object or a copy of it), thus creating a loop where the item or information has no true origin. An example of this is in the movie Back to the Future in which Marty McFly travels back in time and performs the Chuck Berry song "Johnny B. Goode" in 1955 and inspires Chuck Berry to write the same song which he releases in 1958. Another example is Kirk's glasses in Star Trek IV: The Voyage Home, which are an antique gift given to him by Dr. McCoy in Star Trek II and then pawned off in the past so that they can be given as a gift again. The existence of Skynet in the movie The Terminator may also be a bootstrap paradox, since (in Terminator 2) it is claimed that Skynet could not have been invented without help from the salvaged components of the terminator robot from the first film. If this explanation is true, then the knowledge to build Skynet has no origin, as humans could not have done it independently without the robot having been sent back in time in the first place.
- Grandfather Paradox = based on the idea of going back in time to kill your own grandfather and prevent your own conception, this paradox refers to any situation in which a time traveller visiting the past causes an event that would logically prevent that time traveller from ever having gone to the past to begin with (such as preventing his or her own birth or preventing the invention of the time machine). The "reverse grandfather" paradox is related to the predestination paradox, except that it is the time travel itself that allows the time travel to become possible. For instance, Fry becoming his own grandfather in an episode of Futurama while attempting to save the man he thought was his grandfather from being killed is a "reverse grandfather" paradox.
I'm sure you can think of plenty of other examples in stories of similar paradoxes. Sometimes, the story's author tries to explain the paradoxes away. Other times, the existence of the paradox is just ignored, or taken for granted. The occurence of such paradoxes is also often used to argue that a story is flawed or "stupid" because it creates an unresolved paradox.
The need for "Temporal Latency"
Another problem that many depictions of time travel fall victim to is determining when, how, and if the effects of changing the past will be observed in the present. This can often create other logical paradoxes and confusion if you stop to think about it. For example, in The Terminator, Kyle Reese explains that he and John Connor found the time travel device used by Skynet to send the T-800 back in time to kill Connor's mother. They then use that same device to send Reese back in time to stop the robot from completing that task.
OK, sounds simple enough. Right? But if Skynet already sent the T-800 back in time before Connor and Reese found the time machine, then wouldn't the past have already been changed, causing Connor to never exist, and therefore, never be able to send Reese back in time to stop it? Well, no, because then we wouldn't have a movie! Unless the Terminator's mission failed, in which case, only minimal changes may have occured, and there would be no need to try to stop the excursion to the past, and we still wouldn't have a movie.
So if you ever come across an enemy's already-used time machine, and then discover that the machine has already been used to prevent you from being born, then why should you even care? After all, if you still exist, then the enemy's plan must have failed. Otherwise you wouldn't be there right? So why would you even bother going back in time to stop the plan (and risk screwing things up) if you know that it must have failed?
Star Trek First Contact gets around this by having the Enterprise get caught in the borg's temporal vortex prior to the actual time travel event, allowing them to witness the immediate ramifications while simultaneously being protected from its effects. But not all movies have the advantages of temporal vortexes.
There are two explanations for why the effects of changing the past would not be immediately observable in the present:
- Changing the past simply creates another time line that exists independently of yours, and therefore, no changes to the past can possibly effect your observance of the present because your original past has already happened. This is very well explained in the various Back to the Future movies.
- There is some sort of temporal latency whenever the past is changed. Thus, changes to the timeline take some additional time to "ripple" up to the present. For example, if I go back in time 10 years to change the past, and it takes me one week to complete that task, then the changes to the present may take one week before they take effect (or some other arbitrary value). This was also sort of explained in Back to the Future!
It took a comedy to figure out how to get time travel right
Back to the Future might be the best example of time travel in a movie, since very careful attention is paid to making sure that everything stays internally logical and consistent. Any time Doc Brown and Marty make a trip to the past, a "parallel timeline" is created. Once the new timeline has been created (which may or may not have erased the previous timeline), Doc and Marty are then "trapped" within that timeline. Any efforts to travel back to the future result in them going into that timeline's future, instead of the original one from which they came. This is all clearly explained in the movie.
Futhermore, Doc and Marty are given the ability to correct paradoxes in the timeline which they may have created due to the time delay between the past changing and them observing a change happen (such as a photo fading away, newspaper headlines changing, or people phasing out of existence). The changes to the past must "catch up" to them in the present like a wave from a rock being thrown into water takes time to reach the person who threw the rock.
But this idea of temporal latency is only a hypothesis. It is by no means an absolute or correct interpretation of how time travel would have to work.
So for other movies, like The Terminator, we may not have a mechanism that allows the time travel to be possible. In the case of The Terminator, we the viewer have to assume that some sort of delay is in effect. Otherwise, Kyle Reese cannot be sent back in time to begin with, and we have no movie. Some people might point to the lack of an explanation for this paradox as an indication that the whole premise of the movie is "stupid". Those people are stupid.
But there are still paradoxes!
But even if we do assume a temporal latency period in a movie like The Terminator, then we still don't have an explanation for what happens in the case of a paradox. The Terminator is, in fact, full of paradoxes. I cited this movie (and its sequel) as examples for both the predestination paradox and the bootstrap paradox. When Kyle Reese goes back in time and becomes John Connor's father, the future is changed. Kyle Reese is now John Connor's father (was he all along?), and Sarah Connor will raise John Connor with the foreknowledge that the machine uprising will happen. The conditions of John Connor's life change. So the events leading up to Kyle Reese being sent back in time must change. Unless this element of the movie's plot is also within a bootstrap paradox. If Kyle Reese is John Connor's original father, and John Connor was always brought up with the foreknowledge that he will lead a human revolt against the machines, then both those events require that a future timeline already exists in which the terminator and Kyle Reese are sent back in time. John Connor is therefore created out of nothing, because he had no father except for the time traveller Kyle Reese.
The recovery of the T-800's remains from the factory also causes a change, since it allows Cyberdine to invent Skynet. Either the invention of Skynet requires that the robot have been sent back in time (the bootstrap paradox), or the existence of the robot's remains in the past alter the way in which Skynet is built (the predestination paradox).
In either case, the future in which the T-800 and Kyle Reese are sent back in time no longer exists. So the time travel should not have happened because even if the new Skynet still sends back a Terminator (it does), and even if the (now different) John Connor sends a (now different) Kyle Reese back after it (they do), the Kyle Reese that comes back would be a different person. The movie is now in violation of the idea that two things cannot occupy the same space at the same time, since an infinite number of Kyle Reeses (each slightly different) are being sent back in time to the same time and place to perform the same mission.
So there must be multiple timelines
This must mean that the time travel of The Terminator has resulted in the creation of "parallel timelines" as in Back to the Future. In that case, Skynet's attempts to change the past are futile, since it cannot possibly affect its own past, and any changes in the timeline create a new, alternate, timeline that exists independently of the original Skynet's timeline. In addition, John Connor's plan to send Kyle Reese back to stop the Terminator is also futile, since all he's doing is removing Kyle Reese from his original timeline, which will remain unaffected by the Terminator's interference.
So any way you look at it, the plot of The Terminator (and any other similar stories), is fundamentally illogical. Note that I am not saying that The Terminator is a "bad" movie. It's still a great movie! It just doesn't make sense unless you are willing to accept that material and information can be conjured out of nothing as in the bootstrap paradox...
Is there any way to save such stories?
Well actually, there is. And it is, in fact, the whole point of this entire dialogue.
How can Skynet's time travel plan possibly work and be paradox free? Well, in order to be paradox free and still be able to make changes to the past (free of any Novikov or time-stream self-healing theories), the only logical method for time travel to work would be the multiple timeline idea. So in order for its time travel plan to work, Skynet would have to be aware that any attempt to change the past would result in a new parallel timeline that Skynet would not be able to experience. The mathematics necessary to create a functioning time machine would probably expose this reality to Skynet.
Therefore, Skynet would also have to create a "dimensional transporter" or communication device capable of allowing it to enter another timeline.
Since the new timeline doesn't exist until after the temporal intrusion takes place, Skynet would not know in advance how to get to the other timeline, so it may be necessary to equip its terminator robot with some kind of interdimensional homing beacon that will give Skynet the "coordinates" of the new timeline once its mission has been completed.
At that time, Skynet would use its interdimensional transporter to transfer its programming and memory (i.e. its "conscienceness") into its duplicate in the other timeline.
Skynet can now enjoy the benefits of its new and improved timeline without having to worry about the consequences of paradox!
Futhermore, the time traveling agent must remain alive long enough to transmit the dimensional coordinates back to its sender in the future. This allows the writer to maintain the tension of an actual conflict. In the case of The Terminator, the necessity for the T-800 to transmit back to Skynet means that it can't just complete its mission by self destructing itself in a suicide mission the moment it gets anywhere close to Sarah Connor, and we can still draw the events out into a two-hour movie!
This mechanism requires only a tiny addition to the technobabble, but allows for an internally consistent and logical universe as well as the freedom of the writer to do what they want with the story's narrative.