I like the Terminator franchise. I wholeheartedly agree with the more or less general consensus that Terminator 2 is one of the best sci-fi movies ever made.
Of course the movies leave quite many questions about their internal logic (especially related to time-travel and how it affects the timeline) unanswered, and they could be considered outright plotholes. On the other hand, it's a fun exercise to come up with plausible explanations for these potential plotholes.
So here are my speculations in the form of questions and answers. To understand the timelines better, see the main timelines graph page.
I will consider only the four movies here. I won't consider the TV series nor the different video games and other works as part of the "movie canon" (although they could work as separate timelines, except that some details in them contradict my theories below).
In this case, the ontological paradox would be the following: Kyle Reese was sent to the past because John Connor was the leader of the resistance (Connor being the leader of the resistance caused Skynet to send a Terminator to the past to kill his mother, which caused the resistance to send Kyle Reese to protect her), but John Connor existed in the first place because Kyle Reese was sent to the past. Without the time travel there would have not been a "need" for the time travel in the first place; in other words, the time travel itself generated its own need (by causing John Connor to be conceived), making it self-referential.
In this case no information or objects were created ex nihilo (that is, the information or the object having no creation point at all), but the reason for the time travel is: The time travel created its own reason to have been performed in the first place, which is the ontological paradox.
On the other hand, there is an even worse ontological paradox where seemingly ex nihilo information creation happens: The Terminator sent to the past causes Skynet to be created (because the Terminator's CPU was intact enough for it to be reverse-engineered). In this case Skynet caused its own creation by sending a Terminator to the past, and thus information was created from nothing.
There's a fandom-created theory that every time that something is sent to the past in the movies, a new timeline is created, distinct from the original one. While I'm personally not very fond of multiple-timelines theories in movies and TV series involving time travel (because it waters down the original motives for the time traveling), in this particular case it actually fits extremely well.
By this theory Kyle Reese was not the original father of John Connor. Instead, in the original timeline John Connor's father was someone else. When the T-800 and Reese are sent to 1984, the timeline is split and a new alternate timeline is created, and in this new timeline Reese becomes John Connor's father. (Even though now John Connor's history is quite different, he still ends up becoming the leader of the resistance because of ontological intertia.)
Also Skynet was created by regular means in the original timeline, rather than being inspired by the reverse-engineered Terminator CPU.
We never get to know who John's father was in the original timeline because the first Terminator movie happens completely in this alternate timeline. In this timeline it's just some guy somewhere who never even meets Sarah Connor.
This new timeline is different from the original one. Kyle Reese has a different father (and was very probably born on a different day, probably earlier), and the development of skynet changes. However, on the grand scale this timeline is very similar to the original one.
Details about all the timelines are given in the main timelines graph page. I recommend having that graph in another tab or window for reference, while reading the questions and answers below.
From here forward I will use the following terms:
My theory: Whatever the time-traveling physics and technology involved might be, it only allows using certain precisely-defined "time windows" within which the destination can be set. Maybe it has something to do with the position of the Earth with respect to the Sun or galaxy, or something like that. Whatever the reason, the amount of possible destination times for time travel is very limited, either because of some property of physics or because of some limitation of the technology involved.
1984 might have been the earliest possible time window that Skynet's time-travel technology allowed to reach, from 2029 in the original timeline, and it was a one-time shot to that window (because as the Skynet's own time advances forward, 1984 gets farther and farther away to the past, too far to be reached again). It also explains why Skynet didn't send a Terminator even further back in time, to kill Connor's grandfather or whoever.
It's possible that the Skynet in the original timeline also sent a T-1000 to 1995 and a T-X to 2004, but if that happened, they created their own separate and independent timelines which do not affect the ones seen in the movies.
(Sending a T-1000 to 1995 in the original timeline makes sense because it was before Judgment Day, and Skynet attempted to kill the teen John Connor before it. However, why send a T-X to 2004, which was well after judgment day? It still makes sense, though: To kill John Connor before he becomes the resistance leader. Also, the John Connor of 2004 in the original timeline wouldn't be prepared for such an advanced Terminator.)
The Skynet of the 1984-timeline probably also sent a T-800 to 1984, but this simply spawned a separate, independent timeline (separate from the original timeline and the timelines in the movies). Most importantly, it later sent a T-1000 to 1995.
This second time travel spawned a third timeline from 1995 forwards. In this timeline Skynet is created later (because the original Terminator CPU is destroyed and the original developer is killed), but not so much later as to miss the 1984 time window.
(Likewise the Skynet in this new 1995-timeline probably also sent a T-800 to 1984 and later a T-1000 to 1995, spawning their own independent timelines, after which it sent the T-X to 2004.)
The reason to send only one Terminator may have to do with the amount of energy and resources needed to send anything. Perhaps sending something back in time requires such a humongous amount of energy that even with all the available resources only one Terminator can be sent.
It might require so much energy and so many resources that organizing a new time travel could require many years of work (and by the time that enough resources have been gathered for a new time travel, the time window gets too far and no more can be sent to that time.)
This is admittedly a bit more difficult to explain, but here are two possible theories:
Of course the second theory cannot account very well for the Terminators that the resistance sent later, so at least with them the first theory is more plausible.
On the other hand, it would probably be much easier for a Terminator to infiltrate Skynet's facilities ("reverse infiltration") than for a human, making that aspect slightly more plausible.
Maybe the energy and resource requirements for sending something back in time are so humongous that Skynet could barely send one Terminator. Sending weapons with him would have been too much.
The T-800 was the most advanced of infiltrator Terminators (can completely pass for a human) and the smallest one, compared to previous versions (according to Terminator Salvation, the T-600 series was considerably larger and bulkier, besides not being at all human-looking at close range). Skynet had to send a T-800 because it had to find Sarah Connor without the authorities or military stopping it first (which is why Skynet couldn't send any other type of killing machine).
One could also speculate that perhaps energy weapons don't survive the time travel because of their composition (ie. perhaps their energy sources get destroyed, damaging the Terminator), and sending regular projectile weapons alongside the Terminator would be rather pointless because it can acquire such weapons in the destination time.
By the time Skynet had developed the T-X series, it also had clearly developed technology further to allow some kind of weaponry to be sent embedded to the Terminator (which is what we see in the Terminator 3 movie). This development probably happened to the weapons themselves, rather than the time-traveling technology. The latter was clearly still limited to sending only one Terminator.
Polymimetic alloy can be sent as-is through time while harder, more basic metals cannot. Might be related to the physical properties of these materials. The T-X was completely covered by polymimetic alloy when it was sent to the past, protecting the endoskeleton.
It's rather clear in each sequel that the protagonists (mainly Sarah Connor, John Connor and the different Terminators) remember the events of the previous movie. Thus no movie can happen in any of the "possible alternate timelines" of my graph. In those timelines some of the movies have never happened.
They could happen in any of the "alternate timelines" in the graph, but the timeline sequence shown in the graph is the shortest one leading to the events of each movie. Placing any movie on those alternate timelines would require at least one extra "loop". While logically not impossible, it's unnecessary.
(Such "extra loops" could, however, be used to explain some things better. For instance, it could be used to explain how the original Skynet was created in such a short time without the reverse-engineered Terminator CPU. Perhaps the very first Skynet which ever existed was from much, much later, eg. from year 2200 or whatever, it sent Terminators back to preserve its own existence, and through a complicated sequence of splitting timelines ended up causing the creation of Skynet in 1997 in one of these timelines, which ends up being the "original timeline" in the graph. In other words, there always was a reverse-engineered Terminator CPU in 1997, but its ultimate origin was a Skynet developed in the normal way in a much longer timespan.
Also, if there's some glaring continuity error between movies, that could also perhaps be explained by having the movie in an alternate timeline, where the previous event equivalent to the previous movie happened slightly differently. For example, there may be a disparity between the age of John Connor in the second movie as compared to his alleged date of birth. However, it's possible this is an alternate timeline where he was born earlier.)
It cannot be determined.
Every time a timeline reaches a Skynet developing time-traveling technology, it sends a T-800 to 1984 (or to a later time window if the development happened too late), which spawns a new timeline, which potentially ends up having a new Skynet doing the same, etc. This potentially causes an infinite number of timelines.
Of course it's plausible that not all timelines end up with the creation of Skynet. In some timelines Skynet might be destroyed before it develops the time-traveling technology.
So it all depends on how many of the spawned timelines end up having a Skynet sending Terminators to the past. The amount might converge to infinity or to a finite number.
True, it doesn't help the Skynet which sent the Terminators, but Skynet is a machine and might come to a conclusion of maximum self-preservation: It's better for Skynet to preserve itself in as many timelines as possible than to be completely destroyed. In other words, Skynet helps "its alternate self" by sending Terminators to the past. This kind of thinking may be completely logical for a machine.
Yes, that will most probably happen. However, until then I'll keep this theory because it makes most sense to me. It explains why only one terminator is sent at a time, why only to three eras, and why no weapons were sent alongside (except for the T-X), as well as the ontological paradoxes and even possible continuity errors between movies.
This is actually canon.
The full nomenclature of the Schwarzenegger-looking terminator sent to 1984 (and by the resistance to 1995) is "Series T-800, model 101".
The series number "T-800" indicates the type of endoskeleton (the metallic parts), while the model number indicates the type of organic cover. Model 101 looks like Schwarzenegger, while model 102 looks like someone else, and so on. Thus we can have, for instance, a T-800 model 102 which would be the same type of robot with with a different physical appearance.
Likewise we could have a different (but with similar physical proportions) endoskeleton with a model 101 organic cover (thus looking like Schwarzenegger). This is actually the case in the third movie, where the terminator is a T-850 model 101 (the T-850 being a more advanced version of the T-800).
There's an odd misconception held by many that all terminators (at least those with a T-800 type endoskeleton) look like Schwarzenegger, even though this is clearly not the case even in the very first Terminator movie, where we see a flashforward to the future, and a Terminator of a different model there attacking a resistance settlement.
There has been a whole progression of "infiltrator terminators" during the history of Skynet, starting from the T-1 (which doesn't look human at all). Not many of them are featured in the movies. Besides the main characters (ie. T-800, T-850, T-1000 and T-X), only few other terminators of the T-series have been shown, most prominently the T-1 (in the third movie) and the T-600 (mentioned in the first, shown and prominently featured in the fourth).
The T-600 has a rubber covering (and according to the fourth movie, is significantly larger than the later and more human-sized T-800), probably mostly intended to look enough like a human from the distance so that the terminator could get close enough to human settlements without raising alarm, before starting its attack. However, it can not pass for a human at close distance, and its endoskeleton is much weaker than the T-800 (the T-600 could be easily destroyed with bullets, at least when aimed to the head, while the T-800 is virtually impervious to them and significantly harder to destroy).
Ancillary material mention or feature other versions and models, hinting at the whole line of development from the T-1 to the T-800, and forward. (However, as I have mentioned, I only take the movies as "canon" in this article, so I consider those to be more like "semi-canon speculation".)
According to background material to the Terminator: Salvation movie, some of the Terminators seen being manufactured in the factory near the end of the movie are actually T-700's. According to this material, these are very similar to the T-800, except that they are designed to be purely combat droids rather than infiltrators, and hence are not even designed to have an organic cover (probably the organic cover requires special equipment in the endoskeleton to be sustained for long periods of time without decay due to natural processes). The endoskeleton of the T-700 tends to be much darker than the one of the T-800 due to the manufacturing process.
(We can assume from this that many, if not most, of the "fleshless" terminators seen in flashforwards in the first movies are actually T-700's, as they clearly have not been given any organic cover to begin with.)
Naturally Skynet has developed a whole lot of other robot types as well (most prominently featured in the fourth movie), but these are not part of the T-series (which is the series of infiltrator machines), but something else completely.
I think Terminator 2 is one of the best sci-fi movies ever made, and in fact one of the best movies overall. Its storytelling has just the right amount of action, character development and philosophy. Many people share this view. The ending is one of the most touching I have seen. Storywise it also establishes that Judgement Day was actually averted and thus never happens in that timeline.
Thus it's no wonder that many people don't appreciate the third movie, as it's seen as a simple rehash of the exact same ideas, up to repeating numerous ideas from the previous movie almost verbatim. They see it as a completely unnecessary attempt at cashing on the franchise without adding much to it (and, on the contrary, watering down much of what was achieved in the second movie).
However, I like the third movie. No, it doesn't reach the same level of storytelling quality as the second, and yes, it's just a more or less thoughtless action movie without much depth to it, but I still like it. The action scenes are good, and the T-X makes a fine and logical addition to the fictional development of Terminator robots.
So Judgement Day was not averted after all, only delayed? I can live with that. I don't mind the story being continued from where T2 left it. T3 is not a bad sequel. It could have been a lot worse. I'm all for considering it fully canon.
Just because the humans want to think they are not bound to inevitable fate doesn't make it so. No inspirational aphorism is going to change that. It's just wishful thinking.
Also, the T-850 in the third movie, when referring to the inevitability of Judgment Day, might have referred to its inevitability at that point in time (inside that timeline). In other words, it was way too late to try to stop it because Skynet had already become self-aware (as hinted by the end of the movie), or at the very least was going to be at any moment, and Judgment Day was less than a day away. Nothing they could have done would have stopped it, and thus it was indeed inevitable.
The US military surely has their most critical servers running in deep underground bomb shelters in order to ensure that all the critical military systems remain operational even in the event of a nuclear war. (This is most probably the case even in real life.)
Skynet was a control system for the entirety of the United States military forces. Thus it only makes sense that it was running from and had access to all these sheltered secure servers.
If you pay attention to the dialogue in the third movie, it's actually explained: Since Cyberdyne's project was a contract work issued by the US military, the latter regularly made and kept copies of all the data.
Miles Dyson might not have been aware of this. (After all, the military really likes secrecy...)
Yes: John Connor's pin number cracker in Terminator 2.
It's ludicrous even as a device to crack an ATM card pin number (they don't work like that), but its usage later in the film to open the safe is even more ludicrous and is just a huge plot hole.
Miles Dyson already knew the code to the safe, there was no need to crack it. However, the whole building was locked down, so no cards nor codes were working. Why would "cracking" the code with the cracking device help this? Even if it had been possible (which would have been even more ludicrous than cracking an ATM because it was a completely different and incompatible system), inputting the code would have done nothing. The building was locked down, and no codes were working.
What's worst about this is that the whole cracking device serves no role in the plot of any significance whatsoever. It could have been completely cut from the script and nothing would have changed (except for this plot hole, of course).