In the not-so-distant future, a human-created supercomputer, skynet, takes control of the world, but skynet is met with stubborn resistance from humans. Led by an elite human named connor, the surviving humans are getting stronger and stronger, and skynet is getting a little overwhelmed, so it uses a dirty trick.

"Skynet created a time machine and transported a powerful robot back to 1984, whose goal was to "End" Connor's mother before she was born, so that there would be no connor in the future. When connor learns of this, of course, he does not ignore it and uses the time machine to transport one of his best men to 1984 ......

The above is a classic scene from the famous science fiction movie "The terminator". Since the time machine in the movie is quite remarkable, people will think about one question more or less after watching the movie: Just because humans can't build a time machine now doesn't mean that they can't build one in the future.

One speculation on this question has been made: Does the fact that we haven't seen time travellers mean that humans were extinct before they built a time machine?

It should be noted that this is indeed a reasonable speculation, because after all, a time machine is a nebulous machine and it is a huge question mark whether it can be built or not, and even if it can be built, it will take an extremely long time, and as things stand now, humanity is likely to encounter a number of difficult hurdles in the future and risk extinction if it is not careful.

This is a depressing answer, but it is not the only reasonable hypothesis, for example, there is the idea that when time travellers return from the "Future" To the "Present", a new timeline is created which runs parallel to the original timeline and never intersects it. This timeline is parallel to the original timeline and never intersects with a parallel universe running above each of them, and the time traveller can only exist in one of them.

The reason why we do not see the time travellers is because they are not in our timeline, while we in the other timeline see these humans from the future, but everything that happens in that parallel universe is completely unrelated to our parallel universe. It is worth mentioning that, apart from the "Parallel universe theory", there is another speculation that seems even more plausible, as we will see below.

While we are enjoying our time, the earth is actually carrying us around the sun at about 30 km/s, and the sun is not sitting idle as it carries the earth around the centre of the milky way at about 230 km/s, which in turn is moving around the centre of mass of our galaxy group at about 600 km/s.

On larger scales in cosmic space, galaxies and galaxies are again driven away from each other by dark energy, and the farther away they are, the faster this distance from each other, in this case galaxies that are very far away from us can move away from us even faster than the speed of light, and conversely, if we take them as a reference point, then we are moving away from those very distant galaxies at superluminal speeds.

As you can see, everything in the universe is not stationary, it is constantly in motion at every moment. So the question arises, how do we describe the exact position of the earth in the whole of cosmic space at this moment in time? The answer, of course, is that we don't know, because we simply don't know how big the universe is, let alone what kind of a state of motion the earth is in at the level of the entire universe.

To know how big the universe really is, one has to understand what the nature of the universe is, which is the ultimate question, and it is likely that future humans will not be able to gain insight into the mysteries, and therefore they will not be able to pinpoint exactly where the earth was in the whole of cosmic space at a certain point in time in the past.

It is clear that time travellers wanting to return to earth in the past must set a specific point in time in addition to the location of the earth at that moment in time, both of which are indispensable.

The fact that we don't see time travellers doesn't mean that humans were extinct before they built the time machine. It is possible that future humans did build the time machine, but since they couldn't pinpoint the past earth, they couldn't go back to the past earth at all, and even if they insisted on going back in time, they couldn't find the past earth in the location where they were.