For hundreds of years, mankind has been looking for opportunities to travel in time, and up to date, such a method has not been found, occurring only in fantasy novels and movies. Nowadays, this idea theoretically can be realized in the context of a self-consistent single timeline (the Novikov self-consistency principle) or the Many-Worlds Interpretation of quantum theory (the MWI). For many reasons, the Novikov self-consistency principle seems to be a better solution to the problem of time travels to the past than the MWI.
Although today the amount the MWI followers keeps increasing, time travel to the past still seems to be more possible in the context of the Novikov self-consistency principle, even though some can say that it undermines people’s free will.
To make time travel to the past happen, it is necessary to have access to a wormhole one end of which has to be pulled at the speed of light and then to be returned to the original point. As a result, this end or mouth of a wormhole will become younger than another one because of time dilation, and so it will be considered to become the past version of another mouth. However, it is not possible to travel to any point in the past in such a way – the furthest destination point is the one where the mouths were decoupled.
According to the Novikov self-consistency principle, no matter what exactly the time travelers intend to do on their arrival to the past, it will not break the laws of the Universe existence. To prove this theory, Novikov comes up with the famous example of a billiard ball. The ball plunged into a wormhole, will not run into itself but will get back to the wormhole, and, consequently, the natural order of things will not be destroyed. This characteristic of the Novikov principle explains how the world stays the way people get used to seeing, and it is one of the most significant arguments for this theory.
There is another argument for the Novikov principle. Currently, when it is related to time travels to the past, plenty of scientists and ordinary people wonder – if journeys to the past are possible or they are to become possible one day, there should be some marks left in present time by the mythical travelers. However, apart from the doubtful figure of John Titor – one of the most famous contemporary “time travelers,” – it is hard to remember any names. Perhaps, the reason is the block that was put by the Universe to protect cause-and-effect relations in safekeeping.
Nevertheless, the Novikov self-consistency principle has also some disadvantages. For instance, according to the Novikov theory, human free will is illusory and does not actually matter – the future is already written. However, it is easy to dispose of this argument – the Novikov theory does not have a relation to the illusiveness of human free will but to the idea of the Universe’s superiority over human beings and the impossibility of ruining the Universe laws. According to this theory, the Universe protects itself from destruction.
As previously noted, the Novikov self-consistency principle is not the only time theory. The MWI of quantum theory also has plenty of followers. According to it, whenever a choice takes place in the Universe, our world is splitting into two, and, as a result, innumerable universes appear. Some can say that the MWI seems to be more plausible than the Novikov theory – for example, it rejects the idea of time paradoxes in general and the Grandfather paradox in particular. The point is that there is no one single timeline in the world of parallel universes. That is the reason why time travelers can stop worrying about meeting in the past themselves or people they know – undoubtedly, all these actions will take place in a parallel universe, and all the happening events including the grandfather’s murder will not influence anyhow the travelers themselves or their home universe but only the travelers’ doubles and the alternative universe.
However, there is one extremely important argument against time travels to the past in the context of the MWI. There are not any time travel mechanisms in quantum mechanics. To get back in time, travelers need wormholes or some sort of similar instrument, but quantum mechanics does not suggest any worthy variants.
Apart from that, there is one more important question left aside. The substance which is called by the MWI “the past” is a parallel universe. If so, it is hardly possible to use this term while having a conversation about time travels to the past. According to this theory, time travelers to the past will never get to the home universe but to its new version. Virtually the MWI does not prove the possibility of time travel to the past – it is much more likely to remove only the possible time paradoxes, particularly the Grandfather paradox.
Finally, it might be concluded that the idea of time travel to the past is possible, and, apparently, it may spring to life more in the context of the Novikov self-consistent single timeline than the MWI of quantum theory. The Novikov theory occurs to have a better-defined scientific bias, and it is also closer to the Universe image people get used to seeing. As for this theory’s disadvantages, it is easy to dispose of them.