Everyone has made mistakes that they can’t regret, and they can’t wait to go back to the past again. This is one of the reasons why the concept of time travel is so attractive. In sci-fi works, as long as there is a time machine, it seems that nothing is eternal, anyway, you can go back to the past and change everything at any time. But is time travel really possible in our universe? Or just stay in science fiction?
Modern understanding of time and causality comes from general relativity. Einstein combined space and time into one, created the concept of "space-time", and made an extremely complex and delicate explanation for the operation mechanism of the two, which is difficult to compare with other theories. It has been more than a hundred years since the general theory of relativity was proposed, and it has been experimentally verified with extremely high precision, so physicists believe it. General relativity accurately describes the causal structure of the universe.
For decades, physicists have been trying to use general relativity to analyze whether time travel is feasible. Some of the equations they have come up with a show that time travel is perfectly compatible with relativity. But physics doesn't equal mathematics, and these equations are meaningless if they don't correspond to real things.
Time travel battle
There are two major problems that make these equations seem unrealistic. The first is a practical problem: building a time machine may require negative energy-matter. All the substances we see in our daily life are positive energy substances. Quantum mechanics tells us that negative energy-matter can be created in theory, but the quantity is extremely small and the existence time is extremely short.
However, there is no evidence that it is absolutely impossible for us to create sufficient amounts of negative energy. In addition, we may find equations that allow time travel without borrowing these substances. So this problem is ultimately just a result of our current state of the art or our limited understanding of quantum mechanics.
A setting often appears in science fiction works: an event changes the past, but after the past changes, it, in turn, prevents the event from happening, thus creating a paradox. For example, suppose you built a time machine, used it to travel back five minutes, and immediately destroyed the machine. Since the machine has been destroyed, you won't be able to use it after five minutes. But since you can't use the time machine, you can't go back in time to destroy it, so the machine isn't destroyed, so you can go back in time and destroy it. In other words, the time machine can only be destroyed if it has not been destroyed. But it is obviously impossible for a machine to be in two states of "destroyed" and "not destroyed" at the same time. This situation is obviously not self-consistent and contradictory.
Remove the paradox
A common misconception in science fiction is that paradoxes can be "created". Time travelers are often warned not to make too many changes to the past and not to meet their past selves. Examples of this are found in many time travel-themed films (like the Back to the Future trilogy).
But in physics, a paradox is not actually an event that can actually happen, but a purely theoretical concept that refers to the inconsistency of the theory itself. In other words, the self-consistency paradox not only means that time travel is a dangerous move, it also implies that it cannot happen at all.
This is one of the motivations of theoretical physicist Hawking's "chronological protection conjecture". Hawking believes that time travel should be impossible. But the conjecture has not been proven. And, rather than rule out time travel because of the paradox, we could just eliminate the paradox itself, and the universe would be a lot more interesting.
To solve the paradox of time travel, theoretical physicist Igor Dmitrievich Novikov proposed the "self-consistent conjecture". The basic idea of the conjecture is that you can travel into the past, but you cannot change the past. Novikov pointed out that if you went back five minutes and tried to destroy the time machine, you would find that you couldn't do it at all, because the laws of physics would try to step in and protect the self-consistency from being destroyed.
Multiple histories
But what's the point of going back to the past if you can't change the past? Some time travel paradoxes are Novikov's self-consistent conjectures, according to a recent study by Barack Shoshenny, assistant professor of physics at Brock University in Canada, and students Jacob Hauser and Jared Vogan. unsolvable. This brings us back to square one: even if only one paradox cannot be eliminated, time travel is logically impossible.
So, does this mean that time travel is doomed to be impossible? Not necessarily. The research of Shoshenny et al. shows that as long as the existence of multiple histories (or parallel timelines) is allowed, the paradox that Novikov's conjecture cannot solve can be solved. In fact, any paradox can be solved this way.
The idea is very simple. When you step out of the time machine, you enter a different timeline; you can do whatever you want on that timeline (including destroying the time machine), and the timeline you were in won't change. Since you cannot destroy the time machine in the original timeline, there is no paradox.
For the past three years, Shoshenny has been working on the paradox of time travel. Now he is more and more convinced that time travel is possible, but only if our universe allows for the existence of multiple timelines. So the question is, can there be multiple timelines in the universe?
From a quantum mechanical standpoint, the answer seems to be "yes." As Everett's "Many Worlds Interpretation" proposes, history can be "split" into multiple histories, each of which corresponds to a different outcome. For example, Schrödinger's cat may be dead or alive.
But these are just guesses. Shoshenny and his students are currently searching for a theory of time travel that incorporates multiple histories and is fully compatible with general relativity. Of course, even if they did find such a theory, it wouldn't be enough to prove that time travel is possible. But this at least means that time travel is not completely ruled out by the self-consistent paradox.
Time travel and parallel timelines have always appeared together in science fiction, but researchers have now demonstrated that they are equally inseparable in the real world. General relativity and quantum mechanics tell us that time travel is possible; but if time travel is possible, so must multiple histories.
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