In science fiction works, as long as we have a time machine, we can go back to the past at any time. But is time travel really possible in our universe? Or just in science fiction?
$ads={1}
The modern understanding of time and causality is rooted in general relativity. Einstein combined space and time into one, creating the concept of "space-time", which explains the mechanics of both in an extremely complex and subtle way. It has been more than a hundred years since general relativity was born, and it has been experimentally verified with such extreme precision that physicists believe that general relativity is the key to accurately describing the structure of the nucleus. fruit of the universe.
For decades, physicists have tried to use general relativity to analyze whether time travel is possible. Some of the equations they came up with showed that time travel is fully compatible with the theory of relativity. But physics is still based on math, and these equations are meaningless if they don't correspond to real things.
Time travel contradictions
There are two major problems that make these equations seem impossible. The first is a practical matter: Building a time machine might require negative energy matter. All the substances we see in daily life are substances with positive energy (positive energy), however negative energy is something that we have not observed in reality.
However, quantum mechanics tells us that it is theoretically possible to create negative energy matter, but in extremely small quantities and with extremely short lifetimes. At the same time, there is no evidence that we are completely unable to generate negative energy. In addition, we can find equations that allow time travel without borrowing these substances. So the problem lies only with our current state of engineering, or our limited understanding of quantum mechanics.
Second are the paradoxes. A setting often appears in science fiction works: an event occurs that makes us want to change the past, but after the past changes, it prevents the event from happening, thus creating out paradox.
For example, let's say you built a time machine, use it to go back five minutes and instantly destroy it. Since the machine has been destroyed, you will not be able to use it after five minutes. But since you can't use the time machine, you can't go back in time and destroy it, so the machine isn't destroyed, so can you go back in time and destroy it? In other words, the time machine can only be destroyed if it hasn't been destroyed. But it is clear that it is impossible for a machine to be in two states of "destroyed" and "not destroyed" at the same time, which is clearly contradictory.
Eliminate paradox
A common misconception in science fiction is that paradoxes can be "created". Time travelers are often warned against changing too much from their previous past and not seeing their past again. Examples of this are found in many movies about time travel (like the Back to the Future trilogy).
But in physics, a paradox is not really a possible event, but a purely theoretical concept that refers to the contradiction of the theory itself. In other words, the self-consistency paradox not only means that time travel is a dangerous act, it also implies that it may be completely impossible.
This is one of the driving forces of the theoretical physicist Hawking's "chronological guard conjecture". Hawking believes that time travel is impossible. But the conjecture has yet to be proven. And, instead of ruling out time travel because of the paradox, we could eliminate the paradox itself, and the universe would be a lot more interesting.
To solve the time travel paradox, theoretical physicist Igor Dmitrievich Novikov proposed the theory of the "self-consistent conjecture". The basic idea of this theory is that you can travel to the past, but you cannot change the past. Novikov points out that if you go back five minutes and try to destroy the time machine, you'll find that you can't do it at all, because the laws of physics will try to interfere and protect you. self-consistency from being destroyed.
New theory
But what's the point of going back to the past if you can't change the past? According to a recent study by Barack Shoshenny, assistant professor of physics at Brock University in Canada, some of the time travel paradoxes that follow Novikov's self-consistent conjectures would be unsolvable. This would lead to the fact that even if only one paradox cannot be eliminated, time travel is logically impossible.
Shoshenny et al.'s work shows that as long as multiple histories (or parallel timelines) are allowed, we can solve the paradox that Novikov's conjecture cannot solve. Okay. In fact, any paradox can be solved this way.
The idea is very simple. When you step out of the time machine, you enter another timeline; you can do whatever you want on that timeline (including destroying the time machine) and the timeline you're in won't change. Since you cannot destroy the time machine in the original timeline, there is no paradox at all.
For the past three years, Shoshenny has been working on the time travel paradox. He is now increasingly 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 point of view, the answer seems to be "yes".
But these are just conjectures. Shoshenny and his students are currently searching for a theory of time travel that combines multiple histories and is fully compatible with general relativity. Of course, even if they come up with such a theory, it won'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 proven that they are inseparable in the real world. General relativity and quantum mechanics tell us that time travel is possible; but if time travel is possible, there must also be a lot of history.