“There would be a way to do it”

Canadian physicist Barack Shoshany is convinced of this and is working on a theory of parallel chronology that is fully compatible with general relativity.

Time Travel / Credit: Ajaj1818, CC BY – SA 4.0, via Wikimedia Commons

Who hasn’t dreamed, at least once in their life, of stepping back in time and making up for a mistake they made in the past? But is it really possible to travel through time or is it just science fiction? According to physicist Barak Shoshany, an assistant professor at Brock University in St. Catharines, Canada, there is a way to do it, but with one condition. Together with his students, Shoshany is working on a theory of time travel that is fully compatible with our modern understanding of time and causality, which derives from general relativity. The latter, written by Albert Einstein and published in 1916, combines space and time into a single entity, space-time, and provides an extraordinarily complex explanation of how the two work, at a level that has no comparisons by any other established theory. Formulated more than 100 years ago, this theory has been tested experimentally with extremely high accuracy, so physicists are quite certain that it provides an accurate description of the causal structure of our Universe.

For decades, Shoshany explained in a recently published article The conversation, physicists have tried to use general relativity to find out if time travel is possible. “It turned out that we can write equations that describe time travel and that are fully compatible and consistent with relativity,” says the expert. But physics is not mathematics and equations do not make sense if they do not correspond to anything in reality.

The paradoxes of time travel

There are at least two main reasons why we think these equations may be unrealistic. “The first problem is practical: building a time machine seems to require exotic matter, which is a matter of negative energy,” says Shoshany. Any matter we see in our daily lives has positive energy, while negative energy matter is something we simply cannot find. From quantum mechanics, however, we know that this matter can be created theoretically, even in small quantities and for too short a time.

However, as there is no evidence that it is impossible to create enough exotic matter, and given that other equations could be discovered that allow time travel without requiring exotic matter, the expert believes that “this problem can only be a limitation. of our current technology, or our understanding of quantum mechanics “.

The second important objection to hypothetical time travel is less practical, but more significant. “It is the observation that time travel seems to contradict logic, in the form of paradoxes: there are different types, but the most problematic are the paradoxes of coherence. […] that occur whenever there is a particular event that leads to a change in the past, but the change in itself prevents that event from happening in the first place ”.

In other words, the paradox of coherence is what fans of science fiction movies, like the trilogy of Back to the future, they met through the story of Martin McFly, transported by DeLorean in 1955 and ended up interfering in the past of their future parents, so much so that they had to do everything possible to fall in love with them so as not to be erased from the face of the Earth. If he had not succeeded, he and his time travel would not have even existed, and since it is not possible to exist and not exist at the same time, this scenario is inconsistent and paradoxical.

There is a common misconception in science fiction that paradoxes can be “created”, explains Shoshany. In general, time travelers are warned not to make any significant changes in the past and not to end up in the past for the same reason.

In physics, however, a paradox is not an event that can actually occur, as it is a purely theoretical concept, pointing to an inconsistency in the theory itself. “This means that the paradoxes of coherence not only mean that time travel is a dangerous business, but also that it cannot be possible. One way to solve these travel paradoxes has been suggested by theoretical physicist Igor Dmitriyevich Novikov’s Self-Consistency Principle, which basically states that it is possible to travel to the past, but it cannot be changed. So what’s the point of going back in time if the past is unchanging?

The solution of parallel delays

For Shoshany, there are paradoxes of time travel that Novikov’s principle cannot solve (“If not even one paradox can be eliminated, time travel is still logically impossible”). However, in a recent paper, in collaboration with students Jacob Hauser and Jared Wogan, the Canadian physicist showed instead that “allowing multiple stories (or in more colloquial terms, parallel timelines) it can solve the paradoxes of Novikov’s principle. ” . And he can solve any other paradox you put into it. “

This principle, Shoshany points out, is very simple. “When I leave the time machine, I go to a different timeline. In this timeline, I can do whatever I want, including destroying the time machine, without changing anything to the original timeline where it came from. And since I can’t destroy the time machine on the original timeline, which is what time used to be, there’s no paradox. “

After working on the paradoxes of time travel for the past three years, Shoshany is increasingly convinced that time travel is possible “but only if our universe can allow multiple stories to coexist.”

Shoshany and his students are currently working “to find a concrete theory of time travel with parallel timelines that is fully compatible with general relativity. Of course, even if we could arrive at this theory, it would not be enough to prove that time travel is possible, but at least it would mean that time travel is not ruled out by coherence paradoxes. “

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