There is a way to travel in time that could be possible: according to the physicist Barak Shoshany, the definitive solution would be in the creation of multiple histories, or parallel chronologies.
The concept of time travel has fascinated mankind for millennia. However, the idea has had a special boom since the appearance of the science fiction genre, which has been taking advantage of it for a long time. And it was not for less. Surely we all have dreamed of the endless possibilities of time travel, even if it’s just to have the opportunity to make amends for a mistake made in the past.
But beyond dreams and science fiction, how realistic is time travel in our universe? What does science say about it?
Current understanding of the principle of causality
Our current understanding of time and causality is based on the general theory of relativity of the German theoretical physicist Albert Einstein (1879-1955). This theory – which has been around for more than 100 years and which physicists agree fairly accurately describes the causal structures of our universe – combines space and time into a single entity, “space-time”, and offers a extraordinarily intricate explanation, according to Shoshany, of how they both work, on a level that is unmatched by any other established theory.
According to Shoshany, the general theory of relativity has allowed physicists to write equations that describe time travel in a way that is coherent and compatible with relativity. “But the equations don’t make sense if they don’t correspond to anything in reality,” says Shoshany. Why then would these equations be unreal?
Two big problems with time travel
First, according to the physicist, to build a time machine, the equations would require scientists to use exotic matter with negative energy. However, negative energy is not available in every corner. In the current state of knowledge, only quantum mechanics gives us hope, at least in theory, of being able to produce it in very small quantities and for extremely short periods of time.
But Shoshany is optimistic, noting, among other things, the possibility of other equations being discovered that allow time travel without the need for exotic matter. “This problem could simply be a limitation of our current technology or our understanding of quantum mechanics,” he says.
Aside from exotic matter, the other main problem is the observation that time travel seems to contradict logic in the form of consistency paradoxes. To explain it, the physicist gives a surprising example.
“For example, consider a scenario where I enter my time machine, use it to go back in time five minutes, and destroy the machine as soon as I reach the past. Now that I have destroyed the time machine, I would be unable to use it.” five minutes later, Shoshany explains.
This means that if you are in the moment you are, you can’t go back in time and destroy it, which implies that the time machine hasn’t been destroyed. And you can use it to go back in time and destroy it. In other words, it’s a time machine that destroys itself if and only if… it doesn’t. Paradoxical, right?
“Since it cannot be destroyed and not destroyed simultaneously, this hypothesis is inconsistent and paradoxical,” says the physicist, who adds that, unlike science fiction, where there is a misconception that paradoxes can be “created,” a paradox in physics is not an event that can actually happen. Thus, paradoxes, he adds, are a purely theoretical concept that points to an inconsistency in the theory itself.
Does this then mean that time travel becomes simply impossible in the eyes of science? Not necessarily, according to some researchers.
Theoretical physicist Igor Dmitriyevich Novikov attempted to resolve the question of time travel paradoxes with a self-consistency conjecture, which “essentially states that you can travel to the past, but you cannot change it”.
The parallel world approach
However, Professor Shoshany, along with his students Jacob Hauser and Jared Wogan, studied time travel and in recent work discovered that there are time travel paradoxes that the Novikov conjecture cannot resolve.
Thus, Shoshany and his students proposed another approach to solve the paradoxes: the theory of parallel timelines.
“We show that allowing for multiple histories (or in more familiar terms, parallel timelines) can resolve paradoxes that the Novikov conjecture cannot. In fact, it can resolve any paradox that is thrown at it,” says the physicist.
According to his theory, when a person steps out of a time machine, he lands in a different timeline, where he can do whatever he wants, including destroying his time machine five minutes before he’s supposed to use it. In this theory, changes to this new timeline would have no effect on the original timeline.
“Because I can’t destroy the time machine in the original timeline that I actually traveled back in time with, there’s no paradox,” says Shoshany.
“After working on time travel paradoxes for the past three years, I am increasingly convinced that time travel might be possible, but only if our universe can allow multiple histories to coexist,” he adds.
Quantum mechanics seems to lean towards yes
According to Shoshany, splitting into multiple storylines is entirely possible in quantum mechanics. “Quantum mechanics seems to imply that it does, at least if you subscribe to Everett’s ‘many worlds’ interpretation, in which a story can be ‘split’ into multiple stories, one for each possible measurement outcome, for example if Schrödinger’s cat is dead or alive, or whether or not I reached the past,” he says.
Today, Shoshany continues to work intensively with his students on developing a concrete theory of time travel on different timelines that adheres to the law of general relativity.
“Of course, even if we do manage to find such a theory, this would not be enough to prove that time travel is possible, but it would at least mean that time travel is not ruled out by consistency paradoxes,”