Time travel is possible!

the time travel. Admit that you too, it makes you dream a little. The opportunity to repair a mistake made in the past. Or to get a glimpse of the future. Science fiction has long been enjoying it. But what does science say about it? This is what Barak Shoshany proposes to us today to understand, a Brock University physicist (Canada).

Let us first recall that it is the theory of general relativity posed by the physicist German Albert Einstein (1879-1955) which has defined, for more than 100 years now, the links that unite space and time. It thus brings us a complex explanation of the principle of causality. And it turns out that she authorizes to write equations that coherently describe time travel. “But the equations have no meaning if they don’t correspond to anything in reality”emphasizes Barak Shoshany, in The Conversation.

That is to say ? Well first let’s note that to build a time machine, the equations would have physicists use a matter with negative energy. However, negative energy is not found on every street corner. In the present state of our knowledge, there is hardly Quantum mechanics which lets us hope to be able, in theory in any case, to manufacture it in very small quantities and for extremely short times. But Barak Shoshany wants to be optimistic: “This problem could simply be a limitation of our current technology or our understanding of the Quantum mechanics. »

Difficulties to overcome to go back in time

Let us therefore sweep the problem of matter with negative energy. There remains, however, another major obstacle. The one we’ve all heard of before. That of the coherence paradox. The physicist gives a striking example. He imagines using a time machine he just built to go back five minutes in time. There he destroys the machine. However, the destroyed machine should not allow him to use it five minutes later to come back five minutes earlier and destroy it. If he is when he is, that must mean that the time machine has not been destroyed. And he can use it to go back in time and destroy her. In other words, here’s a time machine that’s destroyed if and only if… it’s not destroyed. wow! Do you see the inconsistency?

The science fiction deals with the matter by advising its time travelers to avoid intervening in the past. Or to meet themselves. Problem solved. Except that in physics, a paradox corresponds to a purely theoretical concept. Understand that it highlights an inconsistency in the theory itself. And hell! This is how time travel simply becomes impossible for eyes science.

Impossible, really? Not necessarily, according to some researchers. The Russian theoretical physicist Igor Dmitriyevich Novikov is one of them. Her conjecture of self-coherence was imagined to solve the paradoxes of time travel. Quite simply by forbidding time travelers from… changing the past. Really banning. With laws of physics that somehow conspire to prevent Barak Shoshany from destroying his time machine five minutes before he entered it. But then, what good is it? Isn’t the point of going back to the past precisely to be able to change it?

Ending the Coherence Paradoxes of Time Travel

In addition, the work carried out by Barak Shoshany shows that the self-consistency conjecture does not make it possible to eliminate all the paradoxes of time travel. So, back to square one? With time travel made impossible? Well, not quite, the physicist tells us.

The ultimate solution would be to create multiple stories. Or parallel timelines. Which means ? Quite simply, when the time traveler exits his machine, he enters a separate timeline from his original timeline. In this new timeline, he is free from everything. Including destroying his time machine. And this, without any impact on what will happen, a little later, in the timeline from which it comes. This is the end of paradoxes. And science that is reconciled with science fiction which, already, was fond of stories of parallel chronologies.

“I’ve been working on the paradoxes of time travel for the past three years and I’m becoming more and more convinced that time travel might be possible.says Barak Shoshany. But only if our universe can allow multiple stories to coexist. » Which, of course, raises a new question: can our universe accept parallel timelines?

Quantum mechanics seems to lean towards yes. The theory of multiple worlds imagined by Hugues Everett, an American mathematician (1930-1982), envisages it. The story could split. To allow the Schrodinger’s cat to be alive in a new story and dead in the other.

Barak Shoshany’s team is currently working to establish a concrete theory of time travel with multiple stories compatible with Albert Einstein’s general relativity. A theory that would still not prove that time travel is possible, but at least that it is not made impossible by paradoxes of coherence.