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Time travel? It’s theoretically possible, but very complicated.

However, time travel is not a scientific aberration, even if we are very far from “Back to the Future”, or even from “The Time Machine” by HG Wells.

Traveling to the future!

First of all, traveling to the future should not be too much of a problem, as long as you go fast enough. This is one of the consequences of the theory of relativity, expressed by the famous “paradox of twins”: take two twins, one staying on Earth and the other leaving for a trip at a speed close to the speed of light. If he returns to Earth after two years of travel (for him), the “traveling twin” will find a brother on Earth who will have aged 30 years.

Of course, at the moment, no way has been found to approach the speed of light, but it has already been verified by accurately measuring time in clocks onboard aircraft that this difference in speed does indeed have an effect on the “passage of time”.

It is also conceivable that a person who is put in a slowed-down state of life for enough time could also wake up in the future … Again, with no hope of returning to the “time of origin”. The technologies to enable this “long sleep” are not yet developed, but some are already working on it.

People like us, who believe in physics, know that the distinction between past, present and future is only a stubbornly persistent illusion. Albert Einstein

Time travel

Blackhole and wormhole

Those of you who have seen “Interstellar” will know what black holes and wormholes look like. The former generates an extremely intense gravitational field, which according to relativity could bend space-time to the point where it would form a complete circle, thus returning to its “starting point”: a journey through space-time.

The latter can, still in theory, connect one point to another in space… and even space-time. We could, therefore, conceive of a “wormhole” that would connect two different eras and thus allow time travel. Kip Thorne, the astrophysicist, who was the scientific consultant for the film, has been working on these theories of time travel.

However, just because a wormhole or a black hole would have time travel capabilities does not mean that we could build a device that could withstand the gravitational pull of a black hole, or the environment of a wormhole. Maybe we will have such capabilities in the future, but it is not for tomorrow.

Grandfather, time… and space.

Time travel, if it can exist, does, however, generate a few puzzles of the first order. First of all, the famous “grandfather paradox”: imagine a time traveler who goes back in time and kills his own grandfather. If he has done this, he could not have been born and therefore could not have traveled through time to kill his grandfather… Some theorists solve this paradox by imagining that with each time travel, a new “parallel universe” is created and that there is a multitude of them.

Another element not to be forgotten is to travel in space as much as in time. Indeed, we live in a universe in perpetual motion. So, if you wanted to go back 100 years, it would not be enough to project yourself to the same spatial coordinates a century earlier: you would find yourself in the void.

The solar system is indeed moving at high speed through the galaxy, and the Earth does not pass through the same point every year, contrary to what sun-centered representations might suggest. For example, 100 years ago, a given place on our planet was more than 61 billion kilometers away, more than 10 times the average distance from Earth to Pluto. When you consider that it took New Horizons 9 years to reach Pluto, you can imagine the problem!