#### Time travel is not possible for now… However, from a theoretical point of view, based on the theory of general relativity, a trip to the future would be possible.

Things are different when it comes to traveling to the past. A recent study shows how, at least from a mathematical point of view, such a journey would be possible, on a circular trajectory in variable time.

Who doesn’t remember the movie Back to the Future, where the two cute protagonists traveled to the past, the result being a series of adventures and funny pranks? But is time travel possible?

No one has traveled to either the future or the past, however, the way to travel to the future is pretty clear, while the way to travel to the past is extremely mysterious and ambiguous.

Towards the future we could travel with the help of a spaceship that would accelerate us to speeds close to that of light, after which it would return us, passing through positive and negative accelerations, back to Earth. This situation is well exemplified by the so-called “twin paradox” in which two twins are separated: one makes a trip to the Universe on board a spaceship, while the other stays at home.

In the framework of the theory of special relativity (the one valid only for uniform motions), it follows that each of the two twins believes that the other is younger at the time of the reunion. In reality, the traveling twin is subject to accelerations, which necessitates the application of general relativity, resulting in the traveling twin being younger than the one staying home.

Depending on the acceleration experienced and its duration, the difference can be smaller or larger. In extreme cases, there can be hundreds or even thousands of years of difference between the time spent on a spaceship and the time spent on Earth. An extreme gravitational attraction, that of black holes, makes those who would be near the event horizon, be seen by those from the outside as if the time for them passes with extreme slowness. This effect appears in the movie Interstellar, where those who arrive on a planet near a black hole hurry up, as one hour on this planet represents 7 years for that left at home.

But can we travel to the past? There is no single answer to this question. However, at least from a theoretical point of view, the problem is treated as seriously as possible. A recent study by researchers David Tsang (University of Maryland) and Ben Tippett (University of British Columbia, Canada), published in the journal Classical and Quantum Gravity, shows how, at least mathematically, traveling back in time would be possible.

The two researchers used Einstein’s theory of general relativity to build the mathematical model called the TARDIS (Transversable Acausal Retrograde Domain in Space-time), to generate a circular time warp. What’s more, they proposed a time machine that would take travelers both forward in time and back.

If the Spatio-temporal curvature in a circular trajectory (in four dimensions) were to succeed, those who would travel in this trajectory would go back in time.

A time machine like this would have extremely interesting features. Those outside the car would see that inside in two versions going both forward and backward in time. Those in the time machine would see those outside periodically evolving back and forth in time.

For example, they could see how instead of breaking an egg to prepare an omelette, the egg in the omelette would jump back into the shell and become a whole egg again. He would see them rejuvenating those outside the time box.

At the present time, however, we do not have the slightest idea about the materials that would allow the construction of this time machine. In particular how to generate the gravity that would lead to the curvature in a circular trajectory of time.

The study of space and time is fascinating and problematic, argue the two researchers who proposed this mechanism. The new method could perhaps become a reality in the more or less distant future.