Time travel is a dream that man has always hoped to achieve but which, unfortunately, is currently only achievable in the world of science fiction. In these "parallel worlds" it would be possible to travel through the various epochs with a very simple system, such as pressing a button or using a machine as in the case of "Back to the future". In reality, does physics allow time travel? In theory, yes, but in practice the situation is much more complex. Two distinctions need to be made: Time travel in the future and time travel in the past. As for time travel into the future, the problem is easier to solve. In fact, if we were able to build spaceships capable of traveling at speeds close to those of light, we could exploit the time dilation to create a sort of "Time Travel". For us who are on the spaceship, time would pass much slower than for those on Earth. By taking such an action, we could return to Earth and literally find ourselves several years ahead of the future, depending on the speed at which we traveled in the spaceship. Another solution could be that of getting closer to a body of very high density and mass, for example a black hole, and exploiting the space-time dilation. An example of this situation can be found in Interstellar, where the Austronauts, due to the time spent near a black hole, see their time slow down incredibly so that an hour spent in their reference system corresponds to 7 years spent on the Land! So journeys into the future don't seem to be that difficult from a physical point of view, but the problem arises from journeys into the past. In this case, in fact, there would be a number of problems, the most famous of which would be the grandfather's paradox. Let's imagine going back in time and killing our grandfather before he meets our grandmother. In this case, of course, the grandson would never have been born and therefore would never have been able to undertake the time travel. However, possible solutions to this paradox have been found. The most famous concerns the theory of the multiverse: In this case the killing of the grandfather would lead to a problem only in the parallel universe in which the story would develop in a different way from how we know it. If, on the other hand, the universe is unique, there are two possible answers: For Hawking there must be some unknown reason why time travel into the past is impossible. According to Novikov's principle, on the other hand, time travel in the past would be possible but it would be impossible to change the history and the events that happened.