The possibility of traveling in time, that is, moving to different eras or moments of time as one can move to different places in space, is an idea widely explored in literature (just think of The time machine by Herbert George Wells, published in 1895), in the cinema (for example in Terminators, Back to the future, The Army of the 12 Monkeys) and in TV series (for example Dark). But what does science say about it? For physics, Traveling into the future is possible in principle according to Albert Einstein’s theory of relativity: to do this you just need to move at very high speeds. while the journey into the past is much more problematic, not only due to the lack of “strategies” for going back in time but also due to the onset of logical paradoxes such as the “grandfather paradox”.
Einstein’s relativity and travel into the future
There Einstein’s special relativityformulated in 1905, is the theory that relates space and time in a single physical entity called spacetime. One of the main results of this theory is the fact that time appears to flow at different rates for observers in a different state of motion. In particular, if Alice moves relative to Bob, the latter will observe Alice’s clock ticking more slowly than his. It’s as if Alice arrived in the future “faster” than Bob!
Let’s give an example: if Alice moves with respect to Bob at half the speed of light (150,000 km/s) and measures exactly 1 hour, 1 hour, 9 minutes and 18 seconds have passed in the same time interval for Bob. Translated: in fact it is as if Alice had traveled 9 minutes and 18 seconds into the future compared to Bob. If the relative speed is higher, the effect is more consistent: at 90% the speed of light, 1 hour for Alice corresponds to 2 hours, 18 minutes and 3 seconds for Bob.
In principle, therefore, to travel into the future we could imagine carrying out a high-speed space journey in which the final stage is the return to our Earth. Upon our arrival on the blue planet we would discover that, while a certain amount of time has passed for us, for the inhabitants of our planet a much longer time will have passed. From our point of view, space travel will also be perceived as time travel. (This is the idea behind the so-called “twin paradox,” which despite the name is not a real paradox as the underlying physics is fully understood.)
Doing so, however, remains technological out of our reachespecially due to the colossal amount of energy needed to accelerate a hypothetical spaceship to a speed close to that of light, which is 300,000 km/s.
The extension of the theory of special relativity is the theory of general relativitywhich also includes and describes gravity. According to this theory, an effect similar to the one just described can be experienced not only by traveling at high speeds but also by standing near very massive objects, such as black holes. This idea is used in the film Interstellar by Christopher Nolan, in which the protagonist Joseph Cooperafter a space journey that takes him near a supermassive black hole, Gargantua manages to return to Earth, where he finds his daughter, a child when he left, now elderly.
Is it possible to travel to the past? The “grandfather paradox”
The types of “journey” illustrated so far do not refer to the possibility of traveling to the past. Although we cannot talk about traveling to the past, there is however a way to observe events from the past. In fact, every time we observe our Universe with its stars and galaxies, we observe it as it was in the past. Likewise, an alien capable of observing Earth from millions of light-years away might in principle be able to see the extinction of the dinosaurs “live.”
In order to talk about real journeys into the past we must once again bother Einstein with his theory of relativity. Since this theory was published in 1915, generations of physicists and mathematicians have worked to find ways to make time travel possible, succeeding, at least in part, in their aim.
Many of these solutions, however, are probably only possible from a mathematical point of view, but difficult to implement in physical reality. These ideas can be traced back to what is called in physics closed spacetime curveperhaps practicable through dangerous journeys within rotating black holesor with the creation of wormhole (also exploited in very popular films such as Donnie Darko).
However, the possibility of being able to travel back in time opens up more questions than it closes. One of them is the so-called “grandfather paradox“: what would happen if a hypothetical time traveler, returning to the past, killed his grandfather before he could give birth to your parents? The paradox lies in the fact that this would prevent the time traveler from being born, and consequently from going back in time to kill his grandfather. How is it possible that if the latter dies he cannot be killed? The answer to this question is not clear and depends on who, as he believed Stephen Hawkingthinks that the laws of physics somehow prevent travel into the past, to those who say that doing so would create more timelines.
