Hi friends, here I leave this short note that may be interesting
To travel in time we must also travel in space at high speed, for example, in a spaceship.
Travel to the future must be understood as follows: the traveller does not notice anything special inside his spaceship. For him, time passes as usual.
For those who stay on Earth also time passes as usual.
However, when the traveller compares his moving clock with clocks at rest outside the spaceship, he observes that his clock slows down. For example, if for the traveller passes one minute then for people on Earth will pass several hours, days or even years, depending on the spaceship speed. This effect is known as time dilation.
If the spaceship reaches the speed of light then the spaceship clock would stop but moving clocks outside would not.
In travel to the past something similar happens. The difference here is that the time for the spaceship traveller always goes forward, meanwhile time outside of the spaceship go back. It is as if the spaceship clock always went forward, those clocks outside the spaceship went back.
In this case, as time goes upside down outside of the spaceship, the rays of light outside the spaceship propagate from the observer to the object, instead of from the object to the observer. As a consequence, the traveller will see that everything is darkness outside his spaceship. It is as if he travelled through something like a black hole, with the only difference that it does not engulf the spaceship.
Those who remain on Earth will not observe the spaceship going back in time because this journey happens in the fourth dimension, which is invisible as we saw in the section WHAT IS TIME?
