We have been told by mystics since what we perceive as the beginning of time that time itself is an illusion. Annoying isn't it, when they tell you things that are obvious balderdash. Every one of us experiences time ... well ... all the time!
In the days of the earlier scientists, time and space were separate, set entities like we observe in our everyday experience. Then along came Einstein and he did a funny thought experiment. He asked himself if he were sitting on a light beam, traveling with it at the speed of light and the light hitting his eyes were from a clock face, what would he see? Well, he reasoned, if I am traveling at the same speed and in the same direction a the light coming to me from the clock face, then I would never see the hands move! The image would always be the same image! Does that mean that time would stop????
Excitable guy that he was, he started going off into mental journeys pursuing the implications. Kind of like Lewis Carrol projecting his imagination into Wonderland to see where it took him, Einstein had found his own Wonderland. Most people would have left it at that, a flight of frivolous but fun fantasy, but Einstein wasn't made that way.
Before he was done, he had worked out theories of Special and General Relativity that turned our understanding of what was constant and what was changing totally on its head. The beauty of it was that he created predictions of experiments ALL of which had to come out as he predicted or the entire theory was invalidated!
Rather than go through the proofs, I will give you some of the proven results. An astronaut that has been in orbit is slightly younger than he would have been had he stayed on the ground. It is proven by sychronizing two atomic clocks, putting one in orbit for awhile, and then comparing the two after the landing. And, just precisely as predicted at the turn of the 20th Century, the clock put in orbit would show a time slightly earlier than the one on kept on the ground.
But, just as the mystics have told us in their pronouncements about time, Einstein did not replace certainty with the random. In fact, by Einstein's theories, we have greater accuracy in measurements of time and motion than with Newton's. In fact, I have read that he originally wanted to call his theories the theories of IRRELATIVITY, because they showed what was irrelative to the motion of the observers.
So, this is not an effort to be loose in thinking or to lose ourselves in some fun fantasy, it is an effort to tie down physical truth as far as possible. The underlying, proven truth is that time as we experience it is in fact an illusion. There is instead an unobservable combination called spacetime - and no, space is no more constant than time - and the truth lies below the surface of our five senses. In other words, spacetime is a proven reality, but time as a separate thing is an illusion.
That we can be fooled so completely about the nature of our reality should usher in a profound humility.