Terence McKenna made the claim that once the first time machine was invented everything must happen all at once.
Time machines, though fictional, could be made, but would not be practical.
Let’s say you have one that actually works.
You program it to go back or forward in time by one second.
It works.
You instantly die.
Why?
Because the Universe and everything in it is in motion.
The Earth is rotating on its axis, and is revolving around the Sun.
The Sun and all its retinue of planets, various moons, asteroids, comets and dust are in motion orbiting around the Milky Way Galaxy’s black hole at its center.
The galaxy it self is in motion in some direction away from the ‘center’ of the Universe.
The entire Universe is expanding.
So in that ONE SECOND, forward or backward, you wind up in empty space! Or, if you’re very UNLUCKY the middle of a star somewhere and are instantly vaporized.
So, time travel may be possible, but would be quite unpractical...............
And that’s just ONE reason...................
Another reason is you and the machine itself are made of atoms of various elements that may or may not have existed at the time of your choosing.
You cannot introduce mass into a Universe that already exists or does not yet exist......................