Causality is the relationship between an event (the
cause) and a second event (the effect), where the second event is
understood as a consequence of the first.
The connection between a cause(s) and an effect in this
way can also be referred to as a causal nexus.
Now, without causality, there is no real world. In fact
without causality, nothing could exist at all, including us.
Time travel would deny causality and consequently
everything would just collapse. Causality means that everything is related to
everything else. (Remember the Butterfly Effect.)
On the subject of Time Travel paradoxes, some of these
are more than interesting for the thinking woman/man.
A time traveler goes to the past, and does something that
would prevent him from time travel in the first place. If he does not go back
in time, he does not do anything that would prevent his traveling to the past,
so time travel would be possible for him. However, if he goes back in time and does something that would prevent the time
travel, he will not go back in time. Thus each possibility seems to imply its
own negation - a type of logical paradox.
Also this one: A man travels back in time and falls in
love with and marries a woman, who he later learns was his own mother, who then
gives birth to him. He is therefore his
own father and, because of this, also his own grandfather, great-grandfather,
great-great grandfather, great-great-great grandfather and so on, making his
ancestry infinite, and also giving him no origin for his paternal genetic
material.
In the grandfather paradox a time traveler goes back in time and kills his grandfather at a time before his grandfather met his grandmother. If he did so, then his mother or father never would have been born, and neither would the time traveler himself, in which case the time traveler never would have gone back in time to kill his grandfather.
However If interaction with the past is not possible then the traveler simply becomes an invisible insubstantial phantom unable to interact with the past.
Even worst, The idea that a traveler can go into a machine that sends him to 1865 and step out into the exact same spot on Earth might be said to ignore the issue that Earth is moving through space around the Sun, which is moving in the galaxy, and so on, so that advocates of this argument imagine that "realistically" the time machine should actually reappear in space far away from the Earth's position at that date.
