So even NASA have written up about this subject, and suggest, in essence, that time travel to both the past and future are possible, by manipulating Einstein's theory of Special Relativity. And can theoretically been done by astronauts who leave the earth's gravitational field and travel at an "appreciable fraction of light-speed". However with no way of returning back to the landmass or place they were at, as to do so they would have to break the speed of light, which is, as far as current science knows, impossible.

Now, the theory of gravity suggests the possible existence of wormholes, and as such a 'gateway' that could alter the speed of time and as such make travel backwards in time possible, without having to travel away from a landmass (aka time travel is hypothetically possible, returning to earth in the 'past'. And so in essence bypassing the problems addressed in the first paragraph

Which brought about a question for me; If as people suggest, in the future we have invented time travel, and we can in fact travel to the past, then why have we not witnessed them/been visited by these time travelers yet?

What is everyone's opinions on this topic, as I am aware it is one of controversy,

Discuss.