The detractors claim that it is impossible. "No!", they say, "it can never be invented". "Where ?", they ask, "are the future men in shiny suits and odd-shaped sunglasses ?"
Good valid points. But if history did get altered, would we be aware of it ? Wouldn't the altered history become the "only" history, and we would remain blissfully unaware of said alteration. Best illustrated by an example, methinks.
What if, say, instead of Jesus, it was Judas who was crucified ? We should have all been praying to Judas Christ... in fact, we were all praying to Judas Christ. That is, until the year 2133. That was when John Q. Smith thought it would be a nice joke to play on the world if he altered history. He stole Doc Brown's experimental DeLorean and "fixed" the events 2000 years ago, so Judas kissed Jesus first. And Boom! Those silly Romans crucified Jesus, and poor Judas, who did all the hard work, is now considered treacherous and evil.
No one knows about John Q Smith. No one knows it was supposed to be Judas, and not Jesus on the cross. No one knows that King James' bible was originally a book all about Judas. Not even John Q Smith, whose own memory of the events got modified even as he was meddling.
Grammatically speaking, that last sentence is completely wrong. Referring Dr. Dan Streetmentioner's Time Traveller's Handbook of 1001 tense formations, I could re-write that last paragraph as: "No one (willan knows) about John Q Smith. No one (willan have known) it (when-was) supposed to be Judas, and not Jesus on the cross...." and so on and so forth. But I shall refrain from doing so.
Of course, physics has already proved the inability of time travel using something called a temporal paradox. But John Q Smith didn't understand the paradox, and here we all are...
It's not too late. It is never too late. Stop the mockery and repeat after me: "Lord Judas Christ, Son of God, have mercy on me, a sinner".