If someone was able to travel back into time to alter the events of the past, would there be any way of perceiving the change? If someone in the future traveled to the past and altered an event, will anyone notice? I hope this is not in the GQ territory. From the reading of the SDMB on this subject, I get the feeling that has not been 100% ruled out that we will never achieve the ability to travel back in time.
Nobody knows, unless the answer is ‘no’ in which case some of us know but can’t prove it.
Timecop says yes.
The answer is a definitive ‘no’ except for perhaps the timetraveler himself.
Say you George and Joe are sitting on a couch watching TV. You really hate Joe and excusing yourself to go to the bathroom instead pop back in time to kill Joe’s mother before he is conceived (to dodge murder charges in the present). Joe will wink out of existence and George will be none the wiser. Joe will never have existed as far as George is concerned and won’t have a clue.
The more interesting question is whether the timetraveler will be aware of what was done. Usually sci-fi has it that the timetraveller is somehow ‘outside’ of the cause-effect timestream but I don’t know that it must be so. After killing Joe’s mom it is conceivable you (the timetraveller) might be left standingover a dead body wondering what the hell you are doing there. Since Joe never existed your reason for travelling back in time and killing his mom has disappeared and you’ll be one very confused person.
You could also postulate that changing the timeline just causes you to pop into an alternate universe where whatever you made happen came to be but in this case nobody hs a clue what happened either.
For all you know this has happened thousands of times in your lifetime…friends, loved ones, acquaintances all winking out of existance and you none the wiser. 
There is one philosophically valid way to change the past. If by going into the past you make a change that causes history to go the way it has (in the future) been recorded as going, then your “change” is not a change but a confirmation.
Or so says Paul Nahin in his comprehensive tome Time Machines.
This is correct. Kip Thorne, a well known and respected physicist, at one point got tired of being pestered about time travel questions so he sat down to settle the issue once and for all. He was expecting to find something that prohibited it but after doing all the math he found that there was nothing in Relativity that precluded time travel (the math works in both directions).
Of course the theory of relativity may not have all the answers so there might still be something to prhohibit time travel scientists just haven’t found yet. Given the paradoxes that may occur with time travel I think most scientists figure there should be something to prevent it.
Even if it turns out to be technically possible every proposed method (I’ve seen) for doing so makes time travel a practical impossibility. At least I doubt anyone thinks we will ever be able to make galactic-sized spaceships, infinitely long cylinders, the ability to manipulate masses far greater than our sun and so on. Even if you somehow manage these extreme feats there is a good chance getting near your time machine will kill the chrononaut not to mention that in many cases the mere presence of the chrononaut could destabilize the time machine causing it to not work and quite possibly killing the would be timetraveler (or stranding him god only knows where).
Well, there was that time we had to make the pyramids never been built (for National Security Reasons), but we needed to put them back after people kept asking what that triangular thing on the back of the one-dollar bill was.