The idea of time travel is obviously dependant on what time is. & that is not as defined & universally accepted amongst the scientific community as some may think. There was a very interesting programme about it on UK telly last year presented by Dr Brian Cox that made my head spin!!
In one way time travel already exists, every time we look up into the night sky we are looking into the past due to the amount of time it takes for the light from the stars to reach us we are not seeing them as they appear at the time it is now on earth but as they appeared in the past.
If we accept that the past exists in the traditional sense & previous events have already occurred then by travelling back in time would it be possible to change anything(?)
1. You invent a time machine tomorrow & travel back to 22nd November 1963 to prevent the assassination of JFK . But 22nd November 1963 has already happened, so you have already existed in that time/place even before you were born, grew up, & invented a time machine. Your attempt to prevent JFK’s assassination has already failed before you even thought of doing it! Or maybe your attempts to change the past led to that very event happening?
2. Unless by changing the past you create a new timeline, & when you return to the present you return to a new present created by your previous meddling? But would the ‘original’ timeline exist still in a parallel dimension? Or does it get erased? Either way you would be the only person who knew of the ‘original’ timeline/events. Everyone would think you were mad if you, as in the above example, tried to tell everyone that JFK was assassinated. & what else would have changed from this one single event, for good or ill? It would probably be hard for the time traveller to adjust to this new present. In fact would you be a paradoxical element in a transformed present? Would you be able to return to the present at all?
I’m more inclined to believe option 1 is probably what would happen, if you believe in a logical universe with a defined timeline & a beginning, middle & end (which for the sake of this argument I do). It would almost be like a built in fail safe device for time. So it ultimately makes time travel pointless if the sole reason is to change events.