Grandfather paradox?
The paradox is this: suppose a man traveled back in time and killed his biological grandfather before the latter met the traveller's grandmother. As a result, one of the traveller's parents (and by extension, the traveller himself) would never have been conceived. This would imply that he could not have travelled back in time after all, which in turn implies the grandfather would still be alive, and the traveller would have been conceived, allowing him to travel back in time and kill his grandfather. Thus each possibility seems to imply its own negation, a type of logical paradox.
Comments
It's not a paradox if you accept the premise that it's impossible to go back in time and kill your own grandfather (either because you deny time travel, or because you believe that time travel to the past is "determined" in that you can't change anything.)
if you go back to the past before you were born, and your grandfather died by any means before you were born then the question of the possibility of time travel to the past is not possible because if the grandfather or grandmother paradox happened then why you are there? But there is another hypothesis regarding the existence of a parallel multiverses that defy the grandfather paradox. it tells that there are many earths at different space time dimension and you are not the only you. There are many you at different space time dimension. When you go back in time you'll be able to meet the other you and the other grandfather of the other you. But again this is just an imaginary hypothesis and cannot be proven and probably be impossible to prove or disprove. But science is sure that time travel only leads to the future and that's how light year is being measured by physicist. If you are looking at the star right now, you are seeing the past but if you start travelling going to that star the time will start counting going to the future. once you reach that star you are no longer looking at what you have observed from where you came from. That's how treaky light is.
The answer to this paradox is explained in the sense of parallel universes. If you were to watch the movie series "Back to the Future"*1* or the movie "Deja Vu"*2* the concept is explained.
If you were to go back in time, just the fact of you appearing there would cause an alteration of the timeline. Granted this wouldn't cause too much of a change over the next 100 or maybe even the next thousand years. But lets say for the sake of argument, you go through with your plot to kill your grandfather. This is a slightly larger impact to the timeline compared to you just being there. While this event doesn't affect the general public much, it greatly affects the the people and events that originally led to your birth. Since you have just killed your grandfather, in this timeline, you will never exist in the sense of how you grew up and eventually went to the past. However, you do not disapear as suggested by your paradox. In the timeline you came from, nobody killed your grandfather and he was able to the complete the events leading to your birth. Meaning you do, and will always, exist.
Until you die for real anyway.
*1* In "Back to the Future", the explanation of parallel universes happens over the course of the first two movies.
*2* In "Deja Vu", while not explicitly talked about in the movie, if you go to the special features, they have someone who explains it much better than I did.
It's a paradox, but it is only a thought experiment until someone actually goes through time to test it. Until then, it's just an impossibilty designed to make a head spin.
No, the present only changes once the past has been tampered with. He can go back into time when his grandfather still exists, kill him, and then anyone of that lineage from that year on, including the time traveller, vanishes into thin air.
You didn't think this through, did you?
yes that is why time travel as it is commonly depicted is thought to be impossible. however time travel in which alternate realities are spawned does seem possible given the current understanding of quantum physics
Call me crazy, but I've thought about that a LOT. I tell myself that same sad story every time I think it'd be fun to time travel.
Sounds a little like Back to the Future when Marty McFly starts to disappear before his parents kiss.
That's a crazy conundrum though.
What I think would happen is something like what happened in "It's a Wonderful Life" when George wished he was never born, but he himself still existed. I think maybe it would create an alternate universe where you were never born, but you-yourself (as in you and your own conscious) would still exist in it.
imagine your grandson game from a different dimension and time. He pops in to your world and kills you. That's it. His world still goes on and your world still goes on. There is no canceling out.