I have been thinking about time travel, as one does. The laws of physics don’t say it is impossible, and if something is possible in this Universe, it happens. What makes us think it doesn’t, is simply that we don’t know it does.

Take Stephen Hawking’s famous time travel party. The guests arrived (on time, of course), availed themselves liberally of the refreshments, amused themselves hugely racing Hawking’s wheelchair up and down Trinity Street, thanked their host effusively for the wonderful time, bestowed amazing gifts from the future on him, and left. 

Unfortunately, their time line left with them, leaving Hawking with no evidence or memory whatsoever of the marvellous event. Time travel is certainly possible and it happens literally all the time. Unfortunately you have no point of reference other than your own when it does happen. You can kill your grandfather as often as you like but you will never know about it and he will still always be there smiling when you come back.