You can't change history for the simple reason, grandfather paradox. If you go back in time and kill your grandfather, there will be no you, and what you crossed cannot happen. Since you didn't cross, your grandfather wouldn't have died, so there should be you, so this is a fast knot. According to Novikov's self-consistency principle, everything we see is changed history. If human beings can really cross history, then history is doomed, and it is because of your crossing that history naturally happens. For example, a fire took everything from you, and you want to cross back to put out the fire. When you went back, you found that it was because of your crossing that a fire broke out. Now let's look at the grandfather paradox. If you want to go back in time and kill your grandfather, you might as well set a scene. You crossed into an abandoned house, and it happened that your grandfather fled here to get rid of others' tracking. You went up and stabbed your grandfather, and then you left. The stalker came here to watch your grandfather get stabbed, and then left with a sigh of relief. But your grandfather was lucky to survive with the help of passers-by. When you came back from crossing, you found that nothing had changed. You realized that it was you who stabbed your grandfather and failed. If you hadn't stabbed your grandfather, he might have been killed by the pursuers. With this mentality, you crossed back and looked at your grandfather from a distance. Your grandfather found someone following him and ran away. You follow him immediately, make sure your grandfather will not be killed, and chase him to a thatched cottage. You found yours. See here should be clear, everything is because of you. I'm just talking about one possibility. Since history doomed your grandfather to live, you can't change it. That is to say, you can't kill your grandfather, and your choice to cross seems to be your subjective idea. In fact, it is inevitable that history justifies itself. Similarly, if you don't want to cross, it is inevitable. You can't cheat history. No matter how you try to cross, you will stab your grandfather to death. After crossing, it won't follow your idea. There will be a series of accidents and accidents to stop you from killing your grandfather, and these so-called accidents are actually inevitable. This is history.