If a person really "goes back to the past" and kills his grandmother before she is pregnant with his mother, will the time traveler himself still exist? This problem is obvious. Without your grandmother, there would be no mother. Without your mother, there would be no you. Without you, how can you "go back in time" and kill your grandmother before she is pregnant with his mother? This is the "grandmother paradox". -From the encyclopedia

Extension: Anyway, my grandmother is in two States, either dead or not. Now let's assume that grandma is still alive, so mom can be born and so can you. You can travel through time and space, go back to the past, find your grandmother before you conceived your mother, and then kill her. In this way, your mother can't be born, and neither can you. Of course, you have no chance to go back and kill your grandmother, so your mother can have a baby, you can have a baby, or you can go back and kill your grandmother. So you can't be reborn, and your grandmother won't die ... Look, this is an infinite cycle process, and it seems that grandma's life and death exist at the same time, in a superposition state.

Grandmother paradox can be considered from two different angles: on the one hand, from the perspective of physics, grandmother paradox can be analyzed concretely and quantitatively. The general view is that we can change history, but what we change is not the history of our own universe, but the history of other parallel worlds. Of course, there are countless kinds of parallel worlds, because there are countless possible combinations, as long as possible things will happen in different parallel worlds, and as participants in this quantum game, it seems that we feel very sci-fi. Unbelievable? Indeed, this is as incredible as flying to the moon 100 years ago, but the progress of the times always makes the past impossible a reality within reach.

Another angle is that history cannot be changed. No matter how you try to change history, the result is ultimately to promote history, simple and rude, full of fatalism. —— Excerpted from Haida Agricultural College of WeChat official account.