I think there should be several different situations when going back to the past. Suppose, if time goes back, but only you have not changed, just like the time machine in science fiction, this is the first case. In this case, the time equivalent to the whole world has gone backwards, and "history" has been eliminated, except for you. You are no longer in sync with the world around you at the moment, because no one is in the world you arrived in. There is no "cause" you produce. In other words, this is not the world you live in, and your "cause" will not come to this time from the original time. Even if you kill your parents, you will not disappear, because this world is not your world, your parents have nothing to do with you, and cause and effect have been cut off. This is a parallel universe, but if we go back to the original time at this time, then history will be restored, just like in. This is not time travel, but cross-dimensional travel. In the second case, you go back to the past, but you have a causal relationship with that world, that is, you can have a paradox. If you are in sync with that world, that is, you will age with the passage of time in that world, but at this time, there is only one thing that can avoid the paradox-the disappearance of the future as you know it. You choose to re-create a future, and interference in the past will make the world enter another future. Maybe someone will. If you killed your parents here, how did you come into being? It should be noted that it is also possible to solve this paradox. For example, a seed in the past has become a big tree today. Today, we think: Without the seeds of the past, there would be no trees today, right? But in fact, can anyone find the seeds of the past today when people can contact and know each other? Nobody can find it. On the other hand, in the era of seeds in the past, can anyone find this big tree today? It's impossible to find. But today, without the seeds of the past, the tree still exists, and without the past of today's tree, the seeds will not disappear. Does causality really exist? Do you have to have a cause for the result? Besides the problem of cognitive inertia, what else can prove the inevitability between cause and effect? In the case of going back in time, the causal relationship has been violated, because the result precedes the cause, so what is the point of clinging to the causal paradox? The third situation, seeing the past, cannot be changed. This reason should be between the first and the second, because it should be the same world as your world, just like the second, but after returning to the future, everything is still the same as the first. This situation should be disappointing. Generally speaking, it can be divided into three situations. Of course, if we go deeper, there may be more details, such as the middle of time travel. Whether the time machine will return to the future after leaving and so on. First, the second kind of going back to the past is based on the existence of the "past" world or parallel world, and the third kind is like looking after the light. The means of going back to the past is relatively hard to say. In fact, the way of time and space, black holes and so on are all higher-speed motion modes. I think all inventions and creations through the ages come from some similar rules and laws, but they are similar in time and space. There is still something missing. If the world is a plane, going to another world is equivalent to penetrating this plane. If the world is a line, it is good to go to the end, but what is the world like? What is it like to make a miniature version of the world? If we don't start from this aspect, we can only find things that travel through time and space, or their traces. Otherwise, you can only rely on luck.