Think about it, intelligent humans made time machine and you were the one to be tested out. They sent you 100 years ago in past where the marriage of your grandfather and grandmother was going on. Then you killed your grandfather. What will happen? How are you even supposed to be born? If you were not born then who killed your grandfather?
This paradox is known as the grandfather paradox and this can be explained by physicists.

How can this paradox be explained?

This may be a very confusing paradox if you think time as an arrow as older physicists used to think. Travelling in a fixed direction and with constant speed.
Then our hero Albert Einstein came as described time as a river. Flowing in a direction but somewhere it is slow and somewhere it is fast. And that river is stoppable and can even flow backward. That river can split into two direction and flow separately.

The grandfather paradox can be explained by the last point, the river can split into two parts. Suppose you just went backward in time and killed your grandfather then you will create a separate universe for two things. One is where you are born and another is where there is no grandfather and no you. The universe is called parallel universe. There may be many parallel universe present today and we may be totally unaware about it. Maybe time travelers are looking at you right now and you don't know they are here. This may be creepy but it is true.

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