The time machine would never have happened, therefore deleting the altered past, making it impossible for you to change anything significant that has already happened, as it interferes with your need to do such a thing.
E.g You go back in time and kill someone. This means he didnt exist when you had the time machine in the future, deleting the idea of killing him. It is impossible to change the past, merely observe, if you were able to travel in time.
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