Some people say that if time travel could happen we'd be awash in time travellers. Though there is a certain logic to this the truth is that it is merely not the time for time travel. As the universe expands certain constants yet undiscovered by humanity change slighty. At some point in the future the universe will become favorable to time travel. The travellers cannot go beyond the point of the change so the past that is our present is protected. What happens though to the future is extremely chaotic. The universe has been destroyed since passing that point at least 3,201 times only to be recreated. Don't even ask about family trees.
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