What is the paradox of time?

They are problems. The commonest are the so-called paradoxes. For example, if we could travel through time, imagine what would happen to a time traveller if he (or she) travelled back in time and killed his own grandmother at birth. In theory the time traveller will therefore never be born, so the journey could never have been made in the first place; but if the journey never occurred then the grandmother would be born which means the time traveller would have been born and could make the journey ... and so on and so on. This is a paradox.

There are two possibilities to resolve this paradox. The first is that the past is totally defined, i.e. everything that has happened or must happen, including the time traveler's attempt to kill his grandmother, cannot be altered and nothing will change the course of history. In other words, the time traveller will experience endless "mishaps" in trying to kill his grandmother and will never achieve the murder, thus keeping time (or at least events) intact.

The second possibility is more complex and involves the quantum rules which govern the subatomic level of the universe. Put simply, when the time traveller kills the grandmother he immediately creates a new quantum universe, in essence a parallel universe where the young grandmother never existed and where our time traveller is never born. The original universe still remains. Stephen hawking believes he can explain the origin of our universe in a variation of this parallel-worlds theme.