Paradoxes:

    One of the most intriguing qualities of time travel is its ability to create paradoxes and other conceptually troubling phenomena. Dr. Who has explored time paradoxes in several episodes including Father's Day in which the doctor's companion Rose travels back in time to rescue her father from being run over by a car. As a result, she, her father, the Doctor and several other bystanders become trapped in a paradox and tormented by "Reapers" which resemble huge bats.

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Physicists have puzzled over the possibility of time paradoxes which could be generated as the result of time travel. For example, what would happen if someone traveled back in time and killed their grandfather before he had met the individual's grandmother? The individual could therefor not exist, owing to the fact that one of their parents would not have been born. Several theories have been proposed which would ban such paradoxes from occurring (other than the appearance of man eating bat creatures). One theory is that there is a cosmic law which prevents one from creating paradoxes in the same way that there is a cosmic law preventing someone from walking through walls. Another theory, known as the many worlds theory suggests that traveling back in time would result in the formation of a new timeline. As a result, a new world would be manipulated while the original remained unaffected. The consequence of such a model would be that one could travel back in time and murder a man who is genetically identical to their own grandfather, but in reality is not their relative (5).         Time travel allows for many intriguing possibilities. While it may seem bazaar or even paradoxical, there are no physical laws which ban its existence. In fact, because of time dilation, we are able to time travel in minute amounts without considerable effort. Unfortunately, the leaps achieved by the Doctor as still well outside our grasp.