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Time-Travel

Time-Travel. The grandfather paradox. Back to the Future. Back to the Future. In BttF I , Marty travels back to 1955 and helps unite his parents. In BttF III , Marty travels back to 1855 and rescues Doc Brown.

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Time-Travel

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  1. Time-Travel The grandfather paradox

  2. Back to the Future

  3. Back to the Future • In BttF I, Marty travels back to 1955 and helps unite his parents. • In BttF III, Marty travels back to 1855 and rescues Doc Brown. • Marty helps unite his parents (in BttF I) before he rescues Doc Brown (in BttF III). • Marty rescues Doc Brown (in 1855) before he helps unites his parents (in 1955).

  4. Back to the Future • External time is the series of events as ordered by an objective historian. • Personal time is the series of events as experienced by a particular person. • A time-traveler is someone whose personal time diverges from real time.

  5. The Grandfather Paradox

  6. The Grandfather Paradox

  7. The Grandfather Paradox • Time travel is possible. [Assume for Reductio] • If time travel is possible, then you could go back in time and kill your grandfather when he was a child. • [So] You could go back in time and kill your grandfather when he was a child. • If you could go back in time and kill your grandfather when he was a child, then you would never be born. • If you would never be born, then you could not go back in time and kill your grandfather when he was a child. • [So] You both could and could not go back in time and kill your grandfather when he was a child. [Contradiction] • [So] Time travel isn’t possible.

  8. The Altered Past Paradox • If time travel were possible, it would be possible to change the past. • It’s not possible to change the past. • [So] Time travel isn’t possible.

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