T H E G R A N D FAT H E R PA R A D O X

Time-Travel

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.

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).

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.

The Grandfather Paradox

The Grandfather Paradox

The Grandfather Paradox

1) Time travel is possible. [Assume for Reductio]2) If time travel is possible, then you could go back in time and kill your

grandfather when he was a child. 3) [So] You could go back in time and kill your grandfather when he was a

child.4) If you could go back in time and kill your grandfather when he was a child,

then you would never be born.5) If you would never be born, then you could not go back in time and kill

your grandfather when he was a child.6) [So] You both could and could not go back in time and kill your

grandfather when he was a child. [Contradiction] 7) [So] Time travel isn’t possible.

The Altered Past Paradox

1) If time travel were possible, it would be possible to change the past.

2) It’s not possible to change the past.3) [So] Time travel isn’t possible.