1. The Altered Past Paradox
Time-Travel
The Altered Past 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
If time travel were possible, it would be possible to kill your grandfather before your father is conceived.
It’s not possible to kill your grandfather before your father is conceived.
[So] Time travel isn’t possible.

6. 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.