philosophy paper
Valerielee
GrandfatherParadox.pdf
1/18/2018
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Phil 2: Puzzles and Paradoxes
Prof. Sven Bernecker
University of California, Irvine
The Grandfather
Paradox
Grandfather Paradox
• Grandfather paradoxes arise whenever a time traveler goes into
the past and prevents an event that is a pre-condition of the
traveler making the backward time-journey in the first place.
• Example: You have just finished building a time machine. You
use this time machine to travel back twenty-four hours, cut the
power-supply to your own laboratory so you can’t finish the time
machine that lets you go back in time to cut the power-supply ...
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• Another example: A time traveler goes back in time and kills his
grandfather before the grandfather has fathered children. If the
grandfather dies at this point, then one of the time traveler’s parents
never exists. Hence the traveler can’t be born and travel back to kill
the grandfather ... and so on.
• Worry: It is impossible to kill your own grandfather because it would
violate the law of non-contradiction – the grandfather would somehow
both survive to become a parent and not survive. If contradictory
situations are impossible, and if time travelers could create
contradictory situations, then time travel is impossible.
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• A backward time traveler
seems to be able to change the
past. But it is impossible to
change the past. But since the
past cannot be changed, time
travel is impossible.
1) By ordinary standards of ability, the backward time traveler
can kill his grandfather
2) But the time traveler cannot kill his grandfather. The
grandfather lived, so to kill him would be to change the past.
It is logically impossible to change the past.
3) Premises (1) and (2) contradict each other
C) Therefore, backward time travel is impossible
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David Lewis’s Solution of the
Grandfather Paradox • Premises (1) and (2) are both true. They do not contradict one
another. The term “can“ in both premises mean different things.
The Grandfather paradox rests on an equivocation about the
meanng of “can.“
• When we say that somebody “can” do something, we mean that
they have the capacity, holding certain things fixed. Which
things we hold fixed will depend upon context.
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• Unlike dogs and cats, I can speak a foreign language, like Urdu.
So there is a sense in which I can speak Urdu. But don’t take me
as your translator when you go to Pakistan, because I can’t speak
Urdu. I have never learned it. When we say “Sven can speak
Urdu", we are only holding fixed my brain’s linguistic potentialities
— we are saying that Sven could learn it.
• When we say “Sven can’t speak Urdu", we are holding fixed my
actual knowledge of languages — we are saying that he doesn’t
currently speak it.
• What I can do, relative to one set of facts, I cannot do, relative to
another set of facts.
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• Similarly, holding fixed only the past up to the attempt to kill
the grandfather, the time traveler can kill his grandfather.
However, holding fixed the past following the attempt to kill
the grandfather, the time traveler cannot kill his grandfather.
There is no paradox.
• The argument on slide #5 simply equivocates with respect to
“can.”
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• Q: What will actually prevent the time traveler from killing his grandfather?
• A: The failure to kill his grandfather will be caused by strange coincidences:
His gun jams, a noise distracts him, he slips on a banana peel, he
accidentally shoots someone else, etc.
• To render backward time travel possible we have to assume that a time
traveler cannot change the past even though he can participate in the
past. A time traveler cannot do anything that did not actually happen. But he
can be amongst the people who did make the past the way it was. The
freedom of action of a backward time traveler is severely limited.
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¿Parallel Worlds?
• Imagine that whenever a person travels into the past, the universe
splits into parallel worlds. If this occurs, there is no longer a
contradiction between the grandfather both existing and not existing.
The grandfather may exist in one world but not in the other.
• Parallel worlds allow for coherent scenarios in which the past is
changed
• But this is no longer time travel; it is inter-world travel.
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Add’l Videos about the
Grandfather Paradox
• Short video about the grandfather paradox:
https://www.youtube.com/watch?v=Y6RjjaEy59I
• Slightly longer video about the grandfather paradox
https://www.youtube.com/watch?v=M8oITAoaCr4
• Video explaining Lewis’s solution to the grandfather paradox:
https://www.youtube.com/watch?v=_oZhA5cxUxs
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