I'm boosting Rio I'm a philosophy
professor at MIT and today I want to
tell you about the grandfather paradox
which is a famous paradox of time travel
imagine Bruno Bruno has a grudge against
his grandfather and rightly so his
grandfather was a horrible man amongst
many other things he perpetrated this
fraud that left millions of people in
poverty so there's nothing Bruno would
want more than to kill grandfather
unfortunately grandfather's already dead
grandfather died many years before Bruno
was born but that needn't stop Bruno
because Bruno has constructed a time
machine and intends to travel to the
past to kill grandfather when he was
still a young man in fact he wants to
travel back enough in time
that he can kill grandfather before
grandfather had any children
so picture Bruno he's traveled back in
time he's brought a sniper's rifle with
him and he's trained himself to use it
he's been tracking grandfather's
movements it knows exactly where he goes
for his morning walk he's found a
perfect spot in a nearby building and he
has a clear shot grandfather is walking
he stops to tie his shoelaces
Bruno caresses the trigger and what
happens we know that Bruno will not
succeed in killing grandfather why well
I already told you the grandfather is
runo's grandfather and from that it
follows that grandfather must have lived
long enough to have children but on this
story Bruno has traveled to a time
before the time when grandfather had
children so
if Bruno were to succeed grandfather
would never have any children which
would contradict what I already told you
about maybe you think that the story
could be interpreted differently maybe
you think that we can imagine a
situation in which Bruno kills
grandfather as a result the fraud never
takes place so Bruno travels back to the
present and finds that everyone is much
happier I claim that that doesn't make
any sense but I think it's easy to think
otherwise because there are lots of
movies that tell stories which are kind
of like that one
one good example of this is one of my
favorite films
Back to the Future now I don't know if
you've seen Back to the Future but it's
very easy to give you a quick summary so
according to the film there's this guy
Marty McFly who has a father George and
nobody respects George but Marty gains
access to a time machine which is
especially modified DeLorean he gets
into the DeLorean reaches a certain
speed there's a flash of bright light
and that allows him to travel to the
past to a time when his father was a
young man he interacts with his father
he changes his father's character and
when Marty comes back to the future
he finds his father a changed man he
finds that now everybody respects his
father and his father is very very happy
my colleague Brad scow once taught me a
wonderful way of seeing just why this
story is inconsistent because look at
the beginning of the film were told that
at a certain time 1985 George is
respected by nobody and at the end of
the film were told fed at that very time
in 1985 everyone respects George but
look it doesn't matter whether we're
told one thing at the beginning of the
film in a different thing at the end of
the film it's still inconsistent one way
to see this is to imagine that you're
sitting on a
the witness takes the stand and at the
beginning of her testimony she says that
at the crucial time she was by
cigarettes of the store and towards the
end of her testimony she says that at
that very same time she was instead at
home watching TV you can tell right
there that not everything in the
witness's story is true and that's
because her story is inconsistent and it
doesn't help fit part of the story was
told at the beginning of her testimony
in the other part was told at the end of
her testimony the story is still
inconsistent I think the same thing
happens with Back to the Future just
because we're told one thing at the
beginning of the story in a different
thing at the end of the story that
doesn't mean that the story is
consistent there is a different way of
interpreting back to the future where it
is a consistent story
it's just that I'm this different
interpretation it's not primarily a time
travel story it's really a story about
travel between universes and this
interpretation what happens is that when
Marty McFly boards the DeLorean what
happens is not that he goes to the path
of his own universe what happens is that
he travels to a different universe in
this different universe he meets copies
of his parents interacts with them and
then he goes to the future of that
universe and finds the older copies of
his parents to be very happy and
respected by all notice however that
that's not the interpretation that is
suggested by the movie yes on that
interpretation the movie is a tragedy
think of Marty's poor original father
stuck in our universe it's not only the
case that nobody respects him he was
also abandoned by his son Marty in
summary I think that insofar as we
interpret Back to the Future as an
ordinary bonafide
time travel story we have to conceive it
even though it's a great movie it's
logically inconsistent similarly when we
interpret the story of Bruno and his
grandfather as an ordinary bonafide time
travel story
we cannot say consistently that
grandfather is his grandfather and the
Bruno succeeds in killing him at a time
before he have children this leaves us
with a very tricky question
we know that Bruno will not succeed in
killing grandfather but what stops him
here's some different options for
answering this question one thing you
could say is what stops him is that time
travel is impossible so he could have
never been in that position another
thing you could say is that the universe
contains a special anti paradox force
and that if Bruno was about to succeed
in killing grandfather somehow this
force would come down and intervened I
think there is no need to go in for
either of these options if you ask me
what stops Bruno from killing
grandfather I say I don't know because
you haven't finished telling the story
but probably what stops in' is something
quite mundane in some ways of developing
the story it'll turn out that Bruno was
pointing the rifle but lost his nerve at
the very last minute and other ways of
spelling out the story he got distracted
maybe there was a barking dog in the
background and other ways of telling the
story he shoots
but misses we know that there is some
inconsistent time-travel stories but
that doesn't mean that every time travel
story is inconsistent there are
perfectly consistent ways of telling the
story Bruno and his grandfather and on
some of these there are perfectly
mundane reasons for explaining why Bruno
failed it could be the time travel is
technologically impossible so it could
be that it will remain forever beyond
our reach but that doesn't mean the time
travel itself
logically inconsistent in fact I think
the concept of time travel is perfectly
consistent