The
Grandfather Paradox illustrates why time travel into the past is impossible.
The paradox
goes like this: Sally is an angry young woman who blames her grandfather, and
his dedication to the family business, for most of her problems in life. She is
also an expert sniper and engineer. One day, Sally constructs a machine that
allows her to travel back in time and, using the machine, she journeys 80 years
into the past, when her grandfather was a child. Fueled by hatred of her
grandfather and armed with her trusty rifle, which she brought with her, Sally finds
her grandfather and kills him.
But the
murder of Sally’s grandfather as a young boy, before he fathered any children, means
that one of her biological parents will never be born. This, in turn, means
that Sally will never be born and, thus, cannot travel back in time and kill
her grandfather.
The
Grandfather Paradox thus seems to lead to the conclusion that backwards time
travel is impossible.
Is there a
way out of this paradox?
Perhaps.
One response
to the Grandfather Paradox appeals to the concept of alternate timelines. We might
say that when Sally travels back 80 years, she emerges on a parallel or
alternate timeline. In other words, Sally goes into the past, but lands on a
different plane of time (or possibly spacetime) than the one she left, and so
it is possible for her to kill her grandfather, who exists on the alternate
timeline, without undoing her existence, on her own timeline, in the process.
But what
this scenario describes is not time travel, as the concept is generally
understood. If a traveler jumps 80 years into the past and onto a separate
plane of existence, the traveler has not really traveled through time, because
she has not landed in a time that was prior to the one she left. Rather, she
has traveled to a whole new existence—maybe a whole new universe—that is
apparently 80 years behind her own but identical in all other respects. In
contrast, time travel as we imagine it, and as the Grandfather Paradox
conceives of it, requires a unity of the time sequence being traveled. (Imagine
a bus terminal with several ticket windows. You are in the front of the line at
Window A. Traveling back in time would be like journeying to the beginning of the
Window A line, while traveling onto an alternate timeline at a past time would
be like journeying to the end of the line at Window B.)
Another answer
to the Grandfather Paradox is to say that, while it might preclude some types
of backwards time travel, it need not render all travel to the past impossible.
In other words, the most the Grandfather Paradox shows is that backwards time
travel is impossible only if it is used to upset the conditions that allowed
the backwards time travel to happen. Using
this approach, we can concede that Sally cannot travel back in time to kill her
grandfather (because once she does, there will be no Sally to travel back in
time to kill her grandfather), while still imagining all sorts of travel to the
past that might be possible: Jim travels back 100 years and drinks some water
from an isolated stream; Carol travels back 40 years and pets a cat; Roger
travels back two weeks and moves a book from one end of a bookshelf to another.
This type of explanation is referred to as the “self-consistent” approach. It
says that backwards time travel is possible, so long as anything the time
traveler does is “self-consistent” with the set of circumstances that allowed
the time travel in the first place.
The problem
with this approach is that it is very difficult—if not impossible—to know what
actions in the past will affect the future. The Grandfather Paradox is gripping
because it offers a stark and obvious example of an action in the past that
prevents the time travel itself. But how would it ever be possible to determine
whether any action—a sip of water from a stream, a stroke underneath a cat’s
chin, or moving a book from one place to another—affects the future in some way
that is inconsistent with the time travel that made the action possible? Unless
and until we can say for certain that a particular event will not have such an
effect, the “self-consistent” approach will be an unsatisfying response to the
Grandfather Paradox.
It’s also
possible to explain the Grandfather Paradox by various “plot tricks” that would
be at home in any Hollywood movie. For example, we might imagine that Sally
kills her grandfather and yet is mysteriously not erased from existence. Sally
then travels back to her own time, utterly confused by her continued presence
on Earth, only to discover years later that the man she murdered was actually
not her grandfather. We might also imagine that although Sally succeeds in
killing her actual grandfather, after the murder he is somehow brought back to
life and goes onto father one of Sally’s biological parents. These explanations
are fun and can make for good cinema, but they are ultimately unsatisfying,
since they deny the Grandfather Paradox the very thing that makes it
paradoxical: the idea that one can travel backwards in time and take actions
that affect the future.
