Time travel stories can make fascinating fiction, however the metaphysical implications
of time travel are not typically discussed in popular culture. For example, if backwards time
travel is possible, then it would be possible for events in the future to affect the past. This is
known as backwards causation, and it is very controversial among philosophers. Typically,
people tend to think that only past events can cause future events and never the other way
around. With this view, it would seem that altering the past from the future is impossible. Yet,
several philosophers hold onto the belief that backwards causation is possible, logically and
physically. For them, not only will it one day be physically be able to go back in time, but we
could do so without causing paradoxes.
In order to accept this view, though, it seems that we would have to give up certain
beliefs people generally hold about the world and the effects humans can have on it. Perhaps the
idea of backwards causation seems alien at first is because it contradicts and causes severe
logical problems to any idea of free will. As it will be discussed later, any author who advocates
for the possibility of backwards causations also endorses some form of determinism. Likewise,
free will and the possibility that the future can change is a popular arguments against backwards
causation. Therefore, it seems the problem of backwards causation may have more to do with the
problem of free will than any theory of causation.
In his article addressing paradoxes concerning time travel, David Lewis discusses several
problems people have with time travel. The first paradox he tackles is the local causal loop
(Lewis 148). His example involves a time traveler going to the past to tell his younger self how
to build a time machine. The young time traveler learned this information by talking to his older
self, not from any other source. In the future, the same time traveler, assumingly, would have to
go to the past to make sure he knew how to travel through time. This begs the question of where
the information about time travel came from in the first place. The older time traveler only knew
this information because his older self knew, causing an infinite loop of older time travelers
going back in time to relay this information.
With this paradox, Lewis admits it seems odd that something like this could happen, but
it is not at all impossible. He argues that we already accept certain events despite not knowing
their origin. For example, the universe was brought into existence by God or the Big Bang or
something else, yet people do not argue that something before that had to happen. Therefore, it
does not seem too far-fetched that a stable causal time loop could exist. Lewis even admits that
this is circular reasoning; however it does make sense and fits well with his established theories
of personal and local timelines (148).
While I do not think there is a glaring fault with Lewis’s handling of this paradox, I
believe he missed an opportunity by not addressing any other possibilities of causal loops. It is
equally possible that the time traveler figured out the mechanics of a time machine for himself,
went back in time and told his younger self of his findings. By going back in time to tell his
younger self about the time machine, he caused a change in the time line. This caused the causal
time loop and the seemingly infinite times the older time traveler told his younger self about the
time machine. In order to accept this possibility, not only is acceptance of backwards causation
required, but also free will. It implies that the time traveler was physically and logically able to
change the past in a way that caused a local causal time loop. As it will be discussed later, these
two do not always go well together. However, it should be mentioned that Lewis seems to
assume that free will is a non-issue. Neither was it mentioned in this paradox nor in the next.
The Grandfather Paradox is a well-known problem when dealing with time travel. A man
goes back in time and kills his own grandfather before his father was born. Because the
grandfather is dead, the father is never born, and because the father was not born, the time
traveler was not born. If the time traveler was not born, then he could not go back in time to kill
his grandfather. It seems that the time traveler was capable and not capable of killing his
grandfather at the same time.
Lewis examines this paradox with using two different definitions of “can kill his own
grandfather.” The first is physical “can.” Assuming the time traveler knows how to travel back in
time, knows how to kill a person, knows how to find his grandfather in the past, and is willing to
kill his grandfather, it seems that the time traveler can kill his grandfather. With this sense of
“can” it seems false to say that he cannot kill his grandfather (Lewis 148-149). However, if “can”
is used in the sense of “logically can,” then it seems to be the opposite. There is no logical way
the time traveler can kill his grandfather. Because he was born, his grandfather had to stay alive
long enough to beget the time traveler’s father at least. In this sense, it seems false to say he can
kill grandfather. According to Lewis, the paradox arose because people were using two different
definitions of “can” interchangeably (Lewis 149-150).
This explanation seems to satisfy the Grandfather paradox fairly well, however I feel that
the explanation is not as complete as it could be because it does not address the possibility of free
will very well. If the time traveler is fully capable of killing his grandfather in the past, it implies
that there is the possibility that time traveler successfully kills his grandfather. Yes this would
cause a paradox, but Lewis does not give us an answer as to what would happen if the time
traveler was successful. Lewis is not a hard determinist, so it would seem that he would accept
that there is a possibility that the time traveler could kill his own grandfather. At the very least, it
seems possible to imagine a world in which this could happen. Perhaps if the time traveler was
successful, this would cause some sort of local causal time loop, similar to the time traveler
talking to his younger self. The time traveler would keep going back in time to kill his
grandfather. Or maybe there is some other possibility we cannot conceive when a person creates
a time paradox.
Bryson Brown defends Lewis’ takes on time paradoxes and adds his own view on other
possible problematic outcomes of time travel and backwards causation. One paradox he
discusses is a time machine going back in time that causes the time machine in the past to selfdestruct. Similar to the Lewis example above, this causes a contradiction in the time machine’s
existence. When the time machine was given the order to self-destruct it could not send the selfdestruct order from the future. Brown agrees with the way Lewis handles the paradoxes, and sees
one possible way to avoid this paradox is to say that the time machine may be capable of sending
a self-destruct message in the past, but it is logically impossible for this to happen (Brown 430).
Another way Brown deals with the paradox is discussing the role of free will, if we even
have it. He argues that in order for the typical time paradox to work it assumes that the agents
have freedom to change the past. According to Brown, all these paradoxes prove is that free will
and backwards causation are incompatible with each other. Also, if we did not hold onto the idea
of free will then there would be no argument about whether or not backwards causation is even
possible (Brown 441).
Furthermore, he states that when we discuss the paradoxes we are assuming that the time
travelers have more agency and capabilities than we do in the real world (Bryson 437). If we
were to fairly evaluate the paradoxes, we would have to assume that the agents are under the
same conditions we are, which is to say their actions will be just as predetermined as ours. With
this in mind, a hard determinist defense of backwards causation is the most effective. It is
impossible for any hypothetical time traveler to do anything logically impossible or anything that
would cause a paradox because nothing like that has ever happened, therefore it will never
happen (Bryson 441).
Brown’s defense of backwards causation is inherently tied to deterministic beliefs. The
only way backwards causation could be possible without the chance of any paradoxes happening
is for every event to be predetermined. Brown concedes that a compatibilist defense could work,
but it would be weaker. A compatibilist would argue that agents could act in a paradoxical
manner but they never perform them because the consequences of their actions could are not
externally possible (Bryson 441).
To a determinist, the idea of backwards causation working out will seem plausible,
however it is alienating to libertarians and many compatibilists. Regardless how the latter groups
feel about time travel, they cannot hold onto the belief that agents have free will and the ability
to change the future (or past in these cases) and backwards causation is not problematic. It is
difficult to believe that any person who truly believes in free will change their views on this
important issue so they can believe backwards time travel is possible. However, if someone is a
determinist, then this view of backwards causation will seem like the right path.
Kristie Miller1, on the other hand, argues against backwards causation. Miller’s main
argument involves the “intuitive view” of time. There seems to be fundamental differences
between the past, present, and future. The past is fixed, and the future is open (Miller 173)
People certainly act this way in everyday life, and it seems like common sense. According to
Miller, the future is open in the sense that at any moment there seems to be several possible
directions, or branches, the future can take. These branches become the future. Miller uses this
tree analogy to illustrate her idea of the growing universe model. The branches continue to grow
In her article, Miller also discusses her own causation theory which involves “pseudo-necessities” and “pseudocontingencies” (183). For the purposes of this paper, discussion on this has been omitted.
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with several possibilities, and the past is the unchangeable trunk (Miller 176). In everyday
language we used past-tense propositions, which add to the idea that past is fixed.
Although Miller does not state her view on free will, it seems that she endorses some sort
of free will for humans. The future could only be open if we are able to do things that are not
predetermined. We have the ability to change the direction or which branch the future will take.
A determinist will most likely reject this notion because it suggests that people do have the
ability to do things they otherwise would not have.
Miller also concedes that there are some problems with accepting the intuitive view. As
mentioned before, a person would have to be a libertarian or a compatibilist to agree with
Miller’s take of backwards causation. A hard determinist would never find her argument
persuasive. Also, she acknowledges that if the scientific community ever proves that backwards
causation could be physically possible then we would have to reject the intuitive view. She
writes, “[W]e should reject any metaphysical theory that is at odds with science” (Miller 191). If
scientist unveiled a time machine that would allow people to go back in time and potentially
affect the present, then it would seem that the past is not as fixed as we once thought it was.
Miller’s view about backwards causation is very appealing to libertarians and people who
are doubtful about backwards causation. Skeptics of future events affecting the past may latch
onto her ideas because it seems better than accepting the alternative. One thing Miller does not
address in her article is whether free will could ever be compatible with backwards causation.
Although she acknowledges that there is the possibility of time travel being a reality in the
future, she does not address how this would affect free will directly. If the scientific community
does release a backwards time machine, should everyone become determinist?
It seems that it is possible for backwards causation to be compatible with free will. What
will be suggested is certainly controversial, but backwards causation could allow for free will. If
a person went back in time, they would be able to act freely. This means that not only would
their actions affect the future their actions in the past would not necessarily be predetermined.
These actions would have the possibility of altering the present to be different than how it is
now.
The main point is that we should expect a person to have as much or little free will in the
past as they do in the present. If we believe people to act freely right now, then we should
assume that if they went back in time their actions would be just as free. Free will with
backwards causation could have severe consequences. If someone went back in time right now,
the present could change drastically because of the time traveler’s actions. These changes do not
have to necessarily be paradoxical. As many science fictions story like to explore, little actions in
the past could make the present almost unrecognizable. Potential effects of acting freely in the
past could be more severe than any time travel story, but it is important to acknowledge that a
person could act freely if they travel back in time.
With this in mind, yes it would be possible to produce a paradox. However, we do not
know what would happen if someone did something paradoxical. As Bryson pointed out, there
have been no paradoxes in the past, which means there will likely never be paradoxes in the
future. Is this because of the deterministic laws of nature or other forces making sure paradoxes
never happen? This may not be the case.
It could be that it is logically, and therefore physically, impossible to cause a time
paradox. This may sound like science fiction writing, but it may be there have been no paradoxes
in the past because the laws of nature are set in a way that a paradox could never happen, or at
the very least, if a paradox happened we would never be aware of it. Take the example of the
time traveler who went back in time to kill his grandfather. Let us suppose he actually succeeded
in killing his grandfather. What would happen? Because he killed his grandfather, he was never
born, which means he could have never gone back in time to kill his grandfather. If his
grandfather had not died, then the time traveler would be alive. This would go completely
unnoticed by everyone in the world. The time traveler would not realize that he had gone back in
time. The timeline would be altered after such a paradoxical thing would happen. Granted, once
the time traveler is back at the time he went back originally, it is possible he would keep going
back in time killing his grandfather, only to be back in the present. The laws of nature, God, or
some other force will correct these types of paradoxes.
Free will seems to be deeply connected with the possibility of backwards causation.
Arguments for and against backwards causation seem to bring up free will. However, none of the
arguments reviewed in this paper consider the possibility of some type of free will being possible
if someone were to travel back in time. The consequences of free will being involved in
backwards causation could be very severe, but philosophers should consider the possibility that
this could happen. We should not assume that our agency and free will changes if we are ever
able to go back in time.
Works Cited
Brown, Bryson. “Defending Backwards Causation.” Canadian Journal of Philosophy 22 (1992):
429-43. Print.
Lewis, David. “The Paradoxes of Time Travel.” American Philosophical Quarterly (1976): 14552. Print.
Miller, Kristie. “Backwards Causation, Time, and the Open Future.” Metaphysica 9.2 (2008):
173-91. Print.