## 1. What is Time Travel?


There is a number of rather different scenarios which would seem,
intuitively, to count as 'time travel'--and a number
of scenarios which, while sharing certain features with some of the
time travel cases, seem nevertheless not to count as genuine time
 travel:[1]



> 
> **Time travel**
> 
> 
> * *Doctor*. Doctor Who steps into a machine in
> 2024. Observers outside the machine see it disappear. Inside the
> machine, time seems to Doctor Who to pass for ten minutes. Observers
> in 1984 (or 3072) see the machine appear out of nowhere. Doctor Who
> steps out.[2]
> * *Leap*. The time traveller takes hold of a special device
>  (or steps into a machine) and suddenly disappears; she appears at an
>  earlier (or later) time. Unlike in *Doctor*, the time
>  traveller experiences no lapse of time between her departure and
>  arrival: from her point of view, she instantaneously appears at the
>  destination 
>  time.[3]
> * *Putnam*. Oscar Smith steps into a machine in 2024. From
>  his point of view, things proceed much as in *Doctor* : time
>  seems to Oscar Smith to pass for a while; then he steps out in
>  1984. For observers outside the machine, things proceed
>  differently. Observers of Oscar's arrival in the past see a
>  time machine suddenly appear out of nowhere and immediately divide
>  into two copies of itself: Oscar Smith steps out of one; and (through
>  the window) they see inside the other something that looks just like
>  what they would see if a film of Oscar Smith were played backwards
>  (his hair gets shorter; food comes out of his mouth and goes back
>  into his lunch box in a pristine, uneaten state; etc.). Observers of
>  Oscar's departure from the future do not simply see his time
>  machine disappear after he gets into it: they see it collide with the
>  apparently backwards-running machine just described, in such a way
>  that both are simultaneously
>  annihilated.[4]
> * *Godel*.
>  The time traveller steps into an ordinary rocket ship (not a special
> time machine) and flies off on a certain course. At no point does she
> disappear (as in *Leap*) or 'turn back in time'
> (as in *Putnam*)--yet thanks to the overall structure of
> spacetime (as conceived in the General Theory of Relativity), the
> traveller arrives at a point in the past (or future) of her
> departure. (Compare the way in which someone can travel continuously
> westwards, and arrive to the east of her departure point, thanks to
> the overall curved structure of the surface of the
> earth.)[5]
> * *Einstein*. The time traveller steps into an ordinary
>  rocket ship and flies off at high speed on a round trip. When he
>  returns to Earth, thanks to certain effects predicted by the Special
>  Theory of Relativity, only a very small amount of time has elapsed
>  for him--he has aged only a few months--while a great deal
>  of time has passed on Earth: it is now hundreds of years in the
>  future of his time of 
>  departure.[6]
> 
> 
> **Not time travel**
> 
> 
> * *Sleep*. One is very tired, and falls into a deep
>  sleep. When one awakes twelve hours later, it seems from one's
>  own point of view that hardly any time has passed.
> * *Coma*.
> One is in a coma for a number of years and then awakes, at which point
> it seems from one's own point of view that hardly any time has
> passed.
> * *Cryogenics*. One is cryogenically frozen for hundreds of
> years. Upon being woken, it seems from one's own point of view
> that hardly any time has passed.
> * *Virtual*. One enters a highly realistic, interactive
> virtual reality simulator in which some past era has been recreated
> down to the finest detail.
> * *Crystal*. One looks into a crystal ball and sees what
> happened at some past time, or will happen at some future
> time. (Imagine that the crystal ball really works--like a
> closed-circuit security monitor, except that the vision genuinely
> comes from some past or future time. Even so, the person looking at
> the crystal ball is not thereby a time traveller.)
> * *Waiting*. One enters one's closet and stays there
> for seven hours. When one emerges, one has 'arrived' seven
> hours in the future of one's 'departure'.
> * *Dateline*. One departs at 8pm on Monday, flies for
> fourteen hours, and arrives at 10pm on Monday.
> 
> 
> 


A satisfactory definition of time travel would, at least, need to
classify the cases in the right way. There might be some
surprises--perhaps, on the best definition of 'time
travel', *Cryogenics* turns out to be time travel after
all--but it should certainly be the case, for example,
that *Godel* counts as time travel
and that *Sleep* and *Waiting* do
not.[7]




In fact there is no entirely satisfactory definition of 'time
travel' in the literature. The most popular definition is the
one given by Lewis (1976, 145-6):



>  What is time travel? Inevitably, it involves a
> discrepancy between time and time. Any traveller departs and then
> arrives at his destination; the time elapsed from departure to
> arrival...is the duration of the journey. But if he is a time
> traveller, the separation in time between departure and arrival does
> not equal the duration of his journey....How can it be that the
> same two events, his departure and his arrival, are separated by two
> unequal amounts of time?...I reply by distinguishing time itself,
> external time as I shall also call it, from the personal time of a
> particular time traveller: roughly, that which is measured by his
> wristwatch. His journey takes an hour of his personal time, let us
> say...But the arrival is more than an hour after the departure in
> external time, if he travels toward the future; or the arrival is
> before the departure in external time...if he travels toward the
> past.
> 
> 
> 


This correctly excludes *Waiting*--where the length of
the 'journey' precisely matches the separation between
'arrival' and
'departure'--and *Crystal*, where there is no
journey at all--and it includes *Doctor*. It has trouble
with *Godel*, however--because
when the overall structure of spacetime is as twisted as it is in the
sort of case Godel imagined, the notion of external time
("time itself") loses its grip.




Another definition of time travel that one sometimes encounters in the
literature (Arntzenius, 2006, 602) (Smeenk and Wuthrich, 2011, 5,
26) equates time travel with the existence of CTC's: closed
timelike curves. A curve in this context is a line in spacetime; it is
timelike if it could represent the career of a material object; and it
is closed if it returns to its starting point (i.e. in
spacetime--not merely in space). This now includes
*Godel*--but it excludes *Einstein*.




The lack of an adequate definition of 'time travel' does
not matter for our purposes
 here.[8] 
It suffices that we have clear cases of
(what would count as) time travel--and that these cases give rise
to all the problems that we shall wish to discuss.




Some authors (in philosophy, physics and science fiction) consider
'time travel' scenarios in which there are two temporal
dimensions (e.g. Meiland (1974)), and others consider scenarios in
which there are multiple 'parallel' universes--each
one with its own four-dimensional spacetime (e.g. Deutsch and Lockwood
(1994)). There is a question whether travelling to another version of
2001 (i.e. not the very same version one experienced in the
past)--a version at a different point on the second time
dimension, or in a different parallel universe--is really time
travel, or whether it is more akin to *Virtual*. In any case,
this kind of scenario does not give rise to many of the problems
thrown up by the idea of travelling to the *very same past* one
experienced in one's younger days. It is these problems that
form the primary focus of the present entry, and so we shall not have
much to say about other kinds of 'time travel' scenario in
what follows.


### 1.1 Time Discrepancy


One objection to the possibility of time travel flows directly from
attempts to define it in anything like Lewis's way. The worry is
that because time travel involves "a discrepancy between time
and time", time travel scenarios are simply incoherent. The time
traveller traverses thirty years in one year; she is 51 years old 21
years after her birth; she dies at the age of 90, 200 years before her
birth; and so on. The objection is that these are straightforward
contradictions: the basic description of what time travel involves is
inconsistent; therefore time travel is logically
impossible.[9]




There must be something wrong with this objection, because it would
show *Einstein* to be logically impossible--whereas this
sort of future-directed time travel has actually been observed (albeit
on a much smaller scale--but that does not affect the present
point) (Hafele and Keating, 1972b,a). The most common response to the
objection is that there is no contradiction because the interval of
time traversed by the time traveller and the duration of her journey
are measured with respect to different frames of reference: there is
thus no reason why they should coincide. A similar point applies to
the discrepancy between the time elapsed since the time
traveller's birth and her age upon arrival. There is no more of
a contradiction here than in the fact that Melbourne is both 800
kilometres away from Sydney--along the main highway--and
1200 kilometres away--along the coast
road.[10]


### 1.2 Changing the Past


Before leaving the question 'What is time travel?' we
should note the crucial distinction between changing the past and
participating in (aka affecting or influencing) the
past.[11] In the
popular imagination, backwards time travel would allow one to change
the past: to right the wrongs of history, to prevent one's
younger self doing things one later regretted, and so on. In a model
with a single past, however, this idea is incoherent: the very
description of the case involves a contradiction (e.g. the time
traveller burns all her diaries at midnight on her fortieth birthday
in 1976, and does not burn all her diaries at midnight on her fortieth
birthday in 1976). It is not as if there are two versions of the past:
the original one, without the time traveller present, and then a
second version, with the time traveller playing a role. There is just
one past--and two *perspectives* on it: the perspective of
the younger self, and the perspective of the older time travelling
self. If these perspectives are inconsistent (e.g. an event occurs in
one but not the other) then the time travel scenario is
incoherent.




This means that time travellers can do less than we might have hoped:
they cannot right the wrongs of history; they cannot even stir a speck
of dust on a certain day in the past if, on that day, the speck was in
fact unmoved. But this does *not* mean that time travellers
must be entirely powerless in the past: while they cannot do anything
that did not actually happen, they *can* (in principle) do
anything that *did* happen. Time travellers cannot change the
past: they cannot make it different from the way it was--but they
can participate in it: they can be amongst the people who did make the
past the way it
 was.[12]




What about models involving two temporal dimensions, or parallel
universes--do they allow for coherent scenarios in which the past
is changed?[13] 
There is certainly no contradiction in saying that the
time traveller burns all her diaries at midnight on her fortieth
birthday in 1976 *in universe 1* (or *at
hypertime* *A*), and does not burn all her diaries at
midnight on her fortieth birthday in 1976 *in universe 2*
(or *at* *hypertime* *B*). The question is whether
this kind of story involves *changing* the past in the sense
originally envisaged: righting the wrongs of history, preventing
subsequently regretted actions, and so on. Goddu (2003) and van 
Inwagen (2010) argue that it does (in the context of particular
hypertime models), while Smith
(1997, 365-6; 2015) argues that it does not: that it
involves *avoiding* the past--leaving it untouched while
travelling to a different version of the past in which things proceed
differently.


## 2. The Grandfather Paradox


The most important objection to the logical possibility of
backwards time travel is the so-called Grandfather paradox. This
paradox has actually convinced many people that backwards time travel
is impossible:



>  The dead giveaway that true time-travel is flatly
> impossible arises from the well-known "paradoxes" it
> entails. The classic example is "What if you go back into the
> past and kill your grandfather when he was still a little
> boy?"...So complex and hopeless are the
> paradoxes...that the easiest way out of the irrational chaos that
> results is to suppose that true time-travel is, and forever will be,
> impossible. (Asimov 1995 [2003, 276-7])
> 
> 
> travel into one's past...would seem to give rise to all
> sorts of logical problems, if you were able to change history. For
> example, what would happen if you killed your parents before you were
> born. It might be that one could avoid such paradoxes by some
> modification of the concept of free will. But this will not be
> necessary if what I call the *chronology protection conjecture*
> is correct: *The laws of physics prevent closed timelike curves
> from* *appearing*. (Hawking, 1992,
> 604)[14]
> 
> 
> 


The paradox comes in different forms. Here's one version:



>  If time travel was logically possible then the time
> traveller could return to the past and in a suicidal rage destroy his
> time machine before it was completed and murder his younger self. But
> if this was so a necessary condition for the time trip to have
> occurred at all is removed, and we should then conclude that the time
> trip did not occur. Hence if the time trip did occur, then it did not
> occur. Hence it did not occur, and it is *necessary* that it
> did not occur. To reply, as it is standardly done, that our time
> traveller cannot change the past in this way, is a *petitio
> principii*. Why is it that the time traveller is constrained in
> this way? What mysterious force stills his sudden suicidal rage?
> (Smith, 1985, 58)
> 
> 
> 


The idea is that backwards time travel is impossible because if it
occurred, time travellers would attempt to do things such as kill
their younger selves (or their grandfathers etc.). We know that doing
these things--indeed, changing the past in any way--is
impossible. But were there time travel, there would then be nothing
left to stop these things happening. If we let things get to the stage
where the time traveller is facing Grandfather with a loaded weapon,
then there is nothing left to prevent the impossible from
occurring. So we must draw the line earlier: it must be impossible for
someone to get into this situation at all; that is, backwards time
travel must be impossible.




In order to defend the possibility of time travel in the face of this
argument we need to show that time travel is not a sure route to doing
the impossible. So, given that a time traveller has gone to the past
and is facing Grandfather, what could stop her killing Grandfather?
Some science fiction authors resort to the idea of chaperones or time
guardians who prevent time travellers from changing the past--or
to mysterious forces of logic. But it is hard to take these ideas
seriously--and more importantly, it is hard to make them work in
detail when we remember that changing the past is impossible. (The
chaperone is acting to ensure that the past remains as it
was--but the only reason it ever was that way is because of his
very
 actions.)[15] 
 Fortunately there is a better
response--also to be found in the science fiction literature, and
brought to the attention of philosophers by Lewis (1976). What would
stop the time traveller doing the impossible? She would fail
"for some commonplace reason", as Lewis (1976, 150) puts
it. Her gun might jam, a noise might distract her, she might slip on a
banana peel, etc. Nothing more than such ordinary occurrences is
required to stop the time traveller killing Grandfather. Hence
backwards time travel does not entail the occurrence of impossible
events--and so the above objection is defused.


### 2.1 Can and Cannot


A problem remains. Suppose Tim, a time-traveller, is facing his
grandfather with a loaded gun. Can Tim kill Grandfather? On the one
hand, yes he can. He is an excellent shot; there is no chaperone to
stop him; the laws of logic will not magically stay his hand; he hates
Grandfather and will not hesitate to pull the trigger; etc. On the
other hand, no he can't. To kill Grandfather would be to change
the past, and no-one can do that (not to mention the fact that if
Grandfather died, then Tim would not have been born). So we have a
contradiction: Tim can kill Grandfather and Tim cannot kill
Grandfather. Time travel thus leads to a contradiction: so it is
impossible.




Note the difference between this version of the Grandfather paradox
and the version considered above. In the earlier version, the
contradiction happens if Tim kills Grandfather. The solution was to
say that Tim can go into the past without killing
Grandfather--hence time travel does not entail a
contradiction. In the new version, the contradiction happens as soon
as Tim gets to the past. Of course Tim does not kill
Grandfather--but we still have a contradiction anyway: for he
both can do it, and cannot do it. As Lewis puts it:



>  Could a time traveler change the past? It seems not:
> the events of a past moment could no more change than numbers
> could. Yet it seems that he would be as able as anyone to do things
> that would change the past if he did them. If a time traveler visiting
> the past both could and couldn't do something that would change
> it, then there cannot possibly be such a time traveler. (Lewis, 1976,
> 149)
> 
> 
> 




Lewis's own solution to this problem has been widely
accepted.[16] It
turns on the idea that to say that something *can* happen is to
say that its occurrence is compossible with certain facts, where
context determines (more or less) which facts are the relevant
ones. Tim's killing Grandfather in 1921 is compossible with the
facts about his weapon, training, state of mind, and so on. It is not
compossible with further facts, such as the fact that Grandfather did
not die in 1921. Thus 'Tim can kill Grandfather' is true
in one sense (relative to one set of facts) and false in another sense
(relative to another set of facts)--but there is no single sense
in which it is both true and false. So there is no contradiction
here--merely an equivocation.




Another response is that of Vihvelin (1996), who argues that there is
no contradiction here because 'Tim can kill Grandfather'
is simply false (i.e. contra Lewis, there is no legitimate sense in
which it is true). According to Vihvelin, for 'Tim can kill
Grandfather' to be true, there must be at least some occasions
on which 'If Tim had tried to kill Grandfather, he would or at
least might have succeeded' is true--but, Vihvelin argues,
at any world remotely like ours, the latter counterfactual is always
false.[17]


### 2.2 Improbable Coincidences


Return to the original version of the Grandfather paradox and
Lewis's 'commonplace reasons' response to it. This
response engenders a new objection--due to Horwich
(1987)--not to the possibility but to the *probability* of
backwards time travel.


Think about correlated events in general. Whenever we see two things
frequently occurring together, this is because one of them causes the
other, or some third thing causes both. Horwich calls this the
Principle of V-Correlation:



>  if events of type A and B are associated with one
> another, then either there is always a chain of events between
> them...or else we find an earlier event of type C that links up
> with A and B by two such chains of events. What we do not see
> is...an inverse fork--in which A and B are connected only
> with a characteristic subsequent event, but no preceding
> one. (Horwich, 1987, 97-8)
> 
> 
> 


For example, suppose that two students turn up to class wearing the
same outfits. That could just be a coincidence (i.e. there is no
common cause, and no direct causal link between the two events). If it
happens every week for the whole semester, it is *possible*
that it is a coincidence, but this is *extremely
unlikely*. Normally, we see this sort of extensive correlation
only if either there is a common cause (e.g. both students have
product endorsement deals with the same clothing company, or both
slavishly copy the same fashion blog) or a direct causal link
(e.g. one student is copying the other).




Now consider the time traveller setting off to kill her younger
self. As discussed, no contradiction need ensue--this is
prevented not by chaperones or mysterious forces, but by a run of
ordinary occurrences in which the trigger falls off the time
traveller's gun, a gust of wind pushes her bullet off course,
she slips on a banana peel, and so on. But now consider this run of
ordinary occurrences. Whenever the time traveller contemplates
auto-infanticide, someone nearby will drop a banana peel ready for her
to slip on, or a bird will begin to fly so that it will be in the path
of the time traveller's bullet by the time she fires, and so
on. In general, there will be a correlation between auto-infanticide
attempts and foiling occurrences such as the presence of banana
peels--and this correlation will be of the type that does not
involve a direct causal connection between the correlated events or a
common cause of both. But extensive correlations of this sort are, as
we saw, extremely rare--so backwards time travel will happen
about as often as you will see two people wear the same outfits to
class every day of semester, *without* there being any causal
connection between what one wears and what the other wears.




We can set out Horwich's argument this way:


1. If time travel were ever to occur, we should see extensive
uncaused correlations.
2. It is extremely unlikely that we should ever see extensive
uncaused correlations.
3. Therefore time travel is extremely unlikely to occur.


The conclusion is not that time travel is impossible, but that we
should treat it the way we treat the possibility of, say, tossing a
fair coin and getting heads one thousand times in a row. As Price
(1996, 278 n.7) puts it--in the context of endorsing
Horwich's conclusion: "the hypothesis of time travel can
be made to imply propositions of arbitrarily low probability. This is
not a classical reductio, but it is as close as science ever
gets."




Smith (1997) attacks both premisses of Horwich's
argument. Against the first premise, he argues that backwards time
travel, in itself, does not entail extensive uncaused
correlations. Rather, when we look more closely, we see that time
travel scenarios involving extensive uncaused correlations always
build in prior coincidences which are themselves highly
unlikely. Against the second premise, he argues that, from the fact
that we have never seen extensive uncaused correlations, it does not
follow that we never shall. This is not inductive scepticism: let us
assume (contra the inductive sceptic) that in the absence of any
specific reason for thinking things should be different in the future,
we are entitled to assume they will continue being the same; still we
cannot dismiss a *specific reason* for thinking the future will
be a certain way *simply* on the basis that things have never
been that way in the past. You might reassure an anxious friend that
the sun will certainly rise tomorrow because it always has in the
past--but you cannot similarly refute an astronomer who claims to
have discovered a specific reason for thinking that the earth will
stop rotating overnight.




Sider (2002, 119-20) endorses Smith's second
objection. Dowe (2003) criticises Smith's first objection, but
agrees with the second, concluding overall that time travel has not
been shown to be improbable. Ismael (2003) reaches a similar
conclusion. Goddu (2007) criticises Smith's first objection to
Horwich. Further contributions to the debate include Arntzenius (2006)
and Smeenk and Wuthrich (2011, SS2.2). For a different
argument to the same conclusion as Horwich's--that time
travel is improbable--see Ney (2000).


### 2.3 Inexplicable Occurrences


Return again to the original version of the Grandfather
paradox and Lewis's 'commonplace
reasons' response to it. This response engenders a
further objection. The autoinfanticidal time traveller is
attempting to do something impossible (render herself permanently dead from
an age younger than her age at the time of the attempts). Suppose we
accept that she will not succeed and that what will stop her is
a succession of commonplace occurrences. The previous objection was that
such a succession is *improbable*. The new objection is that the
exclusion of the time traveler from successfully committing
auto-infanticide is mysteriously *inexplicable*. The worry is as
follows. Each particular event that foils the time traveller is explicable
in a perfectly ordinary way; but the inevitable combination of these events
amounts to a ring-fencing of the forbidden zone of
autoinfanticide--and this ring-fencing is
mystifying. It's like a grand conspiracy to stop the time traveler from
doing what she wants to do--and yet there are no
conspirators: no time lords, no magical forces of logic. This is
profoundly perplexing. Riggs (1997, 52) writes:
"Lewis's account may do for a once only attempt,
but is *untenable* as a general explanation of Tim's continual lack
of success if he keeps on trying." Ismael (2003,
308) writes: "Considered individually, there will
be nothing anomalous in the explanations...It is
almost irresistible to suppose, however, that there is something anomalous
in the cases considered collectively, i.e., in our unfailing lack of
success." See also Gorovitz (1964,
366-7), Horwich (1987,
119-21) and Carroll (2010, 86).


There have been two different kinds of defense of time travel
against the objection that it involves mysteriously inexplicable
occurrences. Baron and Colyvan (2016, 70) agree with the objectors
that a purely *causal* explanation of failure--e.g. Tim
fails to kill Grandfather because first he slips on a banana peel,
then his gun jams, and so on--is insufficient. However they
argue that, in addition, Lewis offers a non-causal--a
*logical*--explanation of failure: "What explains
Tim's failure to kill his grandfather, then, is something about logic;
specifically: Tim fails to kill his grandfather because the law of
non-contradiction holds." Smith (2017) argues that the
appearance of inexplicability is illusory. There are no scenarios
satisfying the description 'a time traveller commits
autoinfanticide' (or changes the past in any other way) because
the description is self-contradictory (e.g. it involves the time
traveller permanently dying at 20 and also being alive at 40). So
whatever happens it will not be 'that'. There is
literally no way for the time traveller not to fail. Hence there is
no need for--or even possibility of--a substantive
explanation of why failure invariably occurs, and such failure is not
perplexing.


## 3. Causation


Backwards time travel scenarios give rise to interesting issues
concerning causation. In this section we examine two such issues.


### 3.1 Backwards Causation


Earlier we distinguished changing the past and affecting the past,
and argued that while the former is impossible, backwards time travel
need involve only the latter. Affecting the past would be an example
of backwards causation (i.e. causation where the effect precedes its
cause)--and it has been argued that this too is impossible, or at
least 
 problematic.[18]
 The classic argument against backwards
causation is the *bilking
 argument*.[19] 
Faced with the claim that some event *A* causes an earlier
event *B*, the proponent of the bilking objection recommends an
attempt to decorrelate *A* and *B*--that is, to bring
about *A* in cases in which *B* has not occurred, and to
prevent *A* in cases in which *B* has occurred. If the
attempt is successful, then *B* often occurs despite the
subsequent nonoccurrence of *A*, and *A* often occurs
without *B* occurring, and so *A* cannot be the cause
of *B*. If, on the other hand, the attempt is
unsuccessful--if, that is, *A* cannot be prevented
when *B* has occurred, nor brought about when *B* has not
occurred--then, it is argued, it must be *B* that is the
cause of *A*, rather than vice versa.




The bilking procedure requires repeated manipulation of
event *A*. Thus, it cannot get under way in cases in
which *A* is either unrepeatable or unmanipulable. Furthermore,
the procedure requires us to know whether or not *B* has
occurred, prior to manipulating *A*--and thus, it cannot get
under way in cases in which it cannot be known whether or not *B*
has occurred until after the occurrence or nonoccurrence of *A*
(Dummett, 1964). These three loopholes allow room for many claims of
backwards causation that cannot be touched by the bilking argument,
because the bilking procedure cannot be performed at all. But what
about those cases in which it can be performed? If the procedure
succeeds--that is, *A* and *B* are
decorrelated--then the claim that *A* causes *B* is
refuted, or at least weakened (depending upon the details of the
case). But if the bilking attempt fails, it does not follow that it
must be *B* that is the cause of *A*, rather than vice
versa. Depending upon the situation, that *B* causes *A*
might become a viable alternative to the hypothesis that *A*
causes *B*--but there is no reason to think that this
alternative must always be the superior one. For example, suppose that
I see a photo of you in a paper dated well before your birth,
accompanied by a report of your arrival from the future. I now try to
bilk your upcoming time trip--but I slip on a banana peel while
rushing to push you away from your time machine, my time travel horror
stories only inspire you further, and so on. Or again, suppose that I
know that you were not in Sydney yesterday. I now try to get you to go
there in your time machine--but first I am struck by lightning,
then I fall down a manhole, and so on. What does all this prove?
Surely not that your arrival in the past causes your departure from
the future. Depending upon the details of the case, it seems that we
might well be entitled to describe it as involving backwards time
travel and backwards causation. At least, if we are not so entitled,
this must be because of other facts about the case: it would not
follow simply from the repeated coincidental failures of my bilking
attempts.


### 3.2 Causal Loops


Backwards time travel would apparently allow for the possibility of
causal loops, in which things come from nowhere. The things in
question might be objects--imagine a time traveller who steals a
time machine from the local museum in order to make his time trip and
then donates the time machine to the same museum at the end of the
trip (i.e. in the past). In this case the machine itself is never
built by anyone--it simply exists. The things in question might
be information--imagine a time traveller who explains the theory
behind time travel to her younger self: theory that she herself knows
only because it was explained to her in her youth by her time
travelling older self. The things in question might be
actions. Imagine a time traveller who visits his younger self. When he
encounters his younger self, he suddenly has a vivid memory of being
punched on the nose by a strange visitor. He realises that this is
that very encounter--and resignedly proceeds to punch his younger
self. Why did he do it? Because he knew that it would happen and so
felt that he had to do it--but he only knew it would happen
because he in fact did
 it.[20]




One might think that causal loops are impossible--and hence that
insofar as backwards time travel entails such loops, it too is
impossible.[21]
There are two issues to consider here. First, does backwards time
travel entail causal loops? Lewis (1976, 148) raises the question
whether there must be causal loops whenever there is backwards
causation; in response to the question, he says simply "I am not
sure." Mellor (1998, 131) appears to claim a positive answer to
the question.[22]
 Hanley (2004, 130) defends a negative
answer by telling a time travel story in which there is backwards time
travel and backwards causation, but no causal
loops.[23]
Monton (2009) criticises Hanley's counterexample, but also
defends a negative answer via different counterexamples.




Second, are causal loops impossible, or in some other way
objectionable? One objection is that causal loops
are *inexplicable*. There have been two main kinds of response
to this objection. One is to agree but deny that this is a
problem. Lewis (1976, 149) accepts that a loop (as a whole) would be
inexplicable--but thinks that this inexplicability (like that of
the Big Bang or the decay of a tritium atom) is merely strange, not
impossible. In a similar vein, Meyer (2012, 263) argues that if
someone asked for an explanation of a loop (as a whole), "the
blame would fall on the person asking the question, not on our
inability to answer it." The second kind of response (Hanley,
2004, SS5) is to deny that (all) causal loops are inexplicable. A
second objection to causal loops, due to Mellor (1998, ch.12), is that
in such loops the chances of events would fail to be related to their
frequencies in accordance with the law of large numbers. Berkovitz
(2001) and Dowe (2001) both argue that Mellor's objection fails
to establish the impossibility of causal
loops.[24]


## 4. Time and Change


Godel (1949 [1990a])--in which Godel presents
models of Einstein's General Theory of Relativity in which there
exist CTC's--can well be regarded as initiating the modern
academic literature on time travel, in both philosophy and physics. In
a companion paper, Godel discusses the significance of his
results for more general issues in the philosophy of time (Godel
1949a [1990b]). For the following half century, the time travel
literature focussed predominantly on objections to the possibility (or
probability) of time travel. More recently, however, there has been
renewed interest in the connections between time travel and more
general issues in the metaphysics of time and change. We examine some
of these in the present
 section.[25]




The first thing that we need to do is set up the various metaphysical
positions whose relationships with time travel will then be
discussed. Consider two metaphysical questions:


1. Are the past, present and future equally real?
2. Is there an objective flow or passage of time, and an objective now?


We can label some views on the first question as
follows. *Eternalism* is the view that past and future times,
objects and events are just as real as the present time and present
events and objects. *Nowism* is the view that only the present
time and present events and objects exist. *Now-and-then-ism*
is the view that the past and present exist but the future does
not. We can also label some views on the second
question. The *A-theory* answers in the affirmative: the flow
of time and division of events into past (before now), present (now)
and future (after now) are objective features of reality (as opposed
to mere features of our experience). Furthermore, they are linked: the
objective flow of time arises from the movement, through time, of the
objective now (from the past towards the
future). The *B-theory* answers in the negative: while we
certainly experience now as special, and time as flowing, the B-theory
denies that what is going on here is that we are detecting objective
features of reality in a way that corresponds transparently to how
those features are in themselves. The flow of time and the now are not
objective features of reality; they are merely features of our
experience. By combining answers to our first and second questions we
arrive at positions on the metaphysics of time such
as:[26]


* the *block universe* view: eternalism + B-theory
* the *moving spotlight* view: eternalism + A-theory
* the *presentist* view: nowism + A-theory
* the *growing block* view: now-and-then-ism + A-theory.




So much for positions on time itself. Now for some views on temporal
objects: objects that exist in (and, in general, change over)
time. *Three-dimensionalism* is the view that persons, tables
and other temporal objects are three-dimensional entities. On this
view, what you see in the mirror is a whole
person.[27]
Tomorrow, when you look again, you will see the whole person again. On
this view, persons and other temporal objects are *wholly
present* at every time at which they
exist. *Four-dimensionalism* is the view that persons, tables
and other temporal objects are four-dimensional entities, extending
through three dimensions of space and one dimension of time. On this
view, what you see in the mirror is not a whole person: it is just a
three-dimensional temporal part of a person. Tomorrow, when you look
again, you will see a different such temporal part. Say that an
object *persists* through time if it is around at some time and
still around at a later time. Three- and four-dimensionalists agree
that (some) objects persist, but they differ over how objects
persist. According to three-dimensionalists, objects persist
by *enduring*: an object persists from *t*1
to *t*2 by being wholly present
at *t*1 and *t*2 and every instant in
between. According to four-dimensionalists, objects persist
by *perduring*: an object persists from *t*1
to *t*2 by having temporal parts
at *t*1 and *t*2 and every instant in
between. Perduring can be usefully compared with being extended in
space: a road extends from Melbourne to Sydney not by being wholly
located at every point in between, but by having a spatial part at
every point in between.




It is natural to combine three-dimensionalism with presentism and
four-dimensionalism with the block universe view--but other
combinations of views are certainly possible.


### 4.1 Time Travel and Time


Godel (1949a [1990b]) argues from the possibility of time
travel (more precisely, from the existence of solutions to the field
equations of General Relativity in which there exist CTC's) to
the B-theory: that is, to the conclusion that there is no objective
flow or passage of time and no objective now. Godel begins by
reviewing an argument from Special Relativity to the B-theory: because
the notion of simultaneity becomes a relative one in Special
Relativity, there is no room for the idea of an objective succession
of "nows". He then notes that this argument is disrupted
in the context of General Relativity, because in models of the latter
theory to date, the presence of matter does allow recovery of an
objectively distinguished series of "nows". Godel
then proposes a new model (Godel 1949 [1990a]) in which no such
recovery is possible. (This is the model that contains CTC's.)
Finally, he addresses the issue of how one can infer anything about
the nonexistence of an objective flow of time in *our* universe
from the existence of a merely *possible* universe in which
there is no objectively distinguished series of
"nows". His main response is that while it would not be
straightforwardly contradictory to suppose that the existence of an
objective flow of time depends on the particular, contingent
arrangement and motion of matter in the world, this would nevertheless
be unsatisfactory. Responses to Godel have been of two main
kinds. Some have objected to the claim that there is no objective flow
of time in his model universe (e.g. Savitt (2005); see also Savitt
(1994)). Others have objected to the attempt to transfer conclusions
about that model universe to our own universe (e.g. Earman (1995,
197-200); for a partial response to Earman see Belot (2005,
SS3.4)).[28]




Earlier we posed two questions:


1. Are the past, present and future equally real?
2. Is there an objective flow or passage of time, and an objective now?


Godel's argument is related to the second
question. Let's turn now to the first question. Godfrey-Smith
(1980, 72) writes "The metaphysical picture which underlies time
travel talk is that of the block universe [i.e. eternalism, in the
terminology of the present entry], in which the world is conceived as
extended in time as it is in space." In his report on
the *Analysis* problem to which Godfrey-Smith's paper is
a response, Harrison (1980, 67) replies that he would like an argument
in support of this assertion. Here is an
argument:[29]



>  A fundamental requirement for the possibility of time
> travel is the existence of the destination of the journey. That is, a
> journey into the past or the future would have to presuppose that the
> past or future were somehow real. (Grey, 1999, 56)
> 
> 
> 


Dowe (2000, 442-5) responds that the destination does not
have to exist at the time of departure: it only has to exist at the
time of arrival--and this is quite compatible with non-eternalist
views. And Keller and Nelson (2001, 338) argue that time travel is
compatible with presentism:



>  There is four-dimensional [i.e. eternalist, in the
> terminology of the present entry] time-travel if the appropriate sorts
> of events occur at the appropriate sorts of times; events like people
> hopping into time-machines and disappearing, people reappearing with
> the right sorts of memories, and so on. But the presentist can have
> just the same patterns of events happening at just the same times. Or
> at least, it can be the case on the presentist model that the right
> sorts of events will happen, or did happen, or are happening, at the
> rights sorts of times. If it suffices for four-dimensionalist
> time-travel that Jennifer disappears in 2054 and appears in 1985 with
> the right sorts of memories, then why shouldn't it suffice for
> presentist time-travel that Jennifer will disappear in 2054, and that
> she did appear in 1985 with the right sorts of memories?
> 
> 
> 


Sider (2005) responds that there is still a problem reconciling
presentism with time travel conceived in Lewis's way: that
conception of time travel requires that personal time is similar to
external time--but presentists have trouble allowing
this. Further contributions to the debate whether presentism--and
other versions of the A-theory--are compatible with time travel
include Monton (2003), Daniels (2012) and Hall (2014) on the side of compatibility,
and Miller (2005), Slater (2005), Miller (2008) and Hales (2010) on
the side of incompatibility.


### 4.2 Time Travel and Change


Leibniz's Law says that if *x* = *y* (i.e. *x*
and *y* are identical--one and the same entity)
then *x* and *y* have exactly the same properties. There is
a superficial conflict between this principle of logic and the fact
that things change. If Bill is at one time thin and at another time
not so--and yet it is the very same person both times--it
looks as though the very same entity (Bill) both possesses and fails
to possess the property of being thin. Three-dimensionalists and
four-dimensionalists respond to this problem in different
ways. According to the four-dimensionalist, what is thin is not Bill
(who is a four-dimensional entity) but certain temporal parts of Bill;
and what is not thin are *other* temporal parts of Bill. So
there is no single entity that both possesses and fails to possess the
property of being thin. Three-dimensionalists have several
options. One is to deny that there are such properties as
'thin' (simpliciter): there are only temporally
relativised properties such as 'thin at time *t*'. In
that case, while Bill at *t*1 and Bill
at *t*2 are the very same entity--Bill is wholly
present at each time--there is no single property that this one
entity both possesses and fails to possess: Bill possesses the
property 'thin at *t*1' and lacks the
property 'thin at
 *t*2'.[30]




Now consider the case of a time traveller Ben who encounters his
younger self at time *t*. Suppose that the younger self is thin
and the older self not so. The four-dimensionalist can accommodate
this scenario easily. Just as before, what we have are two different
three-dimensional parts of the same four-dimensional entity, one of
which possesses the property 'thin' and the other of which
does not. The three-dimensionalist, however, faces a problem. Even if
we relativise properties to times, we still get the contradiction that
Ben possesses the property 'thin at *t*' and also
lacks that very same 
property.[31]
 There are several possible options for
the three-dimensionalist here. One is to relativise properties not to
external times but to personal times (Horwich, 1975, 434-5);
another is to relativise properties to spatial locations as well as to
times (or simply to spacetime points). Sider (2001, 101-6)
criticises both options (and others besides), concluding that time
travel is incompatible with three-dimensionalism. Markosian (2004)
responds to Sider's 
argument;[32]
 Miller (2006) also responds to Sider and
argues for the compatibility of time travel and endurantism; Gilmore
(2007) seeks to weaken the case against endurantism by constructing
analogous arguments against perdurantism. Simon (2005) finds problems
with Sider's arguments, but presents different arguments for the
same conclusion; Effingham and Robson (2007) and Benovsky (2011) also
offer new arguments for this 
conclusion.[33]


## 5. Where are the Time Travellers?


We have seen arguments to the conclusions that time travel is impossible,
improbable and inexplicable. Here's
an argument to the conclusion that backwards time travel simply will
not occur. If backwards time travel is *ever* going to occur,
we would already have seen the time travellers--but we have seen
none such.[34]
The argument is a weak 
 one.[35]
 For a start, it is perhaps not beyond
reasonable doubt that time travellers have indeed visited the
 Earth[36]--but
 even granting that they have
not, this is still compatible with the future actuality of backwards
time travel. First, it may be that time travel is very expensive,
difficult or dangerous--or for some other reason quite
rare--and that by the time it is available, our present period of
history is insufficiently high on the list of interesting
destinations. Second, it may be--and indeed existing proposals in
the physics literature have this feature--that backwards time
travel works by creating a CTC that lies entirely in the future: in
this case, backwards time travel becomes possible after the creation
of the CTC, but travel to a time earlier than the time at which the
CTC is created is not
 possible.[37]