philosophical investigation. In science, different models of the
cosmos and the laws of nature governing the universe imply different
possibilities for time travel. Theories about time travel have changed
as the dominant cosmological theories have evolved from classical,
Newtonian conceptions to modern, relativistic and quantum mechanical
conceptions. Philosophers were quick to note some of the implications
of the new physics for venerable issues in metaphysics: the nature of
time, causation and personal identity to name just a few. The subject
continues to produce a fruitful cross-fertilization of ideas between
scientists and philosophers as theorists in both fields struggle to
resolve confounding paradoxes that emerge when time travel is pondered
seriously. This article discusses both the scientific and
philosophical issues relevant to time travel.
1. Introduction
Time travel stories have been a staple of the science fiction genre
for the past century. Good science fiction stories often pay homage to
the fundamentals of scientific knowledge of the time. Thus, we see
time travel stories of the variety typified by H. G. Wells as set
within the context of a Newtonian universe: a three-dimensional
Euclidean spatial manifold that changes along an inexorable arrow of
time. By the early to mid-twentieth century, time travel stories
evolved to take into account the features of an Einsteinian universe:
a four-dimensional spacetime continuum that curves and in which time
has the character of a spatial dimension (that is, there can be local
variations or "warps"). More recently, time travel stories have
incorporated features of quantum theory: phenomena such as
superposition and entanglement suggest the possibility of parallel or
many universes, many minds, or many histories. Indeed, the sometimes
counter-intuitive principles and effects of quantum theory have
invigorated time travel stories. Bizarre phenomena like negative
energy density (the Casimir effect) lend their strangeness to the
already odd character of time travel stories.
In this article, we make a distinction between time travel stories
that might be possible within the canon of known physical laws and
those stories that contravene or go beyond known laws. The former type
of stories, which we shall call natural time travel, exploit the
features or natural topology of spacetime regions. Natural time travel
tends to severely constrain the activities of a time traveler and
entails immense technological challenges. The latter type of stories,
which we shall call Wellsian time travel, enable the time traveler
more freedom and simplify the technological challenges, but at the
expense of the physics. For example, in H. G. Wells' story, the
narrator is a time traveler who constructs a machine that transports
him through time. The time traveler's journey, as he experiences it,
occurs over some nonzero duration of time. Also, the journey is
through some different nonzero duration of time in the world. It is
the latter condition that distinguishes the natural time travel story
from the Wellsian time travel story. Our laws of physics do not allow
travel through a nonzero duration of time in the world (in a sense
that will be made clearer below). Wellsian time travel stories are
mortgaged on our hope or presumption that more fundamental laws of
nature are yet to be discovered beyond the current horizon of
scientific knowledge. Natural time travel stories can be analyzed for
consistency with known physics while Wellsian time travel stories can
be analyzed for consistency with logic. Finally, time travel stories
implicate themselves in a constellation of common philosophical
problems. Among these philosophically related issues we will address
in this article are the metaphysics of time, causality, and personal
identity.
2. Definition
What is time travel? One standard definition is that of David Lewis's:
an object time travels iff the difference between its departure and
arrival times in the surrounding world does not equal the duration of
the journey undergone by the object. This definition applies to both
natural and Wellsian time travel. For example, Jane might be a time
traveler if she travels for one hour but arrives two hours later in
the future (or two hours earlier in the past). In both types of time
travel, the times experienced by a time traveler are different from
the time undergone by their surrounding world.
But what do we mean by the "time" in time travel? And what do we mean
by "travel" in time travel? As the definition for time travel
presently stands, we need to clarify what we mean by the word "time"
(see the next section). While philosophical analysis of time travel
has attended mostly to the difficult issue of time, might there also
be vagueness in the word "travel"? Our use of the word "travel"
implies two places: an origin and a destination. "I'm going to
Morocco," means "I'm departing from my origination point here and I
plan to arrive eventually in Morocco." But when we are speaking of
time travel, where exactly does a time traveler go? The time of origin
is plain enough: the time of the time traveler and the time traveler's
surrounding world coincide at the beginning of the journey. But
"where" does the time traveler arrive? Are we equivocating in our use
of the word 'travel' by simply substituting a when for a where? In
truth, how do we conceive of a "when"—as a place, a locale, or a
region? Different scientific ontologies result in different ideas of
what travel through time might be like. Also, different metaphysical
concepts of time result in different ideas of what kinds of time
travel are possible. It is to the issue of time in philosophy that we
now turn.
3. Time in Philosophy
How is time related to existence? Philosophy offers three primary
answers to this metaphysical question: eternalism, possibilism, and
presentism. The names of these views indicate the ontological status
given to time. The eternalist thinks that time, correctly understood,
is a fourth dimension essentially constitutive of reality together
with space. All times, past, present and future, are actual times just
like all points distributed in space are actual points in space. One
cannot privilege any one moment in the dimension of time as "more"
real than any other moment just like one cannot privilege any point in
space as "more" real than any other point. The universe is thus a
spacetime "block," a view that has philosophical roots at least as far
back as Parmenides. Everything is one; the appearance of things coming
to be and ceasing to be, of time passing or flowing, is simply
phenomenal, not real. Objects from the past and future have equal
ontological status with present objects. Thus, a presently extinct
individual dodo bird exists as equably as a presently existing
individual house finch, and the dodo bird and the house finch exist as
equably as an individual baby sparrow hatched next Saturday. Whether
or not the dodo bird and the baby sparrow are present is irrelevant
ontologically; they simply aren't in our spacetime region right now.
The physicist typically views the relation of time to existence in the
way that the eternalist does. The life of an object in the universe
can be properly shown as:
timetravel1
This diagram shows the spatial movement (in one dimension) of an
object through time. The standard depiction of an object's spacetime
"worldline" in Special Relativity, the Minkowski diagram (see below),
privileges this block view of the universe. Many Wellsian time travel
stories assume the standpoint of eternalism. For example, in Wells'
The Time Machine, the narrator (the time traveler) explains: "There is
no difference between Time and any of the three dimensions of Space
except that our consciousness moves along it." Eternalism fits easily
into the metaphysics of time travel.
The second view is possibilism, also known as the "growing block" or
"growing universe" view. The possibilist thinks that the eternalist's
picture of the universe is correct except for the status of the
future. The past and the present are fixed and actual; the future is
only possible. Or more precisely, the future of an object holds the
possibility of many different worldlines, only one of which will
become actual for the object. If eternalism seems overly
deterministic, eliminating indeterminacies and human free choice, then
possibilism seems to retain some indeterminacy and free choice, at
least as far as the future is concerned. For the possibilist, the
present takes on a special significance that it does not have for the
eternalist. The life of an object according to possibilism might be
shown as:
timetravel2
This diagram shows that the object's worldline is not yet fixed or
complete. (It should be pointed out that the necessity of illustrating
the time axis with a beginning and end should not be construed as an
implicit claim that time itself has a beginning and end.) Some
Wellsian time travel stories make use of possibilism. Stories like
Back to the Future and Terminator suggest that we can change the
outcome of historical events in our world, including our own personal
future, through time travel. The many different possible histories of
an object introduce other philosophical problems of causation and
personal identity, issues that we will consider in greater depth in
later sections of the article.
The third view is presentism. The presentist thinks that only
temporally present objects are real. Whatever is, exists now. The past
was, but exists no longer; the future will be, but does not exist yet.
Objects are scattered throughout space but they are not scattered
throughout time. Presentists do not think that time is a dimension in
the same sense as the three spatial dimensions; they say the block
universe view of the eternalists (and the intermediate view of the
possibilists) gets the metaphysics of time wrong. If eternalism has
its philosophical roots in Parmenides, then presentism can be
understood as having its philosophical roots in Heraclitus. Presently
existing things are the only actuality and only what is now is real.
Each "now" is unique: "You cannot step twice into the same river; for
fresh waters are ever flowing in upon you." The life of an object
according to presentism might be shown as:
timetravel3
Many presentists account for the continuity of time, the timelike
connection of one moment to the next moment, by appealing to the
present intrinsic properties of the world (Bigelow). To fully describe
some of these present intrinsic properties of the world, you need
past- and future-tensed truths to supervene on those properties. For
example, in ordinary language we might make the claim that "George
Washington camped at Valley Forge." This sentence has an implicit
claim to a timeless truth, that is, it was true 500 years ago, it was
true when it was happening, it is true now, and it will be true next
month. But, according to presentism, only presently existing things
are real. Thus, the proper way to understand the truth of this
sentence is to translate it into a more primitive form, where the
tense is captured by an operator. So in our example, the truth of the
sentence supervenes on the present according to the formulation
"WAS(George Washington camps at Valley Forge)." In this way,
presentists can describe events in the past and future as truths that
supervene on the present. It is the basis for their account of
persistence through time in issues like causality and personal
identity.
4. Time in Physics
Since the use of the term 'time' in our definition of time travel
remains ambiguous, we may further distinguish external, or physical
time from personal, or inner time (again, following Lewis). In the
ordinary world, external time and one's personal time coincide with
one another. In the world of the time traveler, they do not. So, with
these two senses of time, we may further clarify time travel to occur
when the duration of the journey according to the personal time of the
time traveler does not equal the duration of the journey in external
time. Most (but not all) philosophy of time concerns external time
(see the encyclopedia entry Time). For the purpose of natural time
travel, we need to examine the scientific understanding of external
time and how it has changed.
a. Newtonian Cosmology
Newton argued that space, time and motion were absolute, that is, that
the entire universe was a single, uniform inertial frame and that time
passed equably throughout it according to an eternally fixed,
immutable and inexorable rate, without relation to anything external.
Natural time travel in the Newtonian universe is impossible; there are
no attributes or topography of space or time that can be exploited for
natural time travel stories. Only time travel stories that exceed the
bounds of Newtonian physics are possible and scenarios described by
some Wellsian time travel stories (most notably like the one Wells
himself wrote) are examples of such unscientific time travel.
Several philosophers and scientists objected to the notion of absolute
space, time and motion, most notably Leibniz, Berkeley and Mach. Mach
rejected Newton's implication that there was anything substantive
about time: "It is utterly beyond our power to measure the changes of
things by time. Quite the contrary, time is an abstraction, at which
we arrive by means of the changes of things" (The Science of
Mechanics, 1883). For Mach, change was more fundamental than the
concept of time. We talk about time "passing" but what we're really
noticing is that things move and change around us. We find it
convenient to talk as if there were some underlying flowing substance
like the water of a river that carries these changes along with it. We
abstract time to have a standard measuring tool by which we can
quantify change. These views of Mach's were influential for the young
Albert Einstein. In 1905, Einstein published his famous paper on
Special Relativity. This theory began the transformation of our
understanding of space, time and motion.
b. Special Relativity
The theory of Special Relativity has two defining principles: the
principle of relativity and the invariance of the speed of light.
Briefly, the principle of relativity states that the laws of physics
are the same for any inertial observer. An observer is an inertial
observer if the observer's trajectory has a constant velocity and
therefore is not under the influence of any force. The second
principle is the invariance of the speed of light. All inertial
observers measure the speed c of light in a vacuum as 3 x 108 m/s,
regardless of their velocities relative to one another. This principle
was implied in Maxwell's equations of electromagnetism (1873) and the
constancy of c was verified by the Michelson-Morley interferometer
experiment (1887).
This second principle profoundly affected the model of the cosmos: the
constancy of c was inconsistent with Newtonian physics. The invariance
of the speed of light according to Special Relativity replaces the
invariance of time and distance in the Newtonian universe. Intervals
of space, like length, and intervals of time (and hence, motion) are
no longer absolute quantities. Instead of speaking of an object in a
particular position independently of a particular time, we now speak
of an event in which position and time are inseparable. We can relate
two events with a new quantity, the spacetime interval. For any pair
of events, the spacetime interval is an absolute quantity (that is,
has the same value) for all inertial observers. To visualize this new
quantity, one constructs spacetime diagrams (Minkowski diagrams) in
which an event is defined by its spatial position (usually restricted
to one dimension, x) and its time (ct). Thus, a spacetime interval
might be null (parallel to the trajectory of light, which, because of
the y-axis units, is shown at a 45° angle), spacelike (little or no
variation in time), or timelike (little or no variation in spatial
position). The following figure shows a Minkowski diagram depicting
the flat spacetime of Special Relativity and three different spacetime
intervals, or worldlines.
timetravel4
What are the consequences of Special Relativity for time travel?
First, we lose the common sense meaning of simultaneity. For example,
the same event happens at two different times if one observer's
inertial frame is stationary relative to another observer's inertial
frame moving at some velocity. Furthermore, an observer in the
stationary inertial frame may determine two events to have happened
simultaneously, but an observer in the second moving inertial frame
would see the same two events happening at different times. Thus,
there is no universal or absolute external time; we can only speak of
external time within one's own frame of reference. The lack of
simultaneity across frames of reference means that we might experience
the phenomenon of time dilation. If your frame of reference is moving
at some fraction of the speed of light, your external time passes more
slowly than the external time in a frame of reference that is
stationary relative to yours. If we imagine that someone in the
stationary frame of reference could peek at a clock in your frame of
reference, they would see your clock run very slowly. So in Special
Relativity, we can find a kind of natural time travel. An example of
Special Relativity time travel is of an astronaut who travels some
distance in the universe at a velocity near the speed of light. The
astronaut's personal time elapses at the same rate it always has. He
travels to his destination and then returns home to find that external
time has passed there quite differently. Everyone he knew has aged
more than he, or perhaps has even been dead for hundreds or thousands
of years.
Such stories are physically consistent with the Einsteinian universe
of Special Relativity, but of course they remain technologically
beyond our present capability. Nevertheless, they are an example of a
natural time travel story—adhering to the known laws of physics—which
do not require exceptions to fundamental scientific principles (for
example, the invariant and inviolable speed of light). But as a time
travel story, they require that the time traveler also be an ordinary
traveler, too, that is, that he travel some distance through space at
extraordinary speeds. Furthermore, this sort of natural time traveler
can only time travel into the future. (Conversely, from the
perspective of those in the originating frame of reference, when the
astronaut returns, they witness the effects of time travel to the past
perhaps because they have a person present among them who was alive in
their distant past.) So natural time travel according to Special
Relativity is perhaps too limited for what we normally mean by time
travel since it requires (considerable) spatial travel in order to
work.
In addition, there are other limitations, not least of which is
mass-energy equivalence. This principle was published by Einstein in
his second paper of 1905, entitled "Does the Inertia of a Body Depend
Upon Its Energy Content?" Mass-energy equivalence was implied by
certain consequences of Special Relativity (other theorists later
discovered that it was suggested by Maxwell's electromagnetism
theory). Mass-energy equivalence is expressed by the famous formula, E
= mc2. It means that there is an energy equivalent to the mass of a
particle at rest. When we harmonize mass-energy equivalence with the
conservation law of energy, we find that if a mass ceases to exist,
its equivalent amount of energy must appear in some form. Mass is
interchangeable with energy. Now only mass-less objects, like photons,
can actually move at the speed of light. They have kinetic energy but
no mass energy. Indeed, all objects with mass at rest, like people and
spaceships cannot, in principle, attain the speed of light. They would
require an infinite amount of energy.
c. General Relativity
In Special Relativity, all inertial frames are equivalent, and while
this is a useful approximation, it does not yet suggest how inertial
frames are to be explained. Mach had stated that the distribution of
matter determines space and time. But how? This was the question
answered by Einstein in his theory of General Relativity (1916).
Special Relativity is actually a subset of General Relativity. General
Relativity takes into account accelerating frames of reference (that
is, non-inertial frames) and thus, the phenomenon of gravity. The
topography of spacetime is created by the distribution of mass.
Spacetime is dynamic, it curves, and matter "tells" a region of
spacetime how to curve. Likewise, the resultant geometry of a
spacetime region determines the motion of matter in it.
The fundamental principle in General Relativity is the equivalence
principle, which states that gravity and acceleration are two names
designating the same phenomenon. If you are accelerating upwards at a
rate g in an elevator located in a region of spacetime without a
gravitational field, the force you would feel and the motion of
objects in the elevator with you would be indistinguishable from an
elevator that is stationary within a downward uniform gravitational
field of magnitude g. To be more precise, there is no "force" of
gravity. When we observe astronauts who are in orbit over the Earth,
it is not true to say that they are in an environment with no gravity.
Rather, they are in free fall within the Earth's gravitational field.
They are in a local inertial frame and thus do not feel the weight of
their own mass.
One curious effect of General Relativity is that light bends when it
travels near objects. This may seem strange when we remember that
light has no mass. How can light be affected by gravity? Light always
travels in straight lines. Light bends because the geometry of
spacetime is non-Euclidean in the vicinity of any mass. The curved
path of light around a massive body is only apparent; it is simply
traveling a geodesic straight line. If we draw the path of an airplane
traveling the shortest international route in only two dimensions
(like on a flat map), the path appears curved; however, because the
earth itself is curved and not flat, the shortest distance, a straight
line, must always follow a geodesic path. Light travels along the
straight path through the various contours of spacetime. Another
curious effect of General Relativity is that gravity affects time.
Imagine a uniformly accelerating frame, like a rocket during an engine
burn. General Relativity predicts that, depending on one's location in
the rocket, one will measure time differently. To an observer at the
bottom or back of the rocket (depending on how you want to visualize
its motion), a clock at the top or front of the rocket will appear to
run faster. According to the principle of equivalence, then, a clock
at sea level on the Earth runs a little slower than a clock at the top
of Mount Everest because the strength of the field is weaker the
further you are from the center of mass.
Are natural time travel stories possible in General Relativity? Yes,
they are, and some of them are quite curious. While most of spacetime
seems to be flat or gently rolling contours, physicists are aware of
spacetime regions with unusual and severe topologies such as rotating
black holes. Black holes are entities that remain from the complete
collapse of stars. Black holes are the triumph of gravity over all
other forces and are predicted by a solution to Einstein's General
Relativity equations (Kerr, 1963). When they rotate, the singularity
of the black hole creates a ring or torus, which might be traversable
(unlike the static black hole, whose singularity would be an
impenetrable point). If an intrepid astronaut were to position herself
near the horizon of the rapidly spinning center of a black hole
(without falling into its center and possibly being annihilated), she
would be treated to a most remarkable form of time travel. In a brief
period of her personal time she would witness an immensely long time
span in the universe beyond the black hole horizon; her spacetime
region would be so far removed from the external time of the
surrounding cosmos that she conceivably could witness thousands,
millions, or billions of years elapse. This is a kind of natural time
travel; however, it severely restricts the activity of the
astronaut/time traveler and she is limited to "travel" into the
future. Are there solutions to General Relativity that allow natural
time travel into the past? Yes, but unlike rotating black holes, they
remain only theoretical possibilities.
Einstein's neighbor in Princeton, Kurt Gödel, developed one such
solution. In 1949, Gödel discovered that some worldlines in closed
spacetime could curve so severely that they curved back onto
themselves, forming a loop in spacetime. These loops are known as
closed timelike curves (CTCs). If you were an object on a CTC
worldline, you would eventually arrive at the same spacetime position
from which you started, that is, your older self would appear at one
of its own earlier spacetime points. Gödel's CTC spacetime describes a
rotating universe; thus, it is an extreme case for a CTC because it is
globally intrinsic to the structure of the universe. It is not
considered a realistic solution since current cosmological theory
states that the universe is expanding, not rotating.
One type of spacetime region that a natural time traveler might
exploit is a wormhole: two black holes whose throats are linked by a
tunnel. Wormholes would connect two regions of space and two regions
of time as well. Physicist Kip Thorne speculated that if one could
trap one of the black holes that comprise the mouths of the wormhole
it would be conceivable to transport it, preferably at speeds near the
speed of light. The moving black hole would age more slowly than the
stationary black hole at the other end of the wormhole because of time
dilation. Eventually, the two black holes would become unsynchronized
and exist in different external times. The natural time traveler could
then enter the stationary black hole and emerge from the wormhole some
years earlier than when he departed. Unfortunately for our time
traveler, if wormholes exist naturally many scientists think that they
are probably quite unstable (particularly if quantum effects are taken
into account). So, any natural wormhole would require augmentation
from exotic phenomena like negative energy in order to be useful as a
time machine.
Another type of CTC suggested by Gott (1991) employs two infinitely
long and very fast moving cosmic "strings" of extremely dense
material. The atom-width strings would have to travel parallel to one
another in opposite directions. As they rush past one another, they
would create severely curved spacetime such that spacetime curved back
on itself. The natural time traveler would be prepared to exploit
these conditions at just the right moment and fly her spaceship around
the two strings. If executed properly, she would return to her
starting point in space but at an earlier time.
One common feature of all CTCs, whether it is the global Gödelian
rotating universe or the local regions of rolled-up spacetime around a
wormhole or cosmic strings, is that they are solutions to General
Relativity that would describe CTCs as already built into the
universe. The natural time traveler would have to seek out these
structures through ordinary travel and then exploit them. So far, we
are not aware of any solution to General Relativity that describes the
evolution of a CTC in a spacetime region where time travel had not
been possible previously; however, it is usually assumed that there
are such solutions to the equations. These solutions would entail
particular physical constraints. One constraint would be the creation
of a singularity in a finite region of spacetime. To enter the region
where time travel might be possible, one would have to cross the
Cauchy horizon, the hourglass-shaped (for two crossing cosmic strings)
boundary of the singularity in which the laws of physics are unknown.
Were such a CTC constructed, a second constraint would limit the
external time that would be accessible to the time traveler. You could
not travel to a time prior to the inception date of the CTC. (For more
on this sort of time travel, see Earman, Smeenk, and Wüthrich, 2002.)
Natural time travel according to General Relativity faces daunting
technological challenges especially if you want to have some control
over the trajectory of your worldline. One problem already mentioned
is that of stability. But equally imposing is the problem of energy.
Fantastic amounts of exotic matter (or structures and conditions
similar to the early moments of the Big Bang, like membranes with
negative tension boundary layers, or gravitational vacuum
polarization) would be needed to construct and manage a usable
wormhole; infinitely long tubes of hyperdense matter would be needed
for cosmic strings. Despite these technological challenges, it should
be pointed out that the possibility of natural time travel into the
past is consistent with General Relativity. But Hawking and other
physicists recognize another problem with actual time travel into the
past along CTCs: maintaining a physically consistent history within
causal loops (see Causation below). One advantage of some
interpretations of relativistic quantum theory is that the logical
requirement for a consistent history in a time travel story is
seemingly avoided by postulating alternative histories (or worlds)
instead of one history of the universe.
d. Quantum Interpretations
Certain aspects of quantum theory are relevant to time travel, in
particular the field of quantum gravity. The fundamental forces of
nature (strong nuclear force, electromagnetic force, weak nuclear
force, and gravitation) have relativistic quantum descriptions;
however, attempts to incorporate gravity in quantum theory have been
unsuccessful to date. On the current standard model of the atom, all
forces are carried by "virtual" particles called gauge bosons
(corresponding to the order given above for the forces: mesons and
gluons, photons, massive W and Z particles, and the hypothetical
graviton). A physicist might say that the photon "carries"
electromagnetic force between "real" particles. The graviton, which
has eluded attempts to detect it, "carries" gravity. This
particle-characterization of gravity in quantum theory is very
different from Einstein's geometrical characterization in General
Relativity. Reconciling these two descriptions is a robust area of
research and many hope that gravity can be understood in the same way
as the other fundamental forces. This might eventually lead to the
formulation of a "theory of everything."
Scientists have proposed several interpretations of quantum theory.
The central issue in interpretations of quantum theory is
entanglement. When two quantum systems enter into temporary physical
interaction, mutually influencing one another through known forces,
and then separate, the two systems cannot be described again in the
same way as when they were first brought together. Microstate and
macrostate entanglement occurs when an observer measures some physical
property, like spin, with some instrumentation. The rule, according to
the orthodox (or Copenhagen) interpretation, is that when observed the
state vector (the equation describing the entangled system) reduces or
jumps from a state of superposition to one of the actually observed
states. But what happens when an entangled state "collapses?" The
orthodox interpretation states that we don't know; all we can say
about it is to describe the observed effects, which is what the wave
equation or state vector does.
Other interpretations claim that that the state vector does not
"collapse" at all. Instead, some no-collapse interpretations claim
that all possible outcomes of the superposition of states become real
outcomes in one way or another. In the many-worlds version of this
interpretation (Everett, 1957), at each such event the universe that
involves the entangled state exfoliates into identical copies of the
universe, save for the values of the properties included in the
formerly entangled state vector. Thus, at any given moment of
"collapse" there exist two or more nearly identical universes,
mutually unobservable yet equally real, that then each divide further
as more and more entangled events evolve. On this view, it is
conceivable that you were both born and not born, depending on which
world we're referring to; indeed, the meaning of 'world' becomes
problematic. The many universes are collectively designated as the
multiverse. There are other variations on the many-worlds
interpretation, including the many minds version (Albert and Loewer,
1988) and the many histories version (Gell-Mann and Hartle, 1989);
however, they all share the central claim that the state vector does
not "collapse."
Many natural time travel stories make use of these many-worlds
conceptions. Some scientists and storytellers speculate that if we
were able to travel through a wormhole that we would not be traversing
a spacetime interval in our own universe, but instead we would be
hopping from "our" universe to an alternative universe. A natural time
traveler in a many-worlds universe would, upon their return trip,
enter a different world history. This possibility has become quite
common in Wellsian time travel stories, for example, in Back to the
Future and Terminator. These types of stories suggest that through
time travel we can change the outcome of historical events in our
world. The idea that the history of the universe can be changed is why
many of the inconsistencies with causation and personal identity
arise. We now turn to these topics to examine the philosophical
implications of time travel stories.
5. Causation
Inconsistencies and incoherence in time travel stories often result
from spurious applications of causation. Causation describes the
connected continuity of events that change. The nature of this
relation between events, for example, whether it is objective or
subjective, is a subject of debate in philosophy. But for our
purposes, we need only notice that events generally appear to have
causes. The distinction made between external and personal time is
crucial now for the difficulties of causation in some time travel
stories.
Imagine Heloise is a time traveler who travels 80 years in the past to
visit Harold. They have a fight and Heloise knocks out one of Harold's
teeth. If we follow the progression of Heloise's personal time (or of
Harold's), the story is consistent; indeed, time travel seems to have
little effect upon the events described. The difficulty arises when we
test the consistency of the story in external time, because it
involves an earlier event being affected by a later event. The
ordinary forward progress of events related to Harold 80 years ago
requires a schism in the connectivity and continuity of those events
to allow the entry of a later event, namely, Heloise's time travel
journey. The activity of Heloise is causally continuous with respect
to her personal time but not with respect to external time (assuming
that the continuity of her personal identity is not in question, as we
shall discuss in the next section). With respect to external time,
this story describes reversed causation, for later events produce
changes in earlier events. How does the story change if Heloise is
homicidal and encounters her own grandfather 80 years ago? This is a
scenario many think show that time travel into the past is
inconsistent and thus impossible.
a. The Grandfather Paradox
Heloise despises her paternal grandfather. Heloise is homicidal and
has been trained in various lethal combat techniques. Despite her
relish at the thought of murdering her grandfather, time has conspired
against her, for her grandfather has been dead for 30 years. As a
crime investigator might say, she has motive and means, but lacks the
opportunity; that is, until she fortuitously comes into the possession
of a time machine. Now Heloise has the opportunity to fulfill her
desire. She makes the necessary settings on the machine and plunges
back into time 80 years. She emerges from the machine and begins to
stalk her grandfather. He suspects nothing. She waits for the perfect
moment and place to strike so that she can enjoy the full satisfaction
of her hatred. At this point, we might pause to observe: "If Heloise
murders her grandfather, she will have prevented him from fathering
any children. That means that Heloise's own father will not be born.
And that means that Heloise will not be born. But if she never comes
into existence, then how is she able to return…?" And so we have the
infamous grandfather paradox. Before we examine what happens next,
let's consider the possible outcomes of her impending action.
First, let's assume that the many-worlds hypothesis correctly
describes the universe. If so, then we avoid the paradox. If Heloise
succeeds in killing her grandfather before her father is conceived,
then the state of the world includes quantum entanglement of the
events involved in Heloise's mind, body, surrounding objects, etc.,
such that when she succeeds in killing her grandfather (or willing his
death just prior to the physical accomplishment of it), the universe
at that moment divides into one universe in which she succeeded and a
second universe in which she did not. So the paradox of causal
continuity in external time does not arise; causation presumably
connects events in the different universes without any inconsistency.
But as we shall see in the next section this quantum interpretation
trades-off a causation paradox for a personal identity paradox.
Next, let's assume that we do not have the many-worlds quantum
interpretation available to us, nor for that matter, any theory of
different worlds. Can Heloise murder her grandfather? As David Lewis
famously remarked, in one sense she can, and in another sense she
can't. The sense in which she can murder her grandfather refers to her
ability, her willingness, and her opportunity to do so. But the sense
in which she cannot murder her grandfather trumps the sense in which
she can. In fact, she does not murder her grandfather because the
moments of external time that have already passed are no longer
separable. Assuming that events 80 years ago did not include Heloise
murdering her grandfather, she cannot create another moment 80 years
ago that does. A set of facts is arranged such that it is perfectly
appropriate to say that, in one sense, Heloise can murder her
grandfather. However, this set of facts is enclosed by the larger set
of facts that include the survival of her grandfather. Were Heloise to
actually succeed in carrying out her murderous desire, this larger set
of facts would contain a contradiction (that her grandfather both is
murdered and is not murdered 80 years ago), which is impossible.
History remains consistent.
This is also related to Stephen Hawking's view (1992). According to
his so-called Chronology Protection Conjecture, he claims that the
laws of physics conspire to prevent macroscopic inconsistencies like
the grandfather paradox. A "Chronology Protection Agency" works
through events like vacuum fluctuations or virtual particles to
prevent closed trajectories of spacetime curvature in the negative
direction (CTCs). If Hawking is right and many-worlds quantum
interpretations are not available, then is time travel to the past
still possible? Hawking's view about consistent history then takes us
to the special case of causation paradoxes: the causal loop.
b. Causal Loops
A causal loop is a chain of causes that closes back on itself. A
causes B, which causes C,…which causes X, which causes A, which causes
B…and so on ad infinitum. This sequence of events is exploited in some
natural and Wellsian time travel stories. It is a point of debate
whether all time travel stories involving travel to the past include
causal loops. As we have seen, causal loops can occur when
extraordinary cosmic structures curve spacetime in a negative
direction. Wellsian time travel stories with causal loops describe
scenarios like the following one by Keller and Nelson (2001).
Jennifer, a young teenager, is visited by an old woman who
materializes in her bedroom. The old woman describes intimate details
that only Jennifer would know and thus convinces Jennifer to pursue a
professional tennis career. Jennifer does exactly as the old woman
suggested and eventually retires, successful and happy. One day she
comes into the possession of a time machine and decides to use it to
travel back in time so that she might try to make her teenage years
happier. Jennifer travels back into the past and stands before a
person she recognizes as her younger self. Jennifer begins to talk to
the teenager about her hidden talents and the bright future before her
as a tennis professional. At the end of their conversation, Jennifer
activates the time machine and returns to her original time. We can
describe the causal loop in Keller and Nelson's story as follows. The
story contained within in the causal loop is presented on the left
side. At event C, the story splits, with the causal loop continuing
along C1, and the exit from the loop beginning at C2. At C2, the
worldline of Jennifer continues outside the causal loop events. Thus:
timetravel5
The events of Jennifer's life include a causal loop: some of those
events have no beginning and no end. What is the problem with the
story? Each moment of the causal sequence is explicable in terms of
the prior events. But where (or when) did the crucial information that
Jennifer would have a successful tennis career come from originally?
While each part of the causal sequence makes sense, the causal loop as
a whole is surprising because it includes information ex nihilo. It is
controversial whether such uncaused causes are possible. Some
philosophers (for example, Mellor, 1998) think that causal loop time
travel stories are impossible because causal loops are themselves
impossible. They argue that time and causality must progress in the
same direction. Other philosophers (for example, Horwich, 1987) argue
that while causal loops are not impossible, they are highly
implausible, and thus spacetime does not permit time travel into the
"local" past (like one's own life) because fantastic amounts of energy
would be required. Still other philosophers (for example, Lewis) think
that causal loops are possible because at least some events, like the
Big Bang, appear to be events without causes, introducing information
ex nihilo.
According to Hawking, causal loop stories that employ CTCs are like
grandfather paradox stories. While backwards causation might be
logically possible, it is not physically possible. The "Chronology
Protection Agency" actively prevents them from occurring. The laws of
physics conspire such that natural time travel into the past thwarts
backwards or reverse causation. In closed spacetime, the Cauchy
horizon of a CTC acts as an impenetrable barrier to a timelike
worldline for objects. If a time traveler could travel to the past,
whether or not that past included their younger self, they are
prevented from interacting with the events of the past.
If causal loops are possible, then the objects may interact with the
events of the past, but only in a consistent way, that is, only in a
way that preserves the already established events of the past. Perhaps
we could call it the CTC prime directive (see Ray Bradbury's short
story "A Sound of Thunder"). Causal loops, like any other aporia of
uncaused causes, occupy the inexplicable perimeter of philosophical
thought about causation. Nevertheless, causal loop stories like that
of Jennifer raise another issue: personal identity.
6. Personal Identity
The old Jennifer travels back in time to talk with her younger self.
Are there two Jennifers or just one Jennifer at event A? At the same
moment in external time, a young Jennifer and an old Jennifer are
separated by a distance of a few feet. At that moment, is there one
person or two? Identity theory involves the relationships between the
mind and the body that attempts to show the connection between mental
states and physical states (see the entry Personal Identity). It
tries, for example, to describe and explain the connection (if any)
between mind and the brain. For Lewis, the mental/physical distinction
is crucial for explaining how a time traveler like Jennifer is one
person, even when she travels back to talk with her younger self. Our
cognitions change according to the requirement of causal continuity.
These mental states occur in personal time. For everyday purposes, we
can ignore the distinction between personal time and external time;
personal time and external time coincide. But for a time traveler like
Jennifer, identity is maintained only by virtue of the traveler's
personal time; their mental states continue like anyone else's and at
any given point in personal time, later mental states do not cause
earlier ones.
In the case of Jennifer, it is therefore proper to say that at event A
in her life, there is only one person, even though it is also true to
say from an external perspective, that she has two different bodies
present at event A. Lewis's distinction between the sense in which you
can and the sense in which you can't has its coda in the subject of
personal identity. In the sense of personal time, Jennifer is one
person who is perceiving another person (from either Jennifer's
perspective). The older Jennifer's materialization into the presence
of the younger Jennifer is strange, to be sure, but in a time travel
story, it is explicable. Regardless, in her personal time, the causal
continuity of her perception (and thus mental states) is consistent.
In the sense of external time, from the perspective of their
surrounding world, there are two Jennifers at event A. The mental
state of the younger Jennifer is not identical to the mental state of
the older Jennifer. But these mental states, these stages of
Jennifer's life are not duplicates of the same stage; rather, two
moments of personal time overlap at one moment of external time. So is
it still proper to say that there are two of her? Lewis argues no, it
is not. In the strange case of a time traveler like Jennifer, her
stages are scattered in such a way that they do not connect in a
continuously forward direction through external time, but they do
connect continuously forward through her personal time. The time
traveler who meets up with her younger self gives the appearance to an
outside observer that she is two different people, but in reality,
there is only one person.
The question of how objects persist through time is the subject of the
endurance and perdurance debate in philosophy. An endurantist is
someone who thinks that objects are wholly present at each moment of
an interval of time. A perdurantist is someone who thinks that objects
only have a temporal part present at each moment of an interval of
time. The perdurantist claims that the identity of the whole object is
identified as the sum of these temporal parts over the lifetime of the
object. It seems that it is impossible for an endurantist to believe
the story about Jennifer because she would have to be wholly present
in two different spatial locations at the same time. The endurantist
can avoid this problem by appealing to the distinction between
personal time and external time. If Jennifer is wholly present at
different locations "at the same time," which kind of time do we mean?
We mean external time. The endurantist can claim that two different
temporal stages in her personal time just so happen to coincide
because she is a time traveler at different locations at a single
moment of external time. For those of us who are not time travelers,
our different temporal stages are also distinct moments in external
time. But in either case, whether time traveler or not, a person is
wholly present at any moment of their personal time.
The perdurantist seems to have an easier way with the problem of
personal identity in time travel stories. Since a person is only
partially present at each moment of external time, it is readily
conceivable that different temporal parts might coincide, but we still
need to appeal to the distinction between personal time and external
time. The two temporal parts of Jennifer's life that occur when the
young and old Jennifer meet and have a conversation are each elements
among many others that in toto form the whole person.
Personal identity is especially problematic in a many-worlds
hypothesis. Consider the case of Heloise and her desire to murder her
grandfather. According to the many-worlds hypothesis, she travels back
in time but by doing so also skips into another universe. Heloise is
free to kill her grandfather because she would not be killing "her"
grandfather, that is, the same grandfather that she knew about before
her time travel journey. Indeed, Heloise herself may have split into
two different persons. Whatever she does after she travels into the
past would be consistent with the history of the alternative universe.
But the question of who exactly Heloise or her grandfather is becomes
problematic, especially if we assume that her actions in the different
universes are physically distinct. Is Heloise the sum of her
appearances in the many worlds? Or is each appearance of Heloise a
unique person?
Also, see the related article Time in this Encyclopedia.
7. References and Further Reading
* Albert, David and Barry Loewer. 1988. Interpreting the many
worlds interpretation. Synthese 77:195-213.
* Bigelow, John. Time travel fiction. In Gerhard Preyer and Frank
Siebelt, eds., Reality and Humean Supervenience. Lanham, MD: Rowan &
Littlefield, 2001. 58-91.
* Bigelow, John. Presentism and properties. In James E. Tomberlin,
ed., Philosophical Perspectives 10. Cambridge, MA: Blackwell
Publishers, 1996. 35-52.
* Bradbury, Ray. 1952. A Sound of Thunder. In R is for Rocket. New
York: Doubleday.
* Earman, John. 1995. Outlawing Time Machines: chronology
protection theorems. Erkenntnis 42(2):125-139.
* Earman, John, Smeenk, Christopher and Wüthrich, Christian. 2002.
Take a ride on a time machine. In R. Jones and P. Ehrlich, eds.,
Reverberations of the Shaky Game: Festschrift for Arthur Fine. Oxford:
Oxford University Press.
* Everett, Hugh. 1957. Relative state formulation of quantum
mechanics. Review of Modern Physics 29:454-62.
* Gell-Mann, Murray and James B. Hartle. 1989. Quantum mechanics
in the light of quantum cosmology. In Proceedings of the 3rd
International Symposium on the Foundations of Quantum Mechanics.
Tokyo, Japan. 321-43.
* Gott, J. Richard. Time Travel in Einstein's Universe: The
Physical Possibilities of Travel Through Time. Boston: Houghton
Mifflin, 2001.
* Hawking, S. W. 1992. Chronology protection conjecture. Physical
Review D 46(2):603-11.
* Horwich, Paul. 1987. Asymmetries in Time: Problems in the
Philosophy of Science. Cambridge, MA: MIT Press.
* Keller, Simon and Michael Nelson. 2001. Presentists should
believe in time-travel. Australasian Journal of Philosophy 79:333-45.
* Lewis, David. 1976. The paradoxes of time travel. American
Philosophical Quarterly 13:145-52.
* Mellor, D. H. Real Time II. London: Routledge, 1998.
* Monton, Bradley. 2003. Presentists can believe in closed
timelike curves. Analysis 63(3).
* Smith, Nicholas J. J. 1997. Bananas enough for time travel?
British Journal of Philosophy 48:363-389
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