The Fabric of Reality David Deutch

12
 
Time Travel
 
It is a natural thought, given the idea that time is in some ways like an
additional, fourth dimension of space, that if it is possible to travel from one
place to another, perhaps it is also possible to travel from one time to
another. We saw in the previous chapter that the idea of ‘moving’ through
time, in the sense in which we move through space, does not make sense.
Nevertheless, it seems clear what one would mean by travelling to the
twenty-fifth century or to the age of the dinosaurs. In science fiction, time
machines are usually envisaged as exotic vehicles. One sets the controls to
the date and time of one’s chosen destination, waits while the vehicle travels
to that date and time (sometimes one can choose the place as well), and
there one is. If one has chosen the distant future, one converses with
conscious robots and marvels at interstellar spacecraft, or (depending on the
political persuasion of the author) one wanders among charred, radioactive
ruins. If one has chosen the distant past, one fights off an attack by a
Tyrannosaurus rex while pterodactyls flutter overhead.
The presence of dinosaurs would be impressive evidence that we really had
reached an earlier era. We should be able to cross-check this evidence, and
determine the date more precisely, by observing some natural long-term
‘calendar’ such as the shapes of the constellations in the night sky or the
relative proportions of various radioactive elements in rocks. Physics
provides many such calendars, and the laws of physics cause them to agree
with one another when suitably calibrated. According to the approximation
that the multiverse consists of a set of parallel spacetimes, each consisting
of a stack of ‘snapshots’ of space, the date defined in this way is a property
of an entire snapshot, and any two snapshots are separated by a time
interval which is the difference between their dates. Time travel is any
process that causes a disparity between, on the one hand, this interval
between two snapshots, and on the other, our own experience of how much
time has elapsed between our being in those two snapshots. We might refer
to a clock that we carry with us, or we might estimate how much thinking we
have had the opportunity to do, or we might measure by physiological criteria
how much our bodies have aged. If we observe that a long time has passed
externally, while by all subjective measures we have experienced a much
shorter time, then we have travelled into the future. If, on the other hand, we
observe the external clocks and calendars indicating a particular time, and
later (subjectively) we observe them consistently indicating an earlier time,
then we have travelled into the past.
Most authors of science fiction realize that future- and past-directed time
travel are radically different sorts of process. I shall not give future-directed
time travel much attention here, because it is by far the less problematic
proposition. Even in everyday life, for example when we sleep and wake up,
our subjectively experienced time can be shorter than the external elapsed
time. People who recover from comas lasting several years could be said to
have travelled that many years into the future, were it not for the fact that
their bodies have aged according to external time rather than the time they
experienced subjectively. So, in principle, a technique similar to that which
we envisaged in Chapter 5 for slowing down a virtual-reality user’s brain

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could be applied to the whole body, and thus could be used for fully fledged
future-directed time travel. A less intrusive method is provided by Einstein’s
special theory of relativity, which says that in general an observer who
accelerates or decelerates experiences less time than an observer who is at
rest or in uniform motion. For example, an astronaut who went on a round-
trip involving acceleration to speeds close to that of light would experience
much less time than an observer who remained on Earth. This effect is
known as 
time dilation. By accelerating enough, one can make the duration
of the flight from the astronaut’s point of view as short as one likes, and the
duration as measured on Earth as long as one likes. Thus one could travel
as far into the future as one likes in a given, subjectively short time. But such
a trip to the future is irreversible. The return journey would require past-
directed time travel, and no amount of time dilation can allow a spaceship to
return from a flight before it took off.
Virtual reality and time travel have this, at least, in common: they both
systematically alter the usual relationship between external reality and the
user’s experience of it. So one might ask this question: if a universal virtual-
reality generator could so easily be programmed to effect future-directed
time travel, is there a way of using it for past-directed time travel? For
instance, if slowing us down would send us into the future, would speeding
us up send us into the past? No; the outside world would merely seem to
slow down. Even at the unattainable limit where the brain operated infinitely
fast, the outside world would appear frozen at a particular moment. That
would still be time travel, by the above definition, but it would not be past-
directed. One might call it ‘present-directed’ time travel. I remember wishing
for a machine capable of present-directed time travel when doing last-minute
revision for exams — what student has not?
Before I discuss past-directed time travel itself, what about the 
rendering of
past-directed time travel? To what extent could a virtual-reality generator be
programmed to give the user the experience of past-directed time travel? We
shall see that the answer to this question, like all questions about the scope
of virtual reality, tells us about physical reality as well.
The distinctive aspects of experiencing a past environment are, by definition,
experiences of certain physical objects or processes — ‘clocks’ and
‘calendars’ — in states that occurred only at past times (that is, in past
snapshots). A virtual-reality generator could, of course, render those objects
in those states. For instance, it could give one the experience of living in the
age of the dinosaurs, or in the trenches of the First World War, and it could
make the constellations, dates on newspapers or whatever, appear correctly
for those times. How correctly? Is there any fundamental limit on how
accurately any given era could be rendered? The Turing principle says that a
universal virtual-reality generator can be built, and could be programmed to
render any physically possible environment, so clearly it could be
programmed to render any environment that did once exist physically.
To render a time machine that had a certain repertoire of past destinations
(and therefore also to render the destinations themselves), the program
would have to include historical records of the environments at those
destinations. In fact, it would need more than mere records, because the
experience of time travel would involve more than merely seeing past events
unfolding around one. Playing recordings of the past to the user would be

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mere image generation, not virtual reality. Since a real time traveller would