- Gleick, James.
Time Travel.
New York: Pantheon Books, 2016.
ISBN 978-0-307-90879-7.
-
In 1895, a young struggling writer who earned his precarious
living by writing short humorous pieces for London magazines,
often published without a byline, buckled down and penned his
first long work, a longish novella of some 33,000 words. When
published, H. G. Wells's
The
Time Machine would not only help to found a new
literary genre—science fiction, but would introduce a
entirely new concept to storytelling: time travel.
Many of the themes of modern fiction can be traced to the
myths of antiquity, but here was something entirely new:
imagining a voyage to the future to see how current trends
would develop, or back into the past, perhaps not just to
observe history unfold and resolve its persistent mysteries,
but possibly to change the past, opening the door to paradoxes
which have been the subject not only of a multitude of
subsequent stories but theories and speculation by serious
scientists. So new was the concept of travel through time that
the phrase “time travel” first appeared in the
English language only in 1914, in a reference to Wells's story.
For much of human history, there was little concept of a linear
progression of time. People lived lives much the same as those
of their ancestors, and expected their descendants to inhabit
much the same kind of world. Their lives seemed to be
governed by a series of cycles: day and night, the phases of
the Moon, the seasons, planting and harvesting, and
successive generations of humans, rather than the
ticking of an inexorable clock. Even great disruptive
events such as wars, plagues, and natural disasters seemed
to recur over time, even if not on a regular, predictable
schedule. This led to the philosophical view of
“eternal
return”, which appears in many ancient cultures
and in Western philosophy from Pythagoras to Neitzsche.
In mathematics, the
Poincaré
recurrence theorem formally demonstrated that an isolated
finite system will eventually (although possibly only after a
time much longer than the age of the universe), return to a given
state and repeat its evolution an infinite number of times.
But nobody (except perhaps a philosopher) who had lived through
the 19th century in Britain could really believe that. Over the
space of a human lifetime, the world and the human condition had
changed radically and seemed to be careening into a future
difficult to envision. Steam power, railroads, industrialisation
of manufacturing, the telegraph and telephone, electricity and
the electric light, anaesthesia, antiseptics, steamships and
global commerce, submarine cables and near-instantaneous
international communications, had all remade the world. The
idea of progress
was not just an abstract concept of the Enlightenment, but something
anybody could see all around them.
But progress through what? In the
fin de siècle milieu
that Wells inhabited, through time: a scroll of history
being written continually by new ideas, inventions, creative
works, and the social changes flowing from these
events which changed the future in profound and often
unknowable ways. The intellectual landscape was fertile
for utopian ideas, many of which Wells championed. Among
the intellectual élite, the fourth dimension was
much in vogue, often a fourth spatial dimension but also the
concept of time as a dimension comparable to those of space.
This concept first appears in the work of Edgar Allan Poe
in 1848, but was fully fleshed out by Wells in The
Time Machine: “ ‘Clearly,’ the
Time Traveller proceeded, ‘any real body must have
extension in four dimensions: it must have Length,
Breadth, Thickness, and—Duration.’ ”
But if we can move freely through the three spatial directions
(although less so in the vertical in Wells's day than the
present), why cannot we also move back and forth in time,
unshackling our consciousness and will from the tyranny of
the timepiece just as the railroad, steamship, and telegraph
had loosened the constraints of locality?
Just ten years after The Time Machine, Einstein's
special theory of
relativity resolved puzzles in electrodynamics and
mechanics by demonstrating that time and space mixed
depending upon the relative states of motion of observers.
In 1908, Hermann
Minkowski reformulated Einstein's theory in terms of
a four dimensional space-time. He declared, “Henceforth
space by itself, and time by itself, are doomed to fade away
into mere shadows, and only a kind of union of the two will
preserve an independent reality.” (Einstein was,
initially, less than impressed with this view, calling it
“überflüssige
Gelehrsamkeit”: superfluous learnedness, but
eventually accepted the perspective and made it central to
his 1915 theory of gravitation.) But further, embedded within
special relativity, was time travel—at least into
the future.
According to the equations of special relativity, which have been
experimentally verified as precisely as anything in science and
are fundamental to the operation of everyday technologies such
as the Global Positioning System, a moving observer will measure
time to flow more slowly than a stationary observer. We don't
observe this effect in everyday life because the phenomenon only
becomes pronounced at velocities which are a substantial fraction
of the speed of light, but even at the modest velocity of orbiting
satellites, it cannot be neglected. Due to this effect of
time dilation,
if you had a space ship
able to accelerate at a constant rate of one Earth gravity
(people on board would experience the same gravity as they do
while standing on the Earth's surface), you would be able to
travel from the Earth to the Andromeda galaxy and back to
Earth, a distance of around four million light years, in a
time, measured by the ship's clock and your own subjective and
biological perception of time, in less than six and a half years.
But when you arrived back at the Earth, you'd discover that in
its reference frame, more than four million years of time would
have elapsed. What wonders would our descendants have accomplished
in that distant future, or would they be digging for grubs with
blunt sticks while living in a sustainable utopia having finally thrown
off the shackles of race, class, and gender which make our present
civilisation a living Hell?
This is genuine time travel into the future and, although it's
far beyond our present technological capabilities, it violates no law of
physics and, to a more modest yet still measurable degree,
happens every time you travel in an automobile or airplane. But
what about travel into the past? Travel into the future doesn't
pose any potential paradoxes. It's entirely equivalent to going
into hibernation and awaking after a long sleep—indeed,
this is a frequently-used literary device in fiction depicting
the future. Travel into the past is another thing entirely. For
example, consider the
grandfather
paradox: suppose you have a time machine able to transport
you into the past. You go back in time and kill your own
grandfather (it's never the grandmother—beats me). Then
who are you, and how did you come into existence in the first
place? The grandfather paradox exists whenever altering an
event in the past changes conditions in the future so as to be
inconsistent with the alteration of that event.
Or consider the bootstrap paradox or
causal loop.
An elderly mathematician (say, age 39), having struggled for
years and finally succeeded in proving a difficult theorem,
travels back in time and provides a key hint to his twenty year
old self to set him on the path to the proof—the same
hint he remembers finding on his desk that morning so many
years before. Where did the idea come from? In 1991, physicist
David Deutsch demonstrated that a computer incorporating
travel back in time (formally, a
closed
timelike curve) could solve
NP problems
in
polynomial
time. I wonder where he got that idea….
All of this would be academic were time travel into the past
just a figment of fictioneers' imagination. This has been the
view of many scientists, and the
chronology
protection conjecture asserts that the laws of physics conspire
to prevent travel to the past which, in the words of a 1992 paper
by Stephen Hawking, “makes the universe safe for historians.”
But the laws of physics, as we understand them today, do not rule
out travel into the past! Einstein's 1915 general theory of relativity,
which so far has withstood every experimental test for over a century,
admits solutions, such as the
Gödel metric,
discovered in 1949 by Einstein's friend and colleague
Kurt Gödel,
which contain closed timelike curves. In the Gödel universe, which
consists of a homogeneous sea of dust particles, rotating around
a centre point and with a nonzero cosmological constant, it is possible,
by travelling on a closed path and never reaching or exceeding the speed of
light, to return to a point in one's own past. Now, the Gödel
solution is highly contrived, and there is no evidence that it
describes the universe we actually inhabit, but the existence of such
a solution leaves the door open that somewhere in the other exotica of
general relativity such as spinning black holes, wormholes, naked
singularities, or cosmic strings, there may be a loophole which allows
travel into the past. If you discover one, could you please pop back and
send me an E-mail about it before I finish this review?
This book is far more about the literary and cultural history of
time travel than scientific explorations of its possibility and
consequences. Thinking about time travel forces one to confront
questions which can usually be swept under the rug: is the future
ours to change, or do we inhabit a
block
universe where our perception of time is just a delusion as
the cursor of our consciousness sweeps out a path in a space-time
whose future is entirely determined by its past? If we have free
will, where does it come from, when according to the laws of
physics the future can be computed entirely from the past? If
we can change the future, why not the past? If we changed the past,
would it change the present for those living in it, or create a fork
in the time line along which a different history would develop?
All of these speculations are rich veins to be mined in literature
and drama, and are explored here. Many technical topics are discussed
only briefly, if at all, for example the
Wheeler-Feynman
absorber theory, which resolves a mystery in electrodynamics
by positing a symmetrical solution to Maxwell's equations in which
the future influences the past just as the present influences the
future. Gleick doesn't go anywhere near my own
experiments with retrocausality or
the “presponse” experiments of investigators
such as
Dick Bierman
and Dean Radin.
I get it—pop culture beats woo-woo on the bestseller list.
The question of time has puzzled people for millennia. Only recently
have we thought seriously about travel in time and its implications
for our place in the universe. Time travel has been, and will doubtless
continue to be the source of speculation and entertainment, and this
book is an excellent survey of its short history as a genre of
fiction and the science upon which it is founded.
August 2017 