Time and Timescape
Shortly after finishing my doctoral thesis
in 1967, I began doing research at the Lawrence Radiation Laboratory, and
resumed my hobby of writing fiction. It had never occurred to me to intertwine
the two. Yet as I read recent papers on tachyons, hypothetical faster-than-light
particles, I realized that they plainly had a science-fictional feel. In a
stroke, my rigorous habits of thought as a physicist mingled with my
speculative, artistic aspects. It was my first experience with how hard SF could
emerge from the experience of "doing" science.
In Newton's worldview, time ticked off in an absolute way, and space was
measured by a rigid universal framework. This image ruled until the late
nineteenth century. H.G. Wells, always a quick study, caught the shifting winds
and jury-rigged a new analogy which equated time with space—made it a fourth
dimension, which a traveller could navigate.
Einstein shattered immutable time,
combining space and time into a single continuum. The velocity of an observer
served to rotate time into space, so that events which seemed simultaneous to
one person would not look so to another who moved with a different speed. None
of this was readily apparent to us, because we all move very much slower than
light, which is thought to be the ultimate speed limit.
That limit separated two realms which
could never interpenetrate, because approaching the barrier from lower speeds
took ever-greater energy. Nothing precluded particles moving faster than light
if they started out that way. The light barrier was weirdly symmetric, too.
Particles moving infinitely fast have zero energy, just like particles with no
velocity on our side of the barrier. Infinity mirrors zero.
Einstein's theory allowed these eerie faster-than-light particles, as he himself
knew. Nobody paid much attention to their theoretical possibility until the
early 1960s, however, when Gerald Feinberg introduced the name "tachyons"
("fast ones" in Greek); by contrast, ordinary matter such as us is made of slow
ones, "tardyons." The last time I saw Gerry (he died in 1992) he reminded me
that the idea had appealed to him because of James Blish's story, "Beep" (1954;
later expanded into The Quincunx of Time, 1973). That tale concerns a
faster-than-light communicator (a "Dirac transmitter," which he used in later
fiction). It works fine, except that the engineers can't eliminate a beep at the
end of each message. It turns out that, stretched out, that beep contains all
messages from all future times—because, as Blish knew, anything which travels
faster than light can be used to send messages backward in time.
Demonstrating this demands space-time
diagrams and a fair amount of physics. You can see it qualitatively by noting
that a tachyon covers more space than time in its trajectory, so in a sense it
has a net debit in its favor—"time to burn." Several physicists had confronted
directly a problem Gerry left for others—the familiar grandfather's paradox.
Most physicists believed then (and
still do) that this paradox rules out tachyons or any other such
backward-in-time trick. Some tried to maintain that tachyons could still exist;
as Richard Feynman pointed out, a particle traveling backward in time can be
redefined as its own antiparticle (made of anti-matter) moving forward in time.
This "reinterpretation principle" would set everything right: apparently
anti-causal events would merely be reinterpreted by other observers as perfectly
This seemed to me a bold finesse from
an empty hand. When this ploy appeared in the scientific literature I discussed
it with two friends and we wrote a quick paper refuting it. Published in
Physical Review D in 1970 (p. 263) under the title "The Tachyonic
Anti-Telephone"—see, even in dry old Phys Rev you can have fun with
titles, if you try—it remains the only scientific paper I have written without a
single equation in it; the argument was logical, not really technical.
We argued that notions like cause and
effect could not be so easily made relative. The Feynman argument worked for one
particle but not if you used two or more. With a minimum of two, whoever sent a
signal could sign it, clearly establishing the origin.
We regarded the whole thing as rather
amusing, so we discussed an example in which Shakespeare sends his newest work
backward to Francis Bacon. At the time Bacon was a leading contender for the
"true" Shakespeare among those who thought that a mere country boy could not
have penned such masterpieces. "If Shakespeare types out Hamlet on his tachyon
transmitter, Bacon receives the transmission at some earlier time. But no amount
of reinterpretation will make Bacon the author of Hamlet. It is Shakespeare, not
Bacon, who exercises control over the content of the message."
He can simply sign it, after all.
Behind all the mathematics in the earlier papers lurked this simple, fatal idea.
Still, I rather liked tachyons. My two
coauthors were David Book and William Newcomb. Newcomb was the grandson of the
famous Simon Newcomb, an astronomer who wrote the infamous paper showing why
airplanes could not fly. When he happened to mention this over a beer, my alarm
bells went off. Was I signing onto a similar blinkered perspective, to be cited
with ridicule generations later?
So I mulled the matter over, with one eye cocked at the steady stream of papers
about time. Could tachyons actually exist? I was urged on by a report from
Australia in 1972 that two experimenters had observed a tachyon. Their particle
detectors, carried aloft in a balloon to catch cosmic rays, had found that a
single event occurred at about 2.5 times light speed. I read their paper with
astonishment. Dozens of papers followed, proposing theories for tachyons. Other
experimenters tried to duplicate the Australian results—and failed. In the
twenty years since, nobody has seen any such event, and statistically they
should have. The Australian data was probably wrong.
Still, I wondered how tachyons—which
Einstein's special theory of relativity clearly allowed—could fit into the world
as we knew it. I essayed an approach in a novelette in Epoch, an
anthology of the mid-1970s. Then over five years I wrote a novel, Timescape
(published 1980), exploring the simplest situation I could imagine—discovery of
tachyons, and the first attempts to probe their properties and use. Rather than
the convenient Wellsian traveler, I used scientists as I knew them, warts and
all, doing what they would—trying to use the new discovery to communicate
something they cared about.
But how to deal with the paradox? I had
always rather liked another theory which resolved the multiple-outcome property
of conventional quantum mechanics. This interpretation of quantum events
supposes that when a given particle, say, passes through a hole in a wall, it
can go in several directions. The wave-like property of matter says that the
same experiment, repeated many times, will give a pattern of impacts on a far
screen. The density of impacts corresponds to the probability that a single
particle would follow that trajectory and make that impression. But a single
particle's trajectory can't be predicted precisely—we can only get the
Enter a fresh view, due to Hugh Everett
of Princeton in the 1950s. Everett said that all the possible outcomes predicted
by the probability analysis of quantum mechanics are separately real. This means
that every time a particle passes through a hole, the entire universe splits
into many possible outcomes.
Envision separable worlds peeling off
from every microscopic event. In our world, the particle smacks into the wall
and that specific outcome defines our world forever more. Other worlds
simultaneously appear, with a slightly different impact point. Every event
generates great handfuls of other worlds—a cosmic plentitude of astronomical
extravagance. I've often wondered whether Everett was influenced by such SF
stories as Murray Leinster's "Sidewise in Time" (1934). Certainly he influenced
later SF writers, including the Larry Niven of "All the Myriad Ways" (1963).
The Everett view was fun to think
about, and logically defensible, but nobody really believed it. But I found it
handy. (Writers are magpies.) I said in my novel that the Everett interpretation
didn't really apply to every event. Instead, I reserved the Everett picture for
only those events which produced a causal paradox. If a physicist sent a tachyon
backward in time and it had no grandfather-killing effects, no problem. If it
did, though, then the universe split into as many versions as it took to cover
all the possibilities. So you could indeed send some grandfather-killing message
(or anything else that made a paradox), and grandfather would die. But not in
the universe you were doomed to inhabit. Instead, another universe would appear,
unknown to you, in which dear old grandfather died, alas, and you never happened
at all. No paradox, since the tachyon which killed gramps came from another
universe, from another you.
This seemed nifty enough to furnish a
solution to my novel, but I did not take it seriously enough to actually work up
a formal quantum field theory. I published the novel and was astonished at its
success. I thought it was quirky, somewhat self-indulgent and, in its
fascination with how it feels to do science, obviously destined for a small
audience. Yet this rather private novel has been my most successful. It has been
cited in several books about causal problems and some scientific papers. Quite
pleasant for a hard SF writer.
Meanwhile, the problem of time continued. Einstein's special relativity applies
to regions of space-time which are "flat" in the sense that gravity is not
significant. Except for introducing the finite speed of light, the theory feels
Newtonian. George Bernard Shaw, in a tongue-in-cheek toast to Einstein, put it
Newton was able to combine a prodigious
mental faculty with the credulities and delusions that would disgrace a rabbit.
As an Englishman, he postulated a rectilinear universe because the English
always use the word "square" to denote honesty, truthfulness, in short:
Einstein's general theory stitches
together small regions of locally flat spacetime into a quilt of truly warped
structure. Powerfully curved spacetime plays hob with causality. One of
Einstein's close friends, Kurt Gödel, produced a model (from Einstein's field
theory) for a universe which spins so fast that time and space get radically
twisted. Zipping around such a universe can return you to the place and time of
your departure. The mathematics, coming from the famous author of Gödel's Proof
in mathematical logic, was impeccable.
Could this happen? Many hoped not. With a sign of relief they noted that there
is no evidence that our universe rotates. So Gödel's case simply doesn't apply
But then in the 1960s several theorists
showed that local rotation of stressed spacetime near black holes could do
similar tricks. Spin a black hole fast enough and the rotation offsets the
gravitational attraction, effectively stripping the guts of the hole bare. The
bowels of the beast are not pretty, with exotic zones such as negative spacetime.
From such regions a traveler could do as Wells' did, slipping backwards in time.
Worse, he might reach a naked singularity, where all physical things (mass,
density, gravitational attraction) became indefinitely large.
Mathematics cannot handle
singularities, so mathematicians would rather that they be decently clothed. No
one has been able to produce suitable garments except by the lo-and-behold
method. When I last discussed this with Stephen Hawking, in 1989, he admitted
that he suspected that we could merely invoke the clothing of singularities as a
rule, beyond proof.
Of course, he pointed out, to explain
why we don't see time travelers as everyday visitors, notice the requirements.
To make a reasonable time machine with a rotating black hole would take just
about the mass of a small galaxy. Generally, time travel seemed to require vast
public works projects.
Since then there have been other ideas,
such as making quantum "wormholes" stable and large—all quite large orders. So
we now have several ideas of how to make such a machine, though we can't afford
one right now.
But why should this matter? If a time
machine is ever built, in principle we should be receiving visitors now. Yet we
haven't seen any. Why?
An adroit answer provided by Larry
Niven supposes that there is nothing at all illogical about time travel, but we
must remember that causality still works going forward in time. Every
paradox-producing message or traveler sent back will change the conditions back
at the origin of the time machine.* Remember Ray
Bradbury's "A Sound of Thunder" (1952), in which a dinosaur-hunting expedition
bagged its quarry, but accidentally trampled a butterfly with a boot—a striking
image. They returned to find the politics and language of their era had shifted.
Imagine that people keep using such a
time machine until an equilibrium sets in between past changes and future
reactions. The simplest steady-state in which no changes occur is one in which
no time machine exists any longer. Events conspire—say, science falls forever
into disfavor, or humanity dies out—to make the time machine erase itself.
This "Niven's Law" follows directly
from a basic picture from wave mechanics. Suppose time signals behave like
waves. Looping into the past and back to the future, a wave can interfere with
itself. Picture ocean waves intersecting, making chop and froth as they cancel
here, reinforce there.
Quantum mechanically, even particles
can act like waves, so it makes sense to speak of time loops as channels for the
propagation of waves of probability. The wave amplitude gives the probability
that a particle will exist. A loop which brings a wave back to exactly cancel
itself means that the entire process cannot occur—probability zero at the very
beginning, where the trip starts.
This picture actually comes from the history of quantum mechanics. One can
predict the energy levels of hydrogen by thinking of its electron as a wave
propagating around a circle, its orbit about the nucleus. Only certain
wavelengths of the wave will fit on the orbital circumference. This quantizing
condition yields the values of energy the electron must have.
Several scientific papers have explored
this interest in quantum effects as the key to time travel—a welcome change from
the gargantuan gravity machines I've already mentioned. In Timescape I tried to
finesse the paradoxes by combining special relativity (tachyons) and quantum
mechanics. Then the fashion in time machines had shifted to general relativity
(Frank Tipler's rotating cylinders, as used by Poul Anderson in The Avatar
), and then to quantum mechanics (wormholes). What about uniting general
relativity and quantum mechanics—a much harder job.
Imagine my surprise when in November of
1992 I came upon a paper in Physical Review D, where our old tachyon
paper had appeared. Titled somewhat forbiddingly `"Quantum Mechanics Near Closed
Timelike Lines," it constructs a theory for effects in highly curved space-time
which contains causal loops—"closed timelike lines," in the jargon. It was
written by David Deutsch, who has been studying these matters for a decade at
Oxford (not Cambridge, the site of the experiments in Timescape).
"Contrary to what has usually been
assumed," Deutsch says, "there is no reason in what we know of fundamental
physics why closed timelike lines should not exist." In twenty pages of quantum
logic calculations, he shows that no obstacle to free will or even grandfather
murder really exists.
It's all done with the Everett
interpretation. In quantum cosmology there is no single history of space-time.
Instead, all possible histories happen simultaneously. For the vast
preponderance of cases, this doesn't matter—the ontological bloat of an
infinitude of worlds has no observable consequences. It's just a way of talking
about quantum mechanics.
Not so for time machines. Then a
quantum description requires a set of `"classical" (ordinary) space-times which
are similar to each other—except in the important history of the paradox-loop.
The causal loop links all the multiple histories.
Think of unending sheets stacked on end
and next to each other, like the pages in this magazine. Timelines flow up them.
A causal loop snakes through these sheets, so the parallel universes become one.
If the grandson goes back in time, he crosses to another time-sheet. There he
shoots granddad, and lives thereafter in that universe. His granddad lived as
before and had grandchildren, one of whom disappears, period.
Quantum mechanics always furnishes as
many linked universes as there would be conflicting outcomes; it's quite
economical. In this view, "it is only ever an approximation to speak of things
happening 'in a universe'. In reality the 'universes' form part of a larger
object...which, according to quantum theory, is the real arena in which things
happen." Cosmic stuff, indeed.
Just now, writing this three months
after Deutsch's paper appeared, I opened Timescape and tracked down my
old thinking. "When a loop was set up, the universe split into two new
universes.... The grandson reappeared in a second universe, having traveled back
in time, where he shot his grandfather and lived out his life, passing through
the years which were forever altered by his act. No one in either universe
thought the world was paradoxical."
I framed my fictional theory this way
because it seemed at least a plausible escape hatch from the genuine problems of
time machines, using quantum logic. But my deeper motivation was to capture the
eerie sense of having altered the past, the age-old dream . . . but for someone
If you know this, then such an act is
the ultimate altruism: you cannot then benefit in any way from usefully
adjusting the past (or suffer, either). Someone exactly like you does benefit
(yes, a twin; and I wonder how much my being an identical twin has led to my
interest in these ideas)—but you will never see him, and cannot know this except
in theory. Most of all, I was struck in writing the closing pages of the novel
with that glimpse of vistas unknown, whole universes beyond our grasp, times
untouched. To me that is the essential SF impulse. Much critical attention paid
the book (such as Susan Stone-Blackburn's, who contributed a critical summary to
the new Bantam edition of the novel) lauds its characterization, perhaps because
the scientific content and metaphors are less obvious and not traditional.
To me, though, beyond the book's puzzles and plots lurks its central driver: a
sense of unchanging immensity, the timescape glimpsed with the flitting
attention of a mortal being. This touches on the often-invoked emotions behind
much hard SF—awe and thinly veiled transcendence. They are the core passions of
Clarke and Stapledon.
In most of my writing I do try to
portray humans as they really are, because I am uncomfortably aware that real
science is done by people with dirt under their fingernails. In hard SF there is
an inevitable tension between conventional short-focus realism and the impact of
the larger landscape (humanity foregrounded against the universe) that is
central to hard SF's ideology and affect.
The usual hard SF protagonist is an
Everyman, who believes in reason and his/her ability to fathom the unknown. Hard
SF is not about ironic distance or individual failure, though that may play a
part in a particular hard SF work. Still less is it about the symptoms of
narrative exhaustion which some term post-modern—pastiche, borrowing, self-aware
recycling of genre materials, and the rearrangement of conceptual deck chairs on
a cultural Titanic. Titles like Mission of Gravity, Gateway, and
Childhood's End are about the great ol' up and out.
It was quite strange to read Deutsch's
neatly couched arguments in Physical Review D. There is a certain
wrenching sensation in having anticipated the qualitative aspects—not the
thickets of equations; Deutsch's quantum logic calculations I find quite
daunting—of a theory which seems to open the way to actual use of time machines,
if we should ever devise them.
Will we? Perhaps. But hard SF is not
about exactly predicting the future. It is about the beauty of a small,
reasoning reed, which can see past its own mortality and wonder at the vistas
beyond. Its essential drama lies in that huge leap of scale.
23 February 1993
Niven, "The Theory and Practice of Time Travel," All the Myriad Ways
(NY: Ballantine, 1971), 110-23..
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