Time is the dimension which is asymmetrical with regard to
entropy. The way acceleration tilts your time‐axis relative
to the dimensions of space in Einsteinian physics may complicate
things slightly, but anyone who thinks this definition isn't
mysterious enough might as well fuck off right now. (Sorry
about that, but there are some real fruitcakes out there.)
Many time travel plots derive their dramatic tension from some
variant of the question “Will this act change history,
and/or will my home timeline survive?” – the
answer to which depends on the fictional world's temporal
structure – and are thus (tacitly) “experiments”
testing the paradox‐proofing of hypothetical causal
frameworks. Some authors like to hide technical details
behind smokescreens of jargon; but I reckon SF is more fun when
you can tell what's going on.
The following chronophysics jargon definitions can be revisited by
clicking on the terms where they occur in the text.
If “formerly” history had a certain course (e.g. Hitler dying in
1945), then “later” I go back and alter things (in 1920), this
implies a before‐and‐after temporal framework (“metatime”) which
is independent of the internal chronology of the alternative
Any process involving chains of cause‐and‐effect which are
discontinuous and/or retrograde. Nearly‐As‐Fast‐As‐Light
starships or the like do not in my book count as “travelling
into the future”.
Any Time Travel device, including wormholes and
Worldline, timestream, possible course of history, “parallel”
universe (often a misnomer).
Any anomalous effect produced by Time
Travel. The term is traditional, but misleading as no
contradictions are involved. See also the detailed
explanations of Closed Loops and
Anyone trying to alter history decisively by (as my standard
example) shooting Hitler in 1920. Any successful case of
Führericide has four possible consequences; to wit…
Divergent history: Nazism evaporates, Stalin invades
Roumania, and there is a Russo–British nuclear war in
1952. Or whatever.
Parallel history: You can eliminate “our” WW2, but
there is still some kind of global conflict leading to a
US/Soviet Cold War, etc.
Convergent history: Imagine Heinrich Himmler stepping in
as Führer and doing exactly the things Adolf would have…
ideally this would lead to a “replacement” 1997 precisely
identical to the “original”.
Reverting history: As above, but with the corrections
taking place over metatime. Saboteurs
find life strangely confusing – perhaps they all
paradox one another into oblivion…
All of these, by the way, should be kept distinct from the
concept I'll call
Fantasy history: Remarkably familiar even though
it's set in a universe subtly unlike ours from the start, where
(e.g.) the laws of physics permit the working of magic.
The following “laws of time” – commonly referred to in
SF – can be very useful for plot purposes, and are
often taken for granted, but they should not be mistaken for
necessary facts about the chronophysics of the real world
(personally, I reckon they're all nonsense; see
Appendix, plus my Star Trek
Rant for more about “quasiscience”). Contributions,
including especially unwitting ones, are welcome.
The River of Time:
Time “flows”, and has all the properties you'd expect from a
river: a velocity (of “one second per second”) relative to fixed
banks; constant new supplies of water (passing, e.g., the 1873
milestone); inertia (so that it resists diversions –
but also tends to meander); turbulence; fish; temperature; width;
sediment; tributaries; depth; wetness; et cetera.
The Moving Absolute‐Present‐Moment:
There is only one “present moment” which travels through history
as a sort of wavefront, containing the only real life and
consciousness. If you visit the past you will find it full
of decaying automata, since the wavefront has “already” passed
on; the future is empty, not “yet” populated. Of course,
you have to carry a mini‐wave of your own to live in.
A variant of the above, which imagines many such
wavefronts moving in synchrony, usually at intervals conveniently
longer than a human lifetime. If for instance they're at
precise hundred‐year intervals it can be the “Absolute
Nineties” – you could visit 1797 or 2397, but 1920 or
1066 would be tricky.
The Grammar‐as‐Physics Fallacy:
Forwards Time Travel is impossible, because the
past already exists but the future hasn't happened yet.
Alternatively, backwards Time Travel is
impossible, because the future is still undecided but the past
has already been set (this may sound more plausible, but it's
Time and Irrelevant Dimensions in Space:
(added 28 Jan 05, with thanks to all the people who've
mailed me their own theories!)
Time Travellers never need to worry about
where they'll end up, even if their launch point is on a
speeding train on a spinning planet. If you're aboard a
spaceship hurtling towards a black hole, when you decide in
desperation to use your experimental warpdrive to hop one day
forwards in time you can be confident that you'll reappear in
whichever of the following locations is dramatically appropriate:
A point one day's travel beyond the black hole.
Somewhere solidly inside the black hole's event horizon (bad
The point you started from just outside the black hole.
The same point taking into account the galaxy's proper motion
(i.e., millions of miles away from the black hole).
This phenomenon may be related to the way Time
Machines built in the USA are liable to spontaneously switch
to a more interesting continent as soon as you take them back
beyond the eighteenth century.
The Law of Isochrony:
(extended 02 Mar 07)
AKA Temporal Equivalence. If you travel back to the
Jurassic for a week, then return, as many days will have elapsed
“in the present” as you spent “elsewhen”. This also makes
it possible for Time Travellers to report in to
Mission Control about the current situation in the Jurassic, and
for Time Management to order them to hurry up and un‐cause all
the paradoxes their monitors are
displaying. Without the Law of Isochrony, “urgent” missions
would be exactly the ones you'd spend the most time over!
An effect will remain in existence only for so long as it is
being “pushed” by a cause (just as, in dynamics, motion continues
only while a force is being applied). Remove that, and the
effect will vanish. But remember, a Time
Machine arriving from the future never has a locally
detectable cause, whether its origin is a self‐cancelled
Time Line or an umpteenth‐century
chronophysicist's practical joke.
AKA (in Doctor Who) “the First Law of Time” or “the Blinovitch
Limitation Effect”. You can never meet yourself, let alone
shoot yourself, because it's logically impossible (or in some
versions just painful) to be in two places at once (sometimes
“due to mass conservation” or even “due to Pauli's exclusion
Where the above doesn't apply, people who are due to become
Time Travellers never recognise their strangely
familiar visitor as their elder self. This may be because
they would automatically find themselves blurting out something
moronic like “Hang on – you aren't you, you're me!”,
which is such an embarrassing thing to hear yourself say that
even the cosmic force of temporal determinism itself isn't enough
to make them go back and live through it again.
Saboteurs will experience bizarre phenomena such
as gradually acquiring memories and features from the replacement
Time Line. The process in a
reverting history technically takes place over
metatime, not over time, but this way of
narrating it is far easier.
(from contributions by Dennis Himes, 01 Jan 98, and a
Mysterious Benefactor, 22 Oct 98)
When time “runs backwards”, everything happens in reverse
except the central character's mental processes.
Similarly, in cases when time gets “reset” to an earlier moment
and everything in the universe returns to its original state, an
exception is made for the parts of the human brain which retain
memories. Actually, in some cases the exceptions are so
wide‐ranging that you can only tell the timewarp's happening by
its effects on timepieces.
Autoinfanticides vanish, regardless of the solidity of their
flesh and blood. Sometimes they disappear suddenly, leaving
a real – noisy – vacuum; other times (e.g.
“Back to the Future”) they slowly evaporate, which is yet
stranger (how do you walk when your legs are half unreal?).
The bullet in the baby's head may or may not follow suit.
Note that this is also a case of metatemporal
People caught in freak Time Travel accidents will
be left older (I wonder – does an egg turn into a
rotten egg, a chicken, or a KFC Bargain Bucket?), or occasionally
younger (just as the victims of stepladder accidents end up two
feet tall, no doubt).
If you were displaced in time you'd begin to forget things that
“haven't happened yet”. The classic case is the episode of
Star Trek (also cited in my Trek Rant)
in which they put an awkward witness back at the point in
space–time where they first found him (but isn't that point
already occupied?) so he forgets it all. (Strangely, this
effect never adds memories…)
AKA “the Space–Time Vortex”, “The Strat”, etc. There
is a “region outside time” (with its own internal chronology),
from which Time‐Cops operate and in which the TARDIS can park.
Eddies in the Space–Time Continuum:
When a saboteur steals a Time
Machine and sets off to alter history, bystanders have a few
moments' grace to set off in pursuit before the change‐wavefront
(or “timequake”) reaches them and their Time Line
The Jonbar Hinge Effect:
It requires a great effort (or at least, a conscious human
decision) to make a historically significant change, but not to
modify something trivial – so, for instance, it's
easier to accidentally cause the chromosomal fluke that turns
Blackie the Cat into Ginger than to turn Adolf Hitler into Adela.
Guest Star Syndrome:
Even if history had gone differently, people from our version
would still have recognisable equivalents in the other.
See for example Harry Harrison's “A Transatlantic Tunnel,
Hurrah!”, in which the Moors win a battle in 1212, so Spain stays
Islamic, so Columbus never sails, etc. But George
Washington still leads an attempted American rebellion, and the
twentieth century even contains an
Arthur C. Clarke. A
daft axiom, but very popular.
Our Time Line is stable because
paradoxes naturally edit themselves out. If
I shot myself as a baby, I would vanish – but this
would destroy the causal basis of the baby's death, and I would
reappear again. I would oscillate in and out of existence
(with random variations each time) till a consistent
Time Line arose. Note that this is neither
a true Closed Loop (which could no more “vary”
than can the beads on a necklace) nor a Loose End
(which need imply no further metatemporal
layering) but a confused amalgam of the two; and a
paradox in which a baby is “both alive and dead”
is assumed to be worse than one where I just come back and look
at it, leaving the kid “both observed and unobserved”.
Compare the following:
In any universe where Time Travel is easy, it
will never be discovered, as any Time Line
involving Time Machines will be prone to
self‐undermining and get abolished.
(Meta‐)sooner or later a Time
Line must arise in which it happens not to be developed; and
being stable, this final version will persist. This is
relatively clever, but I have some reservations:
The final Time Line must contain at least
one Time Machine – the one which
abolished the penultimate version (maybe it crash‐landed on
the professor's cot).
There may well also be stacks of ancient wrecked
Time Machines lying about, and indeed any
number of machines used only for forward hops.
There are four different possible sets of “laws of chronophysics”
for Time Travel plots, although all but the most
pedantic writers tend to blur the distinctions somewhat, and
anything as confused as “Quantum Leap” is unclassifiable. I
suspect that the cosmos we're living in is a Type Zero Travelproof
(AKA Recalcitrant) Time Line – the sort
(excluded from consideration here as no fun) where it's just not
possible to build a Time Machine. But
arguments about this are doomed to be mere axiom‐juggling unless
we can get experimental evidence… and even then it's almost
impossible to disprove types Two and Four.
Futile – you can't get at the Hitler from your home
TYPE ONE PLOTS (deterministic)
Time‐meddlers meet themselves leaving, fail in neatly dovetailing
ways, and worry about free will. Tales of abortive attempts
to alter history (e.g. Terminator 1) often turn out when
examined to be set in a probable Type One Time
Line after all, which is rather anticlimactic. It's less
traumatic if you don't try to “interfere”, so Wellsian
time‐tourist stories commonly use this backdrop. For further
examples see “Dragonflight” (McCaffrey), “The Anubis Gates”
(Powers), and “Twelve Monkeys”.
The special paradox associated with Type One
chrononautics (almost unavoidable when retrograde Time
Travel occurs in such a Time Line) is the
Causal loops – complicated deterministic dances
in which any attempt to escape turns out to cause exactly what
you were trying to prevent.
Informational loops – if Shakespeare's plays
were written by a Time Traveller, where did they
come from? (Answer: they were logically necessary!)
Financial loops – Time
Travellers can start an account using its own interest.
Material loops – if I die in 1920, and bequeath
to my future self a gold ring which I got as an heirloom from
myself, how old is the ring?
Personal loops – e.g. the classic
Heinlein character who is his/her own
father and mother. Is he/she genetically human? If
Such “paradoxes” are nothing to be afraid
of – in fact they can be very handy. Any
intelligent person in a Type One plot can “bootstrap” godlike
powers… indeed, even Bill and Ted can figure it out. In any
tricky situation, your elder self can bail you out; any seeming
disaster can be negated by going back and converting it into a
fake – so when you thought you witnessed your
own death it was really an android, or a hologram, or (cheapest
and simplest) a post‐hypnotic suggestion! Type One
Time Travelling civilisations usually turn out to
have been bootstrapped into existence in the first place.
TYPE TWO PLOTS (elastic)
This is essentially a muddled compromise between Types
One and Three, and requires an
implausibly purposive history‐defending force. But as it
makes modifying the future more of a challenge, it is common in
Time‐Cop and Temporal‐Imperialist scenarios such as “The Legion of
Time” (Williamson), “The End of Eternity”
(Asimov), or “The Big Time” (Leiber), as
well as in comedies such as (recent) Red Dwarf.
Plots set in Type Two Time Lines make use of both
Closed Loop and Loose Endparadoxes, which are logically
incompatible – hence the proliferation of unnatural
causal glitches and duff chronophysics in such stories.
Ancestricides can never be sure what will happen; often they
vanish or mutate to fit the new history, for no clearly apparent
reason beyond literary tradition. Part of the problem here
is the vagueness of motivation in the change‐resisting
force. What are its priorities? Is it trying to
minimise the degree or duration of the divergence, the
improbability of its corrections, or the number of
witnesses? Is it allowed for instance to preserve recorded
history by annihilating all incoming Time Machines
via quantum miracle?
TYPE THREE PLOTS (overwriting)
Changes in the past produce an entirely independent new
history. According to the strict interpretation, the arrival
of any retrograde Time Machine instantly and
permanently abolishes its home future! The commoner lax
version assumes a replacement history implausibly like the old
one, though it isn't clear how (e.g.) dice are meant to remember
how they landed “last time”. Among the few clear examples
are (in order of laxity) “Lest Darkness Fall” (de Camp), “Bring
the Jubilee” (Ward Moore), “Yesterday's Enterprise”
(ST:TNG), and “The Sound of Thunder”
The endemic “paradox” type (inescapable in a
strict Type Three Time Line) is the Loose
End: any Time Machine can quite happily
abolish its own history of origin, and thus its own causal
basis. You can multiply gold bars, do “retakes”, become
World Dictator… but you can never go home, as even if history
repeats itself, it ipso facto produces a new
“you”! Type Three doppelgängers are
bad news: they will never become you, or vice versa; so
either of you can kill the other without ill effects. If you
both have Time Machines, a single Time
Line isn't big enough for both of you. A shoot‐on‐sight
policy is less crazy than it sounds!
TYPE FOUR PLOTS (quantum‐forking)
Every “choice” causes the cosmos to bifurcate: each possible
outcome occurs somewhere. SF plots in Type Four
Time Lines – e.g. “All the Myriad Ways”
(Niven), “The Coming of the Quantum Cats” (Pohl) – tend
simply to show off their “alternative histories”, as any actual
Time Travel within them gets extremely
confusing. See for instance “The Time Ships” (Baxter).
If every collapsing wave‐function (i.e. every non‐deterministic
event) produces forkings in the Time Line, far
more copies of 2020 exist than 1920s. This hardly matters
before Time Travel is invented, but then it's
shattering. Imagine setting your chronoscaph's controls for
a spot from which to shoot Hitler. As you hit “Launch”, some
particle somewhere decays (or not). Now there are two of you
heading for the same grassy knoll. Or more likely, zillions
of you – not to mention time‐tourists, assassins after
his chauffeur, etc.… all appearing at that same point in
space–time. KaPOW! Was that the Berlin
in our past you just nuked?! Limiting
Time Machines to interbranch rather than
intrabranch Time Travel doesn't stop double hops
(USA 2020 to Byzantium 1970 to Berlin 1920),
and even if you can only travel through “probabilities”, not time
(USA 2020 to Byzantium 2020), your doppelgängers
will follow you.
In the strict quantum‐physical version, there is a solution: the
unpredictable (non)appearance of a Time Machine is
itself a world‐forking event, so there is a Time
Line (ours) where it didn't happen; one where only you appear;
and others for every mathematical combination of arriving
Time Travellers, including all the quantum‐miracle
Time Machines from nowhere. This makes the
universe even more alarmingly uncontrollable; furthermore, it
means you can't kill the original Hitler, or return to your home
Time Line by Time Machine.
The Type Four Time Line cannot contain genuine
Closed Loops or Loose Ends (though
it can seem to), but it can still induce bafflement. The
main problem with a cosmos where everything that's even remotely
possible happens somewhere is that it undercuts the concept of
probability, and of “causing” or “preventing” anything (thus
destroying any narrative tension). Many‐Worldsists have
technical fixes for this, but they're inadequate when a
Time Machine can visit any Time
Line regardless of its probability.
THE “TIME AND MOTION” RANT
Some of my favourite SF stories depend heavily on the metaphor of
time as an “ever‐rolling stream”, but it always surprises me that
people take it so literally. Time and a river have at most
two features in common: each has two distinct “ends”, and an
inbuilt asymmetry of direction. Both these features also
apply to, say, an armadillo; whereas rivers also have misleading
characteristics like a rate of flow (see
Axioms). Just what speed is this river meant
to be moving at? The usual answer is “one second per
second”, but what detectable difference would it make if it was
−9 s ⁄ s? We're not only applying the term
“speed” where no displacement is involved, we're assuming a
Newtonian yardstick of Absolute Time to measure against!
Saying that “Mount Everest is moving into the future at a speed of
one second per second” makes no more sense than saying “the road
passes my house at a speed of one metre per metre”, or “my TV
varies in mass at a speed of one kilogram per kilogram”! You
aren't “moving from birth to death” at any speed; you just
extend into the future, in the same way that you extend a
metre or two in height off the floor, and roads extend from A to
B. It may sometimes be appropriate to use “time ratios” to
compare two relativistic frames of reference; but other uses of
“seconds per second” are gibberish.
Now, it can be hard to understand how an immobile dimension of
time (added to three of space) is adequate to explain our
experience of motion, change, and “free will”… but it's not
impossible. Those who fail frequently assume that
hypothesising an extra dimension of “hyper‐time” (over the course
of which the Absolute Present Moment “moves”) will help
somehow. But if adding an immobile dimension didn't work the
first time, how can you stop short of an infinite regress, which
at least pushes your failure of imagination out of sight?
This whole topic is – for obvious reasons –
hard to discuss in everyday language, which takes for granted the
imagery of “moving through time”. In Isaac Newton's diary,
the word “now” meant 1697; in mine, “now” is 1997. But if
this is evidence for a moving Present Moment (something I
have heard people try to argue), then his use of “I” to mean Isaac
Newton must be evidence of reincarnation! Likewise,
metaphors like “in days gone by” or “it came to pass” are no more
literally justified than “sunrise” or “from the heart”.
European languages such as English, or indeed
Esperanto, treat tense – which is
really a context‐dependent “pointer” like “this”, “on your left”,
“here” etc. – as if it was an essential, objective
feature of the event, just as plurality is an attribute of
objects. And tense marking on verbs is obligatory, no matter
how redundant or meaningless this is – whether the
situation described is tenseless (“time is a dimension”,
“seven is prime”), tensed (“I was born in 1967”,
“Hitler shot himself”) or indeed metatensed
(“I will kill Hitler”, “this Time Linehas become unstable”)! The best solution isn't the
Hitchhiker's Guide one of inventing special tenses for
time travellers; it's the normal non‐European
approach of ignoring tense unless it's worth explicitly
Most of this section was actually published – in an
University SF society newsletter – several
years before I compiled the rest of this essay, but for
contorted reasons it's the last part to be added to this HTML
Take half a critical mass of plutonium back to meet itself.
Infest the timestream with time‐beavers.
Shoot the gunsmith.
Take one end of a space–time wormhole and throw it into the
Release cloned Michael Crichtons into the Jurassic.