Type One Plots (Deterministic)
Type Two Plots (Elastic)
Type Three Plots (Overwriting)
Type Four Plots (Quantum-Forking)
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.
formerlyhistory had a certain course (e.g. Hitler dying in 1945), then
laterI 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 histories.
travelling into the future.
paralleluniverse (often a misnomer).
ourWW2, but there is still some kind of global conflict leading to a US/Soviet Cold War, etc.
replacement1997 precisely identical to the
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
Contributions, including especially unwitting ones, are welcome.
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; etcetera.
present momentwhich 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
alreadypassed on; the future is empty, not
yetpopulated. Of course, you have to carry a mini-wave of your own to live in.
Absolute Nineties– you could visit 1797 or 2397, but 1920 or 1066 would be tricky.
in the presentas 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,
urgentmissions would be exactly the ones you'd spend the most time over!
pushedby 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.
the First Law of Timeor
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 conservationor even
due to Pauli's exclusion principle).
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.
runs backwards, everything happens in reverse except the central character's mental processes. Similarly, in cases when time gets
resetto 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.
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 narration (above).
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…)
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.
timequake) reaches them and their Time Line collapses.
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.
varythan 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 deadis 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:
There are four different possible sets of
chronophysics for Time Travel plots,
although all but the most pedantic writers tend to blur the
distinctions somewhat, and anything as confused as
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.
|TYPE ONE||The Deterministic (AKA Permanent) Time Line.|
History is utterly immutable. Attempts to
ad libalways turn out to have been scripted all along.
Trivialchanges are allowed.
|Metatime:||There is no metatime.|
|Paradoxes:||Closed Loops (phenomena created by their own effects).|
|Doppelgängers:||The elder self (a powerful ally).|
|Saboteurs:||Fated to fail (the gun misfires, you sneeze, or whatnot).|
|TYPE TWO||The Elastic (AKA Resilient) Time Line.|
History has a preferred course/direction, and makes
corrections(over time or metatime) for any interference.
|Lax version:||The timestream can be permanently diverted.|
|Metatime:||Warped – histories revert and/or at least converge.|
|Paradoxes:||Both Closed Loops and Loose Ends (inconsistently).|
|Doppelgängers:||The replacement self (into which you may mutate).|
|Saboteurs:||Confused (likely to suffer Anachronist Amnesia etc.).|
|TYPE THREE||The Overwriting (AKA Contingent) Time Line.|
History is highly vulnerable. Any
erasesthe original and replaces it with a freshly generated new version.
|Lax version:||Histories may be reluctant to diverge.|
abolishedTime Lines are accessible only via meta-Time Travel!
|Paradoxes:||Loose Ends (phenomena which prevent their own cause).|
|Doppelgängers:||The ultimate rival (one of you will end up abolished).|
|Saboteurs:||Good hunting – Hitler dies and stays dead.|
|TYPE FOUR||The Quantum-Forking (AKA Multi-Divergent) Time Line.|
Many Worldsinterpretation of Quantum Physics: the cosmos constantly bifurcates into all possible alternatives.
|Lax version:||Countable forkings (at
historic turning points).
|Metatime:||Mindboggling (nothing is ever truly abolished).|
|Paradoxes:||Technically, neither kind can occur, but they can seem to.|
|Doppelgängers:||Hordes of microdiverged timetwins.|
|Saboteurs:||Futile – you can't get at the Hitler from your home Time Line.|
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
Anubis Gates (Powers), and
The special paradox associated with Type One chrononautics (almost unavoidable when retrograde Time Travel occurs in such a Time Line) is the Closed Loop:
paradoxes are nothing to be afraid
of – in fact they can be very handy. Any
intelligent person in a Type One plot can
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.
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
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 End paradoxes, 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?
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
Bring the Jubilee (Ward Moore),
Enterprise (ST:TNG), and
Sound of Thunder (Bradbury).
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
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!
choice causes the cosmos to bifurcate: each possible
outcome occurs somewhere. SF plots in Type Four
Time Lines – e.g.
All the Myriad
The Coming of the Quantum Cats
(Pohl) – tend simply to show off their
histories, as any actual Time Travel within
them gets extremely confusing. See for instance
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
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
Time and MotionRant
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
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
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 1 s ⁄ s makes no more
sense than saying
the road passes my house at a speed of
1 m ⁄ m, or
my TV varies in mass at a
speed of 1 kg ⁄ kg! 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
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
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
This whole topic is – for obvious reasons –
hard to discuss in everyday language, which takes for granted the
moving through time. In Isaac Newton's
diary, the word
now meant 1697; in mine,
1997. But if this is evidence for a moving Present
Moment (something I have heard people try to argue), then his use
I to mean Isaac Newton must be evidence of
reincarnation! Likewise, metaphors like
in days gone
it came to pass are no more literally justified
from the heart. European
languages such as English, or indeed
Esperanto, treat tense – which is
really a context-dependent
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),
I was born in 1967,
himself) or indeed metatensed (
will kill Hitler,
this Time Line
has 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 mentioning.
Most of this section was actually published – in an Edinburgh 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 version…
The Time Machine.
Time Paradoxand hand them their own skull. Repeatedly.