Time travel in William Gibson's 2014 novel The Peripheral seems to obey a relatively straightforward forking model with a few sensible twists which facilitate the plot and bring forward some themes very familiar to those familiar with time travel stories.

I'm going to avoid specifics, but read on with caution.

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More than a century into the future, there is, somewhere, a server of unclear provenance, construction and purpose. Using this server, it is possible to send information back in time and interfere with the past. The first act of interference causes the timeline to fork. The present timeline where our characters dwell remains unaffected. Meanwhile, a new timeline (referred to as a "continuum" or "stub") spins off from the original at the point of the fork, and begins to chart its own history, unconstrained by anything which happened in the original timeline.

As we would guess, it's likely that events in the new timeline will, for the medium-term at least, play out largely as they did before. But invariably specifics will begin to differ, and ultimately cascade. So far so familiar.

New continua are discrete and there aren't too many of them. The Peripheral concerns itself mainly with one particular stub, forking off seventy years prior to the "present" (but still, as of my writing this, the future; a different, less distant future). The narrative jumps back and forth between the characters in the present of the prime timeline and characters in the stub.

The catch is this: the two timelines proceed "parallel" with one another at identical rates. Now using the server as a relay, it is possible to send arbitrary amounts of information bidirectionally between the continua. Phone calls, video calls. Telepresence is possible, using wacky wheeled telepresence robots in the stub or, in the present, the advanced humanoid "peripherals" referred to in the title. The past, from the perspective of the present, is simply a neighbouring country... a country with an eminently predictable future, with dated, backward technology and, though it can never be visited physically, tremendous possibilities of exploitation.

This is a dizzingly powerful basic premise and The Peripheral itself, despite limiting itself to a relatively constrained and focused story within its premise, is a frankly dizzying read.

Characters in the present use their advanced technology to interfere with the stub. At first, the stub is treated as a curio, but before long a character in the stub witnesses (via VR) a bizarre murder in the present and becomes a target herself. Soon it becomes clear that an opposing faction in the present has gained access to the same stub and is interfering with it, with diametrically opposing goals. The interactions from both factions accelerate and become increasingly alarming in their scope. Because, we see, certain aspects of the stub's political future can be predicted. Financial algorithms in the present are advanced enough to run rings around those in the stub, enabling shell corporations inside the stub to accrue improbable wealth in a matter of days, causing staggeringly rapid economic upheaval as well as initiating great violence. At the same time, plans for anachronistically advanced technology can be sent back, giving our characters in the stub something to work with.

The effect is quite... colonial. The world of the stub is upheaved in a manner analogous the world of 15th-century Aztecs, and by the end of the book -- a scant week later in real time -- it is unrecognisable, and plainly on a brand new historical track.

And the theme which emerges is one seen again and again in time travel stories, at all scales: time travel is an absurdly abusable power. It is a force armed with which an actor can radically alter the appearance of their reality, shaping matters in their own favour. Here, the characters in the prime timeline aren't able to directly modify their own history, but they manage to do substantial damage to the stub's reality, which, in practical terms, is essentially an underdeveloped neighbouring country. We see that prescience and decades of technological advances are excellent weapons, and that a vantage point in the future is the highest of all possible ground.

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But much is intentionally left unexamined.

Most obviously, the origins, owners, physical location, specific nature and purpose of the time travel server are deliberately ignored. Not only are they ignored, the narrative does a rather good job of steering the reader away from the implied questions. The stub is essentially a Twilight-Zone-esque magical artifact of unknown provenance, and the mechanism by which it operates is outside the scope of the tale being told. There's enough intensely paced action going on in the main narrative to keep us occupied. Works for me.

Less obviously... It's unclear how easy it is to create stubs, how many there are overall, and what, if any, restrictions there are on when the point of origin for a new stub can be. What about the ninth century? Can we use this technology for historical research? What about five days ago? Can we use it to solve murders? Do stubs expire or do they propagate forever? Is the present-day universe itself a stub?

I'd be fairly sure that the answers to these questions do, privately, exist. But, also, I imagine that the answers simply aren't massively interesting, and/or would distract from the real, solidly focused story.

Of course, I'm still curious, because that's my gimmick. And the shape of these omissions is a little suspicious. It's almost as if the stub technology is being radically underexploited. For what reason?

The clue is this: the stub and the present are separated by some seventy years, and only one character in the whole story exists in both time periods. This character is a figure of mild historical importance in the prime timeline, whose existence in the time period of the stub is a matter of public record. Everybody else in the stub is, from the perspective of the present, an unknown.

My theory, then, is that the stub's universe is a simple simulation, stitched together from historical records, with the gaps filled in using procedural generation. This answers all of our open questions quite easily. You can create as many stubs as the server has capacity for; how much money have you got? They can be terminated at any time. Stubs further into the past aren't available, because historical records are not sufficiently detailed, or if they are, because nobody has gone to the trouble of stitching them together. Stubs nearer the present day aren't available because the records from nearer pasts are relatively incomplete, and obvious deviations would be unavoidable. "This isn't me from five years ago; I would know." "Hey, how come this unsolved murder from five days ago didn't happen in the stub?" And yes, the present is real.

And you see what I mean. This is a dull theory. Imagine trying to cram it into The Peripheral somewhere, making it explicit. What would it give us, narratively? Nothing. It slows the world down. It subtracts tension, by revealing half of our characters as disposable, easily-recreatable simulacra. If it were revealed, it would alter nearly every decision any of them made, derailing the story, so who would you, the author, even reveal it to?

Far better if everything is real.

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Review: The Peripheral is a high-quality read and a dependable Gibson product. I got a kick out of it, although not specifically a time travel-related kick. The technology stays in the background.

The book is a little impenetrable for the first 100 pages. In part that's because of that intentional science fiction device where instead of directly explaining who anybody is or what the hell's going on, which has been deprecated for decades, you just start describing complex events in two totally foreign universes and keep going until the reader pieces it together. In part it's because of Gibson's stylistic choice to drop crucial prepositions and pronouns in nearly every prose sentence. For example, the complete sentence

He walks to the door.

becomes the fragment

Walks to the door.

and "he" has to be inferred from context. Not overly burdensome in a single case, but a hundred straight pages of this becomes quite tiring and confusing, like reading uphill. Thankfully, this eases up in the remaining two-thirds of the book. Perhaps it was as tiring to write as it is to read.

But overall, yeah. A decent adventure, a suitably interesting time travel model, a tedious fan theory. Pages well spent.