Quantum Thief (The Quantum Thief, 2010_16ya; The Fractal Prince, 2012_14ya; The Causal Angel, 2014_12ya; TvTropes) by Hannu Rajaniemi ( Twitter) is a hard SF trilogy set in a post-Singularity Solar System (cf. Eclipse Phase/Orion’s Arm), starring the distant descendant of Maurice Leblanc’s gentleman jewel-thief Arsène Lupin, a certain amnesiac Jean le Flambeur.

It follows him as he escapes from prison and attempts to retrieve his memories hidden in various places throughout the Solar System; each novel focuses on one world-building setpiece: Quantum Thief is cryptopunk Venice; The Fractal Prince is demoscene Baghdad; and The Causal Angel is MMORPG space cities. Le Flambeur succeeds in stealing back his old self, only to discover his old self was entangled in a web of plots by all the major players fighting over a ‘jewel’ of universe-destroying power, destabilizing the fragile balance of power, and sparking a Solar System-wide war which causes the end of the universe—but don’t worry, it all works out OK (mostly).

I read Quantum Thief in May 2014_12ya, and Fractal Prince/Causal Angel in January 2015_11ya, and enjoyed it, but was left too confused to write a proper review. I could only capsule-summarize it as “Accelerando meets Lupin; uncompromising SF, stuffed full of interesting tidbits—the game-theoretic prison at the beginning is only the beginning; self-recommending.” In August 2022, after visiting DEFCON, I caught an unpleasant case of COVID, and while discussing the best remedies¹ with a ML researcher, I recommended Quantum Thief as a good read but admitted I didn’t get it, in part because it was so in media res.

I was not up to my usual work, so I thought I’d take a second whack. I was right, and the trilogy made sense to me the second time around. So, this is my attempt to explain The Quantum Thief Trilogy, its “emotional core” (to quote Hannu), and how it is similar to The Last Unicorn (which helped inspire it).

Stylistically, QT is set relentlessly in media res: neither we nor le Flambeur know why he is in prison, and little is explained thereafter. Hannu makes no concessions to the casual reader, as he mainlines straight into his veins the pre-deep-learning 2010-era transhumanist zeitgeist via Silicon Valley—if it was ever discussed in a late-night bull session after a Singularity University conference, it might pop up here. Hannu crams the novels with blink-and-you’ll-miss-it ideas on the level of Olaf Stapeldon. A conventional Verne gun like Gerald Bull’s is too easy a way of getting to space—how about beating Project Orion by instead using a nuclear space gun (since emulated brains don’t care about high g acceleration)? Or for example, the All-Defector reveals that, since other universes could be rewriting their rules to expand at maximum speed, erasing other universes before they know it, he plans to rewrite our universe’s rule to do so first (ie. he will defect at the multiversal level against all other universes); whereas beginner-level SF like The Three Body Problem would dilate on this for half a book, Hannu grants the All-Defector’s grand reveal all of 2 paragraphs before crashing into the eucatastrophic ending.

For world-building, he drops neologisms left and right, and hard ones at that—few enough American readers will be familiar with the starting premise of “Arsène Lupin in spaaaace!” (probably more are familiar with the anime Lupin The Third these days), but his expectations go far beyond that: the ideal reader of the trilogy is not merely one familiar with the Prisoner’s Dilemma but also with the bizarre zero-determinant PD strategies discovered ~2008, and not just with such basic physics as quantum entanglement or applications like quantum dots, but exotic applications to quantum auctions & game theory (including Prisoner’s Dilemma) & pseudo-telepathy (yes, those are things), and it would definitely be helpful if that reader happened to also be familiar with Eliezer Yudkowsky’s c. 2000s writings on “Coherent Extrapolated Volition”, with a dash of Nikolai Fyodorovich Fyodorov’s Russian Cosmism for seasoning (although only a dash²).

This leads to an irony: I noted while reading Masamune Shirow’s Ghost in the Shell cyberpunk manga that almost everything technical in the GitS manga turned out to be nonsense, despite Shirow’s defensive pretensions to in-depth research & meticulous attention to detail in his self-congratulatory author notes; while in QT, most technical things sound like cyberpunk nonsense (and Hannu doesn’t defend them), but are actually real, and just so arcane even you haven’t heard of them.

This leads to the strange outcome that readers will claim that where they are familiar with the ideas, it is technobabble or “not even wrong”, but then either decline to provide any specific examples or provide examples which show they didn’t understand it in the first place.

For example, some readers accuse Hannu of relying on FTL communication via quantum entanglement, which is indeed bad pop physics; but Hannu does not do that at all! If they had read more closely (similar to the standard reader failure to understand the physics of “Story of Your Life”), they would have noticed that at no point is there communication faster-than-light, only coordination faster-than-light—‘spooky action at a distance’³ Which is not the same. He is instead employing advanced forms of quantum entanglement, which enable things like secret auctions or for coordinated strategies of game-playing. He explains briefly that the zoku use quantum entanglement in these ways, but a reader could easily miss that, given all the other things they are trying to understand and how common ‘quantum woo’ is.⁴

So, in such hard SF, if we do not handwave some sort of magical DRM for AIs, we face serious world-building problems with AIs and mind uploading: assuming we can understand the entities at all⁵, how do we tell meaningful stories about the entities in that world? In a world where minds can be copied & forked indefinitely over time, how do you maintain personal identity—not necessarily in any philosophical sense, but simply pragmatically, as a security matter? Cryptographic keys can be copied, quantum keys can be stolen, memorized passwords extracted, trusted hardware cracked, and so on.

The zoku are anarchic organizations descended from MMORPG ‘clans’.⁶ Their use of quantum entanglement represent one solution: ‘you’ are defined by your unique physically-uncopyable quantum states, which can be used for entanglement & secure communication, and your jewels slowly brainwash you into an appendage of the zoku (immortality, of a sort). The zoku promiscuously use their ‘jewels’, accepting as a fact of life that there will be individual losses such as theft (hence the eponymous “quantum thief”), but the zoku itself lives on.

But jewels (and the sacrifice of an autonomous self) are too restrictive for a thief like le Flambeur, while the Sobornost are committed to using only technologies that allow copies which can be preserved forever (ruling out all quantum technology with religious fervor), enslaving minds forever to whoever runs them.

So Hannu’s answer for them harks back to that classic of American SF TV, Babylon V (1993^–₅1998_28ya). In B5, characters are interrogated by the benevolent status-quo overlords with the “Vorlon Question”: Who are you? And their opposites, the changing Shadows, ask instead the “Shadow Question”: What do you want?

Hannu’s answer to the personal identity question is the Shadow Question—that Jean & the Sobornost Founders & the zoku elders are all defined by what, at their core, they want. Anyone who wants the same thing is, for all (their) intents and purposes, the same person as them; because they want the same unchanging things, they can be trusted as the original. The ‘Founder codes’, and Jean’s final password to unlock his sealed memories, are all memories of what defines their wants: the Founder Sumanguru wants blood & fire & electricity & screaming children, and enemies to destroy; the Founder Chen recall the trauma of livestreaming their father’s assassination, remaining eternally resolved that the last enemy that shall be defeated is death; while seared into the minds of the Founder Joséphine Pellegrinis is the final thought of their founder as she is uploaded, her desperate dying wish that her lover Jean le Flambeur someday return to her… (And the zoku elders are forced to want to empower their zoku clans.) And perhaps you or I would forever treasure the memory of one perfect day.

But even personal identity frays under the power of time: given freedom to change, sooner or later, like the Ship of Theseus, the mind which sets out is not the mind which arrives. So the price of immortality must be that one cannot change: one is condemned to want the same things, forever.⁷ (“There is no prison, except in your mind.”) Joséphine Pellegrini cannot stop seeking after her lost Jean—nor can Jean stop his thieving nor trying to escape her, because le Flambeur, what does Jean le Flambeur remember?

He remembers that every wall has a door; that every system has a flaw; every proof its assumptions, however poor; and whether man or physics, there is no absolute law. He remembers long ago an impoverished boy locked in a desert jail cell, wanting freedom, and certain of only one truth (repeated throughout): “There is always a way out.”