---
title: "Recursive Worlds"
description: "Recursive Worlds is the edited text of a lecture Sara Imari Walker gave in London in July 2024 for Antikythera's Cognitive Infrastructures Studio."
type: "reading-notes"
authors:
  - "Sara Imari Walker"
published: "2025-05-10"
original_url: "https://recursiveworlds.antikythera.org/"
doi: "10.1162/ANTI.5CZX"
canonical_url: "https://antikythera.wiki/work/journal/recursiveworlds"
md_url: "https://antikythera.wiki/md/work/journal/recursiveworlds"
last_updated: "2026-08-23"
site: "Antikythera Wiki"
---

# Recursive Worlds

> Independent, unofficial reading notes from Antikythera Wiki (https://antikythera.wiki). Written with AI from the published text, with citations; not by the work's author and not affiliated with Antikythera (https://antikythera.org). Read the original at the link below.

- **Authors:** Sara Imari Walker
- **Published:** 2025-05-10
- **Kind:** Journal · Journal article
- **DOI:** https://doi.org/10.1162/ANTI.5CZX
- **Original:** https://recursiveworlds.antikythera.org/
- **Length:** 11k words
- **This page:** https://antikythera.wiki/work/journal/recursiveworlds
- **Notes generated:** 2026-08-23

## Summary

Recursive Worlds is the edited text of a lecture [Sara Imari Walker](https://antikythera.wiki/people/sara-imari-walker) gave in London in July 2024 for Antikythera's Cognitive Infrastructures Studio. Walker, a theoretical physicist who works on the origin of life, argues that life is not a kind of chemistry but a kind of physics that standard physics cannot yet describe: the process by which information structures matter across time, so that objects come to exist that could never form spontaneously. Measured by that yardstick, Earth is the largest object we know of, not in space but in time, because four billion years of construction are stacked recursively inside every living thing on it. Assembly theory, developed with the chemist [Lee Cronin](https://antikythera.wiki/people/leroy-cronin), is the formal version of this claim, with a measurable threshold (an assembly index of about fifteen) above which only selection can produce a molecule in abundance. The closing turn is planetary: the technosphere is the newest and deepest object in time, it has just acquired the instruments to perceive its own origins, and if it is a living structure it will eventually reproduce the biosphere that made it.

## The argument

Walker begins from a reversal of scale. We are used to Earth being small in a vast universe, but if existence is measured in time rather than space, Earth is enormous, because it has been accumulating the memory of its own construction for four billion years. Her warrant for the reversal is David Deutsch's remark that base metals are transmuted into gold by stars and by intelligent beings who understand stars, and by nothing else. The two processes differ in kind. One is spontaneous; the other requires knowledge of the regularities of nature, acquired over time, which then makes new things possible. Her two thought experiments, oganesson (element 118, made in abundance only on Earth) and the thousands of artificial satellites in orbit, also make a methodological point: physics as formulated needs an observer outside the universe, and a physics of life must put us inside it.

From this she draws a redefinition of the unit of life. Textbooks say the cell; Walker says the cell is an instance of a temporally structured object whose atoms turn over entirely between birth and death. What persists is the informational pattern, and the pattern is a lineage. Life is lineages of propagating information structuring matter over time. The origin of life then becomes a transition in what she calls the combinatorial space, the space of all objects that could be built from available parts. Below a certain complexity things form by chance; above it they require an informational lineage. The origin of life is the phase transition at which a planet crosses that boundary without dissipating back into noise, after which life carves a contingent trajectory through an exponentially growing space.

Assembly theory is the formalisation. An object is compositionally constructed from parts; its assembly index is the minimum number of recursive joining steps needed to build it, reusing sub-objects already made; its copy number is how many instances exist. High index with high copy number is the signature of selection, because each step into the space multiplies the possibilities that could have been built but were not. The decisive feature, in Walker's account, is that the index is measurable by mass spectrometry, which lets her treat evolved complexity as a physical attribute of a molecule in the way charge is an attribute of an electron. Cronin's measurements found that only biological samples produced molecules above roughly fifteen, which Walker reads as empirical evidence of the life threshold.

The consequences she draws are metaphysical. Objects exist in time and have a size in time; matter is informational; the construction history is the object. Social reality, language, and culture are not emergent or fake but deep-in-time objects as physical as any other. Standard physics pushes all design into initial conditions, which is why the simulation argument and intelligent design share its structure; assembly theory lets design emerge along lineages, so the universe is genuinely creative, undetermined rather than deterministic or random, and its future is larger than its past. The lecture ends with the technosphere as the largest object in time, now equipped with technologies of perception sufficient to look back at the origin of life, and with the prospect that a living technosphere will reproduce, making copies not of cells or humans but of the biosphere itself.

## Section by section

The published text has no internal headings beyond an editorial introduction; the divisions below follow the lecture's own movements.

### Introduction

The journal's editorial paragraph summarises the talk: life redefined as a physical process structuring matter through recursive information, assembly theory as the account of how molecules, technologies, and living systems emerge only through informational lineages, and Earth as the largest known object in time, whose technosphere may continue planetary evolution toward biospheric reproduction.

### Earth as the largest object in time

Walker opens with Einstein's remark that physics is primitive next to what the study of life might teach. Her first reframing is to treat the planet as a self-constructing system and to shift the measure of size from space to time. Because life has been structuring matter here for four billion years, we are "a very, very deep stack of recursive objects," and the combinatorial possibilities that exist on Earth exist nowhere else.

### Stars, intelligent beings, and two thought experiments

Walker insists on the difference between a spontaneous process and one that has knowledge of that process, and argues that the physics of life needs thought experiments with the observer inside. The first experiment runs from nucleosynthesis to oganesson, which is permitted by physics but produced in abundance only by a system that learned nuclear engineering. The second compares Mars's two captured asteroids, a fluke, with Earth's thousands of artificial satellites, which are a reliable, repeatable, "almost programmatic" output of a biosphere that extracted the laws of gravitation. She defends the anthropocentrism of the examples on the grounds that the physics is the same at every scale.

### Lineages, the unit of life, and the origin of life

Here Walker rejects the cell as the fundamental unit in favour of the informational pattern that persists through material turnover, and generalises: life is lineages of propagating information. The origin of life is defined as the moment the combinatorial space changes so that a planet can build complexity recursively. Life emerges when selection permits objects that could not exist without a lineage; artificial satellites need four billion years of "construction time, not clock time." As memory compacts into small volumes, abstraction and a virtual world become possible; evolved objects are larger in time than in space, and a cell looking up at us would see something abstract.

### Technologies of perception

Borrowing the phrase from [Claire Webb](https://antikythera.wiki/people/claire-webb), Walker argues that the boundary of reality is an evolving perceptual horizon. When life began nothing could see; photoreceptors, compound eyes, and then instruments extended the horizon, and we did not know cellular life existed until we built instruments that could resolve it. A camera is itself an evolved object of the biosphere, and the Large Hadron Collider is a giant microscope marking the current horizon of substructure.

### How large the possibility space is

Against the Hubble Deep Field's ten thousand galaxies at a pen tip, Walker sets Taxol, a molecule of 47 carbons, 14 oxygens, 51 hydrogens, and one nitrogen. Enumerating every three-dimensional structure with that formula would fill one and a half universes of material. Taxol exists and the rest do not because selection has constrained the space along one construction history, so that some possibilities are "exponentially receding" and will never exist. Each evolved object therefore carries a folded-up counterfactual space.

### Lego and improbable objects

A Lego universe has rules (blocks stack but do not join sideways without glue), shaking a pile produces a few pairs, and standard physics would say a Lego Hogwarts has very low but nonzero probability, which is the logic of Boltzmann brains. Assembly theory says the probability is zero without a constructed history, because complex objects never fluctuate into existence; the information has to be learned, and it has to come from other objects inside the universe. She then turns to what an object is, noting that physics textbooks never define one, and that what looks fundamental (atoms, then protons, then quarks) is only the current perceptual boundary.

### Defining objects: construction, copy number, and assembly index

Objects within the horizon are compositionally constructed from parts, and they must exist in more than one copy. Regularity (many near-identical humans in a room, sharing most of their DNA) is the signature of a selected mechanism that can rebuild the same object with some error. Molecular assembly theory follows the Lego logic: rebuild Hogwarts by joining pieces and reusing sub-assemblies, and take the shortest path. ATP has an assembly index of 21, and the full variant space of ATP or the nucleobases, if instantiated on Earth, would collapse into a black hole. The light bulb, with Edison's hundred variants and billions of surviving copies but no surviving failures, illustrates why copy number distinguishes selected structure.

### Measurement and the complexity threshold

Charge, Walker notes, is an invented abstraction that becomes real through reliable laboratory regularity. Assembly index is the first complexity measure with this property, inferable from mass spectrometry, NMR, or infrared spectra. Cronin's Glasgow laboratory measured biological and non-biological samples and found a boundary: only biological systems produced molecules above an index of about fifteen. Walker presents this as an empirically validated result, with much still to do. Every object built becomes a substructure of later objects, so once across the boundary every object is recursive and deep in time.

### The four assembly spaces

Returning to Taxol's counterfactual space, Walker introduces a nested hierarchy. The assembly universe is everything imaginable from the parts, including rule-breaking (glued Lego, square circles). Assembly possible is what conforms to physical law. Assembly contingent is the actual history a biosphere has constructed, which constrains what can be built next; transistors and large language models are her examples of inventions that found subsequent technology and exclude other futures. Assembly observed is the tiny fraction that exists as physical objects. Co-constructed objects are related in time, which is why humans can recognise one another as "bifurcating tips" of one structure.

### Time, matter, and the materiality of information

Every theory since Newton has invented its own time; assembly theory's contribution is that objects have a physical size in time. This reinvents matter as informational, and Walker cites Penrose's dislike of "materialist" for presuming we know what matter is. The Madonna aside frames her claim that social reality is real reality, as physical as any other. She restates assembly theory as a theory of objects: finite, constructible in finitely many steps, existing in more than one copy, formed by selection, and identical with their lineages. Laws of physics exist inside objects, not outside the universe. Walker links this to Timothy Morton's hyperobjects, arguing that every evolved structure is larger than we can interact with, mostly in time.

### The technosphere and the future horizon

Michael Lachmann's answer that he is four billion years old introduces the claim that we are now building objects larger in time than ourselves, which is why the present feels like a virtualisation of reality. The technosphere is the newest and largest object in time; the deeper the stack, the larger the future horizon. The two transitions, biosphere and mind, are planetary-scale, since life's natural boundary is the planet rather than a local pocket. Walker then contrasts assembly theory with the prevailing paradigm point by point: design in initial conditions versus along lineages; indivisible fundamentals versus everything breakable; a fixed future versus one that grows; determinism versus an undetermined universe into which structures like us build determinism; time absent versus time fundamental. She ends with the question of whether we are large enough in time to see our own origins and with the prediction that a living technosphere will eventually reproduce the biosphere.

## Key concepts

- **[Assembly Theory](https://antikythera.wiki/terms/assembly-theory)** — Walker and Cronin's theory that objects are defined by their minimal construction history, that complex objects in abundance require selection, and that evolved complexity is a measurable physical attribute.
- **Assembly index** — The minimum number of recursive joining steps needed to construct an object from its parts, reusing sub-objects already made; ATP scores 21, and about 15 marks the threshold only life crosses.
- **Copy number** — The number of instances of an object; combined with a high assembly index it is evidence that a selective mechanism exists to build the object repeatedly, as with light bulbs but not their failed variants.
- **[Size in Time](https://antikythera.wiki/terms/size-in-time)** — Walker's proposal that evolved objects have a physical extent in construction time, which for most of them exceeds their extent in space; the sense in which Earth is the largest known object.
- **[Assembly Universe / Possible / Contingent / Observed](https://antikythera.wiki/terms/assembly-universe-possible-contingent-observed)** — The nested hierarchy from everything imaginable, to what physical law permits, to what one biosphere's history has actually made buildable, to the small fraction that exists.
- **[Technologies of Perception](https://antikythera.wiki/terms/technologies-of-perception)** — [Claire Webb](https://antikythera.wiki/people/claire-webb)'s phrase, adopted by Walker, for the evolved and built apparatus (eyes, microscopes, colliders) that sets the moving boundary of what counts as reality.
- **[Biospheric Reproduction](https://antikythera.wiki/terms/biospheric-reproduction)** — Walker's closing prospect that a living technosphere which understands the origin of life will eventually make copies of the biosphere rather than only of cells or technologies.
- **[Life](https://antikythera.wiki/terms/life)** — For Walker, not a substance or a unit but the physics of information structuring matter over time, occupying the region of combinatorial space that selection alone can reach.

## Connections

- [Speculative Philosophy of Planetary Computation](https://antikythera.wiki/work/journal/spoc) is the clearest point of uptake. Bratton presents assembly theory's two claims, that selection begins with chemistry and that complexity quantifies evolutionary time, as the scaffold for his sequence from autopoiesis through artificialisation to AI, and refers the audience to this lecture. The relation is agreement; the vocabulary of scaffolding is Bratton's, not Walker's.
- [The Long L](https://antikythera.wiki/work/journal/longl) cites Walker's planetary-transition work and her book on life's emergence, and shares the conclusion that intelligence is a planetary-scale process and that the relevant measure is persistence across substrates. The two agree.
- [The Transition from the Biosphere to the Noösphere](https://antikythera.wiki/work/journal/noosphere) is Vernadsky's earlier statement that a planet's evolution passes through a cognitive stage; Walker's technosphere as the newest and deepest object in time is a physicist's restatement of that sequence, in agreement but with selection and construction history in place of Vernadsky's geochemistry. [Infinity Mirror](https://antikythera.wiki/work/journal/infinitymirror) treats the same sequence as a major evolutionary transition, drawing on Frank, Grinspoon, and Walker's "Intelligence as a Planetary Scale Process" rather than on assembly theory.
- [What Is Intelligence?](https://antikythera.wiki/work/book/what-is-intelligence) offers a computational account of life's emergence that Bratton calls "something like assembly theory in action." The two converge on selection preceding biology, but Walker's insistence that information is material and that objects encode their own laws is a different foundation from computational universalism.
- [Substrates Unbound](https://antikythera.wiki/work/journal/substrates) stands in tension with this lecture. Tripaldi argues that the matter a function runs on is itself agential; Walker's framework makes the lineage the agent and treats the atoms as interchangeable carriers that turn over entirely within a cell's life.
- [Xenophylum](https://antikythera.wiki/work/journal/coginfra/xenophylum), within [Cognitive Infrastructures](https://antikythera.wiki/work/journal/coginfra), borrows assembly theory's distinction between assembly and assemblage to imagine synthetic lineages without biological precedent, extending Walker's "assembly contingent" into design.


## Terms used

- [Assembly Theory](https://antikythera.wiki/terms/assembly-theory)
- [Assembly Universe / Possible / Contingent / Observed](https://antikythera.wiki/terms/assembly-universe-possible-contingent-observed)
- [Biospheric Reproduction](https://antikythera.wiki/terms/biospheric-reproduction)
- [Historical Things](https://antikythera.wiki/terms/historical-things)
- [Size in Time](https://antikythera.wiki/terms/size-in-time)
- [Technologies of Perception](https://antikythera.wiki/terms/technologies-of-perception)

## Related works

- [Substrates Unbound](https://antikythera.wiki/work/journal/substrates)
- [The Transition from the Biosphere to the Noösphere](https://antikythera.wiki/work/journal/noosphere)
- [The Long L](https://antikythera.wiki/work/journal/longl)
- [Speculative Philosophy of Planetary Computation](https://antikythera.wiki/work/journal/spoc)
- [What is Intelligence? (Lecture)](https://antikythera.wiki/work/journal/wii-film)