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Sara Imari Walker

Walker is a theoretical physicist and astrobiologist whose central question is what life is — not in the biological-classification sense but as a physics problem. What distinguishes living matter from non-living matter, and can the distinction be made measurable? Her co-development of assembly theory with Lee Cronin gave this question a formal apparatus: the assembly index as a quantitative measure of an object’s construction history, with the claim that high assembly index signals selection. Her broader programme extends the framework into a proposed account of time as a material property of complex objects — the boldest and most contested reach of the theory.

Life

Born in Connecticut. BS in physics (cum laude) from Florida Institute of Technology (2005). PhD in physics and astronomy from Dartmouth College (2010), with thesis work on the emergence of biomolecular homochirality (advisor: Marcelo Gleiser). Postdoctoral research at Georgia Institute of Technology. Joined Arizona State University, where she is Professor in the School of Earth and Space Exploration and the School of Complex Adaptive Systems, and Deputy Director of the Beyond Center for Fundamental Concepts in Science. External professor at the Santa Fe Institute. Berggruen Institute fellow.

Met Cronin at a NASA astrobiology workshop in 2012; the collaboration that became assembly theory began from 2017.

The physics of life

Walker’s programme starts from a dissatisfaction with existing accounts. Biology describes what life does — metabolism, reproduction, evolution — but does not say what life is in terms that physics can work with. Physics describes matter and energy but has no account of why some arrangements of matter are alive and others are not.

The question is whether the distinction is measurable. Not “what are the features of living things?” but “is there a physical quantity that separates living from non-living matter, in the way that temperature separates phases?” Assembly theory’s biosignature claim — that above a certain assembly index threshold, objects require selection-like processes to exist — is how the framework answers it.

The framing is deliberately physical: Walker treats life as a problem for fundamental physics, not for biology operating on its own terms. This positions her work at the boundary between astrobiology (where the question “is this alive?” has practical urgency for life-detection missions) and theoretical physics (where the question “what is life?” connects to foundational issues about time, information, and causation).

Assembly theory

Co-developed with Lee Cronin from 2017 onward. The assembly theory subject pages carry the full treatment.

Walker’s contribution to assembly theory is the theoretical and philosophical framework built on top of Cronin’s chemical apparatus. The conceptual move — relocating complexity from object-properties to object-trajectories — is joint work. The metaphysical extension — time as a material property of objects, the positioning against Newton, Einstein, and thermodynamics, the identity claim that time is memory, information, causation, and selection — is primarily Walker’s.

Walker’s formulation of the core insight: the information is in the path, not in the initial conditions.

The 2023 Aeon essay “Time is an object” (with Cronin) and her book Life As No One Knows It (2024) carry the broadest version of these claims. The unification framing — that assembly theory does for time and matter what Einstein did for space and time — is Walker’s, and is the version most contested by both sympathetic and hostile readers.

Life As No One Knows It

Walker’s book-length popular treatment (Riverhead, 2024). The book develops the argument that existing physics lacks an account of what makes life distinctive; proposes assembly theory as the framework that fills the gap; and extends the claims into cosmology, arguing that the universe’s complexity grows through accumulated construction history.

The book is accessible rather than technical. It brought assembly theory’s claims — particularly the metaphysical claims about time — to a broader audience and generated substantial discussion in popular science and philosophy circles.

The astrobiology connection

Walker’s formation is in astrobiology — the study of life’s origin, evolution, and distribution in the universe. The practical stakes: if assembly theory’s biosignature claim holds, it gives space agencies a tool for life detection that does not depend on knowing what kind of life to look for. A mass spectrometry measurement of molecular assembly index could in principle detect life in a Mars sample or on an icy moon without assuming Earth-like biochemistry.

This gives assembly theory a practical application beyond its theoretical ambitions. Walker’s dual position — at ASU’s School of Earth and Space Exploration and as a Santa Fe Institute external professor — reflects the two sides of her work: the astrobiological application and the broader complexity-science framework.

Where Walker stops

Walker’s programme is theoretical. The empirical apparatus — mass spectrometry, laboratory chemistry, the actual measurement of assembly indices — is Cronin’s laboratory’s work. The extensions of assembly theory beyond chemistry — to language, technology, cultural evolution, cosmology — are proposed framings that Walker has articulated but that lack the formal and empirical development of the molecular case. The question of whether the metaphysical claims (time as material, the Einstein-sequel unification) can be grounded in the same way the biosignature claim is grounded in laboratory measurement is open.

Key works

See also: Assembly theory · Cronin · Hazen · Kauffman