Sara Walker

Astrobiologist and theoretical physicist at Arizona State University. Co-developer (with Lee Cronin) of assembly theory — a framework for quantifying the historical contingency of objects and detecting signatures of life. Author of Life As No One Knows It: The Physics of Life’s Emergence. Previously mentored by Paul Davies. Santa Fe Institute external faculty.

Background

Works at the intersection of physics and biology, attempting to build universal principles that explain life across all possible substrates — not just Earth-based biochemistry. Core claim: life is not a chemical phenomenon but a physical one, the process by which the universe navigates combinatorial spaces too large to exhaust via historically contingent selection. Assembly theory makes this precise and measurable. Active experimental work on chirality as a proxy for the origin-of-life transition.

Known for: the “life is big in time” reframing; the lineage-not-individual unit of life; assembly theory and the assembly index; the chiral phase transition as origin-of-life signature; the technosphere as the largest temporal object on Earth; the Krakauer quote on theories of everything.

Appearances in this wiki

Key positions

• Life cannot be defined at the level of individuals — it must be defined at the level of lineages and causal structures
• Life is not chemical; chemistry is one substrate where the combinatorial space first exceeds the universe’s capacity to explore it
• Living objects are among the largest structures in the universe, measured by temporal extension (causal depth) rather than spatial size
• The technosphere is the largest temporal object currently known
• Assembly theory: the assembly index of an object measures its causal history; high assembly objects require life-adjacent processes
• Chirality is a signature of complex chemical space, not a generic feature of chemistry; the chiral phase transition (~7–11 heavy atoms) coincides with the origin-of-life transition
• There is no theory of everything; there are better and worse theories; no bottom to reality
• Constructed objects — cells, organisms, machines — are more fundamental than elementary particles in the relevant sense