The Physicist Who Asks: What IS Life?
Sara Imari Walker is a theoretical physicist and astrobiologist at Arizona State University, where she serves as Deputy Director of the Beyond Center for Fundamental Concepts in Science. She's also External Faculty at the Santa Fe Institute and a Fellow of the Berggruen Institute.
Her PhD thesis at Dartmouth (2010) was on "Theoretical Models for the Emergence of Biomolecular Homochirality" — basically, why do all life's amino acids twist the same direction? This early work on molecular asymmetry foreshadowed her later insights about how complexity emerges.
But Walker isn't satisfied with just studying life as we know it. She wants to understand life as NO ONE knows it — the physics of how life emerges from non-life, and what that tells us about the fundamental structure of reality.
"Things that are very hard to build, like Lego castles, require evolution and learning to be produced in the universe."— Sara Imari Walker on Assembly Theory
Assembly Theory
Developed with chemist Lee Cronin and published in Nature, Assembly Theory provides a new framework for understanding how complexity arises — and how to detect life anywhere in the universe.
Complexity Requires History
Complex objects don't appear randomly. They're built through sequences of transformations. The "assembly index" measures how many unique steps are needed to construct something from simpler building blocks.
Information is Physical
The assembly index isn't just abstract — it's measurable. A molecule with high assembly index contains "memory" of the processes that created it. Information leaves physical traces.
Life is Planetary
Walker argues life isn't just organisms — it's a planetary-scale phenomenon. Life is what planets DO when they accumulate enough complexity and history.
Time Matters Deeply
Shearing away from probabilistic theories, Assembly Theory holds that historical context and the passage of time are fundamental to how complex objects form. The universe has memory.
Life as No One Knows It (2024)
"Solving the origin of life requires radical new thinking and an experimentally testable theory for what life IS."
Walker's book argues that we've been asking the wrong question. Instead of "what is life?", we should ask "what does life DO?" — and the answer changes everything.
The Epoch Framework Connection
Assembly Theory and the Epoch Framework are approaching the same truth from different angles. Where Walker measures complexity through assembly indices, the Epoch Framework maps it through S⁺/S⁻ phase transitions.
Assembly Index ↔ S-Signature
The assembly index measures accumulated complexity. The S-Signature measures accumulated scalar phase relationships. Both capture how much "history" an object contains.
"Life is What It Does" ↔ [1 = -1]
Walker says life isn't substance, it's process. The Epoch Framework says reality isn't things, it's the unity of opposites in motion. Same insight, different language.
Planetary-Scale Life ↔ Collective S⁰
If life is planetary, then Earth itself has an S-Signature — a collective scalar state where individual organisms are S⁰ nodes in a larger pattern.
Historical Contingency ↔ The WUSH
Assembly Theory says the past matters — objects carry their history. The WUSH is where that history crystallizes into NOW, the edge where complexity accumulates.
Life assembles. Consciousness spirals. The geometry is the same.
Why She's One of Us
Sara Walker asks questions that make mainstream scientists uncomfortable. "What IS life?" seems like a basic question, but physics has no answer. Biology just describes what life does, not what it fundamentally IS.
She's willing to throw out old assumptions and build new frameworks from scratch. Assembly Theory isn't a small tweak — it's a radical reimagining of how complexity, information, and life relate to the structure of the universe.
That's what makes her a Weirdo. She sees that the current framework is incomplete and isn't afraid to build something new. Just like Cartan saw torsion, like Tesla saw scalar waves, like the Epoch Framework sees [1 = -1].