#388 — What Is Life?

Episode Summary

Executive Summary: Sam Harris interviews astrobiologist-theoretical physicist Sarah Walker about the origin of life, arguing that life may be best understood through physics, information, and evolutionary construction rather than traditional biology. They discuss Schrödinger, why standard definitions of life fail, whether technology can be life-like, and Walker’s assembly theory as a framework for identifying when complex objects require historical construction rather than spontaneous emergence.

Main Topics: Sarah Walker’s background and perspective (Priority: 4/5): Walker explains her training in physics and her shift toward origin-of-life and astrobiology questions, emphasizing theoretical rigor and nontraditional applications of physics to reality, life, and emergence. Schrödinger’s influence on origin-of-life thinking (Priority: 4/5): They revisit What is Life?, especially Schrödinger’s idea of an aperiodic crystal and his broader suggestion that life may require currently unknown physical laws. Limits of standard definitions of life (Priority: 5/5): Walker critiques the common astrobiology definition of life as a self-sustaining chemical system capable of Darwinian evolution, pointing to edge cases like viruses, bees, mules, and individuals versus populations. Artificial life, intelligence, and substrate continuity (Priority: 4/5): They debate whether life and intelligence are substrate-dependent, with Walker arguing for continuity between biology and technology and skepticism about sharp wet/dry distinctions. Information as a physical causal feature (Priority: 5/5): Walker argues that information is not merely abstract but physically real and central to the life/non-life boundary because living systems depend on stored, processed, and inherited information. Assembly theory and constructor theory (Priority: 5/5): Walker outlines assembly theory as a framework for explaining how complex objects and life emerge only through prior construction, and compares it to Deutsch’s constructor theory and its focus on possible versus impossible tasks. Implications for cosmology, quantum theory, and abstract objects (Priority: 4/5): The discussion expands to block universe ideas, many-worlds, determinism, and the ontological status of numbers, with Walker treating theories and mathematics as physically embedded constructive processes.

Key Arguments: Traditional definitions of life break down on edge cases such as viruses, mules, honeybee colonies, and the distinction between individuals and populations. Life is better understood as a process in which information becomes a causal physical feature that enables persistent construction and reproduction of complex objects. Artificial life is not a contradiction in principle; it is a test of whether life can be instantiated in non-biological substrates once we understand it deeply enough. Current AI may mirror human intelligence because it was selected by humans to do so, making claims about machine intelligence philosophically and empirically unsettled. Assembly theory claims that complex objects with high assembly index cannot arise by chance alone; they require historical construction and repeated selection. The abundance of complex molecules in living systems suggests a threshold above which objects only appear when constructors and lineages already exist. Constructor theory and assembly theory both aim to reframe physics around what can and cannot be done, rather than only laws of motion from initial conditions. Block-universe and many-worlds interpretations are, in Walker’s view, philosophical overlays that may miss the constructive, time-embedded nature of reality. Mathematics and abstract objects may feel nonphysical because they are very large, temporally extended structures that finite beings apprehend only partially. The knowledge to generate a structure may exist before the structure itself is instantiated, which helps explain why some objects are discoverable before they are physically realized.

Data Points: Podcast timing: about two weeks out from the U.S. presidential election - Harris frames the episode as arriving shortly before an election and says politics will dominate upcoming episodes. Institutional role: deputy director of the Beyond Center and professor at ASU - Walker’s academic positions are introduced in the episode setup. Award: Stanley L. Miller Early Career Award - Mentioned as recognition for Walker’s origin-of-life research. Assembly index: a measure of the minimum number of steps needed to construct an object - Described as the central quantitative idea in assembly theory. Biological timeline: 3.8 billion years - Walker says parts of her body, in an evolutionary sense, are literally that old because they descend from life on Earth over that span. Origin-of-life threshold: a threshold above which molecules are found only in life - Walker describes experiments in which assembly index distinguishes abiotic from biological molecules.

Pivotal Quotes: "life is a self-sustaining chemical system capable of Darwinian evolution" — Sam Harris: He cites the common astrobiology definition before asking Walker where it fails. "I think that's exactly what I think. I think it's very provincial and I think even, I probably don't ascribe the level of intelligence to current algorithms that other people do." — Sarah Walker: Walker responds to the biological/non-biological distinction and challenges strong claims about present-day AI. "the knowledge to generate that structure exists" — Sarah Walker: Walker refines her view of numbers and abstract objects, suggesting that what exists physically is the capacity to construct them, not necessarily their instantiated form.

Implications: The conversation pushes listeners to treat life as an informational-physical process rather than a narrow biological category. If Walker’s framework is right, future searches for alien life, synthetic life, and AI will depend on recognizing constructed complexity, not just chemistry.