Key Moments
What Is Life?: A Conversation with Sara Imari Walker (Episode #388)
Sam HarrisKey Moments
Exploring life scientifically: physics explains emergence, information's role, and assembly theory models complexity.
Key Insights
Physics, particularly through figures like Schrödinger, has historically grappled with defining life, highlighting the need for new frameworks beyond standard laws.
Current definitions of life, such as 'self-sustaining chemical systems capable of Darwinian evolution,' are inadequate due to numerous edge cases and ambiguities.
The concept of 'artificial life' challenges traditional distinctions between biological and non-biological systems, suggesting life might be substrate-independent.
Information plays a crucial, causal role in the physical universe, especially at the boundary between non-life and life, and can be understood as a physical property.
Assembly Theory proposes that complex objects, including life, cannot arise spontaneously but require a history of construction and the reuse of parts over time.
Constructor Theory offers a framework where physical laws are described by what can and cannot be caused to happen, providing a potential language for understanding life's emergence.
THE LIMITATIONS OF CURRENT DEFINITIONS OF LIFE
The conversation begins by examining the challenges in defining life, noting that prevalent definitions like 'self-sustaining chemical systems capable of Darwinian evolution' fall short when scrutinized. Edge cases such as viruses, mules, and even bee colonies highlight the difficulty in drawing a definitive line. The inadequacy of these definitions suggests a need for a more fundamental, perhaps physics-based, conceptualization of what life truly is.
SCHRÖDINGER'S INSIGHTS AND THE NEED FOR NEW PHYSICS
Erwin Schrödinger's seminal work, 'What Is Life?', is discussed as a foundational contribution that asked critical questions about life from a physics perspective. While Schrödinger's concept of an 'aperiodic crystal' presciently hinted at DNA's informational role, his deeper insight was the potential necessity of undiscovered physical laws to fully explain life. This idea motivates the search for a more comprehensive theoretical framework.
ARTIFICIAL LIFE AND SUBSTRATE INDEPENDENCE
The idea of 'artificial life' is explored, revealing a semantic tension between 'artificial' and 'life.' The discussion posits that if life can be understood fundamentally, it could potentially be instantiated in non-biological substrates (dry systems) as well as biological ones (wet systems). This challenges the provincial distinction between 'wet' and 'dry,' suggesting that life's defining characteristics might be substrate-agnostic and evolutionary continuity matters more than material composition.
INFORMATION AS A PHYSICAL CAUSAL AGENT
Information is presented not merely as an abstract property but as a fundamental physical feature of reality, particularly crucial at the boundary between non-life and life. Within assembly theory, information acquisition over time is seen as essential for constructing complex objects. The causal role of information implies that it must be treated as a physical characteristic, enabling the existence of evolutionary objects that store and process information.
ASSEMBLY THEORY: THE PHYSICS OF COMPLEXITY AND ORIGIN OF LIFE
Assembly Theory, a framework developed by Sara Walker and colleagues, proposes that complex objects, including living systems, require a constructive history and cannot arise spontaneously. It suggests a universe with a vast space of possible structures, but a universal boundary in complexity (the 'assembly index') beyond which spontaneous formation is impossible. Objects above this threshold necessitate 'constructors' – persistent physical systems that constrain possibilities and enable replication.
CONSTRUCTOR THEORY AND THE 'CAN' AND 'CAN'T' OF PHYSICS
Constructor Theory, pioneered by David Deutsch, reframes physics in terms of tasks that can and cannot be caused to happen, rather than just initial conditions and laws of motion. This approach focuses on the role of 'constructors' as agents that enable certain outcomes. While Assembly Theory focuses on the physical embodiment of information and complexity, Constructor Theory provides a theoretical language for describing the causally constrained phenomena that underpin life's emergence.
THE ROLE OF COPY NUMBER AND REPRODUCIBILITY
A key aspect of Assembly Theory is the concept of 'copy number,' emphasizing that abundant complex objects are not products of unique spontaneous fluctuations but arise from reproducible construction processes. This highlights the evolutionary necessity of creating many copies of structures over time. The persistence and reliability of these construction processes, embodied in constructors, are essential for building high-complexity objects and are foundational to understanding life's abundance.
CHALLENGING THE BLOCK UNIVERSE AND MANY-WORLDS INTERPRETATION
The conversation touches upon the 'block universe' concept and the Many-Worlds interpretation of quantum mechanics, questioning their applicability to understanding time and reality. Walker suggests that consciousness and our interaction with physical reality happen at the 'tips' of historical lineages, implying that determinism emerges from non-deterministic underlying realities through causal constraints. Living structures, due to their extensive causal histories, are viewed as the most deterministic entities.
THE PHYSICALITY OF ABSTRACT OBJECTS LIKE NUMBERS
The existence and nature of abstract objects, such as numbers, are explored through the lens of time and scale. Walker argues that what we perceive as abstract concepts might be massive physical structures, either in spatial extent or temporal depth, that are simply too large for our finite human perception to grasp fully. The 'knowledge' or 'constructor' to generate these structures, like the next prime number, exists within this larger physical framework.
INFORMATION AS DEEPLY EMBEDDED PHYSICAL STRUCTURES
Walker posits that 'information' in systems like human language, mathematics, or genomes is not ephemeral but deeply tied to physical size and temporal depth. These informational structures are products of history and computation, giving them a physical manifestation. They appear abstract because our finite interaction with them prevents us from observing their entirety simultaneously, suggesting a re-evaluation of abstract objects as complex, temporally extended physical phenomena.
Mentioned in This Episode
●Organizations
●Books
●Concepts
●People Referenced
Common Questions
Sara Imari Walker is a theoretical physicist and astrobiologist who focuses on the origin of life and the potential discovery of alien life. Her work combines physics, chemistry, and evolutionary theory.
Topics
Mentioned in this video
Recipient of the early career award named after this individual for research on the origin of life.
Collaborator with Sara Imari Walker in developing Assembly Theory.
His book 'What is Life?' is discussed as a foundational contribution to the question of life's nature from a physics perspective.
Host of the Making Sense podcast, interviewing Sara Imari Walker about the nature of life.
Walker's affiliation, where she is a professor in the School of Earth and Space Exploration and deputy director of the Beyond Center for Fundamental Concepts in Science.
Astrobiologist and theoretical physicist, guest on the Making Sense podcast, discussing her book 'Life as No One Knows It'.
His work on Constructor Theory is discussed and compared with Sara Walker's Assembly Theory.
Erwin Schrödinger's influential book that explored life from the perspective of physics, particularly regarding genetic heredity and information storage.
Sara Imari Walker's book, which is the focus of the conversation, exploring the definition and understanding of life within physics and chemistry.
Robin Hanson's concept suggesting that a hurdle must be overcome for life to evolve to a stage where it can colonize the galaxy; potential explanations for the Fermi Paradox.
Used as an example of abstract objects, discussing their infinite nature and potential physical instantiation. Their existence is tied to the knowledge and capability to generate them.
A philosophical interpretation of physics where time is seen as a static dimension, and all moments exist simultaneously. Discussed in the context of determinism and the nature of possibility.
Part of a common definition of life in astrobiology, referring to life as a self-sustaining chemical system capable of evolution.
Sara Imari Walker's theoretical framework, developed with Lee Cronin, aimed at explaining the origin of life by focusing on the construction of complex objects over time and the role of information.
The Fermi Paradox, questioning the apparent contradiction between the high probability of extraterrestrial civilizations and the lack of evidence for them.
A theoretical framework proposed by David Deutsch and Chiara Marletto, which reframes physics in terms of possible and impossible tasks, focusing on 'constructors' as causes.
Used as a point of comparison for 'artificial life,' highlighting that intelligence can be instantiated in a substrate-independent way, unlike the perceived contradiction in 'artificial life'.
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