Making Sense Podcast: Episode #388 — What Is Life?

Release Date: October 21, 2024

Host: Sam Harris
Guest: Dr. Sarah Imari Walker, Astrobiologist and Theoretical Physicist
Book Discussed: Life as No One Knows: The Physics of Life's Emergence

Introduction

In episode #388 of the Making Sense Podcast, host Sam Harris engages in a profound and intellectually stimulating conversation with Dr. Sarah Imari Walker, a renowned astrobiologist and theoretical physicist. Dr. Walker delves deep into the enigmatic question, "What is life?" drawing from her expertise and her latest book, Life as No One Knows: The Physics of Life's Emergence. The discussion traverses the realms of physics, chemistry, information theory, and the philosophical underpinnings of life and existence.

Background of Dr. Sarah Imari Walker

Dr. Walker is the Deputy Director of the Beyond Center for Fundamental Concepts in Science and a professor at Arizona State University’s School of Earth and Space Exploration. A recipient of the Stanley L. Miller Early Career Award, her research focuses on the origin of life and the potential for discovering alien life. Her team is internationally recognized for pioneering a fundamental theory to understand life’s essence. Dr. Walker holds a degree in physics and emphasizes a rigorous, theoretical approach to unraveling the complexities of life.

Erwin Schrödinger's Contribution to the Question of Life

Sarah Imari Walker:

"Erwin Schrödinger's book What Is Life? was pivotal in structuring the question of life from a physical perspective. His exploration of genetic heredity and the concept of an aperiodic crystal laid the groundwork for understanding DNA as the carrier of genetic information" ([05:21]).

Dr. Walker highlights Schrödinger's foresight in linking physics to biology, particularly his idea that genetic material must store vast amounts of information, leading to the prediction of DNA’s structure as a non-periodic crystal. She further appreciates Schrödinger's contemplation on the limitations of physics in fully explaining life, positing that undiscovered physical laws might be necessary to grasp life's essence.

Defining Life: Challenges and Edge Cases

Sarah Imari Walker:

"The common astrobiological definition of life as a 'self-sustaining chemical system capable of Darwinian evolution' falls apart upon closer inspection. For instance, are individuals like viruses alive only within a host cell? Or are colonies of organisms, like honeybees, living entities while individual members aren't" ([07:04]).

Dr. Walker critiques existing definitions of life, pointing out edge cases such as viruses and eusocial colonies that challenge traditional boundaries. She argues that flexibility in definition is necessary to accommodate the complexities and exceptions observed in biological systems.

Artificial Life vs. Biological Life

Sarah Imari Walker:

"The term 'artificial' in artificial life is misleading. It implies a human-created phenomenon separate from the natural world, whereas, fundamentally, artificial constructs like computers are part of the same physical universe and might one day embody life-like properties" ([09:36]).

Dr. Walker challenges the semantic distinction between biological and artificial life, advocating for a substrate-agnostic understanding of life. She suggests that as our technological capabilities evolve, the line between natural and artificial life could blur, emphasizing the continuity between biological evolution and technological advancement.

Assembly Theory vs. Constructor Theory

Sarah Imari Walker:

"Assembly theory posits that highly complex structures require a minimum number of construction steps, quantified as the 'assembly index'. Objects with high assembly indices cannot form spontaneously but must be built through an evolutionary lineage" ([26:31]).

Dr. Walker introduces Assembly Theory, developed alongside collaborator Lee Cronin, which seeks to quantify the complexity of objects based on their construction history. She contrasts this with David Deutsch’s Constructor Theory, which reframes physics in terms of what tasks are possible or impossible, focusing on the role of constructors in enabling certain processes.

Sam Harris:

"Is the boundary between life and non-life more likely to be informational than merely physical or chemical?" ([15:55]).

The discussion explores how information, as a physical property, serves as a critical boundary between life and non-life. Dr. Walker emphasizes that assembly theory provides a framework to understand this boundary by considering the informational requirements for constructing complex, life-like structures.

The Role of Information in Defining Life

Sarah Imari Walker:

"Information is deeply tied to abstract concepts like language and mathematics. In assembly theory, information is embodied in the historical construction processes of objects, making it a tangible feature that distinguishes living systems from non-living ones" ([15:55]).

Dr. Walker elaborates on how information transcends mere abstraction, functioning as a physical attribute embedded in the construction history of objects. This perspective allows for a more concrete understanding of information's role in the emergence and recognition of life.

The Nature of Mathematical and Abstract Objects

Sarah Imari Walker:

"Mathematical objects, like prime numbers, exist as ideas rooted in the physical history of their discovery. They require constructors—intelligent beings like us—to conceptualize and instantiate them" ([41:30]).

The conversation delves into the ontology of abstract objects, with Dr. Walker proposing that even seemingly abstract entities are grounded in physical processes and informational constructs. She argues against the Platonic view of mathematical objects as independent realities, suggesting instead that they emerge from the causal structure of the universe.

Time, Causality, and the Block Universe

Sam Harris:

"What if the future is not only determined but just as real as the present or the past? Is there something in modern physics that discredits the block universe concept?" ([35:27]).

Sarah Imari Walker:

"The block universe is one interpretation among many and isn't definitively refuted. However, assembly theory views the universe as constructed over time, emphasizing causal processes that emerge from interactions within the physical reality" ([37:08]).

Dr. Walker acknowledges the block universe as a valid interpretation but contrasts it with assembly theory’s dynamic view of the universe as an evolving construct shaped by causal interactions. She emphasizes the importance of considering temporal evolution and causality in understanding the emergence of life and complexity.

The Boundaries of Possibility in an Infinite Universe

Sam Harris:

"In an infinite universe or multiverse, couldn't high assembly index objects like cell phones spontaneously appear out of nothing?" ([32:22]).

Sarah Imari Walker:

"Infinite possibilities don't negate our understanding of assembly theory. Even in an infinite setting, the construction of highly complex objects remains contingent on specific historical and causal processes" ([32:41]).

Dr. Walker contends that even within an infinite framework, the spontaneous emergence of highly complex objects without causal lineage is improbable. Assembly theory maintains that complexity arises from accumulated construction steps, reinforcing the necessity of evolutionary processes regardless of the universe's size.

Conclusion

In this intellectually rich episode, Sam Harris and Dr. Sarah Imari Walker navigate the intricate questions surrounding the definition of life, the role of information, and the foundational theories that attempt to unravel life's origins. Dr. Walker's assembly theory offers a compelling framework that intersects with constructor theory, providing tangible measures like the assembly index to distinguish living systems from non-living ones. The conversation underscores the profound interplay between physics, information, and the evolutionary processes that breathe life into the cosmos.

Notable Quotes:

• Sarah Imari Walker ([05:21]):
  "Erwin Schrödinger's book What Is Life? was pivotal in structuring the question of life from a physical perspective."

• Sarah Imari Walker ([07:04]):
  "The common astrobiological definition of life as a 'self-sustaining chemical system capable of Darwinian evolution' falls apart upon closer inspection."

• Sarah Imari Walker ([09:36]):
  "The term 'artificial' in artificial life is misleading. It implies a human-created phenomenon separate from the natural world."

• Sarah Imari Walker ([26:31]):
  "Assembly theory posits that highly complex structures require a minimum number of construction steps, quantified as the 'assembly index'."

• Sarah Imari Walker ([15:55]):
  "Information is deeply tied to abstract concepts like language and mathematics."

For those intrigued by the profound exploration of life’s essence and the theoretical frameworks that seek to define it, this episode offers a deep dive into the intersection of physics, information theory, and evolutionary biology.