Sara Walker: Sara Walker is an astrobiologist and theoretical physicist focused on the origin of life and detecting alien life on other worlds. She is the author of the book 'Life as No One Knows It: The Physics of Life's Emergence' and co-developer of assembly theory with chemist Lee Cronin.
What this episode covers
Sara Walker joins Lex Fridman for her third appearance to argue that life cannot be defined by standard criteria like self-reproduction or Darwinian evolution, because every such definition has counterexamples. She proposes instead that life is the process by which information structures matter over time, making living things enormous objects in time rather than space, with billions of years of causal history packed into them. She and Lex explore assembly theory, which measures complexity via copy number and the minimal recursive steps to build an object, identifying roughly an assembly index of 15 as the boundary where life must take over. The conversation ranges across the origin of life as an abrupt phase transition, chirality, alien life and why advanced civilizations might 'virtualize' and become imperceptible, language and large language models as crystallized collective intelligence, beauty and fashion, consciousness, free will, and the technosphere as a planetary-scale living phenomenon poised to reproduce onto other planets.
The guest's own work
Life as No One Knows It: The Physics of Life's Emergence
Sara Walker
“she has written an amazing new upcoming book titled life as no one knows it the physics of life's emergence”— Lex Fridman
Also referenced (named, not recommended)
Life Ascending: The Ten Great Inventions of Evolution
Nick Lane
“Nick Lane has a book called life ascending where he lists the 10 great inventions of evolution the origin of life being first”— Sara Walker
A New Kind of Science
Stephen Wolfram
“Steven Wolfram through all the explorations of like cell aoma type of objects that he did in a new kind of Science”— Lex Fridman
The Eerie Silence
Paul Davies
“he wrote this book years ago about um you know like the Eerie silence and why we're alone”— Sara Walker
Oculus
Meta
“so now you can put like you know an Oculus headset on and think that you're in this world right”— Sara Walker
GoPro
GoPro
“she goes out with like GoPro cameras and like you know films in high resolution all these Firefly flickering”— Sara Walker
Big reveals from this episode
- Walker argues we must abandon the individual as the relevant unit of life; life is interwoven lineages stretching back billions of years, and definitions fail because they fixate on fleeting individuals rather than the deep causal structure.
- Under assembly theory, an individual organism can evolve because the theory is about construction and how the universe selects which things get to exist, broadening evolution far beyond the Darwinian population-level view.
- Walker claims living objects are among the biggest structures in the universe, but they are big in time, not space, and the modern technosphere is the largest object in time we know of.
- The chirality (handedness) of biological molecules breaks symmetry in time, not space, because choosing a left- or right-handed molecule determines all its future reactions; chirality is a signature of complex chemical space.
- Experimentally, organic molecules with an assembly index above roughly 15 are only observed in living systems, marking the boundary life must cross to produce molecules in high abundance.
- Walker now believes the origin of life is an abrupt phase transition, where the possibility space collapses fast onto self-reinforcing structures, rather than a gradual process.
- Walker fears a 'great perceptual filter': sufficiently advanced biospheres may virtualize themselves, becoming deep in time but closed off and effectively imperceptible, like black holes, to others.
- Large language models act like a genetic system for language, crystallizing a dynamic, highly compressed slice of humanity's collective intelligence into a physical artifact.
Worth remembering
- Taxol, a single molecule of molecular weight about 853, if made in every possible three-dimensional shape with its formula, would fill about 1.5 universes in volume, illustrating how vast chemical space is.
- Lex describes an ayahuasca experience where he could see people's 'past selves' as blurred trailing faces, which Walker frames as a glimpse of the temporal object that a person actually is.
- Lee Cronin's lab found the first non-organic autocatalytic set, a self-reproducing molybdenum ring (around 150 atoms) that becomes a template reinforcing its own production.
- Almost every small molecule (under about 7 to 11 heavy atoms) is achiral, but above that threshold nearly every molecule becomes chiral, suggesting the origin-of-life transition coincides with the onset of chirality.
- Most chemistry we study on Earth is already a product of life, which is why chirality seems like a generic feature of chemistry when it is actually tied to the life transition.
- Walker is helping with the Habitable Worlds Observatory mission planning and its biosignatures team, and believes first contact with alien life will effectively be an origin-of-life experiment.
- Walker's lab has an undergrad building a model of alien communication based on fireflies, imagining an alien signal that maximally differentiates itself against a pulsar background.
- Fashion designer Alexander McQueen, whom Walker admires, mixed horror and beauty in his controversial early shows and left his death note written on Darwin's 'The Descent of Man.'
- Walker orders the emergence of phenomena as life, then consciousness, then intelligence, suggesting they may all be flavors of the same underlying thing.
- Asked how much of the big picture science understands, Walker jokes it is about 1.7%, with seven for humor and one for how much mystery she thinks remains.