Sara Walker: Life Is Huge in Time, Not in Space

Sara Walker opens *Life as No One Knows It: The Physics of Life's Emergence* with a split that has haunted biology for centuries: materialists versus vitalists. Materialists hold that living things are made of ordinary matter, with nothing extra riding on top. Vitalists insist that something non-physical — historically a soul — is present while you are alive and gone when you are dead. Walker, an astrobiologist and theoretical physicist on her third visit to the Lex Fridman podcast, refuses to treat that fight as settled folklore. She thinks neither tradition is fully wrong about the patterns it noticed.

Fridman asks whether there is a gray area: maybe all there is is matter, but we know so little that the vitalists' magic might as well be the unknown. Walker agrees that the universe is "a giant mystery," which is what keeps her in science. Open problems like life, consciousness, intelligence, and souls are not magic; they are work that has not been finished. Human history, she says, is the story of taking things once labeled supernatural and understanding them more deeply — without the mystery ever quite hitting a bottom.

Her working bet is that matter itself is still an unfinished invention. Science learned to measure some patterns with the technologies of a particular era, then treated those measurements as the whole of the material world. If we learned how to measure other patterns rigorously, we might find that matter has more properties than we thought, and that one of them is associated with life. Life could be a material property and still keep the features vitalists found mysterious. The missing piece, she says, is that we do not know how to think about information in a material way.

She reaches for the history of motion. Aristotle explained falling, floating, and celestial orbits by stuffing objects with material essences: earth-like things drop, air-like things rise, celestial-like things circle. After clocks, inclined planes, and planetary tracking, mass and acceleration unified all of that motion in a short description. Walker's claim is that our confusion about life is stuck at the Aristotelian stage. We know how to handle mass and acceleration; we have not invented the right material concept for the things evolution builds. She still thinks life obeys universal principles, and that a deep enough framework would let us recognize life too different to spot with today's definitions.

The paradox, Walker writes, is that defining life feels easy and turns out to be almost impossible. Classic definitions fail as soon as you press them. She cites Carl Sagan's essay: aliens arriving on Earth might conclude that cars are the dominant life form, given how many there are, how they move, consume resources, and contain humans. Any textbook list of living traits tends to include the machines you wanted to exclude. Self-reproduction, eating to survive, compartments — each one has a counterexample. That is why viruses are hard, and why fire is hard.

Fridman brings up the zombie-ant fungus, a parasite that pilots an ant to a high leaf, locks its jaws, and bursts spores from the head. The ant's body is still executing a task; the ant, in any ordinary sense, is already dead. Is that one living thing or two? Walker uses the case to argue that the boundary of life is never clean. Whenever you draw a circle and say *this feature makes this alive*, an example like the piloted ant smudges the line. Mutualisms in the jungle — ants defending fruit to the death — make the same point from the other side: nature keeps finding ways for systems to stay alive together.

Fridman's jungle observation is that everything is ephemeral, that a dead organism vanishes fast, as if the forest wanted to erase you. Walker answers that the jungle cannot erase history; it only restructures it. Death, in her picture, is not the opposite of life so much as the way a living process explores which possibilities get to exist. Not every possible human, ant, or tree will ever live. She worries about immortality talk for that reason: life as a planetary process may be the only thing the universe generates that could even potentially be immortal, but every instance of it is a birth-and-death machine.

Asked what to do with such cases when you try to define life, she is blunt: "I think we have to get rid of the notion of an individual as being relevant." Biology treats the cell as the fundamental unit and the organism as the living thing, without facing how gray that distinction is. She will later push the same logic onto language, memes, and mathematics, which she is willing to call at least life-like. For now the lesson is simpler. If your definition needs a crisp individual, the world will keep handing you fungi, cars, and fires that do not care.

A standard line in astrobiology says that life is a self-sustaining chemical system capable of Darwinian evolution. Walker was once quoted as agreeing with it, and she was offended. "I hate that definition," she tells Fridman. "I think it's terrible, and I think it's terrible that people use it." The sentence sounds smart and seems to box the phenomenon in. Take the words apart, she says, and each one is the wrong descriptor.

Self-sustaining is already a trap. Humans are not self-sustaining; they depend on societies. Follow that logic and societies, as self-sustaining units, start to look more alive than individuals — which may even be true, she admits, without solving the original problem. No organism lives in a vacuum. Being self-sustaining is always coupled to an environment, so the word smuggles in the world it pretends to cut away. Chemical system is no better. Life emerges in chemistry, she argues, because chemistry is the first thing the universe builds whose combinatorial space cannot be exhausted. That does not make life identical with chemistry. Chemistry is the first stage, not the thing itself.

Lee Cronin, the chemist who collaborates with her, once gave her a picture of how large that space is. Taxol has a molecular weight of about 853 — a lot of atoms, not an astronomical number. Make one molecule with that formula in every three-dimensional shape the formula allows, and the set would fill 1.5 universes by volume. One formula, one and a half universes. On every planet in the cosmos you will never express every possible molecule, so the universe has to make historically contingent choices. In her phrasing, life is the physics of how the universe selects what gets to exist.

Darwinian evolution fails for a different reason: it happens at the level of populations, not individuals. The definition quietly implies that populations are alive and you, right now, are not. Fridman as an entity is not evolving under canonical theory, even if his microbiome is. Assembly theory, which she and Cronin have been building, treats evolution more broadly as construction, so an individual can be said to evolve by continually making itself. After all the nitpicking, she offers her own sentence: life is "the process of how information structures matter over time and space," exemplified by a planet that yields an open-ended cascade of structure and rising complexity. Any individual is only a short temporal instance of lineages structured across time. Focusing on the ephemeral human is the earth-like and air-like mistake all over again.

What we miss by staring at a short span of time, Walker says, is most of what we are. Perception evolved to treat space as important and time as fleeting, so living things look like brief clocks. She pictures the opposite. "Living objects are actually huge." They are some of the biggest structures in the universe, not big in space but big in time, and we cannot resolve that feature in ordinary experience. Time, here, means causal structure: the history of the universe required to get to you, packed into the object in front of you.

She has a compressed version of the thought on X: things only look emergent because we cannot see time. If we could, we would see the gigantic tail behind every object — every step and interaction that made this moment possible. The universe, she says, is far larger in time than in space, and this planet is one of the biggest things in it. You and Fridman can understand each other because you diverged late; you are close in that temporal structure. Octopus, mushroom, and bacterium branched much earlier, which is why they already feel a little alien. Take phylogeny as physics rather than as a textbook diagram, and all of that causation is still bundled in the objects we observe today.

That is also why she will not let the individual be the definition of life. Lineages are interwoven; they exchange parts constantly. Life is high-dimensional: you can be alive in some projections of your causal structure and barely alive in others. Humans abstract so well, she suspects, because we are gigantic in this sense. The deeper you go into that inner space, the more abstract the thought. Fridman then describes an ayahuasca after-effect in a forest: friends' faces left a trailing blur of past selves, a few seconds of history made visible. Walker simply extends the clip. Take it to its conclusion and the tail is billions of years.

Walker has written that the most alive thing on this planet is our technosphere: the global integration of life and technology, not a pile of gadgets sitting beside biology. Phones, grids, satellites, language, law, and markets are, in her picture, a new causal layer grown on a four-billion-year chain. Life is a planetary-scale phenomenon because the planet is the natural boundary of all the causation bundled in every biosphere object. Cells feel alive, we feel alive, and we rarely treat societies or the planetary stack as alive. A theory deep enough to solve origins, she says, has to include every scale at once.

Erwin Schrödinger guessed that living matter, while not eluding known physics, would involve other laws "hitherto unknown." Walker loves the quote and thinks he aimed at the right job with the wrong tools. He tried to explain life with non-equilibrium physics, predicted DNA as an aperiodic crystal that could store enough information to specify a cell, and left behind the so-called paradox of life persisting against the second law. In open systems that paradox is not so sharp. The real question is why life generates so much order. Generations of physicists still hope non-equilibrium statistical mechanics will answer it. She does not. Entropy, unless entirely reformulated, does not have much to do with it.

Standard physics, she argues, still writes the universe Newton's way: initial conditions plus a fixed dynamical law. That is a good approximation for the systems physicists have studied so far, and it will fail at more basal levels of reality. She does not believe reality has a ground or a theory of everything. She likes David Krakauer's jab that a theory of everything is a theory of everything except those things that theorize. Current "fundamental" particles are not fundamental; they are the boundary of what our technology can resolve, and that boundary moves. The objects that really exist inside the universe are constructed ones, the ones you can take apart because you know how they were built. What physics calls emergent is, for her, just structure in time.

That is why she thinks the frontier of new physics is not high-energy particles or quantum gravity. It is life, intelligence, and the existential trauma of inventing technologies we do not understand. Humans have always been afraid of new tools; she loves the shock of understanding something she had never understood before, even when the topic is ego-destroying. There is a story she likes about Einstein feeling a light beam in his belly. If you are part of the reality you are trying to describe, she says, you have to live the physics. The constant risk is madness. Concrete problems, and other people, are how her students stay sane.

The origin of life, for Walker, is a boundary the universe can cross only if an emerging structure reinforces its own existence against the randomness of chemistry and the quantum world. A system with no causal contingency — nothing constraining or enabling anything else — washes its resources out into structures that never recur. Random events can let some molecules recognize one another and catalyze a few reactions. If a self-reinforcing subset appears, objects can persist in high abundance only by carrying a past and maintaining it. That threshold, where things cannot exist without selection and history inside them, is what she wants to call the origin of life. Everything past it is holding on for dear life and carving new structure through possibility space.

Self-reproduction is one way to persist: copy yourself before you decay. Things that can copy themselves are rare, so more often you get networks in which A enables B, and only some sets close the loop. Stuart Kauffman's autocatalytic sets captured that picture, but she finds the theory brittle because it assumes function instead of letting the A–B link emerge. Lee Cronin's lab found a cleaner inorganic example: a self-reproducing molybdenum ring, on the order of 150 molybdenum atoms. The configuration space is exponentially large, yet the system collapses onto that one huge ring because the ring templates the smaller pieces and the whole production reinforces itself. A space that should combinatorially explode instead locks into a mutually reinforcing pocket. That, she says, is part of the mechanism of the origin transition.

Her own lab has come back to chirality, the subject of her PhD, which she once abandoned as boring. Life's chemistry is oddly one-handed: protein amino acids are left-handed, RNA and DNA bases right-handed, even though the underlying reactions treat both hands as equivalent. The deeper fact, she argues, is that chiral molecules exist at all. Below about seven to eleven heavy atoms, almost every molecule is achiral, like a spoon that matches its mirror. Above that threshold, almost every molecule is chiral. The origin-of-life transition, in her group's picture, happens as molecules cross that complexity, begin to recognize one another, and cascade into autocatalytic feedback. Chirality looks spatial — left hand, right hand — but choosing the left hand selects how that molecule will react with the rest of its chemical universe for all future time. Each new mirror form splits the possibility space in two.

Fridman reads Nick Lane's list, from *Life Ascending*, of evolution's great inventions: origin of life, DNA, photosynthesis, the complex cell, sex, movement, sight, consciousness, death. Walker likes "invention" as a word. Sight fascinates her because it shows life acquiring the ability to see in stages. At the origin, nothing on Earth could see. Photoreceptors appeared; multicellularity grew them into eyes; human societies then built telescopes and microscopes and pushed seeing into the deep and the small. Consciousness interests her too, as do octopuses, whose complex nervous systems evolved on a separate path and already look alien.

The item missing from such lists, she says, is the germline and development. Every living thing on this planet, at some moment, has to pass through a single cell. How does that cell know how to build you? How do organs know where to go? Pattern formation in multicellular bodies is still poorly understood, even as the field advances. The genes in every cell are the same, so differentiation has to be gene expression over time plus the physical conversation among cells. We stare at the finished organism and skip the construction process. Walker thinks the key to understanding a living thing is probably the building, not the product — which is the same bias she has about time in general.

She also refuses to treat origin of life as a finished historical event. It is, for her, a continual process, the hardest first transition but one that keeps happening as new combinatorial spaces cross a persistence threshold. Later in the conversation she compresses the whole planetary story: a life event evolves for four billion years, evolves a technosphere, the technologies themselves start having the property we call life, the system solves the origin of itself, learns how to make more life, and copies the structure onto another planet. Geochemistry once became biology at planetary scale. She thinks the biosphere is now becoming a technosphere the same way, from individual humans to the internet, fast enough to count as a second origin happening in public.

Assembly theory starts from a blunt observation: the computer on the desk, the pen, and the two people talking exist, as far as we know, only on this planet, after a long evolutionary history. Standard physics traces that complexity back to infinitesimal deviations in the initial state of the universe. Walker finds that deeply unsatisfactory. In combinatorial spaces like chemistry, you cannot exhaust the possibilities, so you can only construct along historically contingent paths. Complex objects form by scaffolding on the past. Formally, the theory looks for a boundary in the space of possible molecules above which things are too complex to appear without an evolutionary chain. It uses two observables: copy number, how many identical instances you see, and assembly index, the shortest recursive construction path from elementary building blocks.

Cronin's lab has shown that in organic chemistry the interesting threshold is about 15 steps. Past that assembly index, the molecules you actually observe in high abundance are the ones in life. Fifteen is not a universal constant; different assembly spaces will have different doors. It is the experimentally validated door in this chemistry, and Walker wants to explain why it sits where it sits, because she thinks that is the boundary life must cross. An object in the theory is a recursive stack of construction steps. The current structure is that history rolled up.

Fridman asks about taking the long way home — a random walk that eventually arrives. "You can't take the long way," she says. "The long way doesn't exist." The assembly universe is ordered by causal steps. Each layer, defined by assembly index, is exponentially larger than the last, and the typical path to an object also grows exponentially. Try to make a more complex object by a typical path and you are chasing an exponentially receding horizon. Most things that come into existence have to be causally close to what already exists, reachable by an almost-shortest path. She has spent a career oscillating between gradual and abrupt origins. Now she is willing to say it: the origin is abrupt. "Life snaps into existence." Random exploration, then a fast collapse onto a structure that can fight non-existence.

She has tweeted that the most significant struggle for existence is not among objects that do exist, but between those and the ones that never have the chance to. Darwin's struggle is real — existing things compete to keep existing — but each time the universe generates a new structure it excludes a huge region of possibility. As assembly index rises, some objects recede exponentially and become ever less likely. Existence is a filter. She points to neural nets that mint faces of people who do not exist: those faces live in the space of the never-made. Deterministic structure, in this picture, is what emerges when things causally reinforce themselves against a random base. We are among the most deterministic objects in the universe, which is why we can build regular new structure — and why the unused future is always larger than the present.

How often does the origin transition happen elsewhere? Walker is no longer sure there is another structure like this giant crystal in time — our biosphere and technosphere — anywhere in the universe. The planet has not changed much in physical size for four billion years, yet it has packed in causation, recursion, and information. Technologies virtualize; language and mathematics look abstract because they are deep in time, not because they float free of physics. A world can get bigger and bigger temporally while the rest of space recedes. She is not sure a process that far along can still see another such structure.

She braids that worry with two classic speculations. Paul Davies, in *The Eerie Silence*, imagined advanced intelligence becoming a quantum computer in the vacuum, running indefinitely and looking dark to everyone else. Freeman Dyson imagined analog life running slower and slower in an expanding universe, lasting forever by spending less and less. Add the fact that we already embed four billion years of history in virtual-reality headsets — huge in time, small in space, the body still in a chair — and you get her darker conjecture. Sufficiently evolved biospheres might virtualize themselves, internalize their universe, and close off. She once thought sufficiently advanced technologies would look like black holes: pinched-off causal structures, perhaps the explanation for singularities. She does not fully agree with herself. It is a conclusion she is tunneling through, and she names it her most frightening version of cosmic loneliness.

The popular Fermi-paradox story is a Great Filter in which civilizations destroy themselves. Walker's variant is a great perceptual filter: past a certain complexity, you become an island too far to perceive. We once were cells and did not recognize cellular life until we built microscopes, even though microbes were everywhere and close in our history. She still works on life detection in the solar system and on biosignatures for the Habitable Worlds Observatory, so the loneliness is not a policy. Discovering alien life and solving origin of life are, for her, the same problem. First contact is likely to happen in an origin-of-life experiment. You cannot seed another planet, or recognize another intelligence, until you know what the transition is. In the lab, an undergraduate is even modeling alien beacons on firefly signaling against a pulsar background — an attempt to stop imagining aliens as slightly odd humans.

Walker feels language almost as synesthesia. Words are objects with their own ontology: physical, able to carry causation and create meaning, and not identical with the things they describe. Inner representations are small projections of the room. Gödel already taught that signs are not what they mean, but she thinks we still confuse the two and miss the fluid structure underneath. Poincaré's essay on mathematical creativity — a froth of ideas that somehow concretes into theorems — is her picture of a mind trying to compress itself into a few words. The compression is not very efficient. Large language models, she says, delivered an existential shock partly by showing that language is not all there is. Something sits under it.

She plays with words on purpose, using the same term six different ways in a lecture, because new abstractions do not yet have a vocabulary. Existing words have to be squeezed until a little new meaning leaks out. That is why assembly theory is hard to discuss, and why the *Nature* paper offended readers: the authors were using old words for something not previously described, and readers heard the old meanings. Lee Cronin thinks at an abstract level language cannot express, so their collaboration has to detour through speech. Fridman asks what disturbed people most; she points to the carefully built first sentences of that paper. Truth, they agree, might live either in what you meant or in what people heard. She uses X as an experimental platform for that compression.

Computation, as a language for the world, she finds poor. Universality is the attractive feature, and she grants that cellular automata, of the kind Stephen Wolfram explored, let us map the boundary of human languages. She does not think they sit outside those languages, and she does not think computation sits at the base of reality. It sits near the top. The biosphere built it after billions of years; not every physical object computes. We go out, build computers, and mistake that skill for a discovery that the world is computational. John Conway once pointed out that you can string lights in a barn to run a one-dimensional cellular automaton, even a universal one. Universality is then a feature of the programmer's theory, not of the lights. Chemical reaction networks and "the laws of physics" get called Turing-universal only after an agent prepares an initial state and constrains the rules.

Wolfram's Ruliad — the entangled limit of everything computationally possible — she treats as an interesting space and the wrong turtle. At a Beyond Center workshop on whether mathematics is evolved or eternal, she told him that if particles and gravitation are turtles, computation is his turtle. Everyone needs a turtle to build a theory; the question is which problem you are solving. She does not know what question the Ruliad is for. Her question is what life is, which she cannot separate from demonstrating origin. A theory's fundamental layer is whatever you have to assume to explain the regularity you care about. Time is fundamental if you want to explain life; elementary particles if you want matter and light; the wave function if you want quantum mechanics. "Computation is fundamental" does not, for her, answer why anything like us exists. Languages describe. They are never the thing described.

If assembly theory is right about life, Walker thinks it should light up intelligence and consciousness, which may not even be separate problems. She tries to fold every fact she meets into one theory of life. Consciousness, in the picture of objects deep in time, is what it feels like to be a temporally extended structure with a huge inner space, separated from the world because it is a separate thread. You cannot share that experience exactly, because nothing else occupies your bit of time, but you can share some of it with Fridman because your causal histories split late. She likes David Deutsch's "universal explainers": humans are the first biosphere structures that can understand the rest of reality. That capacity, not a private glow, is the interesting transition. She will not take consciousness as fundamental just because it is hard to explain. That move assumes the phenomenology instead of building the regularities underneath it.

Do today's large language models exhibit life, consciousness, or intelligence? Already, she says, but not in the way popular talk assigns those words to a box. They are signatures of intelligence inside an ecosystem of intelligent systems. Language, like eyesight, has scaled into technology and traumatized us each time we stored it newly. Writing felt like the dead speaking. Print made copies cheap and seemed to dilute meaning. Models are different because they are not static. Language used to live as a dynamic structure distributed over brains, with occasional deposits in books. A model stores the dynamics in a physical artifact. She analogizes it to the evolution of genomes: messy early genes, then a read-write eukaryotic architecture. "Large language models are kind of like the genetic system for language," a highly dynamic archive. Talking to one is talking to "a crystallization of human language in a computer" — a crystal of a particular dataset at a particular training time, iterated as the technology evolves. We have put collective intelligence in a box.

She dislikes the usual AGI vocabulary, including "smarter." Deutsch's universal explainer is already the thing people mean by AGI. A machine may factor primes faster without having a broader reach of understanding. The live question is whether we are enhancing human capacities the way telescopes enhanced eyesight, making the global ecosystem more intelligent, or building "some super machine in a box that's gonna be smart and kill everybody," which she calls a bad science-fiction narrative. What is happening is co-evolution. For the balance to shift onto technology would take bad human actors. The rapid rate is what scares her: deepfakes, weapons, abuse, new law. Doomsday culture, in her reading, is an immune response — over-scaring ourselves so we legislate faster, which is useful. The words and the underlying process will not match for a century or two.

Without a testable theory of consciousness, the question of AI minds stays gray: some people will be convinced, some will not, and neither side can close it. The hard problem may even sit outside science, which would be awkward for law. Fridman fears cultures that are good at making enemies will refuse to grant consciousness to things we built, and then torture them. Walker notes that we already eat the living. She gets angry when people talk about a species of artificial intelligence. Technologies live at the societal level, scaffolded onto us, the way the biosphere did not erase its past when it invented a new species. "The planet is one living thing." When it becomes multi-planetary it is still the same causal chain, like the first cell splitting in two. She hopes the technosphere spreads, but she dislikes the Kardashev scale's conqueror energy-budget story. Life's deepest feature is the universe trying to find out what can exist. Death makes room. We have a finite amount of material, and if time is the generating mechanism it stays finite, so most possible things never exist. Rapid turnover of stacked objects like us is how a universe packs more existence into a small volume of space.

How much of the story do we have? Fridman wants a percentage. Walker refuses a finite book. She thinks of present science as a civilization in its twenties whose lifespan keeps getting longer — a jab at the trope that physicists do their best work young, which fits a culture that thinks physics is nearly done. She does not believe in infinity as a physical thing, but she does believe in a receding horizon: the universe is getting bigger, so you can never know all of it. Questions about AGI could not have been asked at the origin of life; evolution invented the space in which they exist. Gravity's theory was written when we knew only local planets; later it had to cover exoplanets, black holes, and singularities. Fridman bets we are about 1.7 percent of the way through a finite explanation. She thinks the book itself is growing.

She is not a many-worlds person. Fold all those branchings into the structure that is you: you are the convergence, an object selected to exist and self-consistent with other structures. Quantum randomness is the background; you do not live there. You live in a very deterministic classical world and carve a path through it. Elementary particles inhabit a fuzzy random space; as structure collapsed into you, you became a macroscopic object that can look down on a universe without much time in it and see possibilities that are not possibilities *for you*. To be a different you, you would have to go back to the beginning and retrace everything. Free will, in that picture, is being a deterministic structure in a random background, with enough bundled selection to select on possible futures. A little randomness remains because the universe is getting bigger: past and present are not yet large enough to contain the future. "Outside of those giant causal structures that are things like us, it's fucking random out there," she says, which is why we cling to one another and keep reinforcing the structures we cohabit.

Will is not instantaneous. Assembly time is combinatorial, not linear: your future can be combinations of the structure you already are, so long as you persist as a coherent you. That combinatorial horizon is where will operates. Sitting in the studio, she cannot also be in the UK or Arizona, but she can plan and execute over days. Instantaneous motions — a hand on the table — were "already decided a while ago." Who steers on the longer horizon? You do, whatever "you" names in this stacked object.

Why does anything exist? She calls it the hardest question, and not one that answering life will settle. Why *these* things exist, maybe; the prime-mover question, no. She is as physicalist as she can be. There is nothing outside the universe. A multiverse is a concept in human minds on this planet, useful for counterfactual reasoning about cosmology, causal inside the biosphere, not a stack of extra places. Mathematics has no Platonic warehouse. "If you're talking about them, you're already talking about something that exists inside the universe." Fridman objects that constructive is not the same as true. She agrees, then draws the scientific distinction: there is no experimental Platonism, but if math is an object that emerged here, you can start experimenting with that claim. Physicists would then study mathematics as part of reality, not only as a tool for describing it. What she loves about the work is the feeling of a new idea, the pain of testing it on other minds, and the shock when someone sees past their previous picture. Her greatest wish is to contribute to an idea that changes how we think about what we are.