Walker, Sara & Cronin, Lee
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Walker and Cronin push back on Neil deGrasse Tyson's tweeted skepticism that advanced aliens would care about humans, arguing curiosity about "phenomena they are" (i.e., other examples of life and intelligence) is likely a near-universal trait, and that optimism rather than conflict is probably the more successful evolutionary strategy for any civilization that manages to expand into space. The conversation's core is an extended explanation of assembly theory, the framework the two have been developing together: the idea that molecules (and, they argue, any complex object) can be understood by the shortest possible causal history — the "assembly index" — required to build them from elementary components, and that objects whose assembly index is sufficiently high could only plausibly have arisen through a memory-guided, selective process rather than random chance. They describe having experimentally validated this using mass spectrometry, arguing it gives a measurable, domain-general signature of "life-like" processes that doesn't depend on knowing what alien biochemistry looks like.
Central to their shared framework is the idea that life is "how information structures matter across space and time" (Walker) or the transition point where the universe begins accruing causally actionable memory (Cronin), and that this reframes the traditional physics of timeless, low-memory objects as a special case within a broader theory where time and causal history are physically real properties of objects, not just an external measuring device. They argue this dissolves the classic "invented vs. discovered" debate about mathematics (mathematics is invented, but becomes more "discovered"-feeling the less memory/history is required to produce it) and offers a new way to think about goal-directed behavior and free will: an entity has something like a goal or intention if a symmetry-breaking difference in its present state can be attributed to differences in a represented future outcome — illustrated via a thought experiment (inspired by Daniel Dennett's essay on free will and determinism) contrasting a skier deliberately navigating a hill with a rock passively rolling down it.
On the search for extraterrestrial life, both argue detection will likely require inventing new instruments analogous to microscopes or gravitational-wave detectors — that failure to find aliens so far may reflect a "perceptual filter" (lacking the right tools) rather than evidence of a genuine "Great Filter" catastrophe, and that Walker's research program aims to build laboratory "origin of life engines" (large-scale, automated combinatorial chemistry experiments, building on Cronin's "Chemputer" robotic chemistry platform) to bound the probability of abiogenesis empirically, similar to proton-decay-search experiments in particle physics, rather than trying to simulate it computationally (which they argue is likely intractable, since an object's assembly space effectively is its causal history and can't be cheaply reconstructed). They disagree on one specific point: Cronin initially floated the idea that a single planet could host two independent, non-interacting origins of life (two separate biospheres coexisting), while Walker argues any two lineages interacting on one planet would rapidly "co-mingle" their causal histories into a single, extended definition of "life" as a planetary-scale phenomenon — a disagreement Cronin says he ultimately came around to conceding.
They discuss consciousness and imagination as, in their view, the most distinctively human transition beyond ordinary biological memory: not just remembering past states of the universe but imagining and then physically realizing states that have never existed. Walker frames this as deeply tied to free will and creativity, arguing that if the universe is generating genuine novelty (not just replaying a pre-written "big book" of all possible states), agency and imagination become physically real, causally efficacious phenomena rather than illusions layered on top of deterministic physics. Discussing robots and AI (including reactions to encountering a humanoid robot and a robotic dolphin at a conference), they debate whether current large language models like GPT-3 are near creating genuine "novelty" or are simply increasingly convincing pattern-matchers ("fooling" rather than "understanding"); Cronin argues an AI would need an architecture capable of generating true novelty — something like an internal "soul" or reference — to be more than an emulation, while Walker is more open to graded, less privileged notions of what counts as sufficiently novel or conscious.
On mortality and identity, Walker argues that trying to preserve an individual permanently misses the more meaningful goal of maximizing one's lasting causal impact (citing Einstein as an example of someone whose "echoes" persist long after the physical individual), and states that a perfect atomic copy of a person would not truly be that person because the causal history — not just the current configuration — is constitutive of identity. Both describe formative moments of personal risk-taking and mentorship (Walker recounting her divorce as the scariest, most personally clarifying decision of her life, and both crediting generous early mentors — Paul Davies for Walker — with encouraging them to pursue "unanswerable" questions), and close by locating their sense of hope in the creativity and open-ended imagination of children and of "wild minds" generally, framing the human species as still early and unformed — artists who don't yet know what they're painting.
Excerpts
On what assembly theory claims an object's "assembly index" reveals about the universe's history:
“The assembly index is the shortest path in that space — the minimal amount of history that the universe has to undergo in order to assemble that particular object... if that minimal number of steps was sufficiently large, it would indicate that you required a machine or a system that had information about how to assemble that specific object.”
On why they argue the "Great Filter" is a perceptual problem rather than evidence of civilizational collapse:
“I think it's not a physical filter... I think it's literally we don't have the technology to see them. You can see that with microscopes — we didn't know there were microbes on this table... for thousands of years... maybe that's the key technology that's going to allow us to see things like us and see the universe in a totally different way.”
On why Walker argues a perfect atomic copy of a person would not be that person:
“It is not possible to copy somebody unless you copy the causal history also... you can't have two identical [people] — even if we could copy all of your molecules and all their positions, the other you would be there, and you'd have a different position in space; you're distinguishable.”