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Home > Solutions > Scientists > Walker
Sara Imari Walker
(1946-)
Sara Imari Walker is an American theoretical physicist and astrobiologist with research interests in the origins of life, astrobiology, physics of life, emergence, complex and dynamical systems, and artificial life. Walker is deputy director of the Beyond Center for Fundamental Concepts in Science at Arizona State University. The director is Paul Davies.
Walker is the associate director of the ASU-SFI Center for Biosocial Complex Systems and an associate professor at ASU. She is a co-founder of the astrobiology social network SAGANet, and on the board of directors for Blue Marble Space, a nonprofit education and science organization.
With her ASU colleague Leroy (Lee ) Cronin, Walker is co-developer of Assembly Theory, a metric for complex systems as the number of specific steps needed to assemble the system. Beyond the usual physical and chemical steps, assembly theory looks for evidence of natural selection. Assembly Theory was inspired by John von Neumann's idea of a Universal Constructor.
In her 2024 book, Life As No One Knows It: The Physics of Life's Emergence she writes...
Johnny made an analogy between the idea of computation, which is abstract, and what he called construction, which he intended to describe physical operations, rather than computational ones... Inspired by how a universal Turing machine can in principle compute any computable function, Johnny devised a maniac of an idea for another abstract machine he called the universal constructor. The universal constructor (UC) is an abstract device that can in principle assemble any constructable object
Life as No One Knows It, p.208
Assembly theory conceptualizes objects not as point particles, but as entities defined by their possible formation histories. This allows objects to show evidence of selection, within well-defined boundaries of individuals or selected units. Combinatorial objects are important in chemistry, biology and technology, in which most objects of interest (if not all) are hierarchical modular structures. For any object an 'assembly space' can be defined as all recursively assembled pathways that produce this object. The 'assembly index' is the number of steps on a shortest path producing the object. For such shortest path, the assembly space captures the minimal memory, in terms of the minimal number of operations necessary to construct an object based on objects that could have existed in its past. The assembly is defined as "the total amount of selection necessary to produce an ensemble of observed objects."
Life as No One Knows It, ibid.
In her book, Walker says that scientific measurements are assumed to access the physical properties of real objects, but what we really measure is just the interaction with the measurement devices themselves. She calls this the "hard problem of matter."
You might naïvely think that the electron really does have a mass, a charge, and a spin, and that these are intrinsic properties of the electron. But these properties can also be considered to merely describe how electrons interact with certain measuring devices. For example, when an electron interacts with scientific instruments that measure mass, they provide a reliable value we identify as the electron mass. When these same objects interact with devices that measure charge, they reliably report a negative charge. Spin can be inferred in a number of ways, including by measuring magnetic properties of particles or their interactions with other particles. We have no way to know what other properties electrons may have that we haven’t measured or been able to infer by measurement of their interactions with other objects. Nor can we know if electrons still have the properties we know about when they are not being measured. For practical purposes it may be perfectly reasonable to assume they do, but we can never completely confirm it.
Life as No One Knows It: The Physics of Life's Emergence (p. 41)
Measurements of the position and the time of an event are properties of our rulers and our clocks, not space and time themselves. Where recent books like Julian Barbour's The End of Time and Janus Point claim "time is an illusion," they are recognizing Walker's point that we only know about the measurement events, nothing about time itself.
Walker proposes two other "hard problems" (developed with her Ph.D. thesis adviser Cronin), the hard problem of life (with Davies) and the hard problem of consciousness (this the original idea of David Chalmers).
Walker's most recent work stresses the fundamental importance of information, including its causal power ( mind over matter?). She is in sync with Paul Davies, whose recent book Demon in the Machine concludes with...
The search for a ‘missing link’ that can join non-life and life in a unitary framework has led to an entirely new scientific field at the interface of biology, physics, computing and mathematics. It is a field ripe with promise not only for finally explaining life but in opening the way to applications that will transform nanotechnology and lead to sweeping advances in medicine. The unifying concept that underlies this transformation is information, not in its prosaic everyday sense but as an abstract quantity which, like energy, has the ability to animate matter. Patterns of information flow can literally take on a life of their own, surging through cells, swirling around brains and networking across ecosystems and societies, displaying their own systematic dynamics. It is from this rich and complex ferment of information that the concept of agency emerges, with its links to consciousness, free will and other vexing puzzles. It is here, in the way living systems arrange information into organized patterns, that the distinctive order of life emerges from the chaos of the molecular realm.
The Demon in the Machine, p.2
And in their joint article for Interface, a Journal of the Royal Society, Davies and Walker wrote..,
The central position of information in biology is not itself
especially new or radical. What is often sidestepped,
however, is the fact that in biological systems information is
not merely a way to label states, but a property of the
system. To be explicit, biological information is distinctive
because it possesses a type of causal efficacy —it
is the information that determines the current state and
hence the dynamics (and therefore also the future state(s)).
In this paper, we postulate that it is the transition to
context-dependent causation—mediated by the onset of
information control—that is the key defining characteristic
of life. We therefore identify the transition from non-life to
life with a fundamental shift in the causal structure of the
system, specifically a transition to a state in which algorithmic
information gains direct, context-dependent, causal efficacy
over matter. We now turn to the question of how all this
came about. How did information first gain causal purchase
over certain complex systems that we now call living
organisms?
In Walker's Long Now Foundation talk on YouTube " An Informational Theory of Life.," she says...
[T]he question we need to ask is how does the universe generate information and complexity when it has none. That is the origin of life and I think that question has some really interesting consequences which we're going to explore ...
Long Now (14:06)
[W]e have been constructed on this planet across four billion years and we are the reason we're not reducible to our atoms.
Why that fundamental layer doesn't describe us is because when we do that we remove all of that time and causation. And that's actually where we
exist. That's where the causal structure is that is us is embedded in this history of time. And so I think to
understand life we have to think about time in a real physical way. And actually evolved objects, one of the
reasons they're so perplexing for us to see is because they're bigger in time than space. Imagine putting four billion years in this tiny volume of my brain.
Long Now (40:26)
Walker calls "the causal structure that is us...embedded in this history of time"...in our brains our "lineage." Of course that history itself is not in our brains. but our brains are the evolution and development product of that history, so "lineage" is apt.
More from the Journal of Royal Society article -
Although it is notoriously hard to identify precisely what makes life so distinctive and remarkable, there is general agreement that its informational aspect is one key property, and perhaps the key property . The manner in which information flows through and between cells and sub-cellular structures is quite unlike anything else observed in nature. If life is more than just complex chemistry, its unique informational management properties may be the crucial indicator of this distinction.
Unfortunately, the way that information operates in biology is not easily characterized. While standard information theoretic measures, such as Shannon information, have proved useful, biological information has an additional quality which may roughly be called ‘functionality’—or—‘contextuality’—that sets it apart from a collection of mere bits as characterized by its Shannon information content. The information content of DNA, for example, is usually defined by the Shannon (sequential) measure. However, the genome is only a small part of the story. DNA is not a blueprint for an organism: no information is actively processed by DNA alone. Rather, DNA is a (mostly) passive repository for transcription of stored data into RNA, some (but by no means all) of which goes on to be translated into proteins.The biologically relevant information stored in DNA therefore has very little to do with its specific chemical nature (beyond the fact that it is a digital linear polymer). The geneticmaterial could just as easily be a different variety of nucleic acid (or a different molecule altogether), as recently experimentally confirmed. It is the functionality ofthe expressed RNAs and proteins—not the bits—that is biologically important.
Journal of the Royal Society Interface, ibid.
In a most powerful piece of science writing, Walker describes the long struggle of Albert Einstein's "lineage" to give birth to general relativity...
Your lineage takes pencil to paper. Scribbling notes, you—as you are in 1910—are fiercely trying to elucidate how it is that gravity can work in situations where the speed of light is constant. You are frustrated and have many conversations with your collaborator, Marcel Grossmann, a brilliant mathematician and friend. Still, it takes you nearly a decade to formalize your intuition. The idea was there all along. However, translating it so others among you could readily see it—and more importantly test it—was far more challenging than having the intuition itself. Even so, you persevered and did eventually succeed in formalizing your thoughts. Your theory was validated three years later, after the intervention of a global war. That experiment was led by Sir Arthur Eddington, who had the stroke of insight to use the solar eclipse to test your predictions. The eclipse obscured the light of our Sun and allowed direct observation of the bending of the light from distant stars around it, just as your theory had predicted we should observe it.
It would take decades after your death for us to later prove other predictions your theory made. A key implication of your intuition was the existence of gravitational waves—ripples in the fabric of the space-time you invented to describe how our reality works. The first compelling evidence that your gravitational waves might really be there came in 1978 when two humans you would never know, but who knew of you, named Russell Hulse and Joseph Taylor, observed a binary neutron star system and inferred that the missing energy must be dissipated, exactly as you predicted, by gravitational waves. Like you, they were awarded the Nobel Prize; theirs was for confirming some of your predictions. However, their evidence was not direct. Our quest continued to validate what you intuited was there. In the 1990s, roughly forty years after you left us, we started to build interferometers to directly test your predictions. These devices needed to be so precise it took us decades to build them, even with our technology as it exists now, which is exponentially more advanced than anything you had ever witnessed in your lifetime. But we, too, ultimately persevered. On September 14, 2015, we made first contact with the phenomenon your intuition told us existed. We do not have to tell you this is one hundred years after you formalized your ideas! Did you and Marcel think it would take so long? Perhaps you would think this was quick.
The measurement was among the most precise made in our history and indicated the presence of two colliding black holes with masses roughly thirty and thirty-five times our own Sun’s mass. The ring-down of this merger on our detectors, the symphony of a universe describing itself, lasted 0.2 seconds and was sung on our instruments roughly 1.4 billion years after it happened. Albert Einstein was not born yet, and neither were we: the lineage of life we share was not even multicellular then. His theory was such that it predicted features of events that happened in his past, which could be confirmed by us only in his future.
Our universe is such that this is possible. By observing what is happening now, in the present moment, we can infer what has happened already, even billions of years in the past. We can predict and even cause what happens next. We can write down theories that describe how our reality works, and then test them against observations and experiments. Theoretical physics, among human endeavors, has been stunningly successful at this, in no small part because of minds asking the hard questions about the nature of reality. These are minds like that of Albert Einstein, who painted our current understanding of the nature of space-time with his general theory of relativity that led to the prediction of gravitational waves.
Albert was once a part of the lineage that is human, with all that entails. He enjoyed playing violin, he was fiercely curious and intrepid in his pursuit of understanding reality, he believed education should be accessible to all, and he advocated for an end to systemic racism in the United States—often visiting historically black universities. While living, he played a pivotal role in our current understanding of gravity, space, and time, and even laid some of the foundations for quantum mechanics. He also made significant contributions to philosophy, morality, and ethics. Like every human, he also had his flaws—historians have written about how he was a mostly absent father to his two sons and had multipleaffairs over the course of his two marriages. All these details of Albert’s life are fiercely important, as are the details of any life.
They, however, have less relevance to the fundamental physics of what it is to be alive, because we are not looking at what is specific to us as individuals, but what is universal to us all. What part of you is also a part of Albert Einstein? What part is also a feature of the very first life on Earth? By all I really mean all—not just the instances that we exist as today, but what we have been over the last several billion years and what we will become. We are each just one temporary instance of life on this planet—a pattern of information structuring matter across billions of years.
To get to a universal understanding of what life is, we must do what theoretical physics does best—abstract the nature of reality to its most fundamental and powerful explanations. Universal includes us humans, our ancestors—from microbes to multicellular life—artificial life, artificial intelligence, what comes after those, and even aliens.
The most important thing about Albert Einstein for our purposes is that he was not only human, and a theoretical physicist to boot, but also an example of a broader set of phenomena that exists in our universe that we have yet to understand—like you and I, he was once alive. The theories he produced, that tell us so much about the nature of reality, would not exist if there was not a part of the universe—life—that can comprehend the rest.
What we are about to embark on next is a vision of a future where we understand what we are, not based on theoretical physics as it was—the kind of physics Albert Einstein devised, with his predecessors Isaac Newton and James Clerk Maxwell, his contemporaries Marie Curie and Emmy Noether, and his successors Richard Feynman, Steven Weinberg, and Frank Wilczek, among many others. They all studied the nonliving universe: the universe without us.
Instead, we are now stepping into a possible history of what physics will be if we are to explain the most intimate of physical phenomena—the nature of ourselves. You are a part of that lineage, too, and it is only by accruing knowledge over our lineages that we can understand how it is that we came to be.
Life as No One Knows It: The Physics of Life's Emergence (pp. 77-80)
Normal | Teacher | Scholar
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