Sarah Walker
speaker
510 appearances
1 recordings
1 series
first heard Jun 2024
last heard Jun 2024
Sarah Walker’s voice in public audio — every appearance, attributed to the second.
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is a real feature that we should take seriously as one that's deeply embedded in the laws of physics and the structure of the universe that we live in. Standard physics would say that all of that complexity traces back to the infinitesimal deviations and like the initial state of the universe that there was some order there.
I find that deeply unsatisfactory and what assembly theory says that's very different is that The universe is basically constructing itself. And when you get to these common editorial spaces like chemistry, where the space of possibilities is too large to exhaust them all, you can only construct things along historically contingent paths.
Like you basically have causal chains of events that happen. to allow other things to come into existence. And that this is the way that complex objects get formed is basically on scaffolding on the past history of objects, making more complex objects, making more complex objects.
That idea in itself is easy to state and simple, but it has some really radical implications as far as what you think objects is the nature of the physics that would describe life. And so what assembly theory does formally is try to measure the boundary in the space of all things that, you know, chemically could exist, for example, like all possible molecules.
Where is the boundary above which we should say these things are too complex to happen outside of an evolutionary chain of events, outside of selection? And we formalized that with two observables. One of them is the copy number of the object. So how many of the object did you observe? And the second one is what's the minimal number of recursive steps to make it?
So if you start from elementary building blocks like bonds for molecules and you put them together and then you take things you've made already and build up to the object, what's the shortest number of steps you had to take? And what Lee's been able to show in the lab with his team is that for organic chemistry, it's about 15 steps.
And then you only see molecules that, you know, the only molecules that we observe that are past that threshold are ones that are in life. And in fact, one of the things I'm trying to do with this idea of like trying to actually quantify the origin of life as a transition is in like a phase transition assembly theory is actually be able to explain why that boundary is where it is.
Because I think that's actually the boundary that life must cross. So the idea of going back to this thing we were talking about before about these structures that can reinforce their own existence and move past that boundary, 15 seems to be that boundary in chemical space. It's not a universal number. It will be different for different assembly spaces.
But that's what we've experimentally validated so far.
It's 15 or so for the experimental data, yeah.
That's when you have to have that feature in order to observe molecules in high abundance in that space.
Recursive. Yeah. So you can think of objects in assembly theory as basically recursive stacks of the construction steps to build them. So they're like, it's like you take this step and then you make this object and you make it this object and make this object and then you get up to the final object. But that object is all of that history rolled up into the current structure.
You can't take the long way.
The long way doesn't exist.
No, if you look at objects and you, so we define something we call the assembly universe and the assembly universe is ordered in time. It's actually ordered in the causation, the number of steps to produce an object. And so all objects in the universe are in some sense exist at a layer that's defined by their assembly index. And the size of each layer is growing exponentially.
So what you're talking about, if you want to look at the long way of getting to an object, as I'm increasing the assembly index of an object, I'm moving deeper and deeper into an exponentially growing space. And it's actually also the case that the sort of typical path to get to that object is also exponentially growing with respect to the assembly index.
And so if you want to try to make a more and more complex object and you want to do it by a typical path, that's actually an exponentially receding horizon. And so most objects that come into existence have to be causally very similar to the things that exist because they're close by in that space and they can actually get to it by an almost shortest path for that object.
By a lot.
Yeah, imagine yourself- A copy number greater than one. Yeah, I mean, basically we live, the more complex we get, we live in a space that is growing more exponentially large. And the ways of getting to objects in the space are also growing exponentially large.
And so we're this kind of recursively, like, stacked structure of, like, all of these objects that are clinging on to each other for existence. And then they, like, grab something else and are allowed, like, able to bring that thing into existence because it's kind of similar to them.
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