Assembly Theory and Why Existence Is Rare: Sara Imari Walker on Measuring Life

Walker opens with metrology, the science of measurement, a field she says she encountered only recently. She likes to challenge herself by immersing herself in unfamiliar fields. She admits that although she trained as a physicist, she never took a course on what a measurement actually is, and she found the subject "incredibly complex" once she looked into it.

Her current view is that measurement is how we take abstract ideas and map them onto the physical world. We use devices that let us probe the world outside our own minds and look for structural regularities. That is why she considers measurement so important. If a theory does not correspond to a measurement, or if an abstraction cannot be embedded in one, there is no way to test it.

Walker says she came to origin-of-life research reluctantly as a PhD student. She originally wanted to be a cosmologist, which she thought was "very romantic." She changed her mind when she realized that nobody had a conceptual framing for how life begins. That absence of a framework is what made the problem, in her words, "the love of my life" conceptually and intellectually.

At that early stage she believed researchers might come up with theories about the origin of life but would never be able to test them. Given her emphasis on measurement, that was a serious limitation.

The situation changed when she learned that the chemist Lee Cronin was working on the same problem from a very different angle. According to Walker, Cronin objected to how origin-of-life chemistry was usually done because experimenters built too much design into their experiments. Researchers looked for molecular structures found on Earth today and assumed they were relevant to the origin of life as a general process in the universe. Cronin instead wanted to study how unconstrained chemical systems generate complexity without design imposed by the experimenter or anything else.

To do this, Walker says, Cronin is building robots in his lab to explore chemistry that is "as messy as possible." In his view, the messier the better, because he wants to see life emerge in his lab. That goal raised a practical question: if life does emerge there, how would he measure it? Walker presents this question as the foundation of Assembly Theory.

Walker states the central conjecture directly. Life is the only mechanism the universe has for generating complexity. She draws several strong conclusions from this.

First, there are no Boltzmann brains. Objects like the people in the room with her do not spontaneously fluctuate into existence. Second, there is no multiverse of possibilities in which other copies of us exist. She tells the audience that they exist nowhere else in the universe. They exist here, she says, because four billion years of history on this planet were needed to construct them.

She acknowledges that this framing runs "very contrary to current physics." In her description, current physics holds that anything can exist anywhere given enough possibilities. There might be another you somewhere at very low probability, or you might be a brain that fluctuated into existence with experiences that are not real. Walker says none of these ideas are consistent with the paradigm she and her collaborators are building.

Walker explains why she rejects those possibilities. In her team's view, complexity arises through evolution and selection. The space of possible objects is so large, and the universe so small by comparison with everything it could create, that most possible things never come into being. On this account existence is special. Anything that gets to exist has been produced by a specific historical process rather than by chance.

She ends the excerpt on that note. When the audience reacts with enthusiasm, she responds by saying she also likes existing, and that it is great.