What is the Adjacent Possible?

Why our world is not predictable, and perhaps also why it should not be.

Supracritical soup. A rich minestrone.
The adjacent possible is a kind of shadow future, hovering on the edges of the present state of things, a map of all the ways in which the present can reinvent itself. As Steven [Weinberg] notes, the adjacent possible “captures both the limits and the creative potential of change and innovation.” — Eddie Smith (2010)

The “adjacent possible” is the most salient, most shared and perhaps most important of a cacophony of colorful metaphors about biology, information, and networks offered us by Stuart Kauffman in his seminal “At Home in the Universe”. Kauffman is an American theoretical biologist whose work on the mathematics of boolean networks and the biology of genomic regulatory networks in practice has defined our understanding of both the possible origins of life and of the contemporary dynamics of complex adaptive systems, such as the biosphere and the econosphere at scale. So what is the adjacent possible?

A Kind of Shadow Future

Eddie Smith tells us that the “adjacent possible” really is just that. A shadowy inverse of history, exponential in its possibilities the further into the future we go, yet intelligently curtailed at every step by the limitations of the present.

The Beads in the Jaguar

Because I simply could not say it better, let’s take a moment to look at what Kauffman has to say about the adjacent possible of all known Earthly amino acids, and the proteins that they potentially form:

Biological proteins use 20 kinds of amino acids — glycine, alanine, lysine, arginine, and so forth. A protein is a linear sequence of these [, a string of amino acid beads.] Picture 20 colors of beads. A protein of 100 amino acids is like a string of 100 beads. The number of possible strings is just the number of types of beads, here 20, multiplied times itself 100 times. That’s 10¹²⁰, or a 1 with 120 zeroes after it. Even in these days of vast federal deficits, 10¹²⁰ is a really big number. The estimated number of hydrogen molecules in the entire universe is 10⁶⁰. So the number of possible proteins of length 100 is equal to the square of the number of hydrogen molecules in the universe.
We might suppose that, through natural selection, nature has tried out this stunning number of combinations, rejecting all but the small subset that have found their way into earthly life — that there is no reason for us to look any farther than our own genomes. Robert Shapiro estimated the number of trials that might have occurred in all the oceans ever dreamed of by Columbus. I think this part of his argument is reasonable. Assuming that a “trial” occurs in a volume of one cubic micron and takes one microsecond, Shapiro calculated that enough time has elapsed since the earth was born to carry out 10⁵¹ trials, or less. If a new protein were tried in each trial, then only 10⁵¹ possible proteins of length 100 can have been tried in the history of the earth. Thus only a tiny portion of the total diversity of such proteins has ever existed on the earth! Life has explored only an infinitesimal fraction of the possible proteins.
If such a tiny fraction of the potential diversity of proteins of length 100 have ever felt the sun’s warmth, then there is plenty of room for human explorers to roam. Evolution can have sampled only the tiniest reaches of “protein space.” And since selection tends to stick with the useful forms it finds, evolution’s search has probably been even more restrictive.
— Stuart Kauffman, “At Home in the Universe
The shadow of the past looms larger, it seems, at every strike of the clock; for every moment we move forward, we strip from history perhaps 9, perhaps 900 more moments that otherwise could have been.

In effect, there are more possible biological proteins, and hence species of animal, than time to bring them all into existence. There are more possible video games than there are lines of code to generate them. There are more possible technologies than there can be thoughts, and more possible thoughts than neural permutations, than the time that will exist between now and when the Universe meets its inevitable entropic demise. This really has to make you stop and wonder. Is it any surprise at all that we are here? Somewhat expected, at home in the universe?

Replace in our new toy model of the adjacent possible these marvelously simple building blocks of all the vast and limitless biological forms that surround us with, say, bits on a computer’s graphics card, or every minute technological component, spring and polycapillary, patented or otherwise, that make up an airplane; or, perhaps the different kinds of people that you might encounter at a cocktail party. What’s so stunning to me is that the further into the future we go, the more parts that spring into existence at every strike of the clock, the more the subset of “the actual” — what we actually see around us — becomes smaller as a fraction of the super-set of the sum total realizable — “the stuff that otherwise could have been.” — and yet, here we are, surrounded by ever and increasingly more sophisticated ways to destroy ourselves, to reduce ourselves to a forgotten redundancy of history: persisting, alive and well, struggling as we always have for so many eons and hopefully more.

Can we even ask what the future may hold?

Kauffman proposes something of an antidote to the potential malady that is this seemingly inevitable self-destruction brimming at the edge of too much complexity. It appears, he says, that “biospheres on average keep expanding into the adjacent possible,” as if by mandate, a hint perhaps of a new general law governing complex information networks: a Fourth Law of Thermodynamics. It’s possible that the biosphere as a whole, or the network of information under-girding the interacting components that make up any complex system— whether enzymes, gazelles, or Miley Cyrus—actually maximizes the rate of exploration of the adjacent possible in order to increase the diversity of what can happen next, with the caveat that it does so at a rate that it can get away with being boisterously flamboyant and not self-implode.

That the system as a whole would move in a kind of orchestrated synchrony toward the future makes intuitive sense when you consider that while everybody is trying to make a living, we do so in many different ways, and sometimes in ways that contradict each-other. If the agents that shape the complex environments in which they live have a tendency to compete for, and to dominate at the expense of others — if left totally unchecked — a particular niche of “making a living”, it might actually pay to compete with one another in such a way to ensure that the landscape of the game itself is as unpredictable as possible, a kind of implicit contract to incenvitize cooperative engagement. This is Rawls’ veil of ignorance writ large across the biosphere and everything else, a way to ensure that the tides of competition raise all ships, to allow us to take from the Earth at a rate that does not hinder creativity, and yet to the extent that in isolation, all agents, both you and I each, are also not arbitrarily limiting their own time in the sun. That a subconscious yet ultimately directed effort to build risk and unpredictably into the system itself to ensure its own survival might sound like something out of a science fiction novel is not lost on me, and at the prospect of anthropomorphizing the Earth and everything on it, a major scientific faux-pas for sure, it appears that Gaia does have plans for us yet, even if its “purpose” is in fact a completely natural, emergent property of our co-efforts to share in the construction of the worlds we live in.

Concepts mentioned or alluded to here, and related books and articles:

  1. Adjacent possible: The adjacent possible is a kind of shadow future, hovering on the edges of the present state of things, a map of all the ways in which the present can reinvent itself. As Steven notes, the adjacent possible “captures both the limits and the creative potential of change and innovation.”
  2. Smith, Eddie. (2010). The adjacent possible. Practically Efficient blog: https://www.edge.org/conversation/stuart_a_kauffman-the-adjacent-possible
  3. Kauffman, Stuart. (1996). At Home in the Universe: The Search for the Laws of Self-Organization and Complexity. Available at Amazon: https://www.amazon.com/At-Home-Universe-Self-Organization-Complexity/dp/0195111303
  4. Gell-Mann, Murray. (1995). The Quark and the Jaguar: Adventures in the Simple and the Complex. Available at Amazon: https://www.amazon.com/Quark-Jaguar-Adventures-Simple-Complex/dp/0805072535
  5. Veil of ignorance: Rawls suggests that you imagine yourself in an original position behind a veil of ignorance. Behind this veil, you know nothing of yourself and your natural abilities, or your position in society. You know nothing of your sex, race, nationality, or individual tastes. Given this state of affairs, how would you distribute the Earth’s resources among its people if granted the opportunity? Probably, in such a way so as to maximize your own chance of getting a good deal. Since you could not know, in advance, to whom and where you will be born, you would share them as randomly as possible.
  6. Anthropomorphism: The attribution of human characteristics or behavior to a god, animal, or object.