SiobhanRoberts wrote: September 18th, 2020, 9:55 am
Greetings Lifenthusiasts,
On the occasion of the 50th anniversary of the Game of Life, I am researching/searching out "Life Lessons" — ie, What did you learn, scientifically or philosophically or otherwise, from the Game of Life that has stayed with you/that is still resonant and compelling today? Or more generally, what aspect of Life do you think is important to highlight at this moment in time? And, bonus question, what is your favorite Lifeform, and why?
Siobhan
I would add the intriguing possibility that our own universe is simulated by an automaton. This is the view that Stephan Wolfram is always holding, though in recent years his interest shifted to network-based dynamic systems, rather than cellular automata.
When supporting this view, people often cite the emergence of particle-like structures (especially spaceships) in Life and other CAs, the reactions between these structures, and, for certain CAs, the resulting Turing completeness. And I would like to add a new argument, which is rarely (if ever) discussed before. In fact this gives the much stronger claim that: a typical universe simulated by an automaton will probably look the same from the view of intelligent beings that live in it, regardless of which Turing complete rule you are using!
Since a Turing complete system can simulate any other Turing complete system, if you simulate a very dilute and very large soup in Life (or any other Turing complete CA), then theoretically you will observe spontaneous emergence of patterns that simulate every other Turing complete system (including but not limited to CAs) with arbitrary initial conditions. They will themselves give rise to patterns that simulate all possible Turing complete systems, which will themselves carry out their own simulations, ad infinitum. Now consider: of all simulations of Turing complete systems that pop up from the Turing complete system I with the initial condition i, how many will (directly, without simulating another Turing complete system as an intermediate level) simulate some Turing complete system J with the initial condition j? Denote the result by N({I,i},{J,j}). Gathering the N({I,i},{J,j}) values for all combinations of I, i, J, j, we get an infinite matrix N. Thus, the number of instances of Turing complete system-initial condition pair {J,j} that will be simulated by the system-initial condition pair {I,i} is given by the {I,i}'th row of the matrix N. Now the salient point is: by basic linear algebra we find that the {I,i}'th row of the matrix N^2 gives the number of instances of {J,j} that can be simulated by those Turing complete systems simulated by {I,i} (i.e. on the second level of the "simulation hierarchy"), and the {I,i}'th row of the matrix N^3 gives the number of instances of {J,j} that can be simulated by those Turing complete systems simulated by those Turing complete systems simulated by {I,i} (i.e. on the third level of the "simulation hierarchy"), etc. And according to a theorem in linear algebra, whenever the largest eigenvalue of N is greater than 1 and non-degenerate, and that the associated eigenvector has a non-zero {I,i} component, the {I,i}'th row of the matrix N^(infinity) will, apart from an infinitely large multiplicative constant, be independent of {I,i} and proportional to that eigenvector. This means that, under the above assumptions, the proportion of simulations of a particular Turing complete system-initial condition pair {J,j} will be completely independent of the bottom-level Turing complete system-initial condition pair {I,i}!
Some far-reaching consequences are:
(1) The universes simulated by different Turing complete systems will be totally indistinguishable from the viewpoint of the creatures that live in the universes themselves, since the creatures can only probe the top level of the simulation hierarchy (i.e. if A simulated B which then simulated C ... which then simulated Z, they can only know the rule Z and its associated initial condition), and with the above assumptions the top level of the simulation hierarchy is independent of the bottom level of the hierarchy. Thus, they will be totally and forever clueless as to the bottom level reality of their universe, apart from the fact that the eigenvector corresponding to the largest eigenvalue of N has a non-zero entry for the {I,i} pair that corresponds to the true bottom level reality. They cannot even say which {I,i} pair is more likely the right answer than some other {I,i} pair.
(2) Our own universe as we know it is Turing complete too, and it has an initial condition. Thus, the list of {I,i} includes our universe, which we denote as {U,u}. If in addition the eigenvector corresponding to the largest eigenvalue of N has a non-zero entry for {U,u} (and the eigenvalue is larger than 1), then in the far, far future of our universe, there will be computers that simulate other universes, which then give birth to computers that simulate yet other universes ... Moreover since our own universe is also simulated infinitely often in this simulation hierarchy, it is infinitely more likely that we ourselves are simulated than we are real. Then we will be the creatures in (1) - we will never have any idea about the bottom level reality of our own universe (i.e. the aliens that used computers to simulate computers that simulate computers that ... simulate us), other than that it has a non-zero share in the eigenvector of matrix N that corresponds to its largest eigenvalue.
(3) Under the above assumptions, the probability that a Turing complete universe has a certain set of physical rules and initial condition (as viewed by its creatures themselves) is uniquely defined from pure mathematics, since this is nothing but the aforementioned eigenvector of N. Although the probability is very likely uncomputable, suppose that we have a non-trivial estimate of it. Then we can explain why our universe looks like the way it does, by showing that our universe has a relatively large probability than other candidate universes. If however our universe turns out to be a very improbable one, then this must be solely due to anthropic origins, and nothing else. People have tried to define such a probabilistic measure of possible universes using string landscape etc., and then invoke the anthropic principle to explain all the unexplained fine-tunings, but the present approach does not even need the assumption that strings exist at all, and is fully non-empirical.
Of course the above argument is kind of hand-wavy because there are lots of infinities which may easily ruin the whole argument, but I hope that this convinces you that it is an exciting aspect of CAs that is previous overlooked and worths pursuing.
(And, for that matter, my favorite Lifeform is the infinitely dilute soup, if that really counts as a Lifeform; it retains every single possibility of Life patterns but gives them a non-empirical probabilistic measure, and allows us to distinguish the more natural phenomena from largely artificial ones.)