Simulation hypothesis

Origins

There is a long philosophical and scientific history to the underlying thesis that reality is an illusion. This skeptical hypothesis can be traced back to antiquity; for example, to the "Butterfly Dream" of Zhuangzi, or the Indian philosophy of Maya.

Are you living in a computer simulation?

Many works of science fiction as well as some forecasts by serious technologists and futurologists predict that enormous amounts of computing power will be available in the future. Let us suppose for a moment that these predictions are correct. One thing that later generations might do with their super-powerful computers is run detailed simulations of their forebears or of people like their forebears. Because their computers would be so powerful, they could run a great many such simulations. Suppose that these simulated people are conscious (as they would be if the simulations were sufficiently fine-grained and if a certain quite widely accepted position in the philosophy of mind is correct). Then it could be the case that the vast majority of minds like ours do not belong to the original race but rather to people simulated by the advanced descendants of an original race. It is then possible to argue that, if this were the case, we would be rational to think that we are likely among the simulated minds rather than among the original biological ones. Therefore, if we don’t think that we are currently living in a computer simulation, we are not entitled to believe that we will have descendants who will run lots of such simulations of their forebears.

In 2003, philosopher Nick Bostrom proposed a trilemma that he called "the simulation argument". Despite the name, Bostrom's "simulation argument" does not directly argue that we live in a simulation; instead, Bostrom's trilemma argues that one of three unlikely-seeming propositions must be true. The trilemma points out that a technologically mature "posthuman" civilization would have enormous computing power; if even a tiny percentage of them were to run "ancestor simulations" (that is, "high-fidelity" simulations of ancestral life that would be indistinguishable from reality to the simulated ancestor), the total number of simulated ancestors, or "Sims", in the universe (or multiverse, if it exists) would greatly exceed the total number of actual ancestors. Therefore, at least one of the following three propositions is almost certainly true:

1. "The fraction of human-level civilizations that reach a posthuman stage (that is, one capable of running high-fidelity ancestor simulations) is very close to zero", or
2. "The fraction of posthuman civilizations that are interested in running ancestor-simulations is very close to zero", or
3. "The fraction of all people with our kind of experiences that are living in a simulation is very close to one"

Bostrom goes on to use a type of anthropic reasoning to claim that, if the third proposition is the one of those three that is true, and almost all people with our kind of experiences live in simulations, then we are almost certainly living in a simulation.

Bostrom claims his argument goes beyond the classical ancient "skeptical hypothesis", claiming that "...we have interesting empirical reasons to believe that a certain disjunctive claim about the world is true", the third of the three disjunctive propositions being that we are almost certainly living in a simulation. Thus, Bostrom, and writers in agreement with Bostrom such as David Chalmers, argue there might be empirical reasons for the "simulation hypothesis", and that therefore the simulation hypothesis is not a skeptical hypothesis but rather a "metaphysical hypothesis". Bostrom states he personally sees no strong argument for which of the three trilemma propositions is the true one: "If (1) is true, then we will almost certainly go extinct before reaching posthumanity. If (2) is true, then there must be a strong convergence among the courses of advanced civilizations so that virtually none contains any relatively wealthy individuals who desire to run ancestor-simulations and are free to do so. If (3) is true, then we almost certainly live in a simulation. In the dark forest of our current ignorance, it seems sensible to apportion one’s credence roughly evenly between (1), (2), and (3)... I note that people who hear about the simulation argument often react by saying, 'Yes, I accept the argument, and it is obvious that it is possibility #n that obtains.' But different people pick a different n. Some think it obvious that (1) is true, others that (2) is true, yet others that (3) is true."

As a corollary to the trilemma, Bostrom states that "Unless we are now living in a simulation, our descendants will almost certainly never run an ancestor-simulation."

Criticism of Bostrom's anthropic reasoning

Bostrom argues that, if "the fraction of all people with our kind of experiences that are living in a simulation is very close to one", then it follows that we probably live in a simulation. Some philosophers disagree, proposing that perhaps "Sims" do not have conscious experiences the same way that unsimulated humans do, or that it can otherwise be self-evident to a human that they are a human rather than a Sim. Philosopher Barry Dainton modifies Bostrom's trilemma by substituting "neural ancestor simulations" (ranging from literal brains in a vat, to far-future humans with induced high-fidelity hallucinations that they are their own distant ancestors) for Bostrom's "ancestor simulations", on the grounds that every philosophical school of thought can agree that sufficiently high-tech neural ancestor simulation experiences would be indistinguishable from non-simulated experiences. Even if high-fidelity computer Sims are never conscious, Dainton's reasoning leads to the following conclusion: either the fraction of human-level civilizations that reach a posthuman stage and are able and willing to run large numbers of neural ancestor simulations is close to zero, or we are in some kind of (possibly neural) ancestor simulation.

Some scholars categorically reject or are uninterested in anthropic reasoning, dismissing it as "merely philosophical", unfalsifiable, or inherently unscientific.

Some critics reject the block universe view of time that Bostrom implicitly accepts and propose that we could be in the first generation, such that all the simulated people that will one day be created don't yet exist.

Arguments, within the trilemma, against the simulation hypothesis

Some scholars accept the trilemma, and argue that the first or second of the propositions are true, and that the third proposition (the proposition that we live in a simulation) is false. Physicist Paul Davies deploys Bostrom's trilemma as part of one possible argument against a near-infinite multiverse. This argument runs as follows: if there were a near-infinite multiverse, there would be posthuman civilizations running ancestor simulations, and therefore we would come to the untenable and scientifically self-defeating conclusion that we live in a simulation; therefore, by reductio ad absurdum, existing multiverse theories are likely false. (Unlike Bostrom and Chalmers, Davies (among others) considers the simulation hypothesis to be self-defeating.)

Some people argue that creating a sufficiently high-fidelity ancestor simulation is physically impossible or infeasible even for a posthuman civilization, and therefore believe the first proposition is the true one.

Consequences of living in a simulation

Some scholars speculate that the creators of our hypothetical simulation may have limited computing power; if so, after a certain point, the creators would have to deploy some sort of strategy to prevent simulations from themselves indefinitely creating high-fidelity simulations in unbounded regress. One obvious strategy would be to simply terminate the overly-intensive simulation at that point. Therefore, if we are simulations (or simulations of simulations), and if, for example, we were to start massively creating simulations in the year 2050, there could be a risk of termination around that point, as there could be a jump in our simulation's required processing power.

Economist Robin Hanson argues a self-interested high-fidelity Sim should strive to be entertaining and praiseworthy in order to avoid being turned off or being shunted into a non-conscious low-fidelity part of the simulation. Hanson additionally speculates that someone aware he might be a Sim, might care less about others and live more for today: "your motivation to save for retirement, or to help the poor in Ethiopia, might be muted by realizing that in your simulation, you will never retire and there is no Ethiopia."

Testing the hypothesis physically

A long-shot method to test one type of simulation hypothesis was proposed in 2012 in a joint paper by physicists Silas R. Beane from the University of Bonn (now at the University of Washington, Seattle), and Zohreh Davoudi and Martin J. Savage from the University of Washington, Seattle. Under the assumption of finite computational resources, the simulation of the universe would be performed by dividing the continuum space-time into a discrete set of points. In analogy with the mini-simulations that lattice-gauge theorists run today to build up nuclei from the underlying theory of strong interactions (known as Quantum chromodynamics), several observational consequences of a grid-like space-time have been studied in their work. Among proposed signatures is an anisotropy in the distribution of ultra-high-energy cosmic rays, that, if observed, would be consistent with the simulation hypothesis according to these physicists. A multitude of physical observables must be explored before any such scenario could be accepted or rejected as a theory of nature.

Science fiction themes

Science fiction has highlighted themes such as virtual reality, artificial intelligence and computer gaming for more than twenty years. Simulacron-3 (1964) by Daniel F. Galouye (alternative title: Counterfeit World) tells the story of a virtual city developed as a computer simulation for market research purposes, in which the simulated inhabitants possess consciousness; all but one of the inhabitants are unaware of the true nature of their world. In Overdrawn at the Memory Bank, a 1983 television movie, the main character pays to have his mind connected to a simulation. The same theme was repeated in the 1990 movie Total Recall. More recently, the 1999 film The Matrix depicted a world in which artificially intelligent robots enslaved humanity within a simulation set in the modern world.

The 2012 play World of Wires was partially inspired by the Bostrom essay on the simulation hypothesis.