What is the Simulation Argument?

The Simulation Argument, formulated by Oxford philosopher Nick Bostrom, presents the case that there may be a high probability our world is a computer simulation being run by a more advanced, "posthuman" race. More precisely, the Simulation Argument argues that "at least one of the following propositions is true: (1) the human species is very likely to go extinct before reaching a "posthuman" stage; (2) any posthuman civilization is extremely unlikely to run a significant number of simulations of their evolutionary history (or variations thereof); (3) we are almost certainly living in a computer simulation". (Quoted from Bostrom's paper introducing the Simulation Argument, "Are You Living In a Computer Simulation?")

The Simulation Argument begins with the assumption that it is theoretically possible for a highly advanced race to arrange information-processing modules (neurons, circuits, whatever) in configurations that represent self-contained worlds inhabited by conscious beings. In this manner, it would be possible for these advanced races to run ancestor simulations - that is, simulations of their own ancestors or other primitive races simple enough to simulate in their computers. If the total number of simulated worlds in this universe exceeds the number of real worlds, then it is rational to assume that the probability we are in a simulated world is correlated to that ratio, regardless of how "real" our world seems to our human intuition.

Around the advent of the 21st century, all computers in the world could collectively process around a billion billion operations per second (10^18 ops/sec) and hold several petabytes (10^15 bytes) of data. These values are exploding exponentially, with doubling times of about a year for processing power and several months for data.

The human brain, being a machine too, has a quantifiable amount of computing power and data storage. Cognitive scientists estimate human brain processing power at between 100 trillion and 100,000 trillion operations per second, though some estimates are significantly lower. Tom Landauer, a scientist that specializes in human learning, has estimated that human memory is somewhere between 200 and 300 megabytes large.

If the growth rates for computing power and data storage continue to rise exponentially, as they have possibly done so before for alien races undergoing their own computing revolutions, our species will have control of an utterly massive quantity of computing power and storage space. Given the right programs, this space could be utilized to run simulations of the past at such high resolution that the inhabitants of the simulation become conscious beings with their own experiences, plans, hopes, desires, and belief systems. If this is a common developmental milestone among intelligent species in the universe, then there may exist a multitude of ancestor simulations, and our world could be one of them. Plausible alternative hypotheses include the possibility that all intelligent races go extinct before they reach the level of sophistication required to run ancestor simulations, or that there is some universal reason why advanced races do not run ancestor simulations.