Nick Bostrom has argued that we might be sims. A technologically advanced society might use hugely powerful computers, he says, to run "ancestor simulations" containing actually conscious people who think they are living, say, on Earth in the early 21st century but who in fact live entirely inside an advanced computational system. David Chalmers has considered a similar possibility in his well-known commentary on the movie The Matrix.
Neither Bostrom nor Chalmers is inclined to draw skeptical conclusions from this possibility. If we are living in a giant sim, they suggest, that sim is simply our reality: All the people we know still exist (they're sims just like us) and the objects we interact with still exist (fundamentally constructed from computational resources, but still predictable, manipulable, interactive with other such objects, and experienced by us in all their sensory glory). However, it seems quite possible to me that if we are living in a sim, it might well be a small sim -- one run by a child, say, for entertainment. We might live for three hours' time on a game clock, existing mainly as citizens who will give entertaining reactions when, to their surprise, Godzilla tromps through. Or it might be just me and my computer and my room, in an hour-long sim run by a scientist interested in human cognition about philosophical problems.
Bostrom has responded that to really evaluate the case we need a better sense of what are more likely vs. less likely simulation scenarios. One large-sim-friendly thought is this: Maybe the most efficient way to create simulated people is to evolve up a large scale society over a long period of (sim-clock) time. Another is this: Maybe we should expect a technologically advanced society capable of running sims to have enforceable ethical standards against running small sims that contain actually conscious people.
However, I don't see compelling reason to accept such (relatively) comfortable thoughts. Consider the possibility I will call the Many-Branching Sim.
Suppose it turns out the best way to create actually conscious simulated people is to run a whole simulated universe forward billions of years (sim-years on the simulation clock) from a Big Bang, or millions of years on an Earth plus stars, or thousands of years from the formation of human agriculture -- a large-sim scenario. And suppose that some group of researchers actually does this. Consider, now, a second group of researchers who also want to host a society of simulated people. It seems they have a choice: Either they could run a new sim from the ground up, starting at the beginning and clocking forward, or they could take a snapshot of one stage of the first group's sim and make a copy. Which would be more efficient? It's not clear: It depends on how easy it is to take and store a snapshot and implement it on another device. But on the face of it, I don't see why we ought to suppose that copying would take more time or more computational resources than evolving a sim up from ground.
Consider the 21st century game Sim City. If you want a bustling metropolis, you can either grow one from scratch or you can use one of the many copies created by the programmers or users. Or you could grow one from scratch and then save stages of it on your computer, shutting the thing down when things don't go the way you like and starting again from a save point; or you could make copied variants of the same city that grow in different directions.
The Many-Branching Sim scenario is the possibility that there is a root sim that is large and stable, starting from some point in the deep past, and then this root sim was copied into one or more branch sims that start from a save point. If there are many branch sims, it might be that I am in one of them, rather than in a root sim or a non-branching sim. Maybe one company made the root sim for Earth, took a snapshot in November 2013 on the sim clock, then sold thousands or millions of copies to researchers and computer gamers who now run short-term branch sims for whatever purposes they might have. In such a scenario, the future of the branch sim in which I am living might be rather short -- a few minutes or hours or years. The past might be conceptualized either as short or as long, depending on whether the past in the root sim counts as "this world's" past.
Issues of personal identity arise. If the snapshot of the root sim was taken at root sim clock time November 1, 2013, then the root sim contains an "Eric Schwitzgebel" who was 45 years old at the time. The branch sims would also contain many other "Eric Schwitzgebels" developing forward from that point, of which I would be one. How should I think of my relationship to those other Erics? Should I take comfort in the fact that some of them will continue on to full and interesting lives (perhaps of very different sorts) even if most of them, including probably this particular instantiation of me, now in a hotel in New York City, will soon be stopped and deleted? Or to the extent I am interested in my own future rather than merely the future of people similar to me, should I be concerned primarily about what is happening in this particular branch sim? As Godzilla steps down on me, shall I try to take comfort in the possibility that the kid running the show will delete this copy of the sim after he has enjoyed viewing the rampage, then restart from a save point with New York intact? Or would deleting this branch be the destruction of my whole world?