To simulate not just individual minds but also their interactions among themselves and with an evolving enviroment, the computational load would grow orders of magnitude later. But a sophisticated simulation could cut computional corners with minimal impact on quality. Simulated humans on a simulated Earth won't be bothered if the computer simulates only things lying within the cosmic horizon. More boldly, the simulation might simulate stars beyond the sun only during simulated nights, and then only when the simulated local weather resulted in clear skies. When no one's looking, the computer's celestrial simulator routines could take a break from working out the appropriate stimulus to provide each and every person who could look skyward. A sufficiently well-structured program would keep track of the mental states and intentions of its simulated inhabitants, and so would anticipate, and appropriately respond to, any inpending stargazing. The same goes for simulating cells, molecules, and atoms. For the most part they'd be necessary only for simulated specialists of one scientific persuation or another, and then only when such specialists were in the act of studying these exotic realms. A computationally cheaper replica of familiar reality that adjusts the simulation's degree of detail on an as-needed basis would be adequate.
