Simulation, Consciousness, Existence
Like organisms evolved in gentle tide pools, who migrate to freezing oceans or steaming jungles by developing metabolisms, mechanisms and behaviors workable in those harsher and vaster environments, our descendants may develop means to venture far from the comfortable realms we consider reality, into arbitrarily strange volumes of the all-possible library. Their techniques will be as meaningless to us as bicycles are to fish, but perhaps we can stretch our common-sense hobbled imaginations enough to peer a short distance into this odd territory.
Simulation
During the last few centuries, physical science has effectively answered so many questions about the nature of things, and so hugely increased our abilities, that many see it as the only legitimate claimant to the title of true knowledge. Other belief systems may have social utility for the groups that practice them, but ultimately they are just made-up stories. I myself am partial to such physical fundamentalism.
Physical fundamentalists, however, must agree with René Descartes that the world we perceive through our senses could be an elaborate hoax. In the seventeenth century, Descartes had to postulate an improbable evil demon to create the illusion by controlling all that we see and hear (and feel and smell and taste). In the 21st century, physical science itself, through the technology of virtual reality, will provide the means. Enthusiastic cybernauts are already strapping themselves into virtual reality goggles and body suits for brief stints in made-up worlds, whose fundamental mechanisms are completely different than the quantum fields that, best evidence suggests, make up our physical world.
Today's virtual aventurers continue to interact with the physical world: if they bump into real objects, they feel real pain. That link may weaken when direct connections to the nervous system become possible, leading perhaps to the old science-fiction idea of a living brain in a vat. The brain would be sustained physically by life support machinery, and mentally by connections of all the peripheral nerves to an elaborate simulation of not only a surrounding world, but also a body for the brain to inhabit. Brain vats might be medical stopgaps for accident victims with bodies damaged beyond repair, pending the acquisition, growth or manufacture of a new body.
The virtual life of a brain in a vat can still be subtly perturbed by external physical, chemical or electrical effects impinging on the vat. Even these subtle ties to the physical world would fade in more advanced procedures that extended the body simulation to the brain itself. If damaged or endangered parts of the brain, like the body, could be replaced with functionally equivalent simulations, some individuals could survive total physical destruction, to find themselves alive as pure computer simulations in virtual worlds.
A simulated world hosting a simulated person can be a self-contained entity. It might exist as a program on a computer processing data quietly in some dark corner, giving no external hint of the joys and pains, successes and frustrations of the person inside. Inside the simulation, on the other hand, events unfold according to the strict logic of the program, which defines the laws of physics of the simulation. The inhabitant might, by patient experimentation and inference, deduce some representation of the simulation laws, but not the nature, or even existence of the simulating computer. The simulation's internal relationships would be the same if the program were running correctly on any of an endless variety of different computers, slowly, quickly, intermittently or even backwards and forwards in time, with the data stored as charges on chips, marks on a tape or a pulses in delay line, with the simulation's numbers represented in binary, decimal or Roman numerals, compactly or spread widely across the machine.
Today's simulations, say of aircraft flight or the weather, are run to provide answers and images, via additional programs that translate the simulation's internal representations into forms convenient for external human observers. The need to interpret limits how radical a simulation's hardware and software representations can be: making them too different from the form of the answers may make the translation impractically slow and expensive. This practical limit may be irrelevant for simulations, such as the medical rescue imagined above, that contain their own observers. Conscious inhabitants of simulations experience their virtual lives whether or not outsiders manage to view them. They can be implemented in any way at all.
What does it mean for a process to implement, or encode, a simulation? Something is palpably an encoding if there is a way of decoding, or translating, it into some recognizable form of the simulation. The programs used to view existing simulations, for instance to produce pictures of evolving cloud cover from weather simulations, are examples of such decodings, but don't define the limits. A translation that is impractical today may be possible tomorrow given more powerful computers, some yet undiscovered mathematical approach, or perhaps an alien translator. Like people who dismiss speech and signs in unfamiliar foreign languages as meaningless gibberish, we are likely to do be rudely surprised if we dismiss possible interpretations simply because we can't see them at the moment. Alternatively, we might ask, what decodings are mathematically possible, regardless of present or future practicality. This seems a safe, open-minded approach, and I think it is, but it leads to some strange territory.
An interpretation of a simulation is just a mathematical mapping between steps of the simulation process and views of the simulation meaningful to a particular observer. A small, fast program to do this might be needed to make the interpretation practical, but mathematically the job can also be done by a huge theoretical lookup table, which contains an observer's view for every state of the process. The only problem is that there is always a possible table that maps any situation, for instance, the idle passage of time, into any desired simulation. Not just hard-working computers, but anything at all is theoretically viewable as a simulation of any possible world! We are unlikely to experience more than an infinitesimal fraction of the infinity of possible worlds, yet, as our ability to process data increases, more and more of them will become potentially viewable. But independent of which ones we do or don't contact, all the possible worlds are as physically real for their conscious inhabitants as our world is for us.
This line of thought, growing out of the premises and techniques of physical science, has the unexpected consequence of demoting physical existence to a derivative role. A possible world is only as real as conscious observers, inside or outside the world, think it is!