heliocentric theory The theory that the Earth moves round the Sun, and spins on its own axis.
geocentric theory The theory that the Earth is at rest and other astronomical bodies move around it.
realism The theory that an external physical universe exists objectively and affects us through our senses.
Occam’s razor (My formulation) Do not complicate explanations beyond necessity, because if you do, the unnecessary complications themselves will remain unexplained.
Dr Johnson’s criterion (My formulation) If it can kick back, it exists. A more elaborate version is: If, according to the simplest explanation, an entity is complex and autonomous, then that entity is real.
self-similarity Some parts of physical reality (such as symbols, pictures or human thoughts) resemble other parts. The resemblance may be concrete, as when the images in a planetarium resemble the night sky; more importantly, it may be abstract, as when a statement in quantum theory printed in a book correctly explains an aspect of the structure of the multiverse. (Some readers may be familiar with the geometry of fractals; the notion of self-similarity defined here is much broader than the one used in that field.)
complexity theory The branch of computer science concerned with what resources (such as time, memory capacity or energy) are required to perform given classes of computations.
Although solipsism and related doctrines are logically self-consistent, they can be comprehensively refuted simply by taking them seriously as explanations. Although they all claim to be simplified world-views, such an analysis shows them to be indefensible over-elaborations of realism. Real entities behave in a complex and autonomous way, which can be taken as the criterion for reality: if something ‘kicks back’, it exists. Scientific reasoning, which uses observation not as a basis for extrapolation but to distinguish between otherwise equally good explanations, can give us genuine knowledge about reality.
Thus science and other forms of knowledge are made possible by a special self-similarity property of the physical world. Yet it was not physicists who first recognized and studied this property: it was mathematicians and computer theorists, and they called it the universality of computation. The theory of computation is our third strand.
5
Virtual Reality
The theory of computation has traditionally been studied almost entirely in the abstract, as a topic in pure mathematics. This is to miss the point of it. Computers are physical objects, and computations are physical processes. What computers can or cannot compute is determined by the laws of physics alone, and not by pure mathematics. One of the most important concepts of the theory of computation is universality. A universal computer is usually defined as an abstract machine that can mimic the computations of any other abstract machine in a certain well-defined class. However, the significance of universality lies in the fact that universal computers, or at least good approximations to them, can actually be built, and can be used to compute not just each other’s behaviour but the behaviour of interesting physical and abstract entities. The fact that this is possible is part of the self-similarity of physical reality that I mentioned in the previous chapter.
The best-known physical manifestation of universality is an area of technology that has been mooted for decades but is only now beginning to take off, namely virtual reality. The term refers to any situation in which a person is artificially given the experience of being in a specified environment. For example, a flight simulator — a machine that gives pilots the experience of flying an aircraft without their having to leave the ground — is a type of virtual-reality generator. Such a machine (or more precisely, the computer that controls it) can be programmed with the characteristics of a real or imaginary aircraft. The aircraft’s environment, such as the weather and the layout of airports, can also be specified in the program. As the pilot practises flying from one airport to another, the simulator causes the appropriate images to appear at the windows, the appropriate jolts and accelerations to be felt, the corresponding readings to be shown on the instruments, and so on. It can incorporate the effects of, for example, turbulence, mechanical failure and proposed modifications to the aircraft. Thus a flight simulator can give the user a wide range of piloting experiences, including some that no real aircraft could: the simulated aircraft could have performance characteristics that violate the laws of physics: it could, for instance, fly through mountains, faster than light or without fuel.
Since we experience our environment through our senses, any virtual-reality generator must be able to manipulate our senses, overriding their normal functioning so that we can experience the specified environment instead of our actual one. This may sound like something out of Aldous Huxley’s Brave New World, but of course technologies for the artificial control of human sensory experience have been evolving for thousands of years. All techniques of representational art and long-distance communication may be thought of as ‘overriding the normal functioning of the senses’. Even prehistoric cave paintings gave the viewer something of the experience of seeing animals that were not actually there. Today we can do that much more accurately, using movies and sound recordings, though still not accurately enough for the simulated environment to be mistaken for the original.
I shall use the term image generator for any device, such as a planetarium, a hi-fi system or a spice rack, which can generate specifiable sensory input for the user: specified pictures, sounds, odours, and so on all count as ‘images’. For example, to generate the olfactory image (i.e. the smell) of vanilla, one opens the vanilla bottle from the spice rack. To generate the auditory image (i.e. the sound) of Mozart’s 20th piano concerto, one plays the corresponding compact disc on the hi-fi system. Any image generator is a rudimentary sort of virtual-reality generator, but the term ‘virtual reality’ is usually reserved for cases where there is both a wide coverage of the user’s sensory range, and a substantial element of interaction (‘kicking back’) between the user and the simulated entities.
Present-day video games do allow interaction between the player and the game objects, but usually only a small fraction of the user’s sensory range is covered. The rendered ‘environment’ consists of images on a small screen, and a proportion of the sounds that the user hears. But virtual-reality video games more worthy of the term do already exist. Typically, the user wears a helmet with built-in headphones and two television screens, one for each eye, and perhaps special gloves and other clothing lined with electrically controlled effectors (pressure-generating devices). There are also sensors that detect the motion of parts of the user’s body, especially the head. The information about what the user is doing is passed to a computer, which calculates what the user should be seeing, hearing and feeling, and responds by sending appropriate signals to the image generators (Figure 5.1). When the user looks to the left or right, the pictures on the two television screens pan, just as a real field of view would, to show whatever is on the user’s left or right in the simulated world. The user can reach out and pick up a simulated object, and it feels real because the effectors in the glove generate the ‘tactile feedback’ appropriate to whatever position and orientation the object is seen in.