OF LATE, physicists have been getting very interested in multiverses, the idea that there are many different kinds of universe all with differing properties. What sorts could there be? And how might their existence help us to understand the life-supporting features of our own universe that would otherwise appear to be very fortunate coincidences? At root, these questions are not ultimately matters of opinion or idle speculation.
The underlying theory of everything, if it exists, may require many properties of our universe to have been selected at random from a large collection of possibilities. Modern quantum physics even provides ways in which these possible universes that make up the multiverse of all possibilities may actually exist.
But once you take seriously the notion that all possible universes can (or do) exist then a slippery slope opens up before you: another, rather strange consequence that was pointed out by Yale University philosopher Nick Bostrom (New 杏吧原创, 27 July 2002, p 48). In this infinite array of universes there will exist long-lived technical civilisations far more advanced than ourselves that have the desire and the capability to simulate universes. Instead of merely simulating their weather or the formation of galaxies, as we do, they would be able to go further and watch the appearance of stars and planetary systems. Then, having coupled the rules of biochemistry into their astronomical simulations, they would be able to watch the evolution of life and consciousness (all speeded up to occur on whatever timescale was convenient for them). Just as we watch the life cycles of fruit flies, they would be able to follow the evolution of life 鈥� and watch civilisations grow, communicate with each other and argue about whether there exists a Great Programmer in the Sky who created their universe and who could intervene at will in defiance of the laws of nature they habitually observed.
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Within these simulated universes, self-conscious entities can in turn emerge and communicate with one another, and once that capability is reached fake universes will proliferate and will soon greatly outnumber the real ones.
The simulators determine the laws that govern their worlds and can engineer fine-tunings that suit what we call life. And so we end up with a scenario where, statistically, we are more likely to be in a simulated reality than a real one. The physicist Paul Davies of Macquarie University in Sydney, Australia, suggests that this high probability of our living in a simulated reality is a nonsense 鈥� a 鈥渞eductio ad absurdum鈥� for the whole idea of a multiverse of all possibilities.
Faced with this scenario, is there any way to find out the truth? There may be, if we look closely enough.
For a start, the simulators will have been tempted to avoid the complexity of setting up a consistent set of laws of nature in their worlds when it is far easier to patch in realistic effects. When the Walt Disney company makes an animated film that features the reflection of light from the surface of a lake, it does not use the laws of quantum electrodynamics and optics to compute the light scattering. That would require a stupendous amount of computing power and detail. Instead, the simulation of the light scattering is replaced by plausible rules of thumb that are much simpler than the real thing but give a realistic-looking result, as long as no one looks too closely. There would be an economic and practical imperative for simulated realities to stay that way if they were purely for entertainment. But such simplifcations in the simulation鈥檚 programming would presumably cause occasional telltale problems.
Even if the simulators were scrupulous about simulating the laws of nature, there would still be limits to what they could do. And even if the simulators have an advanced knowledge of the laws of nature, it is likely that this knowledge would still be incomplete 鈥� and some philosophers of science argue that this must always be the case. They may know a lot about the physics and programming needed to simulate a universe, but there will be gaps or, worse, errors in their knowledge of the laws of nature. These lacunae would of course be subtle and far from obvious. They would not prevent simulations running smoothly for long periods of time, but gradually the little flaws would begin to build up. Eventually their effects would snowball and the simulated realities would cease to compute. The only escape would be for their creators to intervene to patch up the problems in turn as they arise.
This is a solution that will be very familiar to the owner of any home computer who receives regular updates in order to protect it against new forms of invasion or to repair gaps that its original creators had not foreseen. The creators of a simulation could offer this type of protection, updating the working laws of nature to include extra things they had learned since they started the simulation running.
In this kind of situation, logical contradictions will inevitably arise and the laws in the simulations will appear to break down now and again. The inhabitants of the simulation 鈥� especially the simulated scientists 鈥� will occasionally be puzzled by the experimental results they obtain. The simulated astronomers might, for instance, make observations that show that their so-called constants of nature are very slowly changing.
It is likely there could even be sudden glitches in the laws that govern these simulated realities. That鈥檚 because the simulators would most likely use a technique that has been found effective in all other simulations of complex systems: the use of error-correcting codes to put things back on track. Take our genetic code, for example. If it were left to its own devices we would not last very long. Errors would accumulate, and death and mutation would quickly follow. We are protected from this by the existence of a mechanism for error correction that identifies and corrects mistakes in genetic coding. Many of our complex computer systems possess the same type of internal immune system to guard against the accumulation of errors.
If the simulators used error-correcting computer codes to guard against the fallibility of their simulations as a whole, as well as simulating them on a smaller scale in our genetic code, then every so often a correction would take place to the state of the simulation or the laws governing it. Mysterious changes would occur that would appear to contravene the very laws of nature that the simulated scientists were accustomed to observing and predicting.
So it seems enticing to conclude that if we live in a simulated reality we should expect to come across scientific phenomena, such as occasional glitches in experimental results that we cannot repeat, or very small drifts in the supposed constants and laws of nature that we cannot explain. Tantalisingly, we do have a few such results: the apparent astronomical variation in the fine structure constant by a few parts in a million, for example (New 杏吧原创, 7 September 2002, p 30).
Clearly, finding explanations for these phenomena is something of a priority. If we can鈥檛, then the flaws of nature may turn out to be at least as significant as the laws of nature for our understanding of true reality.