The Fabric of Reality: The Science of Parallel Universes--and Its Implications

by David Deutsch

This is typical of scientific explanation. Scientific theories explain the objects and phenomena of our experience in terms of an underlying reality which we do not experience directly. But the ability of a theory to explain what we experience is not its most valuable attribute. Its most valuable attribute is that it explains the fabric of reality itself.

Prediction – even perfect, universal prediction – is simply no substitute for explanation.

To put that another way: there already is one such oracle out there, namely the physical world. It tells us the result of any possible experiment if we ask it in the right language (i.e. if we do the experiment),

To say that prediction is the purpose of a scientific theory is to confuse means with ends. It is like saying that the purpose of a spaceship is to burn fuel. In fact, burning fuel is only one of many things a spaceship has to do to accomplish its real purpose, which is to transport its payload from one point in space to another. Passing experimental tests is only one of many things a theory has to do to achieve the real purpose of science, which is to explain the world.

So what distinguishes understanding from mere knowing? What is an explanation, as opposed to a mere statement of fact such as a correct description or prediction?

it is hard to give a precise definition of ‘explanation’ or ‘understanding’. Roughly speaking, they are about ‘why’ rather than ‘what’; about the inner workings of things; about how things really are, not just how they appear to be; about what must be so, rather than what merely happens to be so; about laws of nature rather than rules of thumb.

Much of medicine, in other words, is still in the rule-of-thumb era,

Thus to reductionists and instrumentalists, who disregard both the real structure and the real purpose of scientific knowledge, the base of the predictive hierarchy of physics is by definition the ‘theory of everything’. But to everyone else scientific knowledge consists of explanations, and the structure of scientific explanation does not reflect the reductionist hierarchy. There are explanations at every level of the hierarchy.

explanation (roughly) A statement about the nature of things and the reasons for things.

There is no controversy about the results, yet even now some of them are hard to believe.

To put that another way, shadow photons and tangible photons are affected in identical ways when they reach a given barrier, but the barrier itself is not identically affected by the two types of photon.

As I hope to persuade readers who bear with me, understanding the multiverse is a precondition for understanding reality as best we can.

Admittedly, it is right and proper for theoretical physicists such as myself to devote a great deal of effort to trying to understand the formal structure of quantum theory, but not at the expense of losing sight of our primary objective, which is to understand reality.

what is a rare event in any one universe is a common event in the multiverse as a whole.

Newton explained the ellipses through his inverse-square law of gravitational forces, and his theory was later used to predict that the mutual gravitational attraction of planets would cause small deviations from elliptical orbits. The observation of such deviations led to the discovery in 1846 of a new planet, Neptune, one of many discoveries that spectacularly corroborated Newton’s theory.

What justifies the inferences we draw from these patterns? It is certainly not a matter of logical deduction.

Since solipsism, and an infinity of related theories, are logically consistent with your perceiving any possible observational evidence, it follows that you can logically deduce nothing about reality from observational evidence.

If scientific reasoning does not amount to sequences of logical deductions from the evidence, what does it amount to? Why should we accept its conclusions? This is known as the ‘problem of induction’.

It is hard to know where to begin in criticizing the inductivist conception of science – it is so profoundly false in so many different ways.

Or it may be that there have been some surprising observations – such as the wandering of planets – which existing theories did not predict and cannot explain.

At various times in the history of astronomy there appeared to be a mass of unexplained observational evidence;

But every one of them amounted to this: it seemed to someone that the existing explanations could and should be improved upon.

Thus, after a problem presents itself (stage 1), the next stage always involves conjecture: proposing new theories, or modifying or reinterpreting old ones, in the hope of solving the problem (stage 2).

There is no simple way of discovering the true nature of planets, given (say) a critique of the celestial-sphere theory and some additional observations, just as there is no simple way of designing the DNA of a koala bear, given the properties of eucalyptus trees. Evolution, or trial and error – especially the focused, purposeful form of trial and error called scientific discovery – are the only ways.

Both in science and in biological evolution, evolutionary success depends on the creation and survival of objective knowledge, which in biology is called adaptation. That is, the ability of a theory or gene to survive in a niche is not a haphazard function of its structure but depends on whether enough true and useful information about the niche is implicitly or explicitly encoded there.

Many such theories are possible. Indeed, if making the right predictions were our only constraint, we could invent theories which say that anything we please is going on in space. For example, observations alone can never rule out the theory that the Earth is enclosed in a giant planetarium showing us a simulation of a heliocentric solar system; and that outside the planetarium there is anything you like, or nothing at all.

That is why the argument that a theory is indefensible can be so compelling. A prediction, or any assertion, that cannot be defended might still be true, but an explanation that cannot be defended is not an explanation.

But in fact the rebounding depended on what the rock did, such as being in a certain place, which was in turn related to other effects that the rock had, such as being seen, or affecting other people who kicked it. Dr Johnson perceived these effects to be autonomous (independent of himself) and quite complicated. Therefore the realist explanation of why the rock produces the rebounding sensation involves a complicated story about something autonomous.

If, according to the simplest explanation, an entity is complex and autonomous, then that entity is real.

So we can re-express Dr Johnson’s criterion again, in terms of hypothetical computations: If a substantial amount of computation would be required to give us the illusion that a certain entity is real, then that entity is real.

In those symbols — in our planetariums, books, films and computer memories, and in our brains — there are images of physical reality at large, images not just of the appearance of objects, but of the structure of reality. There are laws and explanations, reductive and emergent. There are descriptions and explanations of the Big Bang and of subnuclear particles and processes; there are mathematical abstractions; fiction; art; morality; shadow photons; parallel universes. To the extent that these symbols, images and theories are true — that is, they resemble in appropriate respects the concrete or abstract things they refer to — their existence gives reality a new sort of self-similarity, the self-similarity we call knowledge.

science and other forms of knowledge are made possible by a special self-similarity property of the physical world.

Concerts and conferences will be held without venues; not only will there be savings on the cost of the auditorium, and on accommodation and travel, but there is also the benefit that all the participants could be allowed to sit in the best seats simultaneously.

Therefore they pass the test for reality, and rightly so, for in fact these calculations are physical processes within the computer, and the computer is an ordinary physical object — no less so than an engine — and perfectly real.

(Hunger and thirst, and other sensations such as balance and muscle tension, are perceived as being internal to the body, but they are external to the mind and are therefore potentially within the scope of virtual reality.)

Could one remedy this deficiency in flight simulators by giving them the capacity to simulate free fall on the ground (in which case they could also be used as spaceflight simulators)? Not easily, for the laws of physics get in the way. Known physics provides no way other than free fall, even in principle, of removing an object’s weight. The only way of putting a flight simulator into free fall while it remained stationary on the surface of the Earth would be somehow to suspend a massive body, such as another planet of similar mass, or a black hole, above

To distinguish between a real aircraft and a simulation, a pilot would only have to fly it in a free-fall trajectory and see whether weightlessness occurred or not.

Thus the laws of physics impose no limit on the range and accuracy of image generators.

a short-sighted view of science is that it is all about predicting our sense-impressions. The correct view is that, while sense-impressions always play a role, what science is about is understanding the whole of reality, of which only an infinitesimal proportion is ever experienced.

It is not customary to think of mathematics as being a form of virtual reality.

We make marks on pieces of paper and look at them, or we imagine looking at such marks — indeed, we cannot do mathematics without imagining abstract mathematical entities. But this means imagining an environment whose ‘physics’ embodies the complex and autonomous properties of those entities.

What may not be so obvious is that our ‘direct’ experience of the world through our senses is virtual reality too. For our external experience is never direct; nor do we even experience the signals in our nerves directly — we would not know what to make of the streams of electrical crackles that they carry. What we experience directly is a virtual-reality rendering, conveniently generated for us by our unconscious minds from sensory data plus complex inborn and acquired theories (i.e. programs) about how to interpret them.

Otherwise, the computer causes the brain to slow down (or, if necessary, to stop) until the calculation of what should happen next is complete; it then restores the brain’s normal speed. What would this feel like to the user? By definition, like nothing. The user would experience only the environment specified in the program, without any slowing down, stopping or restarting.

Thinking about futuristic devices based on computer-controlled nerve stimulation makes this obvious — in fact, almost too obvious.

quantum computers can perform computations of which no (human) mathematician will ever, even in principle, be capable.

The laws of physics, by conforming to the Turing principle, make it physically possible for those same laws to become known to physical objects. Thus, the laws of physics may be said to mandate their own comprehensibility.

Logically, we must concede to solipsism and related doctrines that the reality we are learning about might be an unrepresentative portion of a larger, inaccessible or incomprehensible structure.

Reasoning from the premise of one’s own existence is called ‘anthropic’ reasoning.

I think that I have solved a major philosophical problem: the problem of induction. Karl Popper

there is one opposing theory — namely, common sense — which reason requires me to refute in detail wherever it seems to conflict with what I am asserting.