The Fabric of Reality David Deutch

part of the evidence that that entity is real. But it is not sufficient evidence.
We do not, for example, deem our reflections in a mirror to be real people.
Of course, illusions themselves are real physical processes. But the illusory
entities they show us need not be considered real, because they derive their
complexity from somewhere else. They are not 
autonomously complex. Why
do we accept the ‘mirror’ theory of reflections, but reject the ‘planetarium’
theory of the solar system? It is because, given a simple explanation of the
action of mirrors, we can understand that nothing of what we see in them
genuinely lies behind them. No further explanation is needed because the
reflections, though complex, are not autonomous — their complexity is
merely borrowed from our side of the mirror. That is not so for planets. The
theory that the cosmic planetarium is real, and that nothing lies beyond it,
only makes the problem worse. For if we accepted it, then instead of asking
only how the solar system works we should first have to ask how the
planetarium works, and 
then how the solar system it is displaying works. We
could not avoid the latter question, and it is effectively a repetition of what we
were trying to answer in the first place. Now we can rephrase Dr Johnson’s
criterion thus:
If, according to the simplest explanation, an entity is complex and
autonomous, then that entity is real.
Computational 
complexity theory is the branch of computer science that is
concerned with what resources (such as time, memory capacity or energy)
are required to perform given classes of computations. The complexity of a
piece of information is defined in terms of the computational resources (such

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as the length of the program, the number of computational steps or the
amount of memory) that a computer would need if it was to reproduce that
piece of information. Several different definitions of complexity are in use,
each with its own domain of applicability. The exact definitions need not
concern us here, but they are all based on the idea that a complex process
is one that in effect presents us with the results of a substantial computation.
The sense in which the motion of the planets ‘presents us with the results of
a substantial computation’ is well illustrated by a planetarium. Consider a
planetarium controlled by a computer which calculates the exact image that
the projectors should display to represent the night sky. To do this
authentically, the computer has to use the formulae provided by
astronomical theories; in fact the computation is identical to the one that it
would perform if it were calculating predictions of where an observatory
should point its telescopes to see real planets and stars. What we mean by
saying that the appearance of the planetarium is ‘as complex’ as that of the
night sky it depicts is that those two computations — one describing the
night sky, the other describing the planetarium — are largely identical. 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.
If Dr Johnson’s leg invariably rebounded when he extended it, then the
source of his illusions (God, a virtual-reality machine, or whatever) would
need to perform only a simple computation to determine when to give him
the rebounding sensation (something like ‘if leg-is-extended then rebound
…’). But to reproduce what Dr Johnson experienced in a realistic experiment
it would be necessary to take into account where the rock is, and whether Dr
Johnson’s foot is going to hit or miss it, and how heavy, how hard and how
firmly lodged it is, and whether anyone else has just kicked it out of the way,
and so on — a vast computation.
Physicists trying to cling to a single-universe world-view sometimes try to
explain quantum interference phenomena as follows: ‘No shadow photons
exist,’ they say, ‘and what carries the effect of the distant slits to the photon
we see is — nothing. Some sort of Action at a distance (as in Newton’s law
of gravity) simply makes photons change course when a distant slit is
opened.’ But there is nothing ‘simple’ about this supposed action at a
distance. The appropriate physical law would have to say that a photon is
affected by distant objects exactly 
as if something were passing through the
distant gaps and bouncing off the distant mirrors so as to intercept that
photon at the right time and place. Calculating how a photon reacts to these
distant objects would require the same computational effort as working out
the history of large numbers of shadow photons. The computation would
have to work its way through a story of what each shadow photon does: it
bounces off this, is stopped by that, and so on. Therefore, just as with Dr
Johnson’s rock, and just as with Galileo’s planets, a story that is in effect
about shadow photons necessarily appears in any explanation of the
observed effects. The irreducible complexity of that story makes it
philosophically untenable to deny that the objects exist.
The physicist David Bohm constructed a theory with predictions identical to
those of quantum theory, in which a sort of wave accompanies every photon,

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washes over the entire barrier, passes through the slits and interferes with
the photon that we see. Bohm’s theory is often presented as a single-
universe variant of quantum theory. But according to Dr Johnson’s criterion,
that is a mistake. Working out what Bohm’s invisible wave will do requires
the same computations as working out what trillions of shadow photons will
do. Some parts of the wave describe us, the observers, detecting and
reacting to the photons; other parts of the wave describe other versions of
us, reacting to photons in different positions. Bohm’s modest nomenclature
— referring to most of reality as a ‘wave’ — does not change the fact that in
his theory reality consists of large nets of complex entities, each of which
can perceive other entities in its own set, but can only indirectly perceive
entities in other sets. These sets of entities are, in other words, parallel
universes.
I have described Galileo’s new conception of our relationship with external
reality as a great methodological discovery. It gave us a new, reliable form of
reasoning involving observational evidence. That is indeed one aspect of his
discovery: scientific reasoning is reliable, not in the sense that it certifies that
any particular theory will survive unchanged, even until tomorrow, but in the
sense that we are right to rely on it. For we are right to seek solutions to
problems rather than sources of ultimate justification. Observational
evidence is indeed evidence, not in the sense that any theory can be
deduced, induced or in any other way inferred from it, but in the sense that it
can constitute a genuine reason for preferring one theory to another.
But there is another side to Galileo’s discovery which is much less often
appreciated. The reliability of scientific reasoning is not just an attribute of
us, of our knowledge and our relationship with reality. It is also a new fact
about physical reality itself, a fact which Galileo expressed in the phrase ‘the
Book of Nature is written in mathematical symbols’. As I have said, it is
impossible literally to ‘read’ any shred of a theory in nature: that is the
inductivist mistake. But what is genuinely out there is evidence, or, more
precisely, a reality that will respond with evidence if we interact appropriately
with it. Given a shred of a theory, or rather, shreds of several rival theories,
the evidence is available out there to enable us to distinguish between them.
Anyone can search for it, find it and improve upon it if they take the trouble.
They do not need authorization, or initiation, or holy texts. They need only be
looking in the right way — with fertile problems and promising theories in
mind. This open accessibility, not only of evidence but of the whole
mechanism of knowledge acquisition, is a key attribute of Galileo’s
conception of reality.
Galileo may have thought this self-evident, but it is not. It is a substantive
assertion about what physical reality is like. Logically, reality need not have
had this science-friendly property, but it does — and in abundance. Galileo’s
universe is saturated with evidence. Copernicus had assembled evidence for
his heliocentric theory in Poland. Tycho Brahe had collected his evidence in
Denmark, and Kepler had in Germany. And by pointing his telescope at the
skies over Italy, Galileo gained greater access to the same evidence. Every