The Fabric of Reality David Deutch

particles in this universe 
tangible, and particles in other universes shadow
particles.
multiverse The whole of physical reality. It contains many parallel universes.
parallel universes They are ‘parallel’ in the sense that within each universe
particles interact with each other just as they do in the tangible universe, but
each universe affects the others only weakly, through interference
phenomena.
quantum theory The theory of the physics of the multiverse.

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 quantization The property of having a discrete (rather than continuous) set
of possible values. Quantum theory gets its name from its assertion that all
measurable quantities are quantized. However, the most significant quantum
effect is not quantization but interference.
interference The effect of a particle in one universe on its counterpart in
another. Photon interference can cause shadows to be much more
complicated than mere silhouettes of the obstacles causing them.
SUMMARY
In interference experiments there can be places in a shadow-pattern that go
dark when new openings are made in the barrier casting the shadow. This
remains true even when the experiment is performed with individual
particles. A chain of reasoning based on this fact rules out the possibility that
the universe we see around us constitutes the whole of reality. In fact the
whole of physical reality, the multiverse, contains vast numbers of parallel
universes.
Quantum physics is one of the four main strands of explanation. The next
strand is epistemology, the theory of knowledge.

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3 
Problem-solving
 
I do not know which is stranger — the behaviour of shadows itself, or the
fact that contemplating a few patterns of light and shadow can force us to
revise so radically our conception of the structure of reality. The argument I
have outlined in the previous chapter is, notwithstanding its controversial
conclusion, a typical piece of scientific reasoning. It is worth reflecting on the
character of this reasoning, which is itself a natural phenomenon at least as
surprising and full of ramifications as the physics of shadows.
To those who would prefer reality to have a more prosaic structure, it may
seem somehow out of proportion — unfair, even — that such momentous
consequences can flow from the fact that a tiny spot of light on a screen
should be 
here rather than there. Yet they do, and this is by no means the
first time in the history of science that such a thing has happened. In this
respect the discovery of other universes is quite reminiscent of the discovery
of other planets by early astronomers. Before we sent space probes to the
Moon and planets, 
all our information about planets came from spots of light
(or other radiation) being observed in one place rather than another.
Consider how the original, defining fact about planets — the fact that they
are not stars — was discovered. Watching the night sky for a few hours, one
sees that the stars appear to revolve about a particular point in the sky. They
revolve rigidly, holding fixed positions relative to one another. The traditional
explanation was that the night sky was a huge ‘celestial sphere’ revolving
around the fixed Earth, and that the stars were either holes in the sphere or
glowing embedded crystals. However, among the thousands of points of
light in the sky visible to the naked eye, there are a handful of the brightest
which, over longer periods, do not move as if they were fixed on a celestial
sphere. They wander about the sky in more complex motions. They are
called ‘planets’, from the Greek word meaning ‘wanderer’. Their wandering
was a sign that the celestial-sphere explanation was inadequate.
Successive explanations of the motions of planets have played an important
role in the history of science. Copernicus’s 
heliocentric theory placed the
planets and the Earth in circular orbits round the Sun. Kepler discovered that
the orbits are ellipses rather than circles. 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.
Nevertheless, a few decades later Einstein’s general theory of relativity gave
us a fundamentally different explanation of gravity, in terms of curved space
and time, and thereby predicted slightly different motions again. For
instance, it correctly predicted that every year the planet Mercury would drift
by about one ten-thousandth of a degree away from where Newton’s theory
said it should be. It also implied that starlight passing close to the Sun would
be deflected twice as much by gravity as Newton’s theory would predict. The
observation of this deflection by Arthur Eddington in 1919 is often deemed to
mark the moment at which the Newtonian world-view ceased to be rationally
tenable. (Ironically, modern reappraisals of the accuracy of Eddington’s

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experiment suggest that this may have been premature.) The experiment,
which has since been repeated with great accuracy, involved measuring the
positions of spots (the images of stars close to the limb of the Sun during an
eclipse) on a photographic plate.
As astronomical predictions became more accurate, the differences between
what successive theories predicted about the appearance of the night sky
diminished. Ever more powerful telescopes and measuring instruments have
had to be constructed to detect the differences. However, the explanations
underlying these predictions have not been converging. On the contrary, as I
have just outlined, there has been a succession of revolutionary changes.
Thus observations of ever smaller physical effects have been forcing ever
greater changes in our world-view. It may therefore seem that we are
inferring ever grander conclusions from ever scantier evidence. What
justifies these inferences? Can we be sure that just because a star appeared
millimetrically displaced on Eddington’s photographic plate, space and time
must be curved; or that because a photodetector at a certain position does
not register a ‘hit’ in weak light, there must be parallel universes?
Indeed, what I have just said understates both the fragility and the
indirectness of all experimental evidence. For we do not directly perceive the
stars, spots on photographic plates, or any other external objects or events.
We see things only when images of them appear on our retinas, and we do
not perceive even those images until they have given rise to electrical
impulses in our nerves, and those impulses have been received and
interpreted by our brains. Thus the physical evidence that directly sways us,
and causes us to adopt one theory or world-view rather than another, is less
than millimetric: it is measured in thousandths of a millimetre (the separation
of nerve fibres in the optic nerve), and in hundredths of a volt (the change in
electric potential in our nerves that makes the difference between our
perceiving one thing and perceiving another).
However, we do not accord equal significance to all our sensory
impressions. In scientific experiments we go to great lengths to bring to our
perceptions those aspects of external reality that we think might help us to
distinguish between rival theories we are considering. Before we even make
an observation, we decide carefully where and when we should look, and
what we should look for. Often we use complex, specially constructed
instruments, such as telescopes and photomultipliers. Yet however
sophisticated the instruments we use, and however substantial the external
causes to which we attribute their readings, we perceive those readings
exclusively through our own sense organs. There is no getting away from the
fact that we human beings are small creatures with only a few inaccurate,
incomplete channels through which we receive all information from outside
ourselves. We interpret this information as evidence of a large and complex
external universe (or multiverse). But when we are weighing up this
evidence, we are literally contemplating nothing more than patterns of weak
electric current trickling through our own brains.
What justifies the inferences we draw from these patterns? It is certainly not
a matter of logical deduction. There is no way of 
proving from these or from
any other observations that the external universe, or multiverse, exists at all,
let alone that the electric currents received by our brains stand in any