The Fabric of Reality David Deutch

particle interference phenomena tell us that. What we need deep theories for
is to explain and predict such phenomena: to tell us what the other universes
are like, what laws they obey, how they affect one another, and how all this
fits in with the theoretical foundations of other subjects. That is what
quantum theory does. The quantum theory of parallel universes is not the
problem, it is the solution. It is not some troublesome, optional interpretation
emerging from arcane theoretical considerations. It is the explanation — the
only one that is tenable — of a remarkable and counter-intuitive reality.
So far, I have been using temporary terminology which suggests that one of
the many parallel universes differs from the others by being ‘tangible’. It is
time to sever that last link with the classical, single-universe conception of
reality. Let us go back to our frog. We have seen that the story of the frog
that stares at the distant torch for days at a time, waiting for the flicker that
comes on average once a day, is not the whole story, because there must
also be shadow frogs, in shadow universes that co-exist with the tangible
one, also waiting for photons. Suppose that our frog is trained to jump when
it sees a flicker. At the beginning of the experiment, the tangible frog will
have a large set of shadow counterparts, all initially alike. But shortly
afterwards they will no longer all be alike. Any particular one of them is
unlikely to see a photon immediately. But what is a rare event in any one
universe is a common event in the multiverse as a whole. At any instant,
somewhere in the multiverse, there are a few universes in which one of the
photons is currently striking the retina of the frog in that universe. And that
frog jumps.

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Why exactly does it jump? Because within its universe it obeys the same
laws of physics as tangible frogs do, and its shadow retina has been struck
by a shadow photon belonging to that universe. One of the light-sensitive
shadow molecules in that shadow retina has responded by undergoing
complex chemical changes, to which the shadow frog’s optic nerve has in
turn responded. It has transmitted a message to the shadow frog’s brain,
and the frog has consequently experienced the sensation of seeing a flicker.
Or should I say ‘the 
shadow sensation of seeing a flicker’? Surely not. If
‘shadow’ observers, be they frogs or people, are real, then their sensations
must be real too. When they observe what we might call a shadow object,
they observe that it is tangible. They observe this by the same means, and
according to the same definition, as we apply when we say that the universe
we observe is ‘tangible’. Tangibility is relative to a given observer. So
objectively there are not two kinds of photon, tangible and shadow, nor two
kinds of frog, nor two kinds of universe, one tangible and the rest shadow.
There is nothing in the description I have given of the formation of shadows,
or any of the related phenomena, that distinguishes between the ‘tangible’
and the ‘shadow’ objects, apart from the mere assertion that one of the
copies is ‘tangible’. When I introduced tangible and shadow photons I
apparently distinguished them by saying that we can see the former, but not
the latter. But who are ‘we’? While I was writing that, hosts of shadow Davids
were writing it too. They too drew a distinction between tangible and shadow
photons; but the photons they called ‘shadow’ include the ones I called
‘tangible’, and the photons they called ‘tangible’ are among those I called
‘shadow’.
Not only do none of the copies of an object have any privileged position in
the explanation of shadows that I have just outlined, neither do they have a
privileged position in the full mathematical explanation provided by quantum
theory. I may feel subjectively that I am distinguished among the copies as
the ‘tangible’ one, because I can directly perceive myself and not the others,
but I must come to terms with the fact that all the others feel the same about
themselves.
Many of those Davids are at this moment writing these very words. Some
are putting it better. Others have gone for a cup of tea.
TERMINOLOGY
photon A particle of light.
tangible/shadow For the purposes of exposition in this chapter only, I called