The Fabric of Reality David Deutch

partially transparent barriers have the same degree of transparency for
shadow photons as for tangible ones, it follows that not all the shadow atoms
in the path of a particular shadow photon can be involved in blocking its
passage. Each shadow photon encounters much the same sort of barrier as
its tangible counterpart does, a barrier consisting of only a tiny proportion of
all the shadow atoms that are present.
For the same reason, each shadow atom in the barrier can be interacting
with only a small proportion of the other shadow atoms in its vicinity, and the
ones it does interact with form a barrier much like the tangible one. And so
on. All matter, and all physical processes, have this structure. If the tangible
barrier is the frog’s retina, then there must be many shadow retinas, each
capable of stopping only one of the shadow-counterparts of each photon.
Each shadow retina only interacts strongly with the corresponding shadow
photons, and with the corresponding shadow frog, and so on. In other words,
particles are grouped into 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.
Thus we have reached the conclusion of the chain of reasoning that begins
with strangely shaped shadows and ends with parallel universes. Each step
takes the form of noting that the behaviour of objects that we observe can be

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explained only if there are unobserved objects present, and if those
unobserved objects have certain properties. The heart of the argument is
that single-particle interference phenomena unequivocally rule out the
possibility that the tangible universe around us is all that exists. There is no
disputing the fact that such interference phenomena occur. Yet the existence
of the multiverse is still a minority view among physicists. Why?
The answer, I regret to say, does not reflect well upon the majority. I shall
have more to say about this in Chapter 13, but for the moment let me point
out that the arguments I have presented in this chapter are compelling only
to those who seek explanations. Those who are satisfied with mere
prediction, and who have no strong desire to understand how the predicted
outcomes of experiments come about, may if they wish simply deny the
existence of anything other than what I have been calling ‘tangible’ entities.
Some people, such as instrumentalists and positivists, take this line as a
matter of philosophical principle. I have already said what I think of such
principles, and why. Other people just don’t want to think about it. After all, it
is such a 
large conclusion, and such a disturbing one on first hearing. But I
think that those people are making a mistake. As I hope to persuade readers
who bear with me, understanding the multiverse is a precondition for
understanding reality as best we can. Nor is this said in a spirit of grim
determination to seek the truth no matter how unpalatable it may be (though
I hope I would take that attitude if it came to it). It is, on the contrary,
because the resulting world-view is so much more integrated, and makes
more sense in so many ways, than any previous world-view, and certainly
more than the cynical pragmatism which too often nowadays serves as a
surrogate for a world-view among scientists.
‘Why can’t we just say,’ some pragmatic physicists ask, ‘that photons behave
as if they were interacting with invisible entities? Why can we not leave it at
that? Why do we have to go on to take a position about whether those
invisible entities are really there?’ A more exotic variant of what is essentially
the same idea is the following. ‘A tangible photon is real; a shadow photon is
merely a way in which the real photon could possibly have behaved, but did
not. Thus quantum theory is about the 
interaction of the real with the
possible.’ This, at least, sounds suitably profound. But unfortunately the
people who take either of these views — including some eminent scientists
who ought to know better — invariably lapse into mumbo-jumbo at that point.
So let us keep cool heads. The key fact is that a real, tangible photon
behaves differently according to what paths are open, elsewhere in the
apparatus, for something to travel along and eventually intercept the tangible
photon. Something does travel along those paths, and to refuse to call it
‘real’ is merely to play with words. ‘The possible’ cannot interact with the
real: non-existent entities cannot deflect real ones from their paths. If a
photon is deflected, it must have been deflected by something, and I have
called that thing a ‘shadow photon’. Giving it a name does not make it real,
but it cannot be true that an actual event, such as the arrival and detection of
a tangible photon, is caused by an imaginary event such as what that photon
‘could have done’ but did not do. It is only what really happens that can
cause other things really to happen. If the complex motions of the shadow
photons in an interference experiment were mere possibilities that did not in
fact take place, then the interference phenomena we see would not, in fact,

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take place.
The reason why interference effects are usually so weak and hard to detect
can be found in the quantum-mechanical laws that govern them. Two