Technically speaking, this is wrong, because it is in principle still
possible to have wave interference effects between sad Bob and
happy Bob. But such effects are impractical to observe, due to ef-
fects like short wavelengths and decoherence, so what Bob is doing
by “clearing the books” is an extremely good approximation. We
will refer to this approximation by two different names, theCopen-
hagen approximationand themany-worlds approximation, for the
following historical and psychological reasons.
In the early years of quantum mechanics, the school of physicists
centering on Niels Bohr in Copenhagen were horribly confused about
how to interpret quantum mechanics. They had all kinds of wrong
ideas, such as the idea that quantum mechanics applied to individ-
ual atoms but not to light or to macroscopic objects. They didn’t
know about decoherence. They thought there was a clear divid-
ing line between microscopic things and macroscopic things (there
isn’t), and they hypothesized that quantum mechanics only applied
to microscopic ones (it applies to both). They claimed that clearing
the books was an actual physical process, which they described as
the “collapse” of the wave. This has traditionally been referred to
as the Copenhagen “interpretation,” but we can now see that it is
an approximation. There are cases where it is a bad approximation,
e.g., att= 3 ps during the experiment by Leeet al.(p. 886), when
decoherence had started to happen but was only about half-way
complete.
The many-worlds approximation came along a little later.^5 It
consists of making the same “clearing the books” approximation,
but recognizing that there is no physical process of collapse.
Many physicists are philosophically attached to one or the other
of these approximations, and would object to my description of them
as approximations. My main purpose in writing this explanation is
to immunize you against the impression, which can be mistakenly
picked up from many descriptions of quantum mechanics, that a
particular point of view on these topics (often the Copenhagen ap-
proximation) is somehow “standard.”
13.2.5 Photons in three dimensions
Up until now I’ve been sneaky and avoided a full discussion of
the three-dimensional aspects of the probability interpretation. The
example of the carrot in the microwave oven, for example, reduced
to a one-dimensional situation because we were considering three
points along the same line and because we were only comparing
ratios of probabilities.
A typical example of a probability distribution in section 13.1(^5) It was originally proposed in a 1957 PhD thesis by Hugh Everett, who called
it the relative state interpretation of quantum mechanics. Later it began to be
referred to as the many-worlds interpretation.
Section 13.2 Light as a particle 887