“Physics in Canada”
Book Review
“La Physique au Canada”
Critique de livre
Quantum Physics: Illusion or Reality, 2nd Edition, Alastair Rae, Cambridge University
Press, 2004, pp: 155, ISBN 0521542669 (pbk); Price: US$17
While Einstein’s quote about God not playing dice is now one of the clichés of modern
physics, it hardly does justice to the confounding nature of quantum physics. As I was
reading Alastair Rae’s fine and concise exposition, the Einstein quote that came to my mind
most often was recorded by Philip Frank when he visited Einstein in his Prague office (ca.
1911), which overlooked the garden of an insane asylum: “Those are the madmen,” Einstein
said, “who do not occupy themselves with the quantum theory.” Madmen are known for
confusing illusion with reality and it is this dichotomy that forms both the subtitle and theme
of Rae’s book, Quantum Physics: Illusion or Reality.
The book begins with a quick survey of the concepts of quantum physics necessary to
understand why classical determinism as described by Laplace (“We may regard the present
state of the universe as the effect of its past and the cause of its future.”) must be rejected. It
is interesting to see how little one needs to understand before going on to probe the
fundamental nature of reality. Specifically, one can get by with three concepts: (i) the dual
wave-particle nature of light and how both aspects are simultaneously revealed in two-slit
interference experiments; (ii) the concept of superposition, in which a wave can be described
as a sum of other waves; and (iii) the Heisenberg uncertainty principle, which limits our
ability to simultaneously measure two complementary physical quantities, such as the
position and momentum of a photon.
Because there are an unlimited number of outcomes of position and momentum, in
Chapter 2 Rae begins his analysis with simple polarisation experiments, which have only a
small number of outcomes. These experiments, in which a polarised or unpolarised beam of
light is sent into a pair of polarisers that will split the beam into two beams whose
polarisations differ by 90° (either horizontal/vertical or +45°/-45°), and variations thereof,
form the conceptual data of the book.
The intrigue begins right away with a description of a 45°-polarized beam incident on an
H/V (horizontal/vertical) polariser and then a pair of ±45° polarisers. The 45°-polarised
photons emerge as either horizontally or vertically polarised from the H/V polariser. Then
each polarised beam is sent into a separate ±45° polariser, from which they emerge at
random (polarised +45° or -45°), and then they are detected. Because the photons appear to
lose “memory” of their original polarisation, Rae concludes that “polarisation measurements
in general change the polarisation state of the measured photons.” Further, our inability to
predict the final polarisation states of the photons is inherent in the theory.
However, suppose now a reuniting crystal, H / V , is inserted into the apparatus between the
H/V polariser and the ±45° polariser. Surprisingly, the light emerging from right of the
H / V reuniter has the same polarisation as the light incident on the left of the H/V polariser
– why didn’t the photons lose their “memory” as before? Even when the beam intensity is
weakened such that single photons go through the apparatus, and therefore have nothing to
be reunited with, the same result occurs. An individual photon appears to emerge from both
Physics in Canada / La Physique au Canada, Vol. 62, No. 4, July/August 2006, pp. 217-218
channels of the H/V polariser. Or, if it passes along only one channel, do the photons
somehow “know” what they are going to do had they passed through the other?
From these simple experiments, Rae summarizes the two different ways of looking at
quantum phenomena: “Either the physical world is essentially indeterministic – the photons
don’t know what they are going to do… [or] they do know, but conspire to keep this
information hidden from us.” Given Einstein’s abhorrence to a dice-playing God, one can
now see his attraction to this latter interpretation, the so-called hidden variable theories.
It turns out that indeterminism wasn’t the greatest of Einstein’s concerns. Rather, as Rae
describes in Chapter 3, he realized that “quantum physics implies there is a non-local
interaction between separated particles, i.e. they appear to influence each other when far
apart.” This, effectively, was beyond Einstein’s concept of reality and why, together in a
paper with Podolsky and Rosen (the famous EPR paper), he described quantum mechanics
as incomplete. Unfortunately for him, Bell’s theorem (given a neat, minimally mathematical
proof here) shows that there is non-local interaction – hidden variable theories cannot
preserve locality.
For his part in the debate, Bohr responded by assuming the veracity of quantum mechanics
and accepting the instantaneous correlations between the well-separated particles. In the
Copenhagen interpretation, described in Chapter 4, a quantity can only be considered real “if
it has been measured or if it is in a measurement situation where the outcome is predictable.”
Bohr subsequently anticipates difficulties by saying they “arise only when we try to
extrapolate beyond the actual measured reality and to attribute ‘reality’ to the photons before
they interact with the apparatus” – both of which are forbidden by the Copenhagen
interpretation. What comes through here and elsewhere in the book is the notion of to what
degree “reality” can truly be probed.
The rest of the chapter, and the rest of the book, is devoted to the measurement problem –
when can we say the wave function has collapsed into an either/or state (rather than a
superposition)? Rae examines it from the point of view of consciousness (Chapter 5), the
many worlds interpretation (Chapter 6), spontaneous collapse due to interaction with a
macroscopic object (Chapter 7), whether Ilya Prigogine’s work on irreversible processes
offer a way out (Chapters 8 and 9), a new chapter on consistent histories (Chapter 10),
which allow the calculation of probabilities of events even if no measurement is made. In
the final chapter, Rae refreshingly gives his own opinions on the various interpretations.
This book is a crash course on the fundamentals of quantum mechanics. It is clearly written
and features numerous recapitulations to aid the reader. Figures are simple with good,
explanatory captions. While reading the book, I often had the feeling that this is an exciting
time for quantum mechanics research and philosophy, with much more excitement (and
possibly confusion) to come in the years ahead. It is highly recommended.
Daniel Hudon
Boston University
Boston, Massachusetts, USA
Physics in Canada / La Physique au Canada, Vol. 62, No. 4, July/August 2006, pp. 217-218
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