
Preface
Why have I written this book?
Perhaps
a more
burning
question for YOll
is: Why should you read it? .
.
i'
I
wrote
this
book
because in August 1987 I
made
a discovery that
~
shocked me. If,
before
this
date,
you
had
asked me
at
what stage in the
process
of
emission and subsequent detection
of
a
photon
its state
of
polarization
is
established, I would have answered:
At
thc
moment
of
emission,
of
coursel Imagine then
that
two
photons
emitted in rapid suc-
cession from
an
excited calcium
atom
are
obliged, by
the
laws
of
atomic
physics,
to
be emitted in opposite states
of
circular polarization: one left
circularly polarized
and
one
right circularly polarized. Surely, they set
off
from the
atom
towards
their respective detectors already in those
states
of
circular polarization. Yes?
Well,
...
no. I have since learned
that
this
view-the
assumption
that
the
physical states
of
quantum
particles like
photons
are
'real'
before they
are
measured -
is
called (rather disparagingly, I sometimes think) naive
realism. Now in the
J92O$
and 19305, some
of
the most famous figures
in
twentieth century physics were involved in a big debate
about
the
meaning
of
the new
quantum
theory and
its
implications for physical
reality. In August 1987 I knew a lillIe bit
about
this debate. But I had
assumed that
it
had
the
status
of
a philosophical
debate,
with little or no
, relevance
to
practical matters
that
could be settled in the laboratory. I
had been trained as a scientist,
and
although I enjoyed reading
about
philosophy (like I enjoyed listening
to
musk),
I was
too
busy with more
important
matters
to
dig deeply
into
the subject.
In
July
and
August
1987, I
made
a short study visit
to
the University
of
Wisconsin
at
Madison, where I bought a
book
(always dangerous)
from
the
University
bookstore.
This was a book published in celebration
of
the centenary
of
the birth
of
Niels Bohr. In it were lots'
of
articles
about
his
contribution
to
physics
and
his great
debate
with Einstein
on
the
meaning
of
quantum
theory.
One
of
these articles, written by
N. David Mermin, gave me a tremendous
shock. Mermin described the
results
of
experiments that
had
been carried
out
as recently as 198210 test
something called Bell's theorem using
two-photon 'cascade' emission