SCIENCE sciencemag.org 25 SEPTEMBER 2020 • VOL 369 ISSUE 6511 1573PHOTO: JURIK PETER/SHUTTERSTOCK
I
f you read a randomly selected non-
technical account of quantum en-
tanglement, you will likely be told
that measuring a particle in one place
can instantly change another particle
elsewhere, no matter the distance be-
tween the two. Surprisingly, this is some-
thing that Paul Halpern never claims in
his new book, Synchronicity. As the title
implies, the central concept of his century-
spanning tale is “synchronicity,” a term
coined by psychoanalyst Carl Jung, who
was inspired by conversations with physi-
cists Albert Einstein and Wolfgang Pauli,
to connote an “acausal connecting prin-
ciple.” Quantum correlations, in Halpern’s
view, are a special case of synchronicity.
The book weaves together two themes, the
first being the centuries-long development of
the notion of causality. Halpern does not explic-
itly define causality, but it is what Einstein had
in mind when he spoke of the principle of con-
tiguity: influence propagating at finite speed
without leaping over gaps. Electromagnetism
is a paradigm case. The book’s other theme is
the history of various noncausal explanatory
schemes. Some of these veer toward wishfulQUANTUM MECHANICSBy Allen StairsBOOKS et al.
Understanding quantum
cause and effect
thinking (for example, Kepler’s attempt to fit
the orbits of the planets into a scheme based
on the five Platonic solids) or outright woo
(for example, Pythagorean number mysti-
cism or some of Jung’s more extravagant
claims about archetypes). But others pro-
vide deep insights into the physical world.
Halpern devotes a chapter to
symmetry arguments, giving lay
readers a window into a critical
feature of physical theory and
an introduction to the brilliant
mathematician Emmy Noether.
Explanations of connection that
rely on symmetry are noncausal,
but Halpern is also open to other
acausal forms of explanation.
For him, quantum entanglement
begs to be understood as a kind
of acausal connection. Much of
the chapter called “Reality’s Rodeo” is de-
voted to this topic and to John Bell’s cele-
brated theorem, which postulates that ordi-
nary causal explanations cannot account for
quantum correlations.
Correlations between outcomes of mea-
surements performed on entangled par-
ticles inspired the idea of “spooky action
at a distance.” However, correlation at a
distance needn’t necessarily be spooky. If
a particle’s behavior is determined by the
local environment and by properties the
particle acquired at its source—if it satisfiesso-called “local realism”—then it is conceiv-
able that the source could orchestrate cor-
relations between what is inscribed in each
of a pair of particles.
Bell showed that this idea cannot be ex-
tended to quantum mechanics. He uncov-
ered constraints on how probabilistic corre-
lations can be knit together by local realistic
models. Such models predict statistics that
obey what we now call “Bell inequalities,”
but these statistics are inconsistent with
quantum theory, and experimental data
support quantum theory.
Halpern favors an acausal, synchronistic
approach to quantum correlations, although
just what that might amount to is still an
open question. There are serious arguments
to be made for thinking that quantum cor-
relations require superluminal action at a
distance, but that does not sound like syn-
chronicity. David Bohm’s development of
de Broglie’s pilot wave theory, which posits
high-dimensional waves that guide quan-
tum particles, incorporates such influences.
However, Halpern notes that Bohm’s later
work develops the idea of an “implicate or-
der” underlying space-time. This may be a
better fit for the idea of synchronicity. Some
physicists even believe that quantum en-
tanglement might be the ultimate source
of space-time. If so, causality would rest
on entanglement.
More generally, instead of looking for an
explanation for quantum entanglement, we
might consider it part of our basic explana-
tory machinery. Quantum states
fit together in Hilbert space, an
elegant abstraction from ordi-
nary physical space that Halpern
describes fancifully as akin to “a
kind of network of utility cor-
ridors behind the scenes of an
active office building.” If each
of two particles is represented
by a Hilbert space, the only way
to build a Hilbert space for the
pair guarantees the existence of
entangled states, with their at-
tendant unorthodox statistical predictions.
So-called spooky correlations could exist in
such a scenario without causal mechanisms.
Whatever one’s position, Synchronicity is
an entertaining read and provides a valu-
able service. Although they are important,
causal mechanisms are not the only tools
available for understanding the world. The
book invites nonspecialists to widen their
appreciation of the range of what explana-
tion in science could amount to. j10.1126/science.abe0805Quantum entanglement begs to be understood as
a kind of acausal connection, argues Halpern.Synchronicity
Paul Halpern
Basic Books, 2020.
304 pp.The reviewer is at the Department of Philosophy,
University of Maryland, College Park, MD 20742, USA.
Email: [email protected]Correlations at a distance needn’t necessarily be “spooky”