If you read a randomly selected nontechnical account of quantum entanglement, you will likely be told that measuring a particle in one place can instantly change another particle elsewhere, no matter the distance between the two. Surprisingly, this is something 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 physicists Albert Einstein and Wolfgang Pauli, to connote an “acausal connecting principle.” 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 explicitly define causality, but it is what Einstein had in mind when he spoke of the principle of contiguity: 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 wishful 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 mysticism or some of Jung’s more extravagant claims about archetypes). But others provide 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 devoted to this topic and to John Bell’s celebrated theorem, which postulates that ordinary causal explanations cannot account for quantum correlations.

Correlations between outcomes of measurements performed on entangled particles 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 satisfies so-called “local realism”—then it is conceivable that the source could orchestrate correlations between what is inscribed in each of a pair of particles.

Bell showed that this idea cannot be extended to quantum mechanics. He uncovered constraints on how probabilistic correlations 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 correlations require superluminal action at a distance, but that does not sound like synchronicity. David Bohm’s development of de Broglie’s pilot wave theory, which posits high-dimensional waves that guide quantum particles, incorporates such influences. However, Halpern notes that Bohm’s later work develops the idea of an “implicate order” underlying space-time. This may be a better fit for the idea of synchronicity. Some physicists even believe that quantum entanglement 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 explanatory machinery. Quantum states fit together in Hilbert space, an elegant abstraction from ordinary physical space that Halpern describes fancifully as akin to “a kind of network of utility corridors 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 attendant 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 valuable 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 explanation in science could amount to.

#### About the author

*The reviewer is at the Department of Philosophy, University of Maryland, College Park, MD 20742, USA.*