In Synchronicity Paul Halpern tells the little-known story of the unlikely friendship between the Nobel-prize-winning quantum physicist Wolfgang Pauli and the father of psychoanalysis, Carl Jung. In the 1930s, Pauli and Jung began collaborating on a unified theory of quantum and the mind, the result of which was Jung's synchronicity principle-the idea that events connected by meaning need not be explained by causality. Pauli's work on entanglement theory, which allowed for instantaneous cause and effect relationships, was particularly appealing to Jung, as it seemed to give weight to his controversial theory of a collective unconscious.

Casting their relationship within a larger intellectual history of entanglement theory, Halpern poses a question that has mystified physicists and philosophers alike since the times of Aristotle: Is the speed of light finite, as Einstein posited, or is it, as Pauli and the proponents of entanglement theory asserted, variable across time and dimensions? As Halpern works his way through the history of the physics of cause and effect, he shows that this centuries-old debate is not only relevant at the smallest scales of particle physics but also at the largest scales of the cosmos itself.