Intuition is a funny thing. Loosely defined as the ability to know something without recourse to “conscious reasoning”, it is the notion that we can rely on “gut instinct” or the depths of our unconscious mind, to instinctively intuit knowledge. As the world we live in becomes more and more complex, knowing when to trust your gut and when to follow expert, rational discourse (especially of the scientific sort) is a difficult decision for many. Throw into that mix a subject as notoriously complicated, and often (wilfully) misconstrued, as quantum mechanics, and the brightest of human minds can be convinced that there exist connections, correlations and patterns between random events – whether or not they do. Physicist and author Paul Halpern explores this and other themes in his latest popular-science book, Synchronicity: the Epic Quest to Understand the Quantum Nature of Cause and Effect. If you, like me, are a bit unsure as to what exactly the word “synchronicity” means, the concept is rooted in psychology, not physics. First proposed in the 1920s by famed analytical psychologist Carl Jung, synchronicity referrers to the idea that some events are meaningfully linked or related, despite having no causal relationship. Jung wanted to encompass his idea that the collective unconscious of human experience is linked to our dreams, thoughts and behaviours. Indeed, he was “hoping to establish the reality of an acausal connecting principle”, writes Halpern. Jung’s inspiration for this came from the exciting new physics of relativity and quantum mechanics, via Albert Einstein and Wolfgang Pauli. Carl Jung’s inspiration for synchronicity came from the exciting new physics of relativity and quantum mechanics In a letter he sent to Einstein’s first biographer Carl Seelig in 1935, Jung wrote that “It was Einstein who started me thinking of a possible relativity of time as well as space, and their psychic conditionality. More than 30 years later, this stimulus led to my relation with the physicist Professor W Pauli and to my very thesis of psychic synchronicity.” As it is neither testable nor falsifiable, it’s easy to think of synchronicity as pseudoscience – and indeed Wikipedia dubs it so. With its “spooky” correlations and possible “hidden” mechanics, it’s no surprise that quantum theory inspired Jung to dream up a reality in which the laws of cause and effect could be avoided. READ MORE Dreams of a quantum pioneer Add in the complexities of the “observer effect” and the “measurement problem” – which, depending on your quantum-mechanical interpretation of choice, suggests that the mere act of conscious observation changes the outcome of a quantum experiment – and you can see why it’s a ripe playground for a psychologist. (For a detailed discussion on the subject, see the feature “Thirty years of ‘against measurement’”.) But many readers may be surprised to know that Pauli – a seemingly staunch realist – was as keen as Jung to explore the principle, and thought it a credible means of explaining a variety of so-called paranormal phenomena from entanglement to telepathy. Apart from his significant contributions to physics, Pauli is perhaps best known for his lancing wit and brutal take-downs of people and ideas he deemed foolish or careless. It’s hard to reconcile the Pauli who once famously put down a young physicist’s paper with the phrase “not even wrong” (now synonymous with “non-falsifiable”) with the Pauli whom Halpern describes as an “emotional wreck”. By the 1930s, thanks to troubles in the physicist’s personal life, combined with mental health issues and an alcohol problem, Pauli’s father suggested he seek help, in the form of therapy from Jung. The psychoanalyst first and foremost attempted to help Pauli navigate those turbulent times through the medium of a very detailed dream journal. It was a fortuitous coincidence for Jung, who was at the time developing his idea of synchronicity (then mainly based on relativistic concepts), to be able to run his thesis past Pauli. Jung and Pauli Influencers Wolfgang Pauli (right) and Carl Jung inspired each other’s work. (Courtesy: AIP Emilio Segrè Visual Archives, Physics Today Collection; Ortsmuseum Zollikon) From what I’ve mentioned in this review so far, it would be fair to think that Halpern’s book is based purely on the tale of Pauli and Jung, but that’s far from the case. In fact, the dense subject matter of the duo, their relationship and their academic collaboration is all packed into three chapters out of nine in this 300-page book. The rest is devoted to everything from a detailed view of relativity and particle physics, to cosmology and modern-day quantum computing. This is simultaneously both a strength and weakness of Synchronicity. For the uninitiated, the book gives an excellent if broad look at some of the key concepts and ideas of 21st-century physics. Halpern does fall into the seemingly too-tempting pop-sci trap of beginning his book with the ancient Greeks and their ideas of cosmology and philosophy; though it must be said that he is much more discerning in these three chapters than other authors who attempt to squeeze a couple of millennia of scientific advancement in the West into one chapter. For those of us who would much rather have delved deeper into the backgrounds of Jung and his contemporaries, and read about synchronicity, and the Jung–Pauli partnership, the book is more of a jumping-off point, rather than a treatise. Despite these quibbles, Synchronicity is an absolutely fascinating read, and Halpern is an excellent writer and researcher, despite (or perhaps indicated by) the dizzying number of names and dates in the book. As physics stands at the precipice of a technological quantum revolution, many physicists, philosophers and even psychologists are grappling more than ever with how quantum mechanics truly reflects our everyday reality, if at all. While entanglement between photons across thousands of kilometres is an experimental reality, what this means for our ideas on cause and effect, the speed of light in the universe, and “meaningful” correlations remains unclear. Over the course of his career, Jung described synchronicity in a number of ways – “acausal connecting (togetherness) principle”, “meaningful coincidence”, “acausal parallelism” – all of which seem vague, but maybe that’s the point. I couldn’t help but wonder why “correlation does not imply causation” wasn’t more of a guiding philosophy for the Jung (or indeed, latter-day Pauli). As Alanis Morrissette sagely sang about chance and coincidence, “It’s like ten thousand spoons when all you need is a knife; It’s meeting the man of my dreams, and then meeting his beautiful wife; And isn’t it ironic, don’t you think?”