Over the past decade, however, growing evidence suggests that certain biological systems might employ quantum mechanics. In photosynthesis, for example, quantum effects help plants turn sunlight into fuel. Scientists have also proposed that migratory birds have a “quantum compass” enabling them to exploit Earth’s magnetic fields for navigation, or that the human sense of smell could be rooted in quantum mechanics.

Fisher’s notion of quantum processing in the brain broadly fits into this emerging field of quantum biology. Call it quantum neuroscience. He has developed a complicated hypothesis, incorporating nuclear and quantum physics, organic chemistry, neuroscience and biology. While his ideas have met with plenty of justifiable skepticism, some researchers are starting to pay attention. “Those who read his paper (as I hope many will) are bound to conclude: This old guy’s not so crazy,” wrote John Preskill, a physicist at the California Institute of Technology, after Fisher gave a talk there. “He may be on to something. At least he’s raising some very interesting questions.”

Senthil Todadri, a physicist at the Massachusetts Institute of Technology and Fisher’s longtime friend and colleague, is skeptical, but he thinks that Fisher has rephrased the central question—is quantum processing happening in the brain?—in such a way that it lays out a road map to test the hypothesis rigorously. “The general assumption has been that of course there is no quantum information processing that’s possible in the brain,” Todadri said. “He makes the case that there’s precisely one loophole. So the next step is to see if that loophole can be closed.” Indeed, Fisher has begun to bring together a team to do laboratory tests to answer this question once and for all.

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Fisher belongs to something of a physics dynasty: His father, Michael E. Fisher, is a prominent physicist at the University of Maryland, College Park, whose work in statistical physics has garnered numerous honors and awards over the course of his career. His brother, Daniel Fisher, is an applied physicist at Stanford University who specializes in evolutionary dynamics. Matthew Fisher has followed in their footsteps, carving out a highly successful physics career. He shared the prestigious Oliver E. Buckley Prize in 2015 for his research on quantum phase transitions.

So what drove him to move away from mainstream physics and toward the controversial and notoriously messy interface of biology, chemistry, neuroscience and quantum physics? His own struggles with clinical depression.

Fisher vividly remembers that February 1986 day when he woke up feeling numb and jet-lagged, as if he hadn’t slept in a week. “I felt like I had been drugged,” he said. Extra sleep didn’t help. Adjusting his diet and exercise regime proved futile, and blood tests showed nothing amiss. But his condition persisted for two full years. “It felt like a migraine headache over my entire body every waking minute,” he said. It got so bad he contemplated suicide, although the birth of his first daughter gave him a reason to keep fighting through the fog of depression.