One of the main challenges for researchers attempting to prove the existence of the paranormal is to develop an experiment that can be repeated and verified by other scientists. For most of the twentieth century, the most serious criticism of parapsychology—the study of psychic phenomena, often referred to as psi—was that it had not yielded any reliable display of psychic ability that could be replicated by other investigators. In the mid-1970s, however, several researchers adapted a technique from experimental psychology that promised to provide a repeatable demonstration: the ganzfeld procedure.

In the ganzfeld experiment, a person whose psychic ability is being tested (the receiver) is isolated in a room and subjected to a mild form of sensory deprivation. In a separate room, a sender concentrates on a randomly selected video clip or photograph. After thirty...