Science Fact: Psi, the 23rd (penultimate) letter of the Greek alphabet, has many meanings. When you’re using math to describe research results or propose physical mechanisms, you use symbols as shorthand for things so that you can manipulate the equations. In English and Western European scientific writing, Greek letters are a popular choice because they are easily recognized and are obviously a symbol rather than part of the text. And sometimes an arbitrarily chosen symbol will be used in a particular context by so many people that it comes to adopt that as one of its meanings.
This post concerns two of the specific meanings that have been assigned to this symbol and seem to have stuck:
First, quantum physics: Psi represents the wave function – used by Erwin Schrodinger in 1925 (and published in 1926) in developing his wave equation, a key equation in quantum mechanics, which was later interpreted by Max Born as a “probability amplitude.” The absolute square of the probability amplitude is interpreted by physicists as a probability or probability density, which describes the likelihood of obtaining a particular result from a scientific measurement.
Science Speculation: But Psi also has a role in parapsychology, which is often considered anti-science, or at least fringe science: It represents extrasensory perception (ESP) and psychokinesis (PK) — this use is credited to biologist Berthold P. Wiesner and psychologist Robert H. Thouless in 1942.
Therefore: Fact; and Speculation. The two meanings are inherently contradictory:
– Psi as wave function: no one can see it, but scientists believe in it
– Psi as psychic phenomena: some people see it all around us if they look with a credible eye (coincidences, “small world” phenomena, premonitions, dreams), but scientists disbelieve it.
For more about ESP, see my post Physics and Psi.