Psi's two most prominent modern uses sit at opposite ends of respectability within the sciences. In quantum mechanics, physicists conventionally use an uppercase Psi to denote the wave function in the Schrodinger equation, formulated by Erwin Schrodinger in 1925 and 1926, a mathematical object that encodes the probability of finding a particle in a given state and remains one of the central objects of modern physics. In parapsychology, by contrast, researchers from the mid-20th century onward adopted lowercase psi as an umbrella term for claimed psychic phenomena such as telepathy and precognition, a use popularised by the parapsychologist Robert Thouless in 1942 specifically because the letter carried no prior connotation that might bias interpretation of the phenomena under study; mainstream science has never validated the phenomena the term covers, and the psi research programme remains outside the scientific consensus.
Astronomers, following the Bayer star-naming convention of 1603, also use Psi for one of the fainter numbered stars in a constellation, a third, much quieter modern role with no connection to either of the other two. In the Milesian numeral system, discussed elsewhere in this atlas, psi carries the value seven hundred, a value with no bearing on quantum mechanics, parapsychology or star catalogues alike.