In superstring theory, a picture is a choice of Fock space or, equivalently, a choice of ground state that defines a representation of the theory's state space.
Each picture is denoted by a number, such as the 0 picture or −1 picture, and picture-changing operators transform from one representation to another.
[1][2][3] The use of these operators in BRST quantization is credited to Daniel Friedan, Emil Martinec, and Stephen Shenker in the 1980s, though it has a predecessor in the dual models of the early 1970s.
[1][4][5] The difference between the ground states is indicated by the action of the superghost oscillators on them, and the number of the picture (plus 1/2) reflects the highest superghost oscillator which does not annihilate the ground state.
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