I spent a week in Portadown one Sunday
Upon quantisation, the vibrational modes of the classical open string become an infinite number of quantum point particles, each of which corresponds to a state carrying an irreducible representation of the Poincaré group. At the free level these physical states are described in the old covariant approach of chapter 3 by the Virasoro conditions
The norm of a quantum state can be interpreted as a probability density and so for a string theory to be consistent the norms of all the solutions to equation (11.0.1) must be positive. ...