Massless scalar fields in 1+1 dimensions and Krein spaces
  We consider the Krein realization of the Hilbert space for a massless scalar field in 1+1 dimensions. We find convergence criteria and the completion of the space of test functions ${\cal S}$ with the topology induced by the Krein scalar product. Finally, we show that the interpretation for the Fourier components as probability amplitudes for the momentum operator is lost in this case. 
A. Z. Capri, R. Ferrari, V. Montalbano