I have a question in reading Polchinski's string theory vol I p 123, about the "old covariant quantization".
It is said
... $\langle 0;k | 0; k' \rangle = ( 2\pi)^D \delta^D (k-k') \tag{4.1.15}$ as follows from momentum conservation. The timelike excitation has a negative norm.
How to see "the timelike excitation has a negative norm"? Is Dirac delta function in the RHS of Eq. (4.1.15) either $0$ or $\infty$?