# Negative norm for bosons and fermions

How comes negative norm is such a big thing for fermions that Dirac equation was chosen over KG, while the same kind of problem for photons is just ok? In fact, many books in QED do not even mention that the norm has to be indefinite to preserve covariance.

Dirac equation was chosen over KG only in the early days of QFT, when people haven't discovered second quantization yet. The problem was the apparent negative probability of observing a KG particle. Nowadays it is generally accepted that both KG and Dirac give consistent QFTs of particles with spins $0$ and $1/2$ respectively. All states in the 2nd-quantized theory are positive norm.