"(i) Physical states are represented by rays in Hilbert space."
"It has long seemed to me that a much better starting point is Wigner's definition of particles as representations of the inhomogeneous Lorentz group..."
"the underlying theory might not be a theory of fields or particles, but perhaps of something quite different, like strings."
"any relativistic quantum theory will look like a field theory when applied to particles at sufficiently low energy."
"All the states of negative energy are occupied except perhaps a few of small velocity."
"Dirac's analysis of the problem of negative probabilities in Schrodinger's relativistic wave equation would seem to rule out the existence of any particle of zero spin."
"it is difficult to agree that there is anything fundamentally wrong with the relativistic equation for zero spin that forced the development of the Dirac equation."
"we no longer have the picture of a vacuum with negative energy states filled up."