We consider a process in which a neutral spinless particle decays into a charged spin-1 particle ${V}^{\ifmmode\pm\else\textpm\fi{}}$ and a charged spin-0 particle. The latter then decays into a ${V}^{\ensuremath{\mp}}$ and another neutral spinless particle. It is shown that the spins of the
V+ and ${V}^{\ensuremath{-}}$ are correlated. This correlation, however, becomes vanishingly small as soon as it is possible to decide whether the
V+ or the ${V}^{\ensuremath{-}}$ was emitted first.
It is shown that for a free spinless particle described by the Kemmer equation, the Hamiltonian which Kemmer originally presented yields a velocity operator whose expectation value does not agree with the correspondence principle. Hamiltoniaos for a free spinless particle and a spinless particle in an electromagnetic field are constructed from the Kemmer equation. These Hairoi I toniaos differ from the ones originally presented by Kemmer. The free particle Hamiltonian yields a velocity operator whose expectation value is consistent with the correspondence principle.
In this paper, we shall derive a new conserved current for Klein-Gordon equation. The first component of this current is non-negative and reduces to
∣ϕ∣2 in the non-relativistic limit. Therefore, it can be interpreted as a suitable probability density for spinless particles. In addition, this current is time-like and so prevents faster than light particle propagation. We will see the probability density has a considerable deviation from
∣ϕ∣2 providing the uncertainty in momentum is much greater than
m0c.