  Four $SL(2,C)$ spinors are considered within the framework of Wigner's little
groups which dictate internal space-time symmetries of relativistic particles.
It is indicated that the little group for a massive particle at rest is $O(3)$,
while it is $O(3)$-like for a moving massive particle. The little group becomes
like $E(2)$ in the infinite-momentum/zero-mass limit. Spin-$\frac{1}{2}$
particles are studied in detail, and the origin of the gauge degrees of freedom
for massless particles is clarified. There are sixteen different combinations
of direct products of two $SL(2,C)$ spinors for spin-1 and spin-0 particles.
The state vectors for the $O(3)$ and $O(3)$-like little groups are constructed.
It is shown that in the infinite-momentum/zero-mass limit, these state vectors
become scalars, four-potentials and the Maxwell field tensor. It is revealed
that the Maxwell field tensor so obtained corresponds to some of the state
vectors constructed by Weinberg in 1964.
