It is possible to construct representations of the Lorentz group using
four-dimensional harmonic oscillators. This allows us to construct
three-dimensional wave functions with the usual rotational symmetry for
space-like coordinates and one-dimensional wave function for time-like
coordinate. It is then possible to construct a representation of the Poincar\'e
group for a massive particles having the O(3) internal space-time symmetry in
its rest frame. This oscillator can also be separated into two transverse
components and the two-dimensional world of the longitudinal and time-like
coordinates. The transverse components remain unchanged under Lorentz boosts,
while it is possible to construct the squeeze representation of the $O(1,1)$
group in the space of the longitudinal and time-like coordinates. While the
squeeze representation forms the basic language for squeezed states of light,
it can be combined with the transverse components to form the representation of
the Poincar\`e group for relativistic extended particles.