  In 1971, Feynman et al. published a paper on hadronic mass spectra and
transition rates based on the quark model. Their starting point was a
Lorentz-invariant differential equation. This equation can be separated into a
Klein-Gordon equation for the free-moving hadron and a harmonic oscillator
equation for the quarks inside the hadron. However, their solution of the
oscillator equation is not consistent with the existing rules of quantum
mechanics and special relativity. On the other hand, their partial differential
equation has many other solutions depending on boundary conditions. It is noted
that there is a Lorentz-covariant set of solutions totally consistent with
quantum mechanics and special relativity. This set constitutes a representation
of the Poincar\'e group which dictates the fundamental space-time symmetry of
particles in the Lorentz-covariant world. It is then shown that the same set of
solutions can be used as the mathematical basis for two-photon coherent states
or squeezed states in quantum optics. It is thus possible to transmit the
physics of squeezed states into the hadronic world. While the time-like
separation is the most puzzling problem in the covariant oscillator regime,
this variable can be interpreted like the unobserved photon in the two-mode
squeezed state which leads to an entropy increase.
