  Two coupled oscillators provide a mathematical instrument for solving many
problems in modern physics, including squeezed states of light and Lorentz
transformations of quantum bound states. The concept of entanglement can also
be studied within this mathematical framework. For the system of two entangled
photons, it is of interest to study what happens to the remaining photon if the
other photon is not observed. It is pointed out that this problem is an issue
of Feynman's rest of the universe. For quantum bound-state problems, it is
pointed out the longitudinal and time-like coordinates become entangled when
the system becomes boosted. Since time-like oscillations are not observed, the
problem is exactly like the two-photon system where one of the photons is not
observed. While the hadron is a quantum bound state of quarks, it appears quite
differently when it moves rapidly than when it moves slowly. For slow hadrons,
Gell-Mann's quark model is applicable, while Feynman's parton model is
applicable to hadrons with their speeds close to that of light. While observing
the temperature dependence of the speed, it is possible to explain the
quark-to-parton transition as a phase transition.
