  Since the particles such as molecules, atoms and nuclei are composite
particles, it is important to recognize that physics must be invariant for the
composite particles and their constituent particles, this requirement is called
particle invariance in this paper. But difficulties arise immediately because
for fermion we use Dirac equation, for meson we use Klein-Gordon equation and
for classical particle we use Newtonian mechanics, while the connections
between these equations are quite indirect. Thus if the particle invariance is
held in physics, i.e., only one physical formalism exists for any particle, we
can expect to find out the differences between these equations by employing the
particle invariance. As the results, several new relationships between them are
found, the most important result is that the obstacles that cluttered the path
from classical mechanics to quantum mechanics are found, it becomes possible to
derive the quantum wave equations from relativistic mechanics after the
obstacles are removed. An improved model is proposed to gain a better
understanding on elementary particle interactions. This approach offers
enormous advantages, not only for giving the first physically reasonable
interpretation of quantum mechanics, but also for improving quark model.
