Modern physics is an effort to understand the underlying processes of the interactions of matter utilizing the tools of science & engineering. It implies that nineteenth century descriptions of phenomena are not sufficient to describe nature as observed with modern instruments. It is generally assumed that a consistent description of these observations will incorporate elements of quantum mechanics & relativity.
Small velocities and large distances is usually the realm of classical physics. Modern physics often involves extreme conditions; in practice, quantum effects typically involve distances comparable to atoms (roughly 10−9 m), while relativistic effects typically involve velocities comparable to the speed of light (roughly 108 m/s).