Reality → Energy → Motion → Microscopic motions
The botanist Brown was first to observe and study tiny, erratic movements of microscopic pollen floating in water (Brownian motion). Nearly a century later, Einstein came up with the correct explanation: the pollen, as relatively huge balls floating in a ‘sea’ of tiny water molecules, are bumped around by the vibrating water molecules which, in turn, get their movement from the thermal oscillation of their constituent atoms [1] . The kinetic gas theory explains heat and temperature as the result of statistically distributed kinetic energy of moving and colliding molecules. On this basis, it can be computed that, at normal conditions, air (i.e., mainly nitrogen) molecules have a mean velocity of about half the speed of sound, collide about a billion times per second and travel about half a micrometer, or two thousand times their diameter, between two collisions. Thermal movements are also ascribed to the atoms of solids, in which case they occur as vibrations around an atom’s bound position in a crystal [2] . The vibrations never stop, even at absolute zero they are still present [3] .
Einstein’s insight was considered one of the proofs that molecules and atoms really do exist.
While the kinetics of freely bouncing gas molecules follow the law of classical mechanics, the atomic and molecular vibrations in solids are subjected to the rules of quantum physics. They have terahertz-frequencies (about 1012 to 1014 Hz), detectable by infrared spectrography. The vibrations can be explained by a kinetic theory of solids that assumes the existence of phonons. Their study is part of the broader field of condensed matter physics.
Although the laws of thermodynamics tell that absolute zero can never be reached (the lowest temperature achieved so far is 10-7 K), the rules of quantum mechanics tell that there is zero-point energy and related movement of atoms in their ground state with lowest-possible, but non-zero energy.