Quantum mechanics is a body of scientific principles describing the behavior of matter and its interactions on the atomic and subatomic scales.
Many fundamental parts of the universe, such as photons (discrete units of light), behave in some ways like particles and in other ways like waves. Radiators of photons, such as neon lights, have emission spectra, but these spectra are discontinuous in that only certain frequencies are present. The laws of quantum mechanics predict the energies, the colours, and the spectral intensities of electromagnetic radiation.
But the same laws ordain that the more closely one pins down one
measure (such as the position of a particle), the less predictable
another measure pertaining to the same particle (such as its momentum) must become. Put another way, measuring position first and then measuring momentum does not
have the same outcome as measuring momentum first and then measuring
position. Even more disconcerting, pairs of particles can be created as
-- which means that an action that pins down one characteristic of one
particle will instantaneously pin down the same or other characteristic
of its entangled twin, regardless of the distance separating the