Reality → Energy → Particles → Quantization
Planck's discovery of the quantization of black body radiation led to a new understanding of physics at the microscopic level. Unintuitive quantization applies to particles per se and to their properties such as spin and charge [1] . Discrete quantum numbers describe the electron configuration of the modern atomic model [2] . Quantum Electro Dynamics (QED) describe the interaction of matter and light (Feynman describes in his book QED in a captivating way how the particle and wave phenomena of light can be explained by quantum mechanics). In the Quantum Field Theory (QFT), which combines classical field theory and quantum mechanics, even fields are quantized [3] . With the exception of gravitation [4] , all matter, energy, and physical phenomena of our macro world can be reduced to microscopic quanta, listed in systematic arrangement in the standard model. It is an intriguing thought that the weird and unintuitive constructs of quantum mechanics yield very tangible results when applied in modern technologies [5] .
Elementary and composite particles possess spin, an abstract quantum mechanical property that manifests itself like a magnetic dipole in an inhomogeneous magnetic field (see Stern–Gerlach experiment and also nuclear magnetic resonance). The dimensionless spin quantum number indicates multiples of ħ (= h / 2 π = 1.05 × 10-34 Js, see Planck constant). Elementary particles that make up matter (fermions, see Standard model) have spin 1/2, and particles that mediate forces (bosons) have spin 1. Electrical charge occurs in multiples of the elementary charge. The term 'charge' is also attributed to the 'color' property (see color charge) of quarks and gluons, the constituents of protons and neutrons (see also Major discoveries, Note 7).
Four quantum numbers describe location, momentum, energy, and spin of the electron. The Pauli exclusion principle forbids that two electrons with the same four quantum numbers occupy the same orbital. The principle applies to all particles with spin 1/2 (fermions) and explains the rigidity of atoms and matter despite the huge voids inside atoms.
See also Modern Physics, Note 14, on Pauli's work.
In quantum field theory, particles are interpreted as exited states of underlying fields. The mathematics involved are daunting and the whole theory is inaccessible and incomprehensible for laymen. The press release of the 1999 physics Nobel prize contains a somewhat readable account of the mathematical hurdles faced in quantum field theory.
General relativity explains gravitation as a continuous (non-quantized) phenomenon marked by spacetime curved under the influence of mass. The theory also predicts gravitational waves. Many unsuccessful attempts have been made to reconcile gravitation with quantum mechanics (see quantum gravity and graviton).
Lasers, solar cells, medical imaging (MRI and PET scans), and other innovations are brought about by quantum mechanics. Possibly, some modern technologies could also have been developed solely on the basis of experimental discoveries without the underpinning by quantum theory, but the progress would likely have been slower and we would still ask how and why the underlying physical phenomena do occur, questions for which there are apparently no better answers than the weird and unintuitive concepts of quantum mechanics.