Reality → Energy → Light → EM radiation
Maxwell developed the classical theory of electrodynamics which predicted the existence of electromagnetic waves that consist of linked electric and magnetic fields oscillating with equal phase in two planes perpendicular to each other and propagate through space at the speed of light. Hertz was first to produce and measure such waves in lab experiments with a simple apparatus consisting of an open electric oscillator and a primitive dipole antenna [1] , without knowing where his discovery would lead [2] . Today, we use wireless technologies that are based on the use of electromagnetism, without thinking about the intricacies of this fascinating natural phenomenon and its astonishing technical exploitation [3] .
In electrical engineering, a LC-circuit (with inductance L and capacitance C) produces oscillating current and voltage with a phase difference of π/2 and a frequency determined by L and C. If the circuit is opened and fitted with a suitably dimensioned antenna, the electric and magnetic fields associated with the oscillating voltage and current will separate from the antenna and move as free electromagnetic waves through space with the speed of light (the initial phase difference between electric and magnetic fields is lost after a short distance). The emitted waves can then be detected with a receiver antenna (Hertz used a simple loop, a precursor of the dipol antenna).
Hertz only wanted to prove Maxwell's theory and had no idea that his experiments would become an important milestone in the invention of radio, followed by further groundbreaking inventions throughout the 20th century (e.g., TV, aircraft and missile control, satellite telecommunication). Today, in the early 21st century, we experience an enormous expansion of mobile (i.e. wireless) communications and data transfer, with social and political repercussions.
Classical electromagnetism and its field theory are great achievements of theoretical physics. They opened the view for a wide spectrum of electromagnetic radiation and unified the older disciplines of electricity, magnetism, and optics. They also prepared the ground for quantum electrodynamics (QED), a theory built on paradoxical wave-particle duality. QED accurately describes, but not really explains, the intangible electromagnetic field by using complex mathematical tools (e.g., vector analysis, partial derivatives, line integrals, complex numbers, Lagrangians, and Hamiltonians). Based on the discoveries made, we now operate huge wireless networks without having final answers to the mysteries of fields, particles, and waves (see Particles, Note 4).