In the Middle Ages, the quintessence (from the Latin quinta essentia) was a hypothetical element, also called ether (from the Latin æthēr and this from the Greek aithēr). Known in pre-Socratic times as the fifth element, it was often considered along with the four classical elements of nature: earth, water, fire, and air. The ether, or quintessence, was a hypothetical extremely light substance that was believed to occupy all empty spaces as a fluid. It was later used by 19th century physicists as a substrate for the propagation of light waves, in the same way that sound propagates in a material body, or waves in water, since it seemed inconceivable that a wave would propagate in a vacuum without material support. The experiments of Michelson and Morley at the end of the 19th century showed the impossibility of measuring the velocity of the Earth with respect to this hypothetical medium, which led Einstein to propose his theory of special relativity and to banish the concept of the ether as a universal and intangible substrate that permeates empty space.
However, the idea of the ether or quintessence seems to have had a revival in the modern concept of dark energy as the substance responsible for the current acceleration of the universe. However, this does not mean that we are dealing with the same substrate, nor that we have abandoned Einstein’s theory of relativity. The modern ether satisfies the laws of special and general relativity. What has changed is the concept of vacuum. This describes a physical state of absence of particles, a space-time without matter, but possibly with curvature and, therefore, energy.
Such a space-time state is invariant under local Lorentz transformations and is also general covariant. The only distribution of matter that satisfies these conditions is a constant with pressure dimensions, known as the cosmological constant and introduced by Einstein in 1917 in his first cosmological model. Later, with the development of quantum physics and the discovery of Heisenberg’s uncertainty principle, it was realized that the vacuum of particles cannot in fact be empty of energy, since it is always possible to create virtual particle-antiparticle pairs of the vacuum, which disappear again in an infinitesimal time interval, in accordance with the uncertainty principle. This «boiling» of virtual particles contributes to the vacuum energy exactly as a cosmological constant.
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