1) Physics: A hypothetical medium filling all space formerly postulated to
account for the propagation of → electromagnetic radiation
through space. In order to facilitate description and to provide a physical explanation of
various phenomena involving action at a distance and electromagnetism, a medium
had been postulated with mechanical properties adjusted to provide a consistent theory.
In 1887 Michelson and Morley attempted to measure the motion of the Earth
through the ether. No such motion was detected. The
→ Michelson-Morley experiment has been repeated under
different conditions, but the hypothesis of a stationary ether through which the
Earth moves is not verified.
From L. æther "the upper air, pure air," from Gk. aither "upper air," from aithein "to burn, shine."
Fr.: entraînement de l'éther
A hypothesis put forward to explain the null measurement of the → ether drift. According to this hypothesis, the Earth somehow drags the ether with it as our planet rotates on its axis and revolves around the Sun. However, the ether drag hypothesis contradicts results from several experiments, including the → aberration of starlight.
Fr.: dérive de l'éther
1) The hypothetical motion of the supposed → ether relative to
the Earth. The → Michelson-Morley experiment found no
ether drift. An analogy is given by a boat drifting in a fast-flowing river due
to the river's current. With the same power, the speed would be slower
when sailing across the stream. Moreover, in order to reach
directly opposite the starting point, the boat would have to be headed
somewhat upstream. See also the → ether drag.
Fr.: théorème de Noether
A → symmetry in a physical system leads to a → conserved quantity. For example, symmetry under → translation corresponds to conservation of → momentum, symmetry under → rotation to conservation of → angular momentum, and symmetry in → time to conservation of → energy. The Noether symmetry theorem is a fundamental tool of modern theoretical physics and the calculus of variations, allowing to derive conserved quantities from the existence of variational symmetries.
Named in honor of the German-American woman mathematician Amalie Emmy Noether (1182-1935), who published the theorem in 1918 ("Invariante Variationsprobleme," Nachr. D. König. Gesellsch. D. Wiss. Zu Göttingen, Math-phys. Klasse 1918: 235-257).