"Aether and the theory of relativity"[3] was a title used by Einstein in a lecture on general relativity and aether theory. Einstein said that according to general relativity space is endowed with physical properties (the metric field), and one could use the word "ether", if one wished, to refer to this metric field, although he acknowledged that this meaning of the word "differs widely from that of the ether of the mechanical undulatory theory of light". In particular, the metric field of spacetime has no mechanical properties at all, not even a state of motion or rest. Its parts cannot be tracked over time. [4] The general attitude to this amongst physicists[who?] today is that although it is purely a matter of semantics, Einstein's comments stretch the word "aether" too far: it is argued that an "aether" with no mechanical properties doesn't correspond to the historical idea of aether, and so it is potentially misleading to apply this name to the spacetime field of general relativity.[citation needed]
Aether and quantum mechanics
Quantum mechanics can be used to describe spacetime as being "bitty" at extremely small scales, fluctuating and generating particle pairs that appear and disappear incredibly quickly. Instead of being "smooth", the vacuum is described as looking like "quantum foam". It has been suggested that this seething mass of virtual particles may be the equivalent in modern physics of a particulate aether.
Modern derivatives
In physics there is no concept considered exactly analogous to the aether. However, dark energy is sometimes called quintessence due to its similarity to the classical aether. Modern physics is full of concepts such as free space, spin foam, Planck particles, quantum wave state (QWS), zero-point energy, quantum foam, and vacuum energy.
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