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Quantum foam, also referred to as spacetime foam, is a concept in quantum mechanics, devised by John Wheeler in 1955. The foam is a qualitative description of the turbulence that the phenomenon creates at extremely small distances of the order of the Planck length. At such small scales of time and space the uncertainty principle allows particles and energy to briefly come into existence, and then annihilate, without violating conservation laws. As the scale of time and space being discussed shrinks, the energy of the virtual particles increases. Since energy curves spacetime according to Einstein's theory of general relativity, this suggests that at sufficiently small scales the energy of the fluctuations would be large enough to cause significant departures from the smooth spacetime seen at larger scales, giving spacetime a "foamy" character.[1] However, without a theory of quantum gravity it is impossible to be certain what spacetime would look like at these scales, since it is thought that existing theories would no longer give accurate predictions in this domain.

Quantum foam is theorized to be created by virtual particles of very high energy. Virtual particles appear in quantum field theory, where they arise briefly and then annihilate during particle interactions, in such a way that they affect the measured outputs of the interaction even though the virtual particles are themselves never directly observed. They can also appear and annihilate briefly in empty space, and these "vacuum fluctuations" affect the properties of the vacuum, giving it a nonzero energy known as vacuum energy, a type of zero-point energy (however, physicists are uncertain about the magnitude of this energy[2]). The Casimir effect can also be understood in terms of the behavior of virtual particles in the empty space between two parallel plates. Ordinarily quantum field theory does not deal with virtual particles of sufficient energy to curve spacetime significantly, so quantum foam is a speculative extension of these concepts which imagines the consequences of such high-energy virtual particles at very short distances and times.

The "foamy" spacetime would look like a complex turbulent storm-tossed sea. Some physicists theorize the formation of wormholes therein; speculation arising from this includes the possibility of hyperspatial links to other universes.

Reginald Cahill has developed a theory called Process physics, which describes space as a quantum foam system in which gravity is an inhomogeneous flow of the quantum foam into matter. Cahill's version of the quantum foam would have a specific rest frame, analogous to a physical fluid or the outdated idea of the luminiferous aether, so it would lead to a revival of the notion of absolute space which had been eliminated by special relativity; this implication is specific to Cahill's theory, as most other physicists who discuss the "quantum foam" concept do not imagine it would have a preferred frame.

• Dirac sea
• Hawking radiation
• Hyperspace theory
• Planck time
• "Rolling ball" topology
• Vacuum energy
• Wormhole
• Invariance mechanics

• John Archibald Wheeler with Kenneth Ford. Geons, Black Holes, and Quantum Foam. 1995 ISBN 0-393-04642-7.
• Reginald T. Cahill. Gravity as Quantum Foam In-Flow. June 2003.