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Concept
Quantum foam
Summary
Quantum foam or spacetime foam is a theoretical quantum fluctuation of spacetime on very small scales due to quantum mechanics. The theory predicts that at these small scales, particles of matter and antimatter are constantly created and destroyed. These subatomic objects are called virtual particles. The idea was devised by John Wheeler in 1955.
With an incomplete theory of quantum gravity, it is impossible to be certain what spacetime would look like at small scales. However, there is no definitive reason that spacetime needs to be fundamentally smooth. It is possible that instead, in a quantum theory of gravity, spacetime would consist of many small, ever-changing regions in which space and time are not definite, but fluctuate in a foam-like manner.
Wheeler suggested that the uncertainty principle might imply that over sufficiently small distances and sufficiently brief intervals of time, the "very geometry of spacetime fluctuates". These fluctuations could be large enough to cause significant departures from the smooth spacetime seen at macroscopic scales, giving spacetime a "foamy" character.
The experimental proof of the Casimir effect, which is possibly caused by virtual particles, is strong evidence for the existence of virtual particles. The g-2 experiment, which predicts the strength of magnets formed by muons and electrons also supports their existence.
In 2005, during observations of gamma-ray photons arriving from the blazar Markarian 501, MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov) telescopes detected that some of the photons at different energy levels arrived at different times, suggesting that some of the photons had moved more slowly and thus were in violation of special relativity's notion that the speed of light is constant, a discrepancy which could be explained by the irregularity of quantum foam. More recent experiments were, however, unable to confirm the supposed variation on the speed of light due to graininess of space.
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