Quantum foam (orspacetime foam, orspacetime bubble) is a theoreticalquantum fluctuation ofspacetime on very small scales due toquantum mechanics. The theory predicts that at this small scale, particles of matter and antimatter are constantly created and destroyed. These subatomic objects are calledvirtual particles.[1] The idea was devised byJohn Wheeler in 1955.[2][3]
With an incomplete theory ofquantum gravity, it is impossible to be certain whatspacetime looks like at small scales. However, there is no definitive reason that spacetime needs to be fundamentally smooth. It is possible that instead, in aquantum 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.[4]
John Wheeler suggested that theuncertainty principle might imply that over sufficiently small distances and sufficiently brief intervals of time, the "very geometry of spacetime fluctuates".[5] 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 theCasimir effect, which is possibly caused byvirtual particles, is strong evidence for the existence of virtual particles. Theg-2 experiment, which predicts the strength ofmagnets formed bymuons andelectrons, also supports their existence.[1]
In 2005, during observations ofgamma-rayphotons arriving from theblazarMarkarian 501,MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov) telescopes detected that some of thephotons at differentenergy levels arrived at different times, suggesting that some of the photons had moved more slowly and thus were in violation ofspecial relativity's notion that thespeed of light is constant, a discrepancy which could be explained by the irregularity of quantum foam.[6] Subsequent experiments were, however, unable to confirm the supposed variation on the speed of light due to graininess of space.[7][8]
Other experiments involving the polarization of light from distantgamma ray bursts have also produced contradictory results.[9] More Earth-based experiments are ongoing[10][as of?] or proposed.[11]
The fluctuations characteristic of a spacetime foam would be expected to occur on a length scale on the order of thePlanck length (≈ 10−35 m),[12] but some models ofquantum gravity predict much larger fluctuations.
Photons should be slowed by quantum foam, with the rate depending on the wavelength of the photons. This would violateLorentz invariance. But observations of radiation from nearbyquasars by Floyd Stecker ofNASA'sGoddard Space Flight Center failed to find evidence of violation of Lorentz invariance.[13]
A foamy spacetime also sets limits on the accuracy with which distances can be measured because photons should diffuse randomly through a spacetime foam, similar to light diffusing by passing through fog. This should cause the image quality of very distant objects observed through telescopes to degrade. X-ray and gamma-ray observations of quasars using NASA'sChandra X-ray Observatory, theFermi Gamma-ray Space Telescope and ground-based gamma-ray observations from theVery Energetic Radiation Imaging Telescope Array (VERITAS) showed no detectable degradation at the farthest observed distances, implying that spacetime is smooth at least down to distances 1000 times smaller than the nucleus of a hydrogen atom,[14][15][16][17][18] setting a bound on the size of quantum fluctuations of spacetime.
Thevacuum fluctuations providevacuum with a non-zero energy known asvacuum energy.[19]
Spin foam theory is a modern attempt to make Wheeler's ideaquantitative.
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