Editing Quantum foam (section)

== Experimental results ==

The experimental proof of the [[Casimir effect]], which is possibly caused by virtual particles, is strong evidence for the existence of virtual particles. The [[Muon g-2|g-2 experiment]], which predicts the strength of magnets formed by muons and electrons, also supports their existence.<ref name=lincoln-foam>[https://www.youtube.com/watch?v=nYDokJ2A_vU Quantum Foam], [[Don Lincoln]], Fermilab, 2014-10-24.</ref>

In 2005, during observations of [[gamma ray|gamma-ray]] [[photon]]s arriving from the [[blazar]] [[Markarian 501]], [[MAGIC (telescope)|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.<ref>{{cite web|url=http://www.news.ucdavis.edu/search/news_detail.lasso?id=8364|title=Gamma Ray Delay May Be Sign of 'New Physics'|date=3 March 2021 }}</ref> Subsequent experiments were, however, unable to confirm the supposed variation on the speed of light due to graininess of space.<ref>{{cite journal | doi = 10.1038/nphys3270 | volume=11 | title=A Planck-scale limit on spacetime fuzziness and stochastic Lorentz invariance violation | journal=Nature Physics | pages=344–346| bibcode=2015NatPh..11..344V | year=2015 | last1=Vasileiou | first1=Vlasios | last2=Granot | first2=Jonathan | last3=Piran | first3=Tsvi | last4=Amelino-Camelia | first4=Giovanni | issue=4 | doi-access=free }}</ref><ref>{{cite journal | doi = 10.1038/nature.2012.9768 | 
title=Cosmic race ends in a tie | journal=Nature| 
year=2012 | 
last1=Cowen | 
first1=Ron | 
s2cid=120173051 | 
doi-access=free }}</ref>

Other experiments involving the polarization of light from distant gamma ray bursts have also produced contradictory results.<ref>[http://www.esa.int/Our_Activities/Space_Science/Integral_challenges_physics_beyond_Einstein Integral challenges physics beyond Einstein / Space Science / Our Activities / ESA].</ref> More Earth-based experiments are ongoing<ref>{{cite news|url=http://www.scientificamerican.com/article.cfm?id=is-space-digital|title=Is Space Digital?|last=Moyer|first=Michael|date=17 January 2012|work=[[Scientific American]]|access-date=3 February 2013}}</ref> or proposed.<ref>{{cite news|url=http://www.nature.com/news/single-photon-could-detect-quantum-scale-black-holes-1.11871|title=Single photon could detect quantum-scale black holes|last=Cowen|first=Ron|date=22 November 2012|work=[[Nature News]]|access-date=3 February 2013}}</ref>

===Constraints on the size of quantum fluctuations===

The fluctuations characteristic of a spacetime foam would be expected to occur on a length scale on the order of the [[Planck length]] (≈&nbsp;10<sup>−35</sup>&nbsp;m),<ref name="Hawking1978">{{cite journal |last1=Hawking |first1=Stephen W. |date=November 1978 |title=Spacetime foam |journal=Nuclear Physics B |volume=144 |issue=2–3 |pages=349–362 |bibcode=1978NuPhB.144..349H |doi=10.1016/0550-3213(78)90375-9}}</ref> but some models of [[quantum gravity]] predict much larger fluctuations.

Photons should be slowed by quantum foam, with the rate depending on the wavelength of the photons. This would violate [[Lorentz invariance]]. But observations of radiation from nearby [[quasar]]s by Floyd Stecker of [[NASA|NASA's]] [[Goddard Space Flight Center]] failed to find evidence of violation of Lorentz invariance.<ref>{{cite web|url=http://www.nasa.gov/centers/goddard/news/topstory/2003/1212einstein.html|title=Einstein makes extra dimensions toe the line|publisher=NASA|access-date=9 February 2012|archive-date=18 July 2019|archive-url=https://web.archive.org/web/20190718093350/https://www.nasa.gov/centers/goddard/news/topstory/2003/1212einstein.html|url-status=dead}}</ref>

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's [[Chandra X-ray Observatory]], the [[Fermi Gamma-ray Space Telescope]] and ground-based gamma-ray observations from the [[VERITAS|Very 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,<ref>{{cite web | url=https://www.nasa.gov/mission_pages/chandra/nasa-telescopes-set-limits-on-spacetime-quantum-foam.html | title=NASA Telescopes Set Limits on Spacetime Quantum "Foam" | date=28 May 2015 }}</ref><ref>{{Cite web|title = Chandra Press Room :: NASA Telescopes Set Limits on Space-time Quantum "Foam":: 28 May 15|url = http://chandra.si.edu/press/15_releases/press_052815.html|website = chandra.si.edu|access-date = 2015-05-29}}</ref><ref>{{Cite web |title=Chandra X-ray Observatory – NASA's flagship X-ray telescope |url=http://chandra.si.edu/ |access-date=2015-05-29 |website=chandra.si.edu}}</ref><ref>{{Cite journal|title =  New Constraints on Quantum Gravity from X-ray and Gamma-Ray Observations|journal = The Astrophysical Journal|volume = 805|pages = 10|arxiv = 1411.7262|last1 =  Perlman|first1 = Eric S.|last2 =  Rappaport|first2 = Saul A.|last3 =  Christensen|first3 = Wayne A.|last4 =  Jack Ng|first4 = Y.|last5 = DeVore|first5 = John|last6 = Pooley|first6 = David|year = 2014| issue=1 |doi = 10.1088/0004-637X/805/1/10|bibcode=2015ApJ...805...10P|s2cid = 56421821}}</ref><ref>{{Cite web|title = Chandra :: Photo Album :: Space-time Foam :: May 28, 2015|url = http://chandra.si.edu/photo/2015/quantum/|website = chandra.si.edu|access-date = 2015-05-29}}</ref> setting a bound on the size of quantum fluctuations of spacetime.