Editing Quantum foam

{{short description|Fluctuation of spacetime on very small scales}}
{{For|related articles|Quantum vacuum (disambiguation)}}
[[File:Q-foam.jpg|thumb|A graphic representation of Wheeler's calculations of what quantum reality may look like at the Planck length]]
'''Quantum foam''' (or '''spacetime foam''', or '''spacetime bubble''') is a theoretical [[quantum fluctuation]] of [[spacetime]] on very small scales due to [[quantum mechanics]]. The theory predicts that at this small scale, particles of matter and antimatter are constantly created and destroyed. These subatomic objects are called [[virtual particles]].<ref name=lincoln-foam/> The idea was devised by [[John Archibald Wheeler|John Wheeler]] in 1955.<ref>{{cite journal | title=Geons | bibcode=1955PhRv...97..511W | last=Wheeler | first=J. A. | journal=[[Physical Review]] |date=January 1955 | volume=97 | issue=2 | pages=511–536 | doi=10.1103/PhysRev.97.511}}</ref><ref name="VCE-20191024">{{cite news |last=Minsky |first=Carly |title=The Universe Is Made of Tiny Bubbles Containing Mini-Universes, Scientists Say – 'Spacetime foam' might just be the wildest thing in the known universe, and we're just starting to understand it. |url=https://www.vice.com/en_us/article/j5yngp/the-universe-is-made-of-tiny-bubbles-containing-mini-universes-scientists-say |date=24 October 2019 |work=[[Vice (magazine)|Vice]] |access-date=24 October 2019 }}</ref>

==Background==

With an incomplete theory of [[quantum gravity]], it is impossible to be certain what [[spacetime]] looks 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.<ref name="WilczekLecture">[https://www.youtube.com/watch?v=914jzZ4LXcU&t=2887 See Derek Leinweber's QCD animations of spacetime foam, as exhibited in Wilczek lecture].</ref>

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".<ref>{{Cite book|title=Geons, black holes, and quantum foam : a life in physics|last1=Wheeler|first1=John Archibald|last2=Ford|first2=Kenneth Wilson|publisher=W. W. Norton & Company|year=2010|isbn=9780393079487|location=New York|pages=328|oclc=916428720|orig-year=1998}}</ref> These fluctuations could be large enough to cause significant departures from the smooth spacetime seen at macroscopic scales, giving spacetime a "foamy" character.

== 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.

==Relation to other theories==
The [[quantum fluctuation|vacuum fluctuation]]s provide [[vacuum]] with a non-zero energy known as [[vacuum energy]].<ref>{{cite web
  |url = http://math.ucr.edu/home/baez/vacuum.html
  |title =  What's the Energy Density of the Vacuum?
  |access-date = 2007-12-18
  |last = Baez
  |first = John
  |author-link = John C. Baez
  |date = 2006-10-08
}}</ref>

[[Spin foam]] theory is a modern attempt to make Wheeler's idea [[Quantitative research|quantitative]].

==See also==
{{cols|colwidth=21em}}
* [[False vacuum]]
* [[Geon (physics)|Geon]]
* [[Hawking radiation]]
* [[Holographic principle]]
* [[Loop quantum gravity]]
* [[Lorentzian wormhole]]
* [[Planck time]]
* [[Stochastic quantum mechanics]]
* [[String theory]]
* [[Wormhole]]
* [[Virtual black hole]]
{{colend}}

==Notes==
{{Reflist|32em}}

==References==
* Minkel, J. R. (24 November 2003). [http://www.scientificamerican.com/article.cfm?id=borrowed-time-interview-w "Borrowed Time: Interview with Michio Kaku"]. ''[[Scientific American]]''
* Swarup, A. (2006). [https://www.newscientist.com/article/dn8738-sights-set-on-quantum-froth/ "Sights set on quantum froth"]. ''[[New Scientist]]'', 189, p.&nbsp;18, accessed 10 February 2012

{{quantum gravity}}
{{Quantum mechanics topics}}

[[Category:Quantum gravity]]
[[Category:Wormhole theory]]