Physics:Quantum spacetime foam - Revision history

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	'''~~Quantum~~ spacetime foam''' is the idea that spacetime geometry may fluctuate strongly at extremely small distances, especially near the Planck scale. The phrase is associated with the expectation that quantum uncertainty applied to geometry could make spacetime appear smooth only after averaging over microscopic fluctuations.~~<ref>{{cite web \|title=~~Quantum foam ~~\|url=https://en~~.~~wikipedia~~.~~org/wiki/Quantum_foam \|website=Wikipedia \|access~~-~~date=19 May 2026}}</ref><ref>{{cite book \|last=Rovelli \|first=Carlo \|title=Quantum Gravity \|publisher=Cambridge University Press \|year=2004 \|id=ISBN 978-0-521-83733-0}}</ref>~~		'''spacetime foam''' is a Book II topic in the Quantum Collection. Quantum spacetime foam is the idea that spacetime geometry may fluctuate strongly at extremely small distances, especially near the Planck scale. The phrase is associated with the expectation that quantum uncertainty applied to geometry could make spacetime appear smooth only after averaging over microscopic fluctuations. Quantum spacetime foam is the idea that spacetime geometry may fluctuate strongly at extremely small distances, especially near the Planck scale. The phrase is associated with the expectation that quantum uncertainty applied to geometry could make spacetime appear smooth only after averaging over microscopic fluctuations. In ordinary general relativity, spacetime is represented by a smooth geometry. Spacetime-foam ideas question whether that smooth geometry remains meaningful at the shortest scales when quantum fluctuations of the gravitational field are included.
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	== Observable challenge ==		== Observable challenge ==
	Directly testing spacetime foam is difficult because Planck-scale effects are extremely small at accessible energies. Proposed searches often look for cumulative effects in high-energy photons, interferometry, or early-universe signatures.		Directly testing spacetime foam is difficult because Planck-scale effects are extremely small at accessible energies. Proposed searches often look for cumulative effects in high-energy photons, interferometry, or early-universe signatures.

			== Description ==
			'''spacetime foam''' is a matter-scale concept used to organize how quantum theory describes atoms, particles, fields, condensed matter, plasma, or spacetime-related systems. In the Quantum Collection it is placed by scale so the reader can move from materials and molecules down to subatomic degrees of freedom.

			== Quantum context ==
			At this scale, the relevant behavior is controlled by quantized states, interactions, conservation laws, and the way excitations or particles are observed. The concept is normally linked to measurable properties such as energy, momentum, charge, spin, spectra, scattering rates, or collective modes.

			== Role in the collection ==
			This page provides a compact reference point for related pages in Book II. It should be read together with nearby matter-scale topics and the corresponding foundations in [[Physics:Quantum mechanics\|quantum mechanics]].<ref name="matter-wiki">{{cite web \|url=https://en.wikipedia.org/wiki/Quantum_mechanics \|title=Quantum mechanics \|website=Wikipedia \|access-date=2026-05-20}}</ref>

	=See also=		=See also=

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{{Short description|Hypothetical microscopic fluctuation of spacetime geometry}}

{{Quantum matter backlink|Vacuum and spacetime}}

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'''Quantum spacetime foam''' is the idea that spacetime geometry may fluctuate strongly at extremely small distances, especially near the Planck scale. The phrase is associated with the expectation that quantum uncertainty applied to geometry could make spacetime appear smooth only after averaging over microscopic fluctuations.<ref>{{cite web |title=Quantum foam |url=https://en.wikipedia.org/wiki/Quantum_foam |website=Wikipedia |access-date=19 May 2026}}</ref><ref>{{cite book |last=Rovelli |first=Carlo |title=Quantum Gravity |publisher=Cambridge University Press |year=2004 |id=ISBN 978-0-521-83733-0}}</ref>
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[[File:Quantum_spacetime_foam_yellow.png|thumb|280px|Spacetime foam: fluctuating microscopic geometry near the Planck scale.]]
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== Concept ==
In ordinary general relativity, spacetime is represented by a smooth geometry. Spacetime-foam ideas question whether that smooth geometry remains meaningful at the shortest scales when quantum fluctuations of the gravitational field are included.<ref>{{cite web |title=Quantum foam |url=https://en.wikipedia.org/wiki/Quantum_foam |website=Wikipedia |access-date=19 May 2026}}</ref>

== Relation to quantum gravity ==
Different quantum-gravity approaches describe microscopic geometry in different ways. Some emphasize discrete spectra, causal structure, path integrals over geometries, or emergent spacetime rather than literal foam.

== Observable challenge ==
Directly testing spacetime foam is difficult because Planck-scale effects are extremely small at accessible energies. Proposed searches often look for cumulative effects in high-energy photons, interferometry, or early-universe signatures.

=See also=
{{#invoke:PhysicsQC|tocHeadingAndList|Physics:Quantum basics/See also/Matter}}

=References=
{{reflist|3}}

{{Author|Harold Foppele}}

{{Sourceattribution|Physics:Quantum spacetime foam|1}}