Quantum foam

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.

John 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.

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 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. Subsequent experiments were, however, unable to confirm the supposed variation on the speed of light due to graininess of space.{{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 }}

Other experiments involving the polarization of light from distant gamma ray bursts have also produced contradictory results. More Earth-based experiments are ongoing or proposed.

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 (≈ 10−35 m), 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 quasars by Floyd Stecker of NASA's Goddard Space Flight Center failed to find evidence of violation of Lorentz invariance.

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 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, setting a bound on the size of quantum fluctuations of spacetime.

Relation to other theories

The vacuum fluctuations provide vacuum with a non-zero energy known as vacuum energy.{{cite web |access-date = 2007-12-18 |author-link = John C. Baez

Spin foam theory is a modern attempt to make Wheeler's idea quantitative.

Notes

References

• Minkel, J. R. (24 November 2003). "Borrowed Time: Interview with Michio Kaku". Scientific American
• Swarup, A. (2006). "Sights set on quantum froth". New Scientist, 189, p. 18, accessed 10 February 2012

References

1. Wheeler, J. A.. (January 1955). "Geons". [[Physical Review]].
2. Minsky, Carly. (24 October 2019). "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.". [[Vice (magazine).
3. [https://www.youtube.com/watch?v=914jzZ4LXcU&t=2887 See Derek Leinweber's QCD animations of spacetime foam, as exhibited in Wilczek lecture].
4. (2010). "Geons, black holes, and quantum foam : a life in physics". W. W. Norton & Company.
5. [https://www.youtube.com/watch?v=nYDokJ2A_vU Quantum Foam], [[Don Lincoln]], Fermilab, 2014-10-24.
6. (3 March 2021). "Gamma Ray Delay May Be Sign of 'New Physics'".
7. (2015). "A Planck-scale limit on spacetime fuzziness and stochastic Lorentz invariance violation". Nature Physics.
8. [http://www.esa.int/Our_Activities/Space_Science/Integral_challenges_physics_beyond_Einstein Integral challenges physics beyond Einstein / Space Science / Our Activities / ESA].
9. Moyer, Michael. (17 January 2012). "Is Space Digital?". [[Scientific American]].
10. Cowen, Ron. (22 November 2012). "Single photon could detect quantum-scale black holes". [[Nature News]].
11. (November 1978). "Spacetime foam". Nuclear Physics B.
12. "Einstein makes extra dimensions toe the line". NASA.
13. (28 May 2015). "NASA Telescopes Set Limits on Spacetime Quantum "Foam"".
14. "Chandra Press Room :: NASA Telescopes Set Limits on Space-time Quantum "Foam":: 28 May 15".
15. "Chandra X-ray Observatory – NASA's flagship X-ray telescope".
16. (2014). "New Constraints on Quantum Gravity from X-ray and Gamma-Ray Observations". The Astrophysical Journal.
17. "Chandra :: Photo Album :: Space-time Foam :: May 28, 2015".