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=                            Quantum foam                            =
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                            Introduction
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Quantum foam or spacetime foam is the fluctuation of spacetime on very
small scales due to quantum mechanics.  The idea was devised by John
Wheeler in 1955.


                             Background
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With an incomplete theory of quantum gravity, it is impossible to be
certain what spacetime would look like at small scales. However, there
is no 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.

Wheeler suggested that the Heisenberg 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
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In 2009 the two MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov)
telescopes detected that among gamma-ray photons arriving from the
blazar Markarian 501, some photons at different energy levels arrived
at different times, suggesting that some of the photons had moved more
slowly and thus contradicting the theory of general relativity's
notion of the speed of light being constant, a discrepancy which could
be explained by the irregularity of quantum foam. More recent
experiments were, however, unable to confirm the supposed variation on
the speed of light due to graininess of space.

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 and limits
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The large fluctuations characteristic of a spacetime foam would be
expected to occur on a length scale on the order of the Planck length.
A foamy spacetime would have limits on the accuracy with which
distances can be measured because the size of the many quantum bubbles
through which light travels will fluctuate. Depending on the spacetime
model used, the spacetime uncertainties accumulate at different rates
as light travels through the vast distances.

X-ray and gamma-ray observations of quasars used data from 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) show that spacetime is uniform down
to distances 1000 times smaller than the nucleus of a hydrogen atom.

Observations of radiation from nearby quasars by Floyd Stecker of
NASA's Goddard Space Flight Center have placed strong experimental
limits on the possible violations of Einstein's special theory of
relativity implied by the existence of quantum foam. Thus experimental
evidence so far has given a range of values in which scientists can
test for quantum foam.


Random diffusion model
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Chandra's X-ray detection of quasars at distances of billions of light
years rules out the model where photons diffuse randomly through
spacetime foam, similar to a light diffusing by passing through the
fog.


Holographic model
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Measurements of quasars at shorter, gamma-ray wavelengths with Fermi,
and, shorter wavelengths with VERITAS rule out a second model, called
a holographic model with less diffusion.


                     Relation to other theories
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The vacuum fluctuations provide vacuum with a non-zero energy known as
vacuum energy.

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


                              See also
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* Geon
* Hawking radiation
* Holographic principle
* Lorentzian wormhole
* Planck time
* Quantum fluctuation
* Stochastic quantum mechanics
* String theory
* Sub-Planck
* Vacuum energy
* Wormhole


                             References
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* Minkel, JR (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. 18, accessed
10 February 2012.


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Original Article: http://en.wikipedia.org/wiki/Quantum_foam