Quantum foam is a concept in quantum
mechanics devised by John Wheeler in
1955. The foam is supposed to be
conceptualized as the foundation of the
fabric of the universe. Additionally,
quantum foam can be used as a
qualitative description of subatomic
space-time turbulence at extremely small
distances. At such small scales of time
and space, the Heisenberg uncertainty
principle allows energy to briefly decay
into particles and antiparticles and
then annihilate without violating
physical conservation laws. As the scale
of time and space being discussed
shrinks, the energy of the virtual
particles increases. According to
Einstein's theory of general relativity,
energy curves space-time. This suggests
that—at sufficiently small scales—the
energy of these fluctuations would be
large enough to cause significant
departures from the smooth space-time
seen at larger scales, giving space-time
a "foamy" character.
With an incomplete theory of quantum
gravity, it is impossible to be certain
what space-time would look like at these
small scales, because existing theories
of gravity do not give accurate
predictions in that realm. Therefore,
any of the developing theories of
quantum gravity may improve our
understanding of quantum foam as they
are tested. However, 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.
Experimental evidence
The MAGIC telescopes have 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=
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 show that space-time is uniform
down to distances 1000 times smaller
than the nucleus of a hydrogen atom.
Quantum mechanics predicts that
space-time is not smooth, instead
space-time would have a foamy, jittery
nature and would consist of many small,
ever-changing, regions in which space
and time are not definite, but
fluctuate.
The predicted scale of space-time foam
is about ten times a billionth of the
diameter of a hydrogen atom's nucleus,
which cannot be measured directly. A
foamy space-time 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
space-time model used, the space-time
uncertainties accumulate at different
rates as light travels through the vast
distances.
Chandra's X-ray detection of quasars at
distances of billions of light years
rules out the model where photons
diffuse randomly through space-time
foam, similar to light diffusing passing
through fog.
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
Quantum foam is theorized to be the
'fabric' of the Universe, but cannot be
observed yet because it is too small.
Also, quantum foam is theorized to be
created by virtual particles of very
high energy. Virtual particles appear in
quantum field theory, arising briefly
and then annihilating during particle
interactions in such a way that they
affect the measured outputs of the
interaction, even though the virtual
particles are themselves space. These
"vacuum fluctuations" affect the
properties of the vacuum, giving it a
nonzero energy known as vacuum energy,
itself a type of zero-point energy.
However, physicists are uncertain about
the magnitude of this form of energy.
The Casimir effect can also be
understood in terms of the behavior of
virtual particles in the empty space
between two parallel plates. Ordinarily,
quantum field theory does not deal with
virtual particles of sufficient energy
to curve spacetime significantly, so
quantum foam is a speculative extension
of these concepts which imagines the
consequences of such high-energy virtual
particles at very short distances and
times. Spin foam theory is a modern
attempt to make Wheeler's idea
quantitative.
See also
Footnotes
References
John Archibald Wheeler with Kenneth
Ford. Geons, Black Holes, and Quantum
Foam. 1995 ISBN 0-393-04642-7.
Reginald T. Cahill. Gravity as Quantum
Foam In-Flow. June 2003.
Borrowed Time: Interview with Michio
Kaku, Scientific American
Swarup, A 2006, 'Sights set on quantum
froth', New Scientist, 189, p. 18,
Science Full Text Select, EBSCOhost,
viewed 10 February 2012.
