that quantum-gravity noise might show
up in large-scale experiments.
Indeed, Hogan is hoping that two experiments — one ongoing, the other in the
planning stages — may find evidence for
the quantum clatter.
Since 2002, a British-German apparatus called GEO600 has been searching
for the notes of a cosmic symphony — ripples in spacetime known as gravitational
waves — that general relativity says ought
to exist. Such waves should be generated
any time a dense body, such as a neutron
star or black hole, is accelerated.
The experiment, based in Hannover,
Germany, hasn’t found any of these
waves. But part of the signal that GEO600
has detected might be accounted for by
holographic noise, Hogan says.
GEO600, like some other gravitational
wave detectors, uses laser light to measure tiny changes in the relative lengths
of two perpendicular arms of an interferometer. A gravitational wave would alternately expand one arm ever so slightly
while compressing the other. GEO600
uses a single laser beam, split into two
beams by a half-silvered mirror, to measure and compare the two lengths.
BOTH: MAX PLANCK INSTI TUTE FOR GRAVITATIONAL PH YSICS (AEI)/LEIBNIZ UNIVERSITÄT HANNOVER
Because of the experiment’s design,
GE0600 is much more sensitive to the
sideways motion of the beam splitter
than are other large gravitational wave
detectors. And an unexplained sideways
motion of the beam splitter is just “the
effect claimed by Hogan as a consequence
of his holographic noise,” says GEO600
principal investigator Karsten Danzmann
of the Leibniz Universität Hannover.
Scientists have detected small, un-
accounted-for motion in GEO600’s beam
splitter (shown), some of which may
have come from Hogan’s proposed noise.
it will take a much more sensitive experiment to prove the fuzzy, holographic
nature of spacetime, says Schutz.
Such an experiment is now in the planning stages at Fermilab, Hogan says. The
proposed $2-million apparatus would
feature a pair of interferometers, each
with perpendicular arms 40 meters in
length. Initially, the two experiments
would be placed next to each other, testing a key prediction of Hogan’s theory:
The noise recorded by two adjacent
devices ought to be correlated.
“They move together because the
whole of spacetime they are sitting in
is jittering around,” says Hogan. If one
device records a type of motion that
might be attributed to quantum jitter
but the other device does not, Hogan’s
theory would be ruled out.
But if the noise is detected, the next
test would be to move the two interferometers farther apart. At large
separation, the correlation between
the amount of noise in the two detectors should shrink to zero according to
Hogan’s model, says Stephan Meyer of
the University of Chicago.
If it all works, the experiment could
give a first glimpse of the tiniest scraps
of spacetime. But whether Hogan’s noise
will be revealed, only time — make that
spacetime — will tell. s
The GEO600 experiment in Germany may be able to detect a jitter in spacetime
by picking up sideways motion in a beam splitter that divides one laser beam into
two. The beams then travel down perpendicular arms (shown) 600 meters long.
Links to Craig Hogan’s publications: s
March 13, 2010 | SCIENCE NEWS | 29