Science News - March 13, 2010

that quantum-gravity noise might show up in large-scale experiments.

Noisy 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