15 January 2009
from IlluminatiNews Website
From the outside, it doesn't look much: in the corner of a field stands an assortment of boxy temporary buildings, from which two long trenches emerge, at a right angle to each other, covered with corrugated iron.
Underneath the metal sheets, however, lies a detector that stretches for 600 meters (below image).
In the front the central building for the laser and the vacuum tanks can be seen.
The tubes, 600 m in length, run in covered trenches at the edge of the field upwards and to the right.
Buildings for the mirrors are situated at the end of each tube.
Credit: Albert Einstein Institute Hannover
GEO600 has not detected any gravitational waves so far, but
it might inadvertently have made the most important discovery in
physics for half a century.
According to Craig Hogan, a physicist at the Fermilab particle physics lab in Batavia, Illinois, GEO600 has stumbled upon the fundamental limit of space-time - the point where space-time stops behaving like the smooth continuum Einstein described and instead dissolves into "grains", just as a newspaper photograph dissolves into dots as you zoom in.
If this doesn't blow your socks off, then Hogan, who has just been appointed director of Fermilab's Center for Particle Astrophysics, has an even bigger shock in store:
The idea that we live in a hologram
probably sounds absurd, but it is a natural extension of our best
black holes, and something with a pretty firm
theoretical footing. It has also been surprisingly helpful for
physicists wrestling with theories of how the universe works at its
most fundamental level.
In the 1990s physicists Leonard Susskind and Nobel prizewinner
Gerard 't Hooft suggested
that the same principle might apply to the universe as a whole. Our
everyday experience might itself be a holographic projection of
physical processes that take place on a distant, 2D surface.
This poses a puzzle, because Hawking radiation does not convey any information about the interior of a black hole.
When the black hole has gone, all the information about
the star that collapsed to form the black hole has vanished, which
contradicts the widely affirmed principle that information cannot be
destroyed. This is known as the black hole information paradox.
He discovered that a black hole's
entropy - which is synonymous with its information content - is
proportional to the surface area of its event horizon. This is the
theoretical surface that cloaks the black hole and marks the point
of no return for infalling matter or light. Theorists have since
shown that microscopic quantum ripples at the event horizon can
encode the information inside the black hole, so there is no
mysterious information loss as the black hole evaporates.
Susskind and 't Hooft extended the insight to the universe as a whole on the basis that the cosmos has a horizon too - the boundary from beyond which light has not had time to reach us in the 13.7-billion-year lifespan of the universe. What's more, work by several string theorists, most notably Juan Maldacena at the Institute for Advanced Study in Princeton, has confirmed that the idea is on the right track.
He showed that the physics inside a hypothetical universe with five dimensions and shaped like a Pringle is the same as the physics taking place on the four-dimensional boundary.
According to Hogan, the holographic principle radically changes our picture of space-time.
Theoretical physicists have long believed that quantum effects will cause space-time to convulse wildly on the tiniest scales. At this magnification, the fabric of space-time becomes grainy and is ultimately made of tiny units rather like pixels, but a hundred billion billion times smaller than a proton. This distance is known as the Planck length, a mere 10-35 meters.
The Planck length is far beyond the
reach of any conceivable experiment, so nobody dared dream that the
graininess of space-time might be discernable.
Hogan realized that in order to have the same number of bits inside the universe as on the boundary, the world inside must be made up of grains bigger than the Planck length.
This is good news for anyone trying to probe the smallest unit of space-time.
So while the Planck length is too small for experiments to detect, the holographic "projection" of that graininess could be much, much larger, at around 10-16 meters.
When Hogan first realized this, he
wondered if any experiment might be able to detect the holographic
blurriness of space-time. That's where GEO600 comes in.
This divides the light into two beams, which pass down the instrument's 600-metre perpendicular arms and bounce back again. The returning light beams merge together at the beam splitter and create an interference pattern of light and dark regions where the light waves either cancel out or reinforce each other.
Any shift in the position of those regions tells you that the relative lengths of the arms has changed.
So would they be able to detect a holographic projection of grainy space-time? Of the five gravitational wave detectors around the world, Hogan realized that the Anglo-German GEO600 experiment ought to be the most sensitive to what he had in mind.
He predicted that if the experiment's beam splitter is buffeted by the quantum convulsions of space-time, this will show up in its measurements (Physical Review D, vol 77, p 104031).
In June he sent his prediction to the GEO600 team.
GEO600's principal investigator Karsten Danzmann of the Max Planck Institute for Gravitational Physics in Potsdam, Germany, and also the University of Hanover, admits that the excess noise, with frequencies of between 300 and 1500 hertz, had been bothering the team for a long time.
He replied to Hogan and sent him a plot of the noise.
Incredibly, the experiment was picking up unexpected noise - as if quantum convulsions were causing an extra sideways jitter. No one - including Hogan - is yet claiming that GEO600 has found evidence that we live in a holographic universe.
It is far too soon to say.
Gravitational-wave detectors are extremely sensitive, so those who operate them have to work harder than most to rule out noise.
They have to take into account passing clouds, distant traffic, seismological rumbles and many, many other sources that could mask a real signal.
At present there are no clear candidate sources for the noise GEO600 is experiencing.
For a while, the GEO600 team thought the
noise Hogan was interested in was caused by fluctuations in
temperature across the beam splitter. However, the team worked out
that this could account for only one-third of the noise at most.
If GEO600 really has discovered holographic noise from quantum convulsions of space-time, then it presents a double-edged sword for gravitational wave researchers.
Such a situation would not be unprecedented in physics.
Giant detectors built to look for a
hypothetical form of radioactivity in which protons decay never
found such a thing. Instead, they discovered that neutrinos can
change from one type into another - arguably more important because
it could tell us how the universe came to be
filled with matter and
Small price to pay
However Danzmann is cautious about Hogan's proposal and believes more theoretical work needs to be done.
Like many others, Danzmann agrees it is too early to make any definitive claims.
The longer the puzzle remains, however, the stronger the motivation becomes to build a dedicated instrument to probe holographic noise. John Cramer of the University of Washington in Seattle agrees.
One possibility, according to Hogan, would be to use a device called an atom interferometer.
These operate using the same principle
as laser-based detectors but use beams made of ultra-cold atoms
rather than laser light. Because atoms can behave as waves with a
much smaller wavelength than light, atom interferometers are
significantly smaller and therefore cheaper to build than their
That noise turned out to be the cosmic microwave background, the afterglow of the big bang fireball.
Hogan is more specific.
Today the most popular approach to quantum gravity is string theory, which researchers hope could describe happenings in the universe at the most fundamental level.
But it is not the only show in town.
Hogan agrees that if the holographic principle is confirmed, it rules out all approaches to quantum gravity that do not incorporate the holographic principle.
Conversely, it would be a boost for those that do - including some derived from string theory and something called matrix theory.
As serendipitous discoveries go, it's
hard to get more ground-breaking than that.