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Our world may be a giant hologram
Posted by Lev/Christopher on January 18, 2009 at 2:15am in Science & Technology

DRIVING through the countryside south of Hanover, it would be easy to
miss the GEO600 experiment. 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 metres.
For the past seven years, this German set-up has been looking for
gravitational waves - ripples in space-time thrown off by super-dense
astronomical objects such as neutron stars and black holes. 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.
For many months, the GEO600 team-members had been scratching their
heads over inexplicable noise that is plaguing their giant detector.
Then, out of the blue, a researcher approached them with an
explanation. In fact, he had even predicted the noise before he knew
they were detecting it. 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. "It looks like GEO600 is being buffeted by the
microscopic quantum convulsions of space-time," says Hogan.
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: "If the GEO600 result is what I suspect
it is, then we are all living in a giant cosmic hologram."
The idea that we live in a hologram probably sounds absurd, but it is a
natural extension of our best understanding of 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.
The holograms you find on credit cards and banknotes are etched on
two-dimensional plastic films. When light bounces off them, it
recreates the appearance of a 3D image. 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.
The "holographic principle" challenges our sensibilities. It seems hard
to believe that you woke up, brushed your teeth and are reading this
article because of something happening on the boundary of the universe.
No one knows what it would mean for us if we really do live in a
hologram, yet theorists have good reasons to believe that many aspects
of the holographic principle are true.
Susskind and 't Hooft's remarkable idea was motivated by
ground-breaking work on black holes by Jacob Bekenstein of the Hebrew
University of Jerusalem in Israel and Stephen Hawking at the University
of Cambridge. In the mid-1970s, Hawking showed that black holes are in
fact not entirely "black" but instead slowly emit radiation, which
causes them to evaporate and eventually disappear. 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.
Bekenstein's work provided an important clue in resolving the 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.
Crucially, this provides a deep physical insight: the 3D information
about a precursor star can be completely encoded in the 2D horizon of
the subsequent black hole - not unlike the 3D image of an object being
encoded in a 2D hologram. 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 metres. 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.
That is, not until Hogan realised that the holographic principle
changes everything. If space-time is a grainy hologram, then you can
think of the universe as a sphere whose outer surface is papered in
Planck length-sized squares, each containing one bit of information.
The holographic principle says that the amount of information papering
the outside must match the number of bits contained inside the volume
of the universe.
Since the volume of the spherical universe is much bigger than its
outer surface, how could this be true? Hogan realised 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. "Or, to put it another way, a holographic universe is blurry,"
says Hogan.
This is good news for anyone trying to probe the smallest unit of
space-time. "Contrary to all expectations, it brings its microscopic
quantum structure within reach of current experiments," says Hogan. 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 metres. "If you lived inside a hologram, you could tell
by measuring the blurring," he says.
When Hogan first realised this, he wondered if any experiment might be
able to detect the holographic blurriness of space-time. That's where
GEO600 comes in.
Gravitational wave detectors like GEO600 are essentially fantastically
sensitive rulers. The idea is that if a gravitational wave passes
through GEO600, it will alternately stretch space in one direction and
squeeze it in another. To measure this, the GEO600 team fires a single
laser through a half-silvered mirror called a beam splitter. 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.
"The key thing is that such experiments are sensitive to changes in the
length of the rulers that are far smaller than the diameter of a
proton," says Hogan.
So would they be able to detect a holographic projection of grainy
space-time? Of the five gravitational wave detectors around the world,
Hogan realised 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). "This random jitter would cause noise in the laser
light signal," says Hogan.
In June he sent his prediction to the GEO600 team. "Incredibly, I
discovered that the experiment was picking up unexpected noise," says
Hogan. 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.
"It looked exactly the same as my prediction," says Hogan. "It was as
if the beam splitter had an extra sideways jitter."
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. "There could still be a mundane source of the noise," Hogan admits.
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.
"The daily business of improving the sensitivity of these experiments
always throws up some excess noise," says Danzmann. "We work to
identify its cause, get rid of it and tackle the next source of excess
noise." At present there are no clear candidate sources for the noise
GEO600 is experiencing. "In this respect I would consider the present
situation unpleasant, but not really worrying."
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.
Danzmann says several planned upgrades should improve the sensitivity
of GEO600 and eliminate some possible experimental sources of excess
noise. "If the noise remains where it is now after these measures, then
we have to think again," he says.
If GEO600 really has discovered holographic noise from quantum
convulsions of space-time, then it presents a double-edged sword for
gravitational wave researchers. One on hand, the noise will handicap
their attempts to detect gravitational waves. On the other, it could
represent an even more fundamental discovery.
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 not
antimatter (New Scientist, 12 April 2008, p 26).
It would be ironic if an instrument built to detect something as vast
as astrophysical sources of gravitational waves inadvertently detected
the minuscule graininess of space-time. "Speaking as a fundamental
physicist, I see discovering holographic noise as far more
interesting," says Hogan.
Small price to pay
Despite the fact that if Hogan is right, and holographic noise will
spoil GEO600's ability to detect gravitational waves, Danzmann is
upbeat. "Even if it limits GEO600's sensitivity in some frequency
range, it would be a price we would be happy to pay in return for the
first detection of the graininess of space-time." he says. "You bet we
would be pleased. It would be one of the most remarkable discoveries in
a long time."
However Danzmann is cautious about Hogan's proposal and believes more
theoretical work needs to be done. "It's intriguing," he says. "But
it's not really a theory yet, more just an idea." Like many others,
Danzmann agrees it is too early to make any definitive claims. "Let's
wait and see," he says. "We think it's at least a year too early to get
excited."
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. It was a
"lucky accident" that Hogan's predictions could be connected to the
GEO600 experiment, he says. "It seems clear that much better
experimental investigations could be mounted if they were focused
specifically on the measurement and characterisation of holographic
noise and related phenomena."
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 ultracold 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 gravitational-wave-detector
counterparts.
So what would it mean it if holographic noise has been found? Cramer
likens it to the discovery of unexpected noise by an antenna at Bell
Labs in New Jersey in 1964. That noise turned out to be the cosmic
microwave background, the afterglow of the big bang fireball. "Not only
did it earn Arno Penzias and Robert Wilson a Nobel prize, but it
confirmed the big bang and opened up a whole field of cosmology," says
Cramer.
Hogan is more specific. "Forget Quantum of Solace, we would have
directly observed the quantum of time," says Hogan. "It's the smallest
possible interval of time - the Planck length divided by the speed of
light."
More importantly, confirming the holographic principle would be a big
help to researchers trying to unite quantum mechanics and Einstein's
theory of gravity. 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. "Holographic space-time is used in certain approaches to
quantising gravity that have a strong connection to string theory,"
says Cramer. "Consequently, some quantum gravity theories might be
falsified and others reinforced."
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. "Ultimately, we may have our first indication of how
space-time emerges out of quantum theory." As serendipitous discoveries
go, it's hard to get more ground-breaking than that.
Check out other weird cosmology features from New Scientist
Marcus Chown is the author of Quantum Theory Cannot Hurt You (Faber, 2008)
http://www.newscientist.com/article/mg20126911.300
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Updated on 5 May 2010
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