Our world may be a giant hologram

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OUR WORLD MAY BE A GIANT HOLOGRAM

15 January 2009 by Marcus Chown

 

 

www.newscientist.com/article/mg20126911.300-our-world-may-be-a-giant-

hologram.html

 

 

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.

Marcus Chown is the author of Quantum Theory Cannot Hurt You (Faber,

2008)