For the past seven years, a German experiment just south of Hanover—the GEO600—has been searching 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 have inadvertently made the most important discovery in physics for half a century.
For many months, the GEO600 team had been puzzling 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, the newly appointed Director at the Fermilab’s Center for Particle Astrophysics in Batavia, Illinois, GEO600 may have 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 dis-solves into dots as you zoom in. “It looks like GEO600 is being buffeted by the microscopic quantum convulsions of space-time. If the GEO600 result is what I suspect it is, then we are all living in a giant cosmic hologram.”1
The idea that we live in a hologram may sound strange, 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. In the 1990s physicists Leonard Susskind and Nobel prizewinner Gerard ‘t Hooft suggested that a holographic principle might apply to the universe as a whole. Susskind and 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 en-code the information inside the black hole, so there is no mysterious information loss as the black hole evaporates.
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 discernible.
That is, not until Hogan realized 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 realized that in order to have the same number of bits inside the uni-verse 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 meters. “If you lived inside a hologram, you could tell by measuring the blurring,” he says. 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.
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-meter 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 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.2 “This random jitter would cause noise in the laser light signal,” says Hogan.
In June he sent his prediction to the GEO600 team. “In-credibly, I discovered that the experiment was picking up un-expected 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 Ho-gan. “It was as if the beam splitter had 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.
Stay tuned. In the meantime here are some suggestions for additional reading about the holographic universe:
Bohm, David, Wholeness and the Implicate Order, Routledge Reissue edition, 1996: David Bohm’s model of explicate (enfolded order), tangible everyday physical reality; and, implicate (unfolded) order, more primary, deeper, underlying reality. This model is supported by Roger Penrose of Oxford, the creator of the modern theory of black holes; Bernard d’Espagnat of the University of Paris; leading authorities on foundations of quantum theory; and, Brian Josephson of the University of Cambridge, winner of the 1973 Nobel Prize in physics.
Missler, Chuck, Cosmic Codes - Hidden Messages From the Edge of Eternity, Chapter 23, Koinonia House, 1999.
“Our Holographic Universe,” Personal Update, February 2002.
“Information in the Holographic Universe,” Scientific American, August 2003.