By BBC Focus – FERMILAB, THE AMERICAN particle accelerator facility near Chicago, has just seen the completion of a peculiar scientific instrument. Its purpose is to probe space-time and demonstrate that, far from being smooth, as Einstein believed, it is grainy like a newspaper photograph viewed close-up. If the Fermilab ‘Holometer’ succeeds, incredibly, it may reveal that the Universe is actually a hologram – a 3D representation of an underlying 2D reality. It would mean that we – and the planet on which we live – are holograms.
Over the coming months the Holometer will be switched on, beams of laser light racing along tubes to measure space-time at the smallest of scales. It will show whether space and time stand still, or whether they shift around a tiny bit. If there’s movement or some ‘blurring’, it would show that the Universe is a hologram – just like the blurred security image on a credit card.
The suggestion that we are living in a giant hologram is surely one of the most bizarre ideas to come out of modern physics – proof, if proof were needed, that science is indeed far stranger than science fiction. Even stranger, the idea has its origin not in cosmology – the study of the large-scale Universe – but in the esoteric field of black holes.
Black holes are born when a massive star reaches the end of its life and shrinks catastrophically, crushing it to a point-like ‘singularity’. Not even light can escape the gravitational clutches of a black hole, hence its name. The vanishing of a star in such a dramatic way was not a problem for physics until, in the 1970s, Stephen Hawking showed that, paradoxically, black holes are not completely back. They radiate into space so-called Hawking radiation until there is nothing left of them and they disappear.
The trouble is that Hawking’s discovery implies that, when a black hole ‘evaporates’, all information about the star that shrunk to create the black hole -the type and location of all its atoms, for instance – would seem to disappear too. This contradicts a fundamental edict of physics that ‘information’ can never be created or destroyed. (Holographic Universe? This May Be the Greatest Revolution of the 21st Century)
A clue to the resolution of the black hole information paradox, as it became known, came from Israeli physicist Professor Jacob Bekenstein. He discovered something profound about the ‘horizon’ – the imaginary membrane that surrounds a black hole and marks the point of no return for in-falling matter. Bekenstein found that the surface area of the horizon is related to the ‘entropy’ of the black hole. In physics, a body’s entropy is synonymous with its microscopic disorder. Crucially, entropy is intimately related to information.
This is the key clue to resolving the black hole information paradox. In 1997, string theorist Professor Juan Maldacena of Princeton’s Institute for Advanced Study in the US showed that it is in the horizon that the information that describes the star may be stored – as microscopic lumps and bumps. So, when the black hole sends out Hawking radiation from the vicinity of its horizon, the radiation has impressed on it information about the star, just as the radio waves from BBC Radio One have pop music impressed on them. So, when the black hole disappears, the ‘song’ of the star is not lost at all. It is broadcast to the Universe as Hawking radiation. No information is ever lost.
But all this implies that a 2D surface – the horizon of a black hole – can store sufficient information to describe a 3D object – a star. This is exactly what a hologram, such as the one on your credit card, does.
This might all seem an esoteric result about an esoteric type of celestial body – a black hole. But, in the late 1990s, Professor Leonard Susskind of Stanford University in California made a surprising connection. The Universe, like a black hole, is surrounded by a horizon. It is a horizon in time rather than in space but it is a horizon nonetheless. So, reasoned Susskind, the information describing the 3D Universe might be stored in its horizon.
What this means is open to a wide range of interpretations. A conservative interpretation is simply that the Universe contains a lot less information than we imagined. A more extreme interpretation is that the Universe is truly a hologram – a 2D object stored on the cosmic horizon which creates the illusion of a 3D universe. In some sense, ‘you’ would be a 3D projection of a flat you. Bekenstein, however, is sceptical of Susskind’s extension of his black hole idea to the Universe. “My result is not applicable to horizons which are not event horizons,” he says. However, there is other evidence that the Universe contains less information than its three space dimensions would imply, and this is now generally accepted by physicists. (Introduction to the Holographic Universe Theory)
GRAINS OF TRUTH
If Susskind and others are right about the Universe being holographic, one thing would appear to follow. A hologram is more blurry than the object it depicts -just like the image on a credit card. And this has implications for space-time.
It turns out that space is like the ocean. Observed from a great height – from the perspective of a bird for instance – it looks smooth. Observed close-up, from the perspective of a boat, it is rough and choppy. In fact, space-time is believed to become so choppy that it loses its smoothness entirely at the size of the ‘Planck scale’, which is about 10(on)35 metres, or about 10 trillion trillion times smaller than a typical atom. But, if the Universe is a hologram, it should be much grainier. It would mean we wouldn’t have to zoom into a scale of 10 on-35 metres to see things get choppy – making the graininess easier to detect.
For a time in 2008, it did indeed seem as if the graininess of space-time had been detected, proving the idea of the holographic Universe. An Anglo-German experiment called GEO600 near Hanover in Germany was looking for gravitational waves. These are ripples in the fabric of space-time, predicted by Einstein’s theory of gravity, General Relativity, and thought to be radiated by violent events such as the explosion of stars and the birth of black holes. As gravitational waves pass by, they alternately squeeze a stretch space in perpendicular directions. The GEO600 interferometer was looking for this distortion with two perpendicular 600m ‘rulers’ made from laser light. The light travels back and forth along the arms of the interferometer, bounced repeatedly from suspended mirrors. And it was these mirrors that revealed something odd. They seemed to be jittering, like a branch being buffeted by the wind. And nobody could figure out why.
Enter Professor Craig Hogan of the University of Chicago and the nearby Fermi National Accelerator Laboratory (Fermilab). When he heard about the unexplained jitter, or ‘noise’, he immediately thought GE0600 had accidentally found the graininess of space-time. The mirrors were being jostled, he reasoned, by its choppiness. The Universe was indeed a hologram.
Unfortunately, the GE0600 experimenters later found the source of the noise and it turned out to be mundane. It was simply an artefact of the way the experimenters recorded the light from their instrument. When they changed their readout method, the noise went away.
But Hogan was undeterred. The idea grew in his mind of building an instrument hugely more sensitive than GE0600 and specifically dedicated to finding the trembling of space-time at the Planck scale itself. Thus the Fermilab Holometer was born.
Hogan did not even assume the reasoning of Susskind – that the Universe’s horizon contains all its information. He simply assumed that, at the sub-microscopic, choppy level, space-time is fundamentally 2D. On the larger scale, a third dimension emerges. Emergent phenomena are seen everywhere in nature. The atoms that make up a wall painted blue are in no sense blue. But when there are large numbers of atoms clumped together, the property of blue emerges.
The Holometer consist of two perpendicular arms, each 40m long. Laser light enters the instrument, is split by a ‘beam-splitter’, travels down each arm, before bouncing off a suspended mirror and travelling back the way it came. The two beams then recombine.
If the two arms keep a constant length, the two light waves will combine and create a constant intensity with time. But, if space-time is choppy the arms will fluctuate in length from moment to moment. This will cause the light waves to go in and out of phase, one instant reinforcing, the next partially cancelling, causing the intensity of the combined light to fluctuate.
The Holometer, built using a tunnel that was part of an old Fermilab experiment, will actually consist of two interferometers, one on top of the other. If they both show exactly the same fluctuation with time, the experiment will know it is not an artefact – a mundane vibration in the Fermilab building, or fluctuations from discrete laser photons. It would be something caused by fundamentally new space-time physics. (Article Our Universe May Be a Giant Hologram)
“I’m glad the Holometer has been constructed at Fermilab and is running,” says Professor John Cramer of the University of Washington in Seattle. “Its cost is very small for an experiment testing such a fundamental question.” At roughly USD 1 million (GBP 623,000), the cost of the Holometer is a drop in the ocean compared with the GBP 4.4 billion price tag of CERN’s Large Hadron Collider.
“The Holometer should be operating and collecting data in a few months,” says Hogan. But it will take a while to reach its optimum performance. “Sources of environmental interference will have to be tracked down.” For instance, the experimenters will have to understand spurious causes of vibration such as the tremors caused by cars going by on the street – so they can subtract them. “We hope it will be working at its optimum level in about two years’ time,” says Hogan.
Cramer has high hopes. “Experimentalists are optimists by natural selection, because otherwise they would never be foolish enough to do experiments,” he says. Hogan is trying to keep a lid on his enthusiasm. “I try not to get carried away thinking about the results,” he says. “This is a journey into the unknown. There are no well-tested ideas to tell us what we will find.”