You 're holding a magazine. It feels solid; it seems to have some kind of independent existence in space. Ditto the objects around you -perhaps a cup of coffee, a computer. They all seem real and out there somewhere. But it 's all an illusion. Those supposedly solid objects are mere projections, Emanating from a shifting kaleidoscopic pattern living on the boundary of our Universe. The world is a hologram, Says J R Minkel

This idea is disturbing and confusing, even to the experts, but they can 't shake it off. It seems to be an inevitable result of mixing some of the dominant strands of modern physics -gravity, quantum theory and thermodynamics. And it also seems to be vitally important -even though scientists cannot agree what it means. Many physicists think it could be a clue to their longed-for theory of everything. Others think it means that there must be something wrong with quantum mechanics. Some go even further, claiming that it implies space and time are nothing but rivers of information.

And it 's that slippery concept, information, that led to the holographic principle. Physicists often talk of the world in terms of information. If you want to describe a piece of matter fully, they say, you must specify the motions of the microscopic pieces that make up nature, be they atoms, photons or more esoteric entities. That 's a lot of information. But just how much?

That depends on how many fundamental parts of an object there are -the number of bits depends on the number of pieces. And to describe the smallest pieces you have to zoom in way past the atomic scale to examine the fine structure of space-time. Einstein showed that matter can twist and bend the fabric of space-time, and that this warping shows itself as gravity. Then quantum mechanics came along and demanded that anything capable of moving about -including space-time -must come in little pieces that wiggle of their own accord. "One key feature of quantum gravity is that space-time should be thought of as made of some constituents," says Finn Larsen of the University of Michigan.

Just what the pieces are, nobody yet knows. Physicists have searched for a quantum description of gravity for decades without finding the answer. But they do have a few pointers -they know that whatever makes up space-time should come in bite-size chunks measuring just 10 ^-35 metres, the so-called Planck length. So if you broke space-time up into little boxes, each a Planck-sized cube, you 'd expect there to be roughly one bit of information per box.

But this picture is shredded by black holes. Instead of having one bit of information for every little volume, they seem to have one bit per patch of surface area.

Pack enough matter or energy into a small enough volume of space and it will collapse into a ball of intense gravitational attraction -a black hole. You might expect a black hole to be featureless, as if all the information in whatever formed it -stars, elephants, encyclopedias, civilisations -had been destroyed. But most physicists believe that black holes do hold information. Because if they didn 't, their very existence would undermine one of science 's most cherished laws, the second law of thermodynamics.

This law says that the amount of disorder in the world can never decrease. A display of stacked-up boxes in a supermarket is always in danger of falling down, but once fallen it is never in danger of righting itself. It takes fewer bits of data to specify the positions of those boxes if they 're stacked up in a pyramid than if they 're littering the floor. Information grows along with disorder.

If black holes obey the second law, they can 't just wipe out information. Where do they store it all? Well, black holes have something else that can never decrease -their surface area. Jacob Bekenstein, then at Princeton University, and Stephen Hawking of the University of Cambridge worked out that the surface area and the disorder in a black hole must be proportional. In information terms, there is roughly one bit per Planck area of the hole -that is, for each square that measures 10 ^-35 metres on a side.

But that 's vastly less than one piece for every Planck volume. So when a volume of space is crumpled into a black hole, a huge chunk of information is seemingly wiped out of existence. Indeed, that 's what Hawking maintains. But it means undermining quantum mechanics -a theory in which information is always preserved -and abandoning the link between disorder and information. Most theorists weren 't sure what to make of this tension, but some would simply not hear of such a flouting of the laws.

In 1993, there came a radical explanation. Working independently, both Leonard Susskind at Stanford University, and Gerard 't Hooft at Utrecht University saw that information might be preserved if it "lives" in just two dimensions of space, as opposed to the obvious, common-sense choice of three.

The idea certainly seems consistent with what we know of black holes. But if true, Susskind and 't Hooft realised, it has to apply to everything, or else you hit a horrible contradiction. Say you could pack more information into some region than a black hole of the same surface area would hold. Then you could keep throwing in more material, increasing the amount of information and the mass. Eventually there is so much mass in there that a black hole forms. If you believe that information can 't be reduced, you have a contradiction -the new black hole holds less information than the material that went into it. "You get punished by gravitational collapse," says Raphael Bousso of the Institute for Theoretical Physics at the University of California Santa Barbara.

The practical upshot is that the information limit for a black hole applies to everything. You really cannot get more than a surface-worth of information into any volume. But how can that be?

Maybe, Susskind and 't Hooft proposed, nature is storing the data about its most basic building blocks like a hologram. In a conventional hologram, a laser beam bouncing off an object is mixed with another laser beam and the resulting interference pattern is recorded on a flat surface. Shine new light onto the recording, and a lifelike three-dimensional image leaps out. If nature works like this, then information somehow lives on the boundary of any region of space-time. The material stuff within that boundary, the objects that we perceive and touch, is just the unpacked, higher-dimensional manifestation of that hologram. That is the holographic principle.

It means nature is remarkably concise. In a single cubic centimetre, there are 10 ⁹⁹ Planck boxes to stick bits of information into. The surface of that cube has space for a mere 10 ⁶⁶ bits. "That 's an outrageous reduction in complexity," says Bousso.

With larger volumes, the reduction is even greater. That 's because if an object gets bigger, its volume increases as the cube of its linear measurements -length, height, whatever -while its surface area increases only as the square of those measurements. It 's why an elephant loses proportionally less body heat, generated throughout its body volume, from its skin surface than a mouse does.

So if you take a cube of space, work out how much information it can hold and then put eight of these cubes together, the new volume of space can hold only four times as much information as the original cube (see Graphic). As you look at ever-bigger regions of space, the density of information goes down and down. So at the level of quantum gravity, there is no consistent way to count the amount of information inside the three-dimensional objects that we see and touch.

This blows away a concept that physicists have found quite useful for the last 150 years, says Don Marolf of Syracuse University in New York. Locality is the idea that points in space are separated and distinct from each other and that forces have to travel between them. "The holographic principle just flies in the face of that," Marolf notes.

And the holographic description of nature is distressingly awkward, says Stephen Shenker, also at Stanford. Say you 're looking out of a window and you see a pair of kids riding their bikes. In three dimensions, it 's easy to decide when the two bicycles are side by side -you can see it clearly. But take a hologram of them and the information about their position and how it changes over time becomes so much static. "The hologram is just a bunch of noisy, random marks," says Susskind. Presumably that is why we perceive a three-dimensional space with clearly separated objects in it : unless you look on the fine scales of quantum gravity, that picture is a lot easier to handle.

Even so, physicists hope to make sense of the holographic principle, because the potential pay-off is huge. To many, the idea means that we shouldn 't be looking for a fundamental theory in the here and now of ordinary space, but in a stranger place. Perhaps the truest, most economical theory is one that does not operate in terms of conventional space-time, but somehow lives on space-time 's edge.

In that case, to describe nature properly, we need to find a theory that lives in a two-dimensional space but can reproduce events in three spatial dimensions. Physicists were persuaded of this possibility only in 1998, when Juan Maldacena, then at Harvard, found a real theory that was holographic.

He was working on a leading candidate for a theory of quantum gravity, called string theory. This supposes that particles such as the electron, quark and photon are not point-like, but deep down are one-dimensional objects -strings.

Maldacena was trying to work out how a black hole could be made of strings -a good test of whether string theory really does work for gravity. He was working in a bizarrely curved space-time in five dimensions, because although it may sound unlikely, the mathematics is easier that way than for our own four-dimensional space-time. Even so, he had got stuck in a mathematical rocky patch, just as other string theorists had.

But Maldacena found a holographic way out. He conjectured that a string theory in one outlandish kind of five-dimensional space-time could be described by chains of quark-like particles swimming in the four-dimensional boundary of that space-time. There would be a precise but tortuously complex correspondence between the two theories. Susskind and Ed Witten, of the Institute for Advanced Study in Princeton, New Jersey, showed that this would obey the holographic principle.

The quarky hologram has one big virtue for theorists : instead of string theory, which is very hard to calculate with, you have a relatively simple quantum theory that describes the hologram. So if you want to work out what happens for certain situations in the five-dimensional space, you just translate it to four dimensions, do the calculations, and translate it back again.

"That 's when everybody started going, 'Oh my God, this is what we should be doing'," Bousso recalls. Maldacena 's result was exciting, but had one big flaw : he 'd found a holographic theory for a hypothetical space-time radially different from our own.

So the string theorists are now looking for a way to deal with the space-time we actually live in. Progress is slow. Perhaps the only big step in the right direction is an answer to the question, "What boundary are we talking about?" Our Universe is big, quite possibly infinite, and if you ask a cosmologist if it has a boundary, they will almost certainly say no. Where then does this hologram live?

Bousso, building on work by Susskind and Willy Fischler of the University of Texas at Austin, has concluded that it must be the boundary of the biggest region of space-time anyone in our Universe could ever observe. The actual size of that region depends on the speed of light and where you are in the Universe -for us, it 's about 15 billion light years. But what this tells us about the exact nature of the pieces of space-time still isn 't clear.

And despite all the hoopla, holography leaves some physicists flat. "I am not even sure I really understand it," says Carlo Rovelli of the Centre for Theoretical Physics in Marseille and Pittsburgh University. "Every time I discuss it with a different person I get a different version of the principle, and even from the same person I get different stories at different times." Marolf, though also unconvinced, has a harder time dismissing it. "It 's terribly self-consistent," he remarks. "Often when an idea is wrong it sort of conflicts with itself and you can demonstrate this very quickly." Perhaps, he says, the burden of proof now rests on those who would oppose the holographic principle.

Strangely, after helping to set this chain of events in motion, 't Hooft seems to be in that camp. "Rather than a 'principle', I now consider holography as being a problem," he says. He thinks holographic explanations could be avoided, and the concept of locality rescued, if quantum gravity is derived from a deeper principle that does not obey the usual rules of quantum mechanics. Instead of dealing in probabilities, as quantum theory does, this deeper mechanism would follow a predictable course while giving the appearance of randomness that we see in quantum events. Such a theory would also account for the "missing" information that holography was invoked to explain.

Susskind is also doing his best to salvage locality, but doesn 't want to give up holography completely. He and Shenker are trying to find a theory that contains holography in its tool kit alongside other, more convenient ways of describing nature. In this approach, space-time would have a full volume 's worth of constituents, but nothing save an area 's worth would have any effect on the physics within the volume.

Maybe the holographic principle is pointing the way to a different conceptual shift, according to Fotini Markopoulou and Lee Smolin of the Perimeter Institute in Waterloo, Ontario. In their approach to quantum gravity, called loop quantum gravity, space-time is built out of a mathematical network, each basic piece of which has an information and an area associated with it. Smolin and Markopoulou have suggested that a hologram act as a limit on the information that can pass across a surface in space-time, rather than as a limit on the total amount of possible information. In this view, we will have to stop thinking about "things" as fundamental features of reality. Instead of things, reality would be made of processes, such as information flow.

For Smolin, the holographic principle must be on the right track because of the way it has changed theorising about quantum gravity. "Everybody who has tried to think about this has come up with something that 's shocking from the point of view of 10 years ago," he says. "That means this is really important."

J R Minkel
J R Minkel is a freelance writer based in New York City