The Cosmic Landscape (53 page)

What would we have to do to faithfully store the full three-dimensional data, including depth information, as well as the “blood-and-guts” data about the interior of objects? The answer is obvious: instead of a collection of pixels filling two dimensions, we would need a space-filling collection of “voxels,” tiny elements that fill the
volume
of space.

Filling space with voxels is far more costly than filling a surface with pixels. For example, if your computer screen is a thousand pixels on a side, the total number of pixels is one thousand squared, or one million. But if we want to fill a volume of the same size with voxels, the required number would be one thousand cubed, or one billion.

That’s what makes holograms so surprising. A hologram is a two-dimensional image—an image on a piece of film—that allows you to unambiguously reconstruct full-blown three-dimensional images. You can walk around the reconstructed holographic image and see it from all sides. Your powers of depth perception allow you to determine which object in a hologram is closer or farther away. Indeed, if you move, the farther object can become the closer object. A hologram is a two-dimensional image, but one that has the full information of a three-dimensional scene. However, if you actually look closely at the two-dimensional film that contains the information, you see absolutely nothing recognizable. The image is thoroughly scrambled.

The information on a hologram, although scrambled, could be located on pixels. Of course nothing is for free. To describe the volume of space one thousand pixels on a side, the hologram would have to be composed of one billion pixels, not one million.

One of the strangest discoveries of modern physics is that the world is a kind of holographic image. But even more surprising, the number of pixels the hologram comprises is proportional only to the
area
of the region being described, not the volume. It is as though the full three-dimensional content of a region, one billion voxels in volume, can be described on a computer screen containing only a million pixels! Picture yourself in an enormous room bounded by walls, a ceiling, and a floor. Better yet, think of yourself as being in a large spherical space. According to the Holographic Principle, that fly just in front of your nose is really a kind of holographic image of data stored on the two-dimensional boundary of the room. In fact you and everything else in the room are images of data stored on a
quantum hologram
located on the boundary. The hologram is a two-dimensional array of tiny pixels—not voxels—each as big as the Planck length! Of course the nature of the quantum hologram and the way it codes three-dimensional data is very different from the way ordinary holograms work. But they do have in common that the three-dimensional world is completely scrambled.

What does this have to do with black holes? Let’s place a black hole in our large spherical room. Everything—black hole, space traveler, mother ship—is stored as information on the holographic walls of the space. The two different pictures that black hole complementarity tries to reconcile are simply two different reconstructions of the same hologram by two different reconstruction algorithms!

The Holographic Principle was not widely accepted when ’t Hooft and I put it forward in the early 1990s. My own view was that it was correct but that it would take many decades until we knew enough about quantum mechanics and gravity to confirm it in a precise way. But just three years later, in 1997, all that changed when a young theoretical physicist—Juan Maldacena—electrified the physics world with a paper titled “The Large N Limit of Superconformal Field Theories and Supergravity.” Never mind what the words mean. Maldacena, by cleverly using String Theory and Polchinski’s D-branes, had discovered a completely explicit holographic description of, if not our world, a world similar enough to make a convincing case for the Holographic Principle. Slightly later Ed Witten put his stamp of approval on the Holographic Principle with a follow-up to Maldacena’s paper titled “Anti De Sitter Space and Holography.” Since then the Holographic Principle has matured into one of the cornerstones of modern theoretical physics. It has been used in many ways to illuminate problems that, on the face of them, have nothing to do with black holes.

What does the Holographic Principle have to do with black hole complementarity? The answer is
everything.
Holograms are incredible scrambles of data that have to be decoded. That can be done by a mathematical algorithm or by shining laser light on the hologram. The laser light implements the mathematical algorithm.

Imagine a scene containing a large black hole and other things that might fall into the black hole as well as the radiation coming out. The entire scene can be described by a quantum hologram localized far away on some distant boundary of space. But now there are two possible ways—two algorithms—for decoding the hologram. The first reconstructs the scene as seen from outside the black hole, with the Hawking radiation carrying away all the bits that fell in. But the second reconstruction shows the scene as it would be seen by someone falling into the black hole—one hologram, but two ways to reconstruct its content.

It’s probably too much to say that the three-dimensional world is a complete illusion. But the idea that the location of a bit of information is not necessarily where you might expect is now a widely accepted fact. What are its implications for the bubble bath universe of chapter 11? Let me remind you where we left off at the end of that chapter.

In the last chapter I explained the two views of history, one series and one parallel. According to the series view, every observer sees at most a small portion of the entire megaverse. The rest will never be seen because it is moving away so fast that light cannot bridge the gap. The boundary between what can and cannot be seen is the horizon. Unfortunately, the rest of the megaverse of pocket universes is all in this never-never land beyond the horizon. According to the classical principles of general relativity, we can wonder all we want about the existence and reality of these other worlds, but we can never know. They are irrelevant. They are meaningless in the scientific sense. They are metaphysics, not physics.

But exactly the same conclusion was incorrectly drawn about black hole horizons. Indeed, the cosmic event horizon of an eternally inflating universe is mathematically very similar to the horizon of a black hole. Let’s return to the infinite lake filled with boats and observers. The black hole was very much like the dangerous drain, the horizon being the point of no return. Let’s compare that situation with the eternally inflating lake, i.e., the lake fed by feeder tubes so that the floating observers all separate according to Hubble’s Law. If the lake is fed at a constant rate, it provides a precise analog for Eternal Inflation.

Any particular boat will be surrounded by a boundary similar to the point of no return that surrounded the drain. Imagine a dinghy hovering around its parent vessel. If by accident or design it gets beyond the point of no return, it simply cannot get back or even communicate with the parent vessel. The only difference between the black hole horizon and the cosmic horizon of inflating space is that in one case we are on the outside looking in, and in the other case, we are inside looking out. But in every other way, the black hole and cosmic horizons are the same.

To someone outside a black hole, the events in the life of the trans-horizon explorer are behind the horizon. But those events are physics, not metaphysics. They are telegraphed to the outside in scrambled holographic code in the form of Hawking radiation. Like the prisoner’s message, it doesn’t matter if the code is lost—or even whether we ever had it. The message is in the cards.

Are there also “cards” coming from behind the cosmic horizon with messages from billions of pocket universes? Cosmic horizons are not nearly as well understood as black holes. But if the obvious similarity between them is any guide, cosmic horizons do yield such cards, and they are very much like the photons Hawking radiation comprises. By now you may have guessed that they are the photons of the cosmic microwave background radiation that bathe us from every direction and for all time. Messengers from the cosmic horizon, they are also coded messages from the megaverse.

George Smoot, one of the leaders in cosmic microwave detection, in an overenthusiastic moment likened a cosmic microwave map of the sky to “the face of God.” I think for inquiring minds curious about the world a scrambled hologram of an infinity of pocket universes is a far more interesting and accurate image.

One theme has threaded its way through our long and winding tour from Feynman diagrams to bubbling universes: our own universe is an extraordinary place that appears to be fantastically well designed for our own existence. This specialness is not something that we can attribute to lucky accidents, which is far too unlikely. The apparent coincidences cry out for an explanation.

An immensely popular story, not only among the general public but also many scientists, is that a “superarchitect” designed the universe with some benevolent purpose in mind.
¹
The advocates of this view, intelligent design, say that it is quite scientific and perfectly fits the facts of cosmology as well as biology. The intelligent designer not only chose excellent Laws of Physics for its purpose but also guided biological evolution through its unlikely chain, from bacteria to Homo sapiens. But this is an intellectually unsatisfying, if emotionally comforting, explanation. Left unanswered are: who designed the designer, by what mechanism the designer intervenes to guide evolution, whether the designer violates the Laws of Physics to accomplish its goals, and whether the designer is subject to the laws of quantum mechanics.

One hundred and fifty years ago, Charles Darwin proposed an answer for the life sciences that has become the keystone of modern biology—a mechanism that needs no designer and no purpose. Random mutation, combined with competition to reproduce, explains the proliferation of species that eventually fill every niche, including creatures that survive by their wit. But physics, astronomy, and cosmology lagged behind. Darwinism may explain the human brain, but the specialness of the Laws of Physics has remained a puzzle. That puzzle may be yielding, at last, to physical theories that parallel Darwin’s biological theory.

The physical mechanisms that I have explained in this book share two key ingredients with Darwin’s theory. The first is a huge Landscape of possibilities—an enormously rich space of possible designs.
²
There are more than 10,000 species of birds, 300,000 species of beetles, and millions of species of bacteria. The total number of
possible
species is undoubtedly immeasurably larger.

Is the number of biological designs as large as the number of universe designs? That depends on exactly what we mean by a biological design. One way of listing all biological possibilities is to enumerate the ways of assigning the base pairs in a large DNA molecule. A human DNA strand has about a billion base pairs, and there are four possibilities for each. The total number of possibilities is the ridiculously large number 4
^1000000000
(or 10
^600000000
). This is much bigger than the 10
⁵⁰⁰
(similarly obtained by counting the number of ways of assigning flux integers) that string theorists guess for the number of valleys of the Landscape, but of course almost all of these do not correspond to viable life forms. On the other hand, most of the 10
⁵⁰⁰
vacuums are also dead ends. In any case both numbers are so large that they are far beyond our powers of visualization.

The second key ingredient is a superprolific mechanism to turn the blueprint designs into huge numbers of real entities. Darwin’s mechanism involved replication, competition, and lots and lots of carbon, oxygen, and hydrogen on which these mechanisms operate. Eternal Inflation also involves exponential replication—but of volumes of space.

As I discussed in chapter 11, the process of populating the Landscape does have its similarities with biological evolution, but it also has at least two very big differences. The first was discussed in chapter 11. Biological evolution along a given line of descent is through minute, undetectable changes from generation to generation. But descent through a series of bubble nucleations involves, at each stage, large changes of vacuum energy, particle masses, and the rest of the Laws of Physics. Biologically, if only such large changes were possible, Darwinian evolution would be impossible. The mutated monsters would be at such a disadvantage relative to normal offspring that their survival in a competitive world would be impossible.

How then does the megaverse become populated with diversity if biological evolution, under the same conditions, would stagnate? The answer lies in the second big difference between the two kinds of evolution: there is no competition for resources among pocket universes. It’s interesting to contemplate an imaginary world in which biological evolution takes place in an environment where resources are so unbounded that there is no need for competition. Would intelligent life evolve in such a world? In most descriptions of Darwinian evolution, competition is a key ingredient. What would happen without it? Let’s take a particular case, the final step in the evolution of our own species. About 100,000 years ago Cro-Magnons were in a struggle for survival with Neanderthals. Cro-Magnons won because they were smarter, bigger, stronger, or sexier. Thus, the average genetic stock of the human race was improved. But suppose resources were unbounded and that sex was unnecessary for reproduction. Would there be fewer Cro-Magnons? Not at all. Everyone who survived would survive more easily without competition. And many who did not survive would do so as well. But there would also be more Neanderthals. In fact there would be more of everyone. All populations would increase exponentially. In a world of unbounded resources, lack of competition would not have slowed the evolution of the smartest creatures, but it would have made a lot more dumb ones.