Introduction

First a word of thanks. When I first came across the papers of John Archibald Wheeler on the foundations of quantum mechanics, most of them reprinted in Wheeler and Zurek (1983), I could not believe what I read. Finally here was a colleague of worldwide reputation, given his many contributions to theoretical physics, who was not afraid to discuss openly the conceptual problems of quantum mechanics. The outstanding feature of Professor Wheeler's viewpoint is his realization that the implications of quantum mechanics are so far-reaching that they require a completely novel approach in our view of reality and in the way we see our role in the universe. This distinguishes him from many others who in one way or another tried to save pre-quantum viewpoints, particularly the obviously wrong notion of a reality independent of us.

Particularly remarkable is Professor Wheeler's austerity in thinking. He tries to use as few concepts as possible and to build on this the whole of physics. A fascinating case in point is the title of one of his papers “Law without law,” the attempt to arrive at the laws of nature without assuming any law a priori.

For me personally his work on fundamental issues in quantum mechanics has been particularly inspiring. The questions he raises are exceptionally far-reaching and some of his concepts in the foundations of physics are so radical that calling them revolutionary would not do them justice.

This book project began as part of a special program, Science and Ultimate Reality, developed in honor of the ninetieth birthday of renowned theoretical physicist John Archibald Wheeler. Having long yearned for a comprehensive, integrated understanding of the nature of the universe, Wheeler has blended scientific rigor with an unusually adventurous approach to research in physics and cosmology over a career spanning almost 70 years. Known for investigating many of the most fundamental and challenging issues in physics, Wheeler has often worked at the frontiers of knowledge where science and philosophy meet, probing the deep nature of physical reality. His vision, shaped in part by his influential mentor Niels Bohr, still flourishes today amid ongoing research activities pursued by several generations of those he has influenced over the course of much of the twentieth century.

With Wheeler as its inspiration, the Science and Ultimate Reality program was developed with a focus on the future. It brought together a carefully selected group of outstanding contemporary research leaders in the physics community to explore the frontiers of knowledge in areas of interest to Wheeler and to map out major domains and possibilities for far-reaching future exploration. Its two principal components – (1) this book and (2) a previously held symposium – were developed to take Wheeler's vision forward into a new century of expanding discovery.

Introduction

Once, while visiting the University of Texas in 1981, I joined John Wheeler and a group of students and postdocs for lunch. As he often did, John posed a provocative question for discussion. This time he asked something like the following, “Perhaps when we die, Saint Peter gives us a physics test to determine if our time spent on earth searching for knowledge for our fellow human beings has been well spent. Because the experience can be traumatic, and we are likely to forget details, we are allowed to bring along a crib sheet, to jog our memories. But as the point of having laws of physics is that they must be simple and general, the crib sheet is only allowed to be a 3 by 5 inch file card. What would you write down on your card?”

Of course, beyond the theological issues, John was making a simple and fundamental pedagogical point. If we believe that the laws of nature are simple, a measure of our understanding of them is the compactness with which they can be expressed. As individuals and as a community, the better we understand the laws of physics, the less the space that will be required to write them.

Exploring quantum reality?

Twentieth-century physics bequeaths us an unruly enigma in the equivocal dichotomy between quantum and classical. Mesoscopic systems: which are they, or when? To some this distinction is but a matter of modeling convenience; to others, the partition bears ontological weight. Whichever one's stance, debates on this issue sharpen our introspection on “Why the quantum?” by demanding rigorous justification for choices of calculative consequence, intuitively made on every day in every field of physics.

The limits seem clear. For few particles, left to their own devices, quantum mechanics runs rampant with its nonclassical phenomenology, viz. superposition, tunneling, and entanglement. But for the largish objects of our everyday experience, the sensory familiarity of classical mechanics holds sway: each object has its (singular) place, and every obstacle must be gone round or over. Strange, then, to ponder how big things are made from small! Somehow the assemblage of perceivable matter inevitably converts quantum constituents to classical collective, as if the ordering of the universe were ruled by atoms' aversion to the public embarrassment of quantum behavior writ large. It's a shame in a sense, for the senses slighted of paranormal experience, but superposition … what would it look like anyway?

Emergence, some say, is merely a philosophical concept, unfit for scientific consumption. Or, others predict, when subjected to empirical testing it will turn out to be nothing more than shorthand for a whole batch of discrete phenomena involving novelty, which is, if you will, nothing novel. Perhaps science can study emergences, the critics continue, but not emergence as such.

It's too soon to tell. But certainly there is a place for those, such as the scientist to whom this volume is dedicated, who attempt to look ahead, trying to gauge what are Nature's broadest patterns and hence where present scientific resources can best be invested. John Archibald Wheeler formulated an important motif of emergence in 1989:

Wheeler summarized his idea – the observer–participant who is both the result of an evolutionary process and, in some sense, the cause of his own emergence – in two ways: in the famous sketch given in Fig. 26.1 and in the maxim “It from bit.” In the attempt to summarize this chapter's thesis with an equal economy of words I offer the corresponding maxim, “Us from it.”

History will judge John Archibald Wheeler as one of the towering intellects of the twentieth century. His career spanned the transition from the celebrated Golden Age of physics to the New Physics associated with the Space Age, the information revolution and the technological triumphs of quantum and particle physics. His contributions, ranging from trailblazing work in nuclear physics to general relativity and astrophysics, are too numerous to list here. His influence on three generations of physicists is immense.

But Wheeler has been more than a brilliant and influential theoretical physicist. The decision to hold a symposium Science and Ultimate Reality in his honor reflects the fact that he is also an inspiring visionary who brought to physics and cosmology a unique style of thought and mode of reasoning, compared by Jaroslav Pelikan in this volume to that of the Greek philosopher Heraclitus.

“Progress in science,” Wheeler once remarked to me, “owes more to the clash of ideas than the steady accumulation of facts.” Wheeler has always loved contradiction. After all, the Golden Age of physics was founded on them. The theory of relativity sprang from the inconsistency between the principle of relativity of uniform motion, dating back to Galileo, and Maxwell's equations of electromagnetism, which predicted a fixed speed of light. Quantum mechanics emerged from the incompatibility of thermodynamics with the continuous nature of radiation energy.

A “Young Researchers Competition” was held in conjunction with the Science and Ultimate Reality symposium that took place in Princeton, New Jersey, USA, in March 2002. Like the entire Science and Ultimate Reality program, the competition was focused on the future of innovative research into the nature of “quantum reality” and related challenges inspired by Wheeler's “Really Big Questions” (see the Editors' Preface at the front of this book).

Of the 64 original applicants who submitted abstracts in an open competition worldwide, the applications of the 15 young research scientists born on or after January 1, 1970 that were chosen as finalists demonstrated work that is innovative and substantively engaged with the ideas raised by Wheeler's questions related to quantum reality. They also, therefore, related to one or more of the four main themes on which both this book and the symposium were based. The finalists made their presentations in 12-minute time slots (8 minutes plus 4 minutes for questions and answers) at the symposium on Sunday, March 17, 2002.

After evaluating the finalists' research accomplishments, records of achievement, and symposium presentations, appointed judges selected from among the symposium participants (all of whom contributed chapters to this volume) awarded eight prizes, six of $5000 each and a first-place prize of $7500 shared by the top two presenters, on the last day of the symposium on Monday, March 18.

Introduction: about quanta, atoms, photons, and cats

Experiments which manipulate and study isolated quantum systems have come of age. We can now trap single atoms or photons in a box, entangle them together, observe directly their quantum jumps, and realize in this way some of the thought-experiments imagined by the founding fathers of quantum physics. Schrödinger, who believed that observing an atom so to speak in vivo would remain forever impossible (Schrödinger 1952), would have been amazed, could he have seen what experimenters now achieve by manipulating atoms with lasers. These experiments are not just textbook illustrations of quantum concepts. They are considered by many as first steps towards harnessing the quantum world and realizing classically impossible tasks A quantum computer, for instance, would be a machine using quantum interference effects at a macroscopic scale in order to perform massive parallelism in computation (Nielsen and Chuang 2000). It would achieve an exponential speed-up to solve some problems such as the factoring of large numbers (Shor 1994). Such a machine would manipulate large ensembles of “quantum bits” made of atoms, molecules, or photons. Each bit would evolve in a superposition of two states labeled as “0” and “1”. These bits would be entangled together by quantum gates exploiting electromagnetic interactions between them. The behavior of this machine would be strange and counterintuitive. It would be a system made of thousands of two-level particles following during the calculation a huge number of different routes among which it remains coherently suspended.

Introduction

In the July 1957 issue of the Reviews of Modern Physics: Hugh Everett iii put forward a new interpretation of quantum mechanics (Everett 1957). John Wheeler, Everett's thesis adviser, published, in the same issue, an accompanying paper supporting Everett's views (Wheeler 1957). Everett's aim was to cut through the fuzzy thinking displayed by many authors, some of them quite prominent, who in previous years had written incredibly dull papers on how they understood quantum mechanics. Everett's idea was simply to assume that quantum mechanics provides a description of reality in exactly the same sense as classical mechanics was once thought to do.

This is a shocking idea, for it leads to a multiplicity of “realities.” Few physicists in 1957 were prepared to accept it. And yet it can be shown to work. It is the purpose of this article to expand on Everett's original demonstration and to reveal the courage John Wheeler displayed in betting that his student was right.

Classical theory of measurement

System, apparatus, coupling

Our starting point is the standard theory of measurement, which we shall first examine classically. In its simplest form a measurement involves just two dynamical entities: a system and an apparatus. It is the role of the apparatus to record the value of some system observables. For this purpose system and apparatus must be coupled together for a certain period of time, which will be taken to be finite.

Introduction

Almost a century after its birth, quantum theory remains odd and counterintuitive. As Richard Feynman wrote, it seems that nobody really understands it. This is indeed true if by understanding we mean being able to explain it using our common sense and everyday experience. The development of some kind of “quantum common sense” has been very slow even though our everyday life is being continuously influenced by technologies whose roots lie in quantum laws. In recent years the growing field of quantum information and quantum computation became a fruitful playground for physicists, mathematicians, computer scientists, and researchers from other fields who developed new interesting ways of storing, transmitting, and processing information using quantum mechanics at its best. Thus, both theoretical and experimental research on multiparticle entanglement, on the manipulation of individual quantum systems, on decoherence, and on the transition from quantum to classical are subjects of interest not only for their basic relevance but also for their potential practical significance as they might be of help for the development of a real quantum computer. Even though this technology may be far in the future, it is interesting to speculate about what lessons on quantum reality could be learned from quantum computation (eg., what would happen if one could operate a quantum computer).

Quantum mechanics occupies a unique position in the history of science. It has survived all experimental tests to date, culminating with the most precise comparison of any measurement to any theory – a 1987 measurement of the electron's magnetic moment, or gyromagnetic ratio ge = 2.002 319 304 39 (Van Dyck et al. 1987), agreeing with QED theory to 12 digits. Despite this and other dramatic successes of quantum mechanics, its foundations are often questioned, owing to the glaring difficulties in reconciling quantum physics with the classical laws of physics that govern macroscopic bodies. If quantum mechanics is indeed a complete theory of nature, why does it not apply to everyday life? Even Richard Feynman (1982), a fierce defender of quantum mechanics, memorably stated that:

In the dawn of the twenty-first century, John A. Wheeler's big question “Why the quantum?” has returned to the forefront of physics with full steam. Advances in experimental physics are beginning to realize the same thought-experiments that proved helpful to Einstein, Bohr, Heisenberg, Schrödinger, and the other founders of quantum mechanics.

Introduction

Suppose the usual description of spacetime as a 3 + 1 manifold breaks down at the scale where a quantum theory of gravity is expected to describe the world, generally agreed to be the Planck scale, lp = 10–35m. Can we still construct sensible theoretical models of the universe? Are they testable? Do they lead to a consistent quantum cosmology? Is this cosmology different than the standard one? The answer is yes, to all these questions, assuming that quantum theory is still valid. After 80 years work on quantum gravity, we do have the first detailed models for the microscopic structure of spacetime: spin foams.

The first spin foam models (Reisenberger 1994, 1997; Markopoulou and Smolin 1997; Reisenberger and Rovelli 1997; Baez 1998) were based on the predictions of loop quantum gravity, namely the quantization of general relativity, for the quantum geometry at Planck scale. A main result of loop quantum gravity is that the quantum operators for spatial areas and volumes have discrete spectra. (Rovelli and Smolin 1995; for a recent detailed review of loop quantum gravity see Thiemann (2001), and for a nontechnical review of the field see Smolin (2001)). Discreteness is central to spin foams, which are discrete models of spacetime at Planck scale.

Writing in Dublin in 1944, Erwin Schrödinger sought the source of order in biological systems. Given the recent radiation mutagenic evidence on the target size of a gene showing that a gene had at most a few thousand atoms, Schrödinger argued that the familiar order due to square root N fluctuations around an equilibrium was insufficient because the fluctuations were too large to account for the hereditary order seen in biology. He argued that quantum mechanics, via stable chemical bonds, was essential for that order. Then he made his brilliant leap. Noting that a periodic crystal could not “say” very much, he opted for genes as aperiodic crystals which, via the aperiodicity, would carry a microcode specifying the ontogeny of the organism. It was a mere two decades until the structure of the aperiodic double helix of DNA and much of the genetic code were known.

But did Schrödinger's book, What Is Life? actually answer his core question? I think not, and the aim of this chapter is to propose a different definition, one concerning what I call an “autonomous agent,” that may have stumbled on an adequate definition of life. I will not insist that I have succeeded, but at a minimum the definition leads in many useful and unexpected directions with import for physics, chemistry, biology, and beyond.

I am immensely pleased with this wonderful volume, and humbled by it. It demonstrates the incredible vibrancy of fundamental physics, both theoretical and experimental, as a new century gets under way. Just as unimagined vistas of the physical world were revealed in the early years of the twentieth century, so too we are encountering unimagined wonders a hundred years later. If there is an end to physics, an end to understanding the reasons for existence, it lies far in the future.

Who would have guessed in 1925, or even in 1950, that quantum mechanics would remain for so many decades such a fertile field of research? Who would have guessed then that its reason for being would remain mysterious for so long? Like many of the authors in this book, I remain convinced that some deeper reason for quantum mechanics will one day emerge, that eventually we will have an answer to the question, “How come the quantum?” And to the companion question, “How come existence?”

And who could have guessed in 1975 – when the black hole was coming to be accepted, when an explanation of pulsars was at hand, when primordial black-body radiation had been identified – who could have guessed then that an incredible confluence of deep thinking and stunning experimental techniques would push our understanding of cosmology – of the beginnings, the history, and the fate of the universe – to its present astonishing state?

The Science and Ultimate Reality program began with the symposium Science and Ultimate Reality: Celebrating the Vision of John Archibald Wheeler, held March 15–18, 2002 in Princeton, New Jersey, USA. The members of the Program Oversight Committee and the four Program Development Committees, many of whom are contributors to this volume, are listed below.