The role that time plays in Einstein's theory of gravity and in quantum mechanics is described, and the difficulties that these conflicting roles present for a quantum theory of gravity are discussed.

Gravity and time

The relation of any fundamental theory to time is crucial as was evident from the earliest days of physics. If we go back to Newton's Principia, in which he established a general theoretical structure for the field of physics, we find an odd series of sentences in the first few pages. He tells us that it is unnecessary to define time as it is obvious to all, but then proceeds to do just that. And his definition is, certainly to modern eyes, rather strange. To quote

Reading this definition today, it is hard to see what the fuss is about. Time for us common folk is exactly Newton's true time.

Introduction

In this chapter I attempt to review a few somewhat loosely connected themes, which figure in recent work by physicists on decoherence and irreversibility in quantum mechanics. The looseness is forced upon us by the subject – any proper discussion inevitably drags in questions about thermodynamic reversibility and the various arrows of time, about the foundations of quantum mechanics and quantum measurement, about the nature of quantum coherence, and about the relationship between quantum and classical mechanics. There are inevitably many gaps in our understanding of these questions, both for the physicist and for the philosopher. Some philosophers may feel that the subject is too immature to be interesting to them – that it is not yet ripe for philosophical analysis. Indeed I have recently heard the opinion expressed that quantum mechanics in its entirety was still not amenable to such analysis – that if the physicists could not yet understand it, there was no point in philosophers trying! I hope this feeling is not too widespread amongst philosophers of science – I suspect that it may arise in part because the philosophical community (along with a good part of the scientific community) is unaware of some important advances that have been made in the last decade in understanding the physics of quantum processes at the macroscopic level, as well as the relation between classical and quantum mechanics.

Much of the chapter is therefore unashamedly tutorial in nature; however, given the wide range of topics covered, I imagine that even physicists will find at least some of the material to be novel.

Most of the contributors to this volume are philosophers of science, but three are physicists (Leggett, Stamp, and Unruh) and one is a mathematician (Douglas). Their chapters are intended to be original contributions towards answers to, rather than comprehensive discussions of, some of the oddly exasperating and fascinating questions known collectively as the problem of the direction of time, but the chapters were written with an eye towards communicating their results to the scientifically literate non-specialist.

Most of the work in this volume was presented at the ‘Time's Arrows Today’ Conference held on the campus of the University of British Columbia in June, 1992. The exceptions are the papers by Douglas and Earman and Sklar's ‘The elusive object of desire: in pursuit of the kinetic equations and the Second Law’. I am grateful to the Social Sciences and Humanities Research Council of Canada and to the Dean of the Faculty of Arts and the President of the University of British Columbia for their generous support of that conference.

In addition I wish to express my gratitude to Philip Stamp, whose advice and assistance have been invaluable, to Roy Douglas, who has long provided for me a model of rigorous thinking, and to William Unruh, without whose help my discussion of Penrose's thought experiment in the Introduction would have been much easier to write.

On rereading the text of my Tarner Lectures I was at first tempted to expand the arguments, to tone down the style, to present a range of alternative interpretations in the discussion of quantum mechanics, to add many more footnotes and references and other scholarly paraphernalia – in short to turn the Lectures into a monograph. But on reflection I decided to do none of these things. I came to Cambridge in 1987 with a certain missionary enthusiasm, to defend the objectivity and rationality of science, and to develop the new discipline of the philosophy of physics, and the Tarner Lectures afforded an opportunity to make a sharp, if at times simplistic, statement that would represent to a wider audience the sort of thing that I and my colleagues and students were trying to do.

So I have left the text more or less as a verbatim report of the Lectures as I delivered them. I leave it to the reader to decide whether I would have produced a better book if I had written a different book.

In his bookThe View from Nowhere the philosopher Tom Nagel argues that while each one of us, as a potential scientist or protoscientist, has a particular, personal and subjective perspective from which we view and interpret the world, the object of science is to abstract from these subjective perspectives and produce the objective view from nowhere, as he puts it.

This idea is really implicit in the doctrines of the ancient atomists, and was given a sharp formulation in the seventeenth century with the distinction drawn by Galileo and particularly emphasized in a somewhat different way by Locke, between what are usually called primary and secondary qualities. In the objective world ‘out there’ matter has extension, shape and motion and it is by virtue of these primary qualities that matter has the power to produce in our subjective experience the colours, tastes, smells, the ideas of secondary qualities, which we falsely impute to matter itself in our everyday thinking. Science is concerned only with the objective primary qualities, and, hence, abstracts from the diverse richness of subjective experience just those features which are objective, independent of subjective experience, independent of the observer, as we may call her. In the seventeenth century it was natural to think in terms of a dualism of mind and matter, the Cartesian res cogitans and res extensa. Science was concerned then with res extensa.

I am very grateful to the Master and Fellows of Trinity College for inviting me to give the Tarner Lectures.

What I want to do in this book, stated boldly and baldly, is to consider the relevance of developments in modern theoretical physics to metaphysical questions about the ultimate nature of reality.

By metaphysical questions I mean the very general sorts of questions that arise out of a critical examination of the principles, concepts and fundamental presuppositions that lie behind modern physics. How should we interpret the claims that physicists seem to be making about the world? Are they intended to be understood as literally true, for example, and exactly what are those claims, given that the pages of theoretical physics monographs are largely filled with abstract mathematical formalism?

In the first chapter, I shall be looking at a range of philosophical points of view in the framework of which such questions can be addressed, and attempt the difficult task of adjudicating between the different approaches. At the end of the day I shall be arguing for a pretty straightforward metaphysical realism, that the external world exists independently of our knowledge of it.

In the second chapter I shall look at arguments from various branches of modern physics that are sometimes claimed to bring in an essential role for subjectivity in physics, bringing the observer into a central role, creating his own reality.

Is it possible to bring experiments to bear on metaphysical theses such as realism? In recent developments in quantum mechanics particularly associated with the late John Bell it has been claimed that a certain type of realism, what is known as local realism, has indeed been refuted by a series of experiments, culminating in those carried out by Alain Aspect and his collaborators in Paris in 1982. Crudely local realism means in this context that atomic and subatomic entities, the subject matter of quantum mechanics, possess definite sharp values for all their attributes at all times, and, in addition, these attributes cannot be affected instantaneously by operations such as measurements performed on other microentities at different spatial locations from the one whose attributes are in question. In other words, realism is supplemented with a denial of the possibility of instantaneous action-at-a-distance. Now relativity theory, the other great pillar of modern theoretical physics, is standardly understood as committed to such a denial. So an experimental refutation of local realism would force us to give up either locality (the no-instantaneous-action-at-a-distance principle) or the realist thesis itself. The first horn of this dilemma is closed off by appeal to relativity theory, and so the conclusion of the argument would appear to be the refutation of realism.

In 1980 Stephen Hawking delivered his inaugural lecture as Lucasian Professor at Cambridge with the title ‘Is the End in Sight for Theoretical Physics?’ Hawking was cautiously optimistic that by the end of the twentieth century the answer might have turned out to be ‘yes’. Since we have now reached beyond the half-way mark, I thought it might be appropriate to take stock.

The view I am going to discuss is that physics and, more generally, experimental science has developed at an increasingly rapid rate since its inception in the seventeenth century, and will simply burn itself out in a final frenzy of intellectual effort. It is rather like the exploration of the physical globe – there is only one globe, and once all the continents, mountains, rivers, etc., have been discovered, the work is essentially finished, there is just detail to fill in. Although it may be granted that there are infinitely many exact locations on the globe, there is a real sense in which the job of map-making on any ‘interesting’ scale is finite, completable and has in fact been completed over a total span of less than 500 years.

Towards the end of the nineteenth century Lord Kelvin is famously supposed to have claimed that all that remained in physics was the job of filling in the next decimal place. Matter, and energy in its diverse forms, were governed by mechanical laws and the principles of thermodynamics.

One must admit that many physicists would dismiss the sort of question that philosophers of physics tackle as irrelevant to what they see themselves as doing, viz. producing simple, unified, empirically adequate theories about the world. Either these metaphysical questions arise, they would say, as the result of philosophers involving themselves with the technicalities of theoretical physics, which they, the philosophers, never really understand, or it is the physicists themselves who in some cases get side-tracked and ensnared by the temptation to indulge in the subtle sophistry of the philosophers, posing unanswerable questions, a subject where there is no discernible progress, where there is no general agreement on premisses from which an argument could be launched, where every conceivable position has been argued for by some group of philosophers and equally refuted by another group. In short, philosophy, like religion, abounds in isms and schisms, which it is a waste of time to try and sort out. Much better to keep one's nose to the grindstone, and produce good physics, than to indulge in idle fancy and speculation.

These typical reactions of physicists to the philosophy of physics show how deeply the divide between the two cultures still runs, between the hard sciences and the soft humanities. But let us step over the wall, so to speak, and look back at the standpoint of the physicists from the point of view of the philosophers.

Fritz London went to Berlin at the start of the winter semester of 1927–1928 and left for Oxford at the end of August 1933. He was Schrödinger's assistant and became a Privatdozent in 1928. During this time he was away for a month in Leningrad and for six months in Rome. In the joint paper with Walter Heitler, written when both were at Zürich in 1927, it was shown that the formation of the hydrogen molecule was a purely quantum mechanical phenomenon, whose understanding depended on the Pauli exclusion principle. In his Berlin years, London exploited fully the possibilities provided by the notion of a purely quantum phenomenon, and developed the rather abstruse group theoretical method to deal with polyelectronic molecules and with the difficulties of calculations involving many bodies. He also developed a theory of chemical reactions as activation processes. In 1930, together with Eisenschitz, he started to investigate the characteristics of the molecular forces. He found out that they could be understood only as manifestations of the uncertainty principle – another purely quantum mechanical notion. In Berlin, he was in an intellectually stimulating environment and was highly appreciated by all the holy men there: Einstein, von Laue, Schrödinger and the retired Planck. He signed a contract with Springer to write a book about atomic and molecular forces. He met Edith Caspary and they were married in 1929. He placed his younger brother Heinz in good hands; but after a short stay with Gerlach in Berlin, Heinz went to Breslau to work with Franz Simon.

It took almost half a century after the discovery of superconductivity by Kamerlingh Onnes in 1911 before a satisfactory explanation at the microscopic level was given by Robert Schrieffer, Leon Cooper and me in 1957. The theory was the result of many years of effort by many people in both theory and experiment. The first theories were phenomenological: equations were proposed to account for a range of experiments without an understanding of how they could be derived from the equations of motion of the electrons and ions that constitute a superconducting metal.

By far the most important step towards understanding the phenomena was the recognition by Fritz London that both superconductors and superfluid helium are macroscopic quantum systems. Quantum theory was derived to account for the properties of atoms and molecules at the microscopic level. It was Fritz London who first recognized that superconductivity and superfluid flow result from manifestations of quantum phenomena on the scale of large objects.

Perhaps the most striking illustration is that the magnetic flux threading a superconducting ring is an integral multiple of a small flux unit, hc/2e. In a footnote in his book published in 1950, London predicted such a relation (with e rather than 2e in the denominator). Flux quantization was first observed experimentally eleven years later. Quantization is a direct result of the de Broglie relation between momentum and wave length, p = h/λ, and the fact that there must be an integral number of wave lengths around the ring, nλ = 2πr, where r is the radius.

Fritz London's first published paper in a professional journal was in philosophy. In 1921, the year he graduated from the University of Munich, whilst supervised by one of the most well-known phenomenologists, Alexander Pfänder, he wrote a thesis that dealt with deductive systems. It was among the very first attempts to investigate ideas about philosophy of science expressed by the founder of the phenomenological movement in philosophy, Edmund Husserl. It was a remarkable piece of work for someone who was 21 years old. In this work, London developed an antipositivist and antireductionist view. This is all the more surprising, given London's knowledge of and interest in science, and the appeal of positivism to many scientists. London also intervened in the controversy between Richard Tolman and Tatiana Ehrenfest-Afanassjewa about the possibility of finding physical laws by dimensional considerations alone. Many of the ideas elaborated by London in his later researches, including his insightful suggestions and discussions of macroscopic quantum phenomena, can, indeed, be traced back to these early philosophical wanderings.

Right after his graduation from the University he started teaching in the Gymnasium and, when he was ready to matriculate as a Gymnasium teacher, he resigned and went to Max Born, who was at the University of Göttingen, to work in philosophy. Born did his utmost to discourage him, but to no avail. Born's only hope to persuade the young London to do an actual calculation, like all others beginning a career in physics, was to convince him to go to Munich and to study with Arnold Sommerfeld.