In the 1920s, physics was shaken by the quantum revolution. Experimental data could not be reconciled with the older classical paradigms. A new view was required, along with a radical revision to concepts that had seemed completely secure. Arguably, the quantum revolution still rumbles on, although the aftershocks are somewhat more muted now. While we have a complete mathematical grasp of quantum theory, its interpretation remains open to debate (although not that many working physicists engage with the debate, taking a more “shut up and calculate” approach). It is entirely possible that the ambiguity over interpretation is a signal that we are still not in possession of the correct fundamental concepts. That would also chime with the difficulties inherent with reconciling the quantum and relativistic pictures. Modern theoretical developments towards a quantum gravity may also shed some light on entropy, or at least information. Consequently, for a variety of reasons, we need to engage with the quantum description of the microworld and to see how that impacts on our understanding of entropy .
In the 1920s, physics was shaken by the quantum revolution. Experimental data could not be reconciled with the older classical paradigms. A new view was required, along with a radical revision to concepts that had seemed completely secure. Arguably, the quantum revolution still rumbles on, although the aftershocks are somewhat more muted now. While we have a complete mathematical grasp of quantum theory, its interpretation remains open to debate (although not that many working physicists engage with the debate, taking a more “shut up and calculate” approach). It is entirely possible that the ambiguity over interpretation is a signal that we are still not in possession of the correct fundamental concepts. That would also chime with the difficulties inherent with reconciling the quantum and relativistic pictures. Modern theoretical developments towards a quantum gravity may also shed some light on entropy, or at least information. Consequently, for a variety of reasons, we need to engage with the quantum description of the microworld and to see how that impacts on our understanding of entropy .
By the end of the nineteenth century all the important elements of classical physics were fully mature. Newton's Laws had been applied with success to everything from industrial machines to planetary orbits. Classical thermodynamics had extended energy conservation arguments into thermal processes, and scientists including J. Willard Gibbs and Ludwig Boltzmann had developed the statistical interpretation of entropy. James Clerk Maxwell had explained all electromagnetic phenomena, from AC generators/motors to light, with the four Maxwell's equations and three material-dependent constitutive relations. There was serious discussion that physics was on the verge of being “finished” – becoming a closed system of knowledge like classical geometry with only essentially routine tasks left of determining how the known laws could be applied to engineering problems. Yet by 1930 the foundations of mechanics, electromagnetics, and atomic-scale systems had been shaken to the core by the new revolutions in the Theory of Relativity, the photon picture of light, and quantum physics. We are still reaping the benefits of the application of these new scientific paradigms in our technology today, including semiconductor electronic devices, lasers, and GPS systems. But the description of reality that these theories provide is still deeply anti-intuitive and disturbing. Nevertheless, quantum physics is one of the most extensively experimentally verified theories that we know. Even when quantum theory and common sense seem to diverge, quantum theory gives the right answers when tested by experiment. In the next chapter we will develop the framework of quantum theory that has been so successful when people “shut up and calculate,” as one scientist suggested as the way through the philosophical thicket of quantum physics. But in this chapter, we will spend a brief time considering the historical development and continuing paradoxical implications of quantum theory. The photoelectric effect and photons In 1905, Albert Einstein, who was a key figure in the revolution of modern physics, was working as a clerk in the patent office in Zurich, Switzerland. Einstein had finished his studies in physics, but had not obtained a coveted position as a university professor. He was intensely interested in the latest developments in classical physics and was aware of a few of the remaining loose ends that were resisting explanation in the classical model.
Abstract The development of quantum theory was driven by a consistently repeated failure of classical physics to accommodate the results of ever more sophisticated experiments on the microphysical world of radiation, elementary particles, atoms, and molecules. Each break-through in the development of the theory’s conceptual structure was followed (sometimes very rapidly) by new experimental data that just did not fit, pointing to the need for yet further refinements. As is typical in such a radical change in the approach to acquiring scientific understanding, old and often unsuitable concepts were forced into service to meet the needs of the new description. Confusion reigned. The major figures in the theory’s early development took up radically different positions on the theory’s interpretation, and, as we will see, the debate became polarized in ways that remain relevant today.
Despite scholarly consensus on the empirical facts, ever since its inception in the 1920s there has been deep disagreement about how to interpret quantum theory. “To interpret a physical theory is to say what the world would be like, if the theory were true.” All scientific theories require interpretation, since strictly speaking what they describe are our experiences of the world rather than the world itself. Thus, whether we are trying to explain or merely describe the world, we are always engaged in inference. To make inferences we need a context for interpretation. For any given claim this context is provided locally by related theories, more broadly by disciplinary paradigms, and ultimately by worldview claims about the nature of reality. The challenge in interpreting scientific theories is to integrate them into this hierarchy of knowledge. Usually this is not too difficult. New theories often require adjustments to nearby theories, but in doing so we can rely on paradigms to make them cohere. More rarely paradigmatic assumptions are themselves challenged, but in that case scientists can fall back on their worldview to make sense of the needed changes. By implication, the most difficult interpretive problems arise when our worldview is called into question, which leaves us without any frame of reference on which to fall back. Quantum theory poses a worldview problem. Prior to the quantum revolution the classical worldview could not be tested scientifically and so it was essentially metaphysical. Even though it corresponded to our experience of material objects, we lacked proof that it adequately represented the deep structure of reality. By giving us access to that deep structure for the first time, quantum mechanics made it possible to do such a test, or “experimental metaphysics.” Unfortunately, the results seemed to contradict classical assumptions, at least on the sub-atomic level.
The difficulties of the old quantum theory were largely dispelled by the development of quantum mechanics in the 1920s. First Heisenberg (1925) developed the rather abstract matrix mechanics and soon after Schro¨dinger (1926; see also Eckart, 1926) developed the physically more appealing wave mechanics. Subsequently he showed them to be mathematically equivalent. The physicists were greatly relieved; now they had once again a mathematical formalism that enabled the results of experiments to be calculated. It was possible to move forward. It is certainly a very elegant and indeed beautiful formalism, and does enable the results of many physical measurements to be calculated with great accuracy. It is used daily by atomic and nuclear physicists; there is no other way to make calculations. From the earliest days there has been intense debate about the interpretation of quantum mechanics, and it shows no signs of being settled; if anything it is becoming more intense over the years. As Penrose has remarked, no one understands quantum mechanics. Feynman (1967; see also Sykes, 1994) also admitted that ‘nobody understands quantum mechanics’, and Einstein once remarked: ‘I have thought a hundred times as much about the quantum problems as I have about general relativity theory’ (Pais, 1994, p. 57). It should therefore be evident that the most that can be done here is to try to show where the real difficulties lie, and how the failure to recognize that quantum mechanics is an essentially statistical theory has led to a series of confusing quantum paradoxes. It is just these paradoxes that have often been used to support theological speculations. It is useful to ask what ‘understanding quantum mechanics’ actually means. Certainly it means being able to calculate the results of many measurements. The same applies to Newton’s theory of gravitation, which we understand in the sense that we can use it as a basis of calculation, although we do not really understand the gravitational force itself. Very often, especially in modern physics, the mathematical formalism does not give us the sort of intuitive physical feeling for what is going on that we often obtain from classical mechanics. Perhaps we can never achieve more than this for quantum mechanics. In classical mechanics we can assume that a property that is measured objectively exists prior to the interaction of the measuring apparatus with the observed system. Quantum mechanics, however, ‘is incompatible with the proposition that measurement discovers some unknown but pre-existing property’ (Peres, 1997, p. 14). There is an important distinction between mysteries and absurdities. We may be forced to live with mysteries, but there is no reason why we should put up with absurdities such as the wave-particle duality, tunnelling through potential barriers, acausal events and the collapse of the wave function. These are the quantum paradoxes that will be discussed later on. They arise within the Copenhagen interpretation of quantum mechanics and are solved by other interpretations. These alternative interpretations enable us to make conjectures about what is going on, even though we cannot understand it in detail. This is acceptable and a stimulus to further research. It is certainly difficult enough to understand quantum mechanics, but there is no need to try to visualize logical absurdities in the effort to do so. Quantum mechanics is a mathematical formalism that makes it possible to calculate the results of some experiments. Anything more than this, such as questions about the interpretation of what is measured and whether it tells us what is actually going on in the system being studied, belongs to philosophy. Thus we are concerned here with what is essentially a philosophical debate; if it were just a matter of physics the debate would have been settled long ago. One of the difficulties is that it invokes the results of experiments and uses the language of quantum mechanics. Inevitably this language uses philosophical terms and then it is easy to give the impression that the philosophical propositions are in some way entailed by the physical results. These in turn can influence theological thinking. In order to be quite clear which arguments are justified and which are not, it is essential to separate the three levels of discourse: physical, philosophical and theological. Our task is to relate these together, and this immediately raises the traditional problems of the philosophy of science, including the relation of experiment to theory, and of theory to explanation. We begin by discussing experiments, distinguishing between those that are real and those that are imaginary.
In 1935, the Einstein-Podolsky-Rosen paper introduced a new kind of two-particles quantum state, which was named 'entangled' by Schrödinger.Showing that measurements on both particles were strongly correlated even if the particles were widely separated, Einstein argued that the formalism did not describe all the elements of the physical reality, and he concluded that Quantum Mechanics was not complete.Bohr immediately opposed this reasoning, claiming that Quantum Mechanics was consistent as it was, and that it did not need to be completed, and even that it should not be completed.The discussion between Einstein and Bohr lasted until the death of the two giants, leaving the question unsettled.It was considered by most of the physicists only as an epistemological debate, holding on questions of interpretation, but without any practical consequence on how to use Quantum Mechanics.The situation changed in 1964, when Bell discovered that taking Einstein's point of view led to consequences in contradiction with the predictions of Quantum Mechanics in some (rare) situations.The debate was then displaced from the realm of epistemology to the one of physics, since it could be settled by experiments.After a series of experiments closer and closer to the ideal Gedankenexperiment, more and more physicists realized that entanglement was definitely weirder than any previous concept, and that it might be used for new ways of processing and transmitting information.This prompted the development of a new field, quantum information, which brings us into a new quantum age, and may change our society as dramatically as the first quantum revolution, which gave us integrated circuits and lasers, responsible for the development of the information and communication society.
Abstract Quantum theory has presented scientists and philosophers of science with a series of perplexing questions. It seems to many that any attempt to comprehend a world described by quantum theory will require revision in our understanding of the nature of things far more radical than the revision in our understanding of the nature of space and time demanded by the theories of relativity. It has been claimed that to comprehend quantum theory we must revise our very understanding of such matters as the objective nature of reality and its independence from our perception of it, the nature of a complex system and its relation to its components, and the nature of causal and other kinds of determination in the world.