State collapse is a very peculiar process, and I will have a lot more to say about it through this book. However, it is not the only way that quantum states can change. For most of the time, the various quantum systems in nature wander about the universe not being involved in experiments, yet clearly things can and do change. Any self-respecting theory must be capable of describing the ordinary business of a world in which things interact with one another, change, and develop. Part of our dealings in this chapter will be to develop the machinery that quantum theory uses to portray the changes of ordinary development and the, rather more mysterious, changes due to measurement.
State collapse is a very peculiar process, and I will have a lot more to say about it through this book. However, it is not the only way that quantum states can change. For most of the time, the various quantum systems in nature wander about the universe not being involved in experiments, yet clearly things can and do change. Any self-respecting theory must be capable of describing the ordinary business of a world in which things interact with one another, change, and develop. Part of our dealings in this chapter will be to develop the machinery that quantum theory uses to portray the changes of ordinary development and the, rather more mysterious, changes due to measurement.
Change is built into the structure of a quantum state. State collapse is a very particular and distinctive process, but it is not the only way that quantum states can change. For most of the time, various quantum systems that exist in nature wander about the universe without being involved in experiments, yet clearly, things can and do change. Eigenstates describe systems as having a definite fixed value of a given physical property. Such states are very important. If we know that an object is in an eigenstate, we can predict with absolute certainty what a measurement is going to produce. It’s very important to be able to tell if a state is a mixed state or an eigenstate with reference to a given measurement. There are many different operators in quantum theory, which have different jobs to do. Perhaps the most important ones are the operators that represent the process of measuring a physical property.
A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it. – Max Planck, 1936 The only way quantum theory allows us to interact with quantum systems is via quantum processes. Thus, the only way we can extract information about the state of a quantum system is to apply some non-deterministic process and observe which of the branches happened. For that reason, many people refer to the act of applying certain quantum processes as measurement and sometimes even as observation . A consequence of this unfortunate terminology is that one of the most touted ‘strange’ features of quantum theory is often misleadingly described as follows: ‘The mere act of observing a quantum state changes it.’ ‘Misleading’, since the concept of measurement described in the previous paragraph is far from the passive concept of observation familiar from our macroscopic world, but is rather a non-trivial process that will almost always drastically affect the quantum state. So, the mysterious aspect of quantum theory is not that ‘observation’ alters the quantum state but, rather, that it is impossible, even in principle, to ‘observe’ a quantum system in the classical sense. Given this fundamental restriction on how we can extract information from a quantum system, what can we learn about that system by means of a quantum process? The answer is, in fact, very little! First, performing a particular measurement could, for the vast majority of quantum states, yield any outcome i with a non-zero probability. In that case, obtaining an outcome i doesn't tell us much about what the state of the system was. Second, we say ‘was’, because the measurement will moreover irreversibly change the state according to the outcome. So rather than revealing the state of a system, a quantum measurement typically erases that state from history! Nonetheless, these quantum measurements are crucial to quantum theory since they constitute the only interface between us, in our classical world, and the quantum world. The great insight of the quantum computing community is that the non-deterministic changes induced by these quantum measurements are not a nuisance, but rather an extremely useful resource.
Quantum paradoxes The next few chapters are devoted to resolving a number of quantum paradoxes in the sense of giving a reasonable explanation of a seemingly paradoxical result in terms of the principles of quantum theory discussed earlier in this book. None of these paradoxes indicates a defect in quantum theory. Instead, when they have been properly understood, they show us that the quantum world is rather different from the world of our everyday experience and of classical physics, in a way somewhat analogous to that in which relativity theory has shown us that the laws appropriate for describing the behavior of objects moving at high speed differ in significant ways from those of pre-relativistic physics. An inadequate theory of quantum measurements is at the root of several quantum paradoxes. In particular, the notion that wave function collapse is a physical effect produced by a measurement, rather than a method of calculation, see Sec. 18.2, has given rise to a certain amount of confusion. Smuggling rules for classical reasoning into the quantum domain where they do not belong and where they give rise to logical inconsistencies is another common source of confusion. In particular, many paradoxes involve mixing the results from incompatible quantum frameworks. Certain quantum paradoxes have given rise to the idea that the quantum world is permeated by mysterious influences that propagate faster than the speed of light, in conflict with the theory of relativity. They are mysterious in that they cannot be used to transmit signals, which means that they are, at least in any direct sense, experimentally unobservable.
Scope of this book Quantum mechanics is a difficult subject, and this book is intended to help the reader overcome the main difficulties in the way to understanding it. The first part of the book, Chs. 2–16, contains a systematic presentation of the basic principles of quantum theory, along with a number of examples which illustrate how these principles apply to particular quantum systems. The applications are, for the most part, limited to toy models whose simple structure allows one to see what is going on without using complicated mathematics or lengthy formulas. The principles themselves, however, are formulated in such a way that they can be applied to (almost) any nonrelativistic quantum system. In the second part of the book, Chs. 17–25, these principles are applied to quantum measurements and various quantum paradoxes, subjects which give rise to serious conceptual problems when they are not treated in a fully consistent manner. The final chapters are of a somewhat different character. Chapter 26 on decoherence and the classical limit of quantum theory is a very sketchy introduction to these important topics along with some indication as to how the basic principles presented in the first part of the book can be used for understanding them. Chapter 27 on quantum theory and reality belongs to the interface between physics and philosophy and indicates why quantum theory is compatible with a real world whose existence is not dependent on what scientists think and believe, or the experiments they choose to carry out. The Bibliography contains references for those interested in further reading or in tracing the origin of some of the ideas presented in earlier chapters.
Quantum computing is the first digital support technology we explore in this book in more detail. It is sometimes portrayed as brilliant scientists exploring the most obscure natural phenomena in exotic laboratories and aiming to build very powerful science fiction supercomputers that are practically capable of doing anything. But this public perception has been changing lately since various tech companies around the world have invested in this technology to apply it to real-life business problems and explore potential use cases. Among them are Google, Microsoft, IBM, and Honeywell, for instance, just to mention a few pioneers in this ongoing field of industrial research. They frequently report on their latest achievements, highlight current business applications, and emphasize the great prospects of this digital technology. Even The New York Times picked up this trend lately and described quantum computing as one of the most “jazziest and most mysterious concepts of modern science” [1].
What does it mean to say that an event is physically possible?One might say, in the spirit of everyday usage, that an event is physically possible if its occurrence is consistent with the laws of physics.But which laws of physics, starting from when?Here things get tricky.In a Laplacian universe governed by the fully-deterministic laws of classical electrodynamics and mechanics, exactly one future is consistent with any complete and precise specification of initial conditions.Given an incomplete, imprecise specification -the best that human observers can achieve, even in principle -many futures may appear to be physically possible, but only one of them actually is.In this kind of universe, our intuitive notion that many different outcomes of an experiment, for example, are physically possible is just an ignorancedriven delusion.Philosopher of physics Ruth Kastner's new book, The Transactional Interpretation of Quantum Mechanics, subtitled The Reality of Possibility, takes on this question of the meaning of physical possibility in the theoretical context defined by relativistic quantum field theory.Lest this seem daunting, be assured that this is a philosophical book; aside from Chapters 5 and 6 dealing with technical objections, it emphasizes ontological questions and employs the physics -all of which is clearly presented for the non-expert -as a way to raise them.The ontological questions that it raises, not just 'what is possibility?' but also 'what is actuality?' and 'what are space and time?' underlie not just physics but any theory of action.The Transactional Interpretation of Quantum Mechanics is, therefore, not just about the interpretation of quantum mechanics; it is about how 'nature makes its choice' in any circumstance, and how 'the actual arises from the potential' (205) as a result.After briefly introducing quantum theory and its various interpretations in Chapter 1, Kastner addresses, in Chapter 2, the essential preliminary question of what is to count as an acceptable scientific explanation.The divide between realists and instrumentalists about quantum theory -or between 'ontic ' and 'non-ontic' approaches