In this chapter, we develop some of the most central principles of quantum mechanics. We especially develop the quantum probability rules that predict from the wave function of a quantum system the likelihoods of each of the possible outcomes of experiments which measure the values of the observables of the system.
Quantum mechanics has been enormously successful in explaining the behavior of microsystems such as molecules, atoms, and nuclei. We have seen that quantum mechanics describes a quantum system by the wave function ψ, which is the solution of the Schro¨dinger equation. The wave function determines the probability amplitudes (whose modulus squared gives the probability) for all possible outcomes of a measurement corresponding to an observable on the system. In general, the wave function does not uniquely predict the result of a measurement; it only provides a probability distribution for all possible results of the measurement on a quantum system. Only if the system happens to be in an eigenstate corresponding to the observable being measured, is the result of a measurement of this observable completely predictable. On the other hand, the results of measurements corresponding to a conjugate or non-commuting observable, on the same system in the same state, are indeterminate. Quantum mechanical description of physical systems raises profound questions: How are the wave function and the probabilities for different outcomes of a measurement to be interpreted? Does the wave function represent a single system or an ensemble of systems? Does the physical system actually have the attribute corresponding to the measurement in question or does the measurement itself create the attribute? These questions led to different interpretations of quantum mechanics, which were vigorously debated during the years of its discovery as also in the following decades. The new idea of indeterminacy was especially difficult to reconcile with the long-held view that a physical theory should aim at predicting accurately, and without ambiguity, the result of any measurement on any system, big or small. These debates not only gave us a deeper understanding of quantum mechanics but also changed profoundly the way we view the micro-world.
The behavior of classical macroscopic particles – from baseballs to planets – is governed by Newtonian mechanics as we have described in the first chapters of this book. The behavior of very small particles, on the other hand, is governed by an entirely different collection of equations and rules that go under the name of “quantum mechanics” and these particles are referred to as quantum particles. These rules have analogies to classical mechanics, but are fundamentally different and somewhat mysterious as we discussed in Chapter 8. The fundamental principles of quantum mechanics cannot be proven to be correct. They were developed based on the results of experimental observation, by analogy with classical mechanics, and through the consideration of logical implications of the formalisms used. There is no way to derive the fundamental elements of quantum mechanics, any more than one could predict from first principles that Newtonian forces should add as vectors. On the other hand, quantum mechanics is among the most rigorously tested theories in science, and the predictions of quantum mechanics have, so far, always been in agreement with experiment. In this chapter, we will present quantum mechanics as a series of postulates and examine their implications in application to simple one-dimensional systems. In the next chapter we will examine quantum effects in real systems including atoms, molecules, crystals, and artificially engineered nano-structures. Postulates of quantum mechanics QM Postulate 1: A quantum particle is described by a wave function Ψ( x , y , z , t ) which is a function of spatial coordinates and time. All physically determinable quantities can be derived from the wave function Ψ( x , y , z , t ) ; quantities which cannot be found from the wave function are not physically meaningful. Note that some things that one could ask about a quantum particle, for example the exact position and simultaneous exact momentum of a particle, are not derivable from the wave function and are therefore considered indeterminate (or uncertain ). These are things which cannot ever be known about a particle, and, as such, they are meaningless to contemplate.
Abstract Measurement in the quantum realm is not deterministic as in the classical one, and many of the quantum principles delineated in Chapter 1 are expanded. The results of individual quantum measurements cannot be predicted; quantum mechanics gives us only the probabilities of observing each of the possible measurement outcomes. Anchored in the concrete example of light polarization, Chapter 5 takes on the Born rule or the probability principle of quantum mechanics involving the square of state amplitudes. What is known before a measurement and what is known after (measurement principle) explains the no-cloning theorem. The standard terminology of eigenvectors and eigenstates is also introduced.
This chapter briefly explains the two elementary points concerning quantum mechanics: the mean value rule and the Heisenberg picture. The Schrodinger and Heisenberg pictures are two different but equivalent sets of rules. In other words, when applied to a given physical situation they lead to the same observable predictions. In some respects the Heisenberg picture has definite technical advantages. It is particularly convenient, for instance, in quantum field theory. For describing pure cases the use of wave functions in place of kets is sometimes convenient. The mean value of an observable A and the probability that a measurement of A yields eigenvalue a k can be expressed by means of wave functions. The state vector of a system that does not undergo a measurement does not change with time, obviously this state vector is suddenly modified—also in the Heisenberg picture—when a measurement takes place.
The objective of Chapter 4 is to give an overview of the basic concepts of quantum mechanics and their interpretation. Thus this chapter functions as a theoretical preparation for a better understanding of the quantum world and of the knowledge measurement process. First, the terminology of the word “quantum” and its impact on the derivation of the term “knowledge quantum” is explained in Chapter 4.1. Then, the discussion moves to the historical development of quantum mechanics. The key issues and current research field for quantum theory will be given in an overview. The following chapters show some insights into key quantum mechanical concepts such as slit-experiments, wave-particle duality, the Copenhagener interpretation, and quantum probability. Chapter 4 closes with an explanation of the Uncertainty Principle of Werner Heisenberg, especially since the Uncertainty Principle is used to derive the knowledge measurement process through the method of analogical reasoning. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
As noted in Section 1.5 , the average values of the observables of a quantum system are determined by its wave function Ψ . https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429155741/5d159368-dc20-4763-957b-88b0e101c959/content/C002_equ_0001.tif"/> In this chapter, we introduce some of the mathematical concepts and techniques needed to analyze quantum wave functions. Especially, we will develop the theory of Hermitian eigenvalue problems . Such problems as we will see occur nearly universally in applications of quantum mechanics to chemical problems.