A central concept of quantum mechanics is that particles spread through space like waves. These waves are expressed mathematically as a “wavefunction,” which is obtained by solving the “Schrödinger equation.” This Topic shows how that equation is set up and solved and the wavefunction interpreted. It also introduces the “uncertainty principle,” one of the most profound departures of quantum mechanics from classical mechanics.
A central concept of quantum mechanics is that particles spread through space like waves. These waves are expressed mathematically as a “wavefunction,” which is obtained by solving the “Schrödinger equation.” This Topic shows how that equation is set up and solved and the wavefunction interpreted. It also introduces the “uncertainty principle,” one of the most profound departures of quantum mechanics from classical mechanics.
This chapter discusses the central concepts and equations of quantum theory which demonstrates how particles spread through space like waves and are described mathematically by a wavefunction. It examines how the wavefunction is interpreted and introduces the uncertainty principle, which is one of the most profound departures of quantum mechanics from classical mechanics. It also highlights the dynamical properties of a system that are contained in the wavefunction and are obtained by solving the Schrödinger equation. The chapter reconciles the facts that atoms and molecules can possess only certain energies, that waves exhibit the properties of particles, and that particles exhibit the properties of waves. It reviews the classical concept that have been accommodated by the development of quantum mechanics, in which equations are set up to treat a particle as spread through space like a wave.
Quantum mechanics is introduced here as a fundamental theory that must treat particles equally often as waves. Here, classical waves are introduced in some detail, with discussions of the important concepts of waves: medium, amplitude, frequency, wavelength, period, harmonics, interference, standing waves, etc. Waves can be oscillatory waves with a repeating pattern, or pulse waves, which are localized in space. All of the elements of waves are important in understanding how the Schrödinger equation describes particles. Classical standing waves must have wave lengths that fit between boundaries giving them discrete allowed frequencies, a harbinger of quantized energies in quantum mechanics. Complicated waves can be analyzed in terms of their component frequencies, the fundamental idea of Fourier analysis. This discussion shows that a classical wave has a limit in how accurately one can specify simultaneously the wave’s position and frequency, in analogy with Heisenberg uncertainty principle of quantum mechanics.
Quantum mechanics — central not only to physics, but also to chemistry, materials science, and other fields — is notoriously abstract and difficult. This book is a concise, explanatory book that focuses on the fundamental structure, concepts, and methods of quantum mechanics, filling the gap between introductory and advanced studies, between popularizations and technical treatises. Although physical understanding is emphasized, the mathematical sophistication necessary to achieve that understanding is also developed. A modern perspective is adopted throughout, the goal being to gain entry into quantum mechanics as used by practicing scientists. This perspective includes a focus on quantum states generally, not just wavefunctions, and a strong emphasis on Dirac notation. Many standard topics are developed, such as the Schrödinger equation, quantum states, operators, and matrix mechanics. Some non-standard topics are also discussed, such as the complex nature of quantum states, and various interpretations of the uncertainty relations. Discussion of applications is limited, and focuses on elucidating the structure of quantum mechanics.
The uncertainty principle comes into force because of the wave like properties of “particles” like electrons and protons. The waves tell the dynamics of the particle – its momentum, its energy and even its angular momentum. Wave packets are made by adding together many simple waves that run forever. If electrons have the wave properties then there must be a wave equation and a wave function to describe the electron waves just as the waves of light, sound and strings are described. The success of wave mechanics was well demonstrated by Erwin Schrödinger before an acceptable interpretation of the wave function was known. Max Born utilized the probability concepts of the uncertainty principle to give us the presently accepted ideas of the wave function.
For systems which obey quantum mechanics, the formulation of the problem of treating large numbers of particles is, of course, somewhat different than it is for classical systems. The microscopic description of the system is provided by a wave function which (in the absence of spin) is a function of the classical coordinates { q i }. The mathematical model is provided by a Hamiltonian operator H which is often obtained from the corresponding classical Hamiltonian by the replacement p i → ( ħ /i)(∂/∂ q i ). In other cases the form of the Hamiltonian operator is simply postulated. The microscopic dynamics are provided by the Schrödinger equation i ħ (∂Ψ/∂ t ) = H Ψ which requires as initial condition the knowledge of the wave function Ψ({ q i }, t ) at some initial time t 0 . (Boundary conditions on Ψ({ q i }, t ) must be specified as part of the description of the model as well.) The results of experiments in quantum mechanics are characterized by operators, usually obtained, like the Hamiltonian, from their classical forms and termed observables. Operators associated with observables must be Hermitian. In general, the various operators corresponding to observables do not commute with one another. It is possible to find sets of commuting operators whose mutual eigenstates span the Hilbert space in which the wave function is confined by the Schrödinger equation and the boundary conditions. A set of such (time independent) eigenstates, termed ψ ν ( q ), is a basis for the Hilbert space.
Classical mechanics is usually valid at the macro scale. The changes in the position and the momentum of the object over time are described by the Hamiltonian equation of motion. The wave function in quantum mechanics, if unobserved, evolves in a smooth and continuous way according to the Schrodinger equation, which is related to the Hamiltonian equation of motion. This equation describes a linear superposition of different states at time. A quantum mechanical description of a physical system is related to a probabilistic representation; it is described by a vector in Hilbert space. This description extends the two- or three-dimensional Euclidean space into spaces that have any finite or infinite number of dimensions. The state of a compound system is projected to the subspace that corresponds to the observed state and the vector representing the state is renormalized to the unit length. An observable describes a subspace for some dimensions with a special case of one dimension.