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Journal of Physics B Atomic and Molecular Physics

On the momentum representation of the Dirac relativistic wavefunctions

Fadei Fadeevich Komarov, A P Novikov

Published Dec 14, 1978• 7 citations
Publisher

Abstract

A momentum representation of the Dirac relativistic wavefunctions is obtained by means of Fourier transformation. A final expression for the momentum wavefunctions is presented in the substantial form convenient for numerical calculations.
DOI
10.1088/0022-3700/11/23/001

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This chapter focuses on a basic but systematic treatment of wavefunctions. By comparison with a spin 1/2 system, the concept of a wavefunction as a collection of coefficients is developed, followed by discussions of eigenstates, operators, and representations in the context of wavefunctions. Transformation from position to momentum representation is effected by a Fourier transform. This is developed next, and illustrated for a Gaussian. The discussion of phases and time evolution in earlier chapters is now applied to wavefunctions, and the time evolution of a free particle discussed. The concept of a wavepacket and its suitability for representing particles follows. A topic of some subtlety — and one which is discussed in detail — is the use of bra-ket (Dirac) notation for wavefunctions. The chapter closes with a short epilogue.
Hilbert space and inner product In Ch. 2 it was noted that quantum wave functions form a linear space in the sense that multiplying a function by a complex number or adding two wave functions together produces another wave function. It was also pointed out that a particular quantum state can be represented either by a wave function ψ( x ) which depends upon the position variable x , or by an alternative function ψ( p ) of the momentum variable p . It is convenient to employ the Dirac symbol |ψ〉, known as a “ket”, to denote a quantum state without referring to the particular function used to represent it. The kets, which we shall also refer to as vectors to distinguish them from scalars , which are complex numbers, are the elements of the quantum Hilbert space H . (The real numbers form a subset of the complex numbers, so that when a scalar is referred to as a “complex number”, this includes the possibility that it might be a real number.) If α is any scalar (complex number), the ket corresponding to the wave function ɑψ( x ) is denoted by α|ψ〉, or sometimes by |ψ〉α, and the ket corresponding to ø( x )+ψ( x ) is denoted by |ø〉+|ψ〉 or |ψ〉+|ø〉, and so forth. This correspondence could equally well be expressed using momentum wave functions, because the Fourier transform, (2.15) or (2.16), is a linear relationship between ψ( x ) and ( p ), so that αø( x )+βψ( x ) and αϕ( p ) + βψ( p ) correspond to the same quantum state α|ψ〉+β|ø〉.
The relativistic energy-momentum relationship is far more subtle than it seems to be at a first sight. As concerns the relativistic quantum mechanics, its underlying equations were deduced from, or in relation with the relativistic energy-momentum relationship by means of the principle of correspondence. Without a clear physical role and meaning associated, the matrices of the Dirac equation seemed to confirm that the principle of the physical determination of equations would not be proper to the new quantum mechanics. Therefore, we have to search for genuine physical information in the terms of the underlying equations of the relativistic quantum mechanics. This information concerns a level of structure of matter “even below that on which nuclear transformations take place.”
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