Article Outline
Glossary
Definition of the Subject
Introduction
Presentation of the Problem and an Example
Perturbation of Point Spectra: Nondegenerate Case
Perturbation of Point Spectra: Degenerate Case
The Brillouin–Wigner Method
Symmetry and Degeneracy
Problems with the Perturbation Series
Perturbation of the Continuous Spectrum
Time Dependent Perturbations
Future Directions
Bibliography
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- Hilbert space:
-
A Hilbert space \({\mathcal{H}}\) is a normed complex vector space with a Hermitian scalar product. If \({\varphi,\,\psi\in\mathcal H}\) the scalar product between φ and ψ is written as \({(\varphi,\psi)\equiv (\psi,\varphi)^*}\) and is taken to be linear in ψ and antilinear in φ: if \({a,\,b\in \mathbb{C}}\), the scalar product between \({a\, \varphi}\) and \({b\, \psi}\) is \({a^*b(\varphi,\psi)}\). The norm of ψ is defined as \({\Vert \psi\Vert\equiv \sqrt{(\psi,\psi)}}\). With respect to the norm \({\Vert\cdot\Vert}\), \({\mathcal H}\) is a complete metric space. In the following \({\mathcal{H}}\) will be assumed to be separable, that is any complete orthonormal set of vectors is countable.
- States and observables:
-
In quantum mechanics the states of a system are represented as vectors in a Hilbert space \({\mathcal H}\), with the convention that proportional vectors represent the same state. Physicists mostly use Dirac's notation: the elements of \({\mathcal H}\) are represented by \( \,\vert\, \cdot\,\rangle \) (“ket”) and the scalar product between \( \,\vert\, \varphi\,\rangle \) and \( \,\vert\, \psi\,\rangle \) is written as \( \langle\, \varphi\mid \psi\,\rangle \) (“braket”). The observables, i. e. the physical quantities that can be measured, are represented by linear Hermitian (more precisely: self‐adjoint) operators on \({\mathcal H}\). The eigenvalues of an observable are the only possible results of the measurement of the observable. The observables of a system are generally the same of the corresponding classical system: energy, angular momentum, etc., i. e. they are of the form \({f(q,p)}\), with \( q\equiv (q_1,\dots,q_n), p\equiv (p_1,\dots,p_n) \) the position and momentum canonical variables of the system: q i and p i are observables, i. e. operators, which satisfy the commutation relations \( [q_i,q_j]\equiv q_iq_j-q_jq_i=0 \), \( [p_i,p_j]=0 \), \( [q_i,p_j]=i\hbar\,\delta_{ij}\), with \({\hbar}\) the Planck's constant h divided by \({2\pi}\).
- Representations:
-
Since separable Hilbert spaces are isomorphic, it is always possible to represent the elements of \({\mathcal H}\) as elements of l 2, the space of the sequences \( \{u_i\},\; {u_i\in\mathbb{C}}\), with the scalar product \( (v,u)\equiv \sum_i v_i^*u_i \). This can be done by choosing an orthonormal basis of vectors e i in \( \mathcal H : (e_i,e_j)=\delta_{ij}\) and defining \( u_i=(e_i,u) \); with Dirac's notations \( \,\vert\, A\,\rangle \to \{a_i\},\ a_i=\langle\, e_i\mid A\,\rangle \). Linear operators ξ are then represented by \( \{\xi_{ij}\}, \xi_{ij}= (e_i,\xi\, e_j)\equiv \langle\, e_i\mid \xi\mid e_j\,\rangle \). The ξ ij are called “matrix elements” of ξ in the representation e i . If \({\xi^\dagger}\) is the Hermitian‐conjugate of ξ, then \( (\xi^\dagger)_{ij}=\xi_{ji}^* \). If the e i are eigenvectors of ξ then the (infinite) matrix ξ ij is diagonal, the diagonal elements being the eigenvalues of ξ.
- Schrödinger representation:
-
A different possibility is to represent the elements of \({\mathcal H}\) as elements of \({L^2[\mathbb{R}^n]}\), the space of the square‐integrable functions on \({\mathbb{R}^n}\), where n is the number of degrees of freedom of the system. This can be done by assigning how the operators q i and p i act on the functions of \({L^2[\mathbb{R}^n]}\): in the Schrödinger representation the q i are taken to act as multiplication by x i and the p i as \({-i\hbar\partial/\partial x_i}\): if \({\,\vert\, A\,\rangle\to \psi_A(x_1,\dots,x_n)}\), then
$$ \begin{aligned} q_i\vert A\rangle&\to x_i\psi_A(x_1,\dots,x_n)\:, \\ p_i\vert A\rangle &\to -i\hbar\partial\psi_A(x_1,\dots,x_n)/\partial x_i\:. \end{aligned}$$ - Schrödinger equation:
-
Among the observables, the Hamiltonian H plays a special role. It determines the time evolution of the system through the time dependent Schrödinger equation
$$ i\hbar\frac{\partial \psi}{\partial t} =H\psi\:, $$and its eigenvalues are the energy levels of the system. The eigenvalue equation \({H\psi=E\psi}\) is called the Schrödinger equation.
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Picasso, L.E., Bracci, L., d'Emilio, E. (2012). Perturbation Theory in Quantum Mechanics. In: Meyers, R. (eds) Mathematics of Complexity and Dynamical Systems. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-1806-1_85
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