Differences

This shows you the differences between two versions of the page.

--- hamburgminicourse2017:why_self-adjoint_operators [2017/04/02 15:52] – markus
+++ hamburgminicourse2017:why_self-adjoint_operators [2017/04/03 16:12] (current) – markus
@@ Line 12: / Line 12: @@
 \]
-The operator thus fails to be symmetric. On $H_0^1([0,1])$ instead it //is// symmetric, but having $g \in H_0^1([0,1])$ is enough for the boundary terms to vanish, so $f \in D((i\partial_x)^*) \supsetneq D(i\partial_x)$. To get a self-adjoint operator we must make $D(i\partial_x)$ larger which lets $D((i\partial_x)^*)$ shrink, until they match. A boundary condition $f(1)=\alpha f(0), \alpha \in \mathbb{C}$ yields
+The operator thus fails to be symmetric. On $H_0^1([0,1])$ instead it //is// symmetric, but having $g \in H_0^1([0,1])$ is enough for the boundary terms to vanish, so $f \in D((i\partial_x)^*) \supsetneq D(i\partial_x)$. To get a self-adjoint operator we must make $D(i\partial_x)$ larger which lets $D((i\partial_x)^*)$ shrink, until they match. A boundary condition $f(1)=\alpha f(0)$, $\alpha \in \mathbb{C}$, yields
 \[
 \bar{f}(1)g(1)-\bar{f}(0)g(0)=|\alpha|^2\bar{f}(0)g(0)-\bar{f}(0)g(0)
@@ Line 22: / Line 22: @@
 **Why self-adjoint operators?** Sometimes it is argued that observables are represented by self-adjoint operators because they have //real// eigenvalues exclusively that are identified as the possible outcomes of a measurement. But why should a $z\in\mathbb{C}$ not be the outcome of a measurement, e.g. the pointer position of a clock? On the other side a non-self-adjoint operator may have only real eigenvalues. So this cannot really be the reason.
-Looking back at the old matrix mechanics of Heisenberg one identifies the central method of diagonalization. For an arbitrary linear operator $A$ on a Hilbert space it is asked, whether there is a unitary $U$ such that $UAU^*$ is diagonal, i.e. a multiplication operator (a function $x \mapsto f(x)$, after all it's //functional analysis//) on $L^2(\Omega,\mathcal{A},\mu)$ on some measure space $(\Omega,\mathcal{A},\mu)$. In the case of $A=i\partial_x$ this can be achieved through Fourier transform. In general the diagonalization is achieved through a **resolution of identity** (the generalization of an orthonormal basis), i.e. //every Hilbert space state can be assigned to one or more specific outcomes//.
+Looking back at the old matrix mechanics of Heisenberg one identifies the central method of diagonalization. For an arbitrary linear operator $A$ on a Hilbert space it is asked, whether there is a unitary $U$ such that $UAU^*$ is diagonal, i.e. a multiplication operator (a function $x \mapsto f(x)$, after all it's //functional analysis//) on $L^2(\Omega,\mathcal{A},\mu)$ on some measure space $(\Omega,\mathcal{A},\mu)$. In the case of $A=i\partial_x$ this can be achieved through Fourier transform. In general the diagonalization is achieved through a **resolution of identity** (the generalization of an orthonormal eigenbasis), i.e. //every Hilbert space state can be assigned to one or more specific outcomes//.
 The mathematical result telling us which operators can be diagonalized is called the **spectral theorem**. Needless to say self-adjoint operators allow for such a diagonalization but also the more general //normal// operators do. They are to self-adjoint operators what complex numbers are to reals: