Propagator for the Harmonic Oscillator

Quantum Mechanics A

Schrödinger Equation The most fundamental equation of quantum mechanics; given a Hamiltonian ${\mathcal {H}}$, it describes how a state $|\Psi \rangle$ evolves in time.

Physical Basis of Quantum Mechanics

Basic Concepts and Theory of Motion UV Catastrophe (Black-Body Radiation) Photoelectric Effect Stability of Matter Double Slit Experiment Stern-Gerlach Experiment The Principle of Complementarity The Correspondence Principle The Philosophy of Quantum Theory

Schrödinger Equation

Brief Derivation of Schrödinger Equation Relation Between the Wave Function and Probability Density Stationary States Heisenberg Uncertainty Principle Some Consequences of the Uncertainty Principle

Operators, Eigenfunctions, and Symmetry

Linear Vector Spaces and Operators Commutation Relations and Simultaneous Eigenvalues The Schrödinger Equation in Dirac Notation Transformations of Operators and Symmetry Time Evolution of Expectation Values and Ehrenfest's Theorem

Motion in One Dimension

One-Dimensional Bound States Oscillation Theorem The Dirac Delta Function Potential Scattering States, Transmission and Reflection Motion in a Periodic Potential Summary of One-Dimensional Systems

Discrete Eigenvalues and Bound States

Harmonic Oscillator Spectrum and Eigenstates Analytical Method for Solving the Simple Harmonic Oscillator Coherent States Charged Particles in an Electromagnetic Field WKB Approximation

Time Evolution and the Pictures of Quantum Mechanics

The Heisenberg Picture: Equations of Motion for Operators The Interaction Picture The Virial Theorem

Angular Momentum

Commutation Relations Angular Momentum as a Generator of Rotations in 3D Spherical Coordinates Eigenvalue Quantization Orbital Angular Momentum Eigenfunctions

Central Forces

General Formalism Free Particle in Spherical Coordinates Spherical Well Isotropic Harmonic Oscillator Hydrogen Atom WKB in Spherical Coordinates

The Path Integral Formulation of Quantum Mechanics

Feynman Path Integrals The Free-Particle Propagator Propagator for the Harmonic Oscillator

Continuous Eigenvalues and Collision Theory

Differential Cross Section and the Green's Function Formulation of Scattering Central Potential Scattering and Phase Shifts Coulomb Potential Scattering

The classical action $S$ can be evaluated as follows:

$S=\int _{0}^{t}(KE-PE)dt$

Where $KE\!$ is the kinetic engergy and $PE\!$ is the potential energy.

Equation of motion for harmonic oscillator: $x_{cl}(t')=A\cos(\omega t')+B\sin(\omega t')\!$
$A\!$ and $B\!$ are constants.

At $t'=0\!$ (starting point),$x_{cl}(0)=x_{0}\rightarrow A=x_{0}$.

At $t'=t\!$ (final point), $x_{cl}(t)=x\rightarrow B={\frac {x-x_{0}\cos(\omega t)}{\sin(\omega t)}}$ . </math>

is a symbolic way of saying "integrate over all paths connecting ${x_{0}}$ and ${x_{N}}$ (in the interval ${t_{0}}$ and ${t_{N}}$)." Now, a path ${x_{t}}$ is fully specified by an infinity of numbers ${x(t_{0})}$,..., ${x(t)}$, ...,${x(t_{N})}$, namely, the values of the function ${x(t)}$ at every point ${t}$ is the interval ${t_{0}}$ to ${t_{N}}$. To sum over all paths, we must integrate over all possible values of these infinite variables, except of course ${x(t_{0})}$ and ${x(t_{N})}$, which will be kept fixed at ${x_{0}}$ and ${x_{N}}$, respectively. To tackle this problem, we follow this idea that was used in section 1.10: we trade the function ${x_{t}}$ for a discrete approximation which agrees with ${x_{t}}$ at the ${N+1}$ points. Substitute:

$x_{cl}(t')=x_{0}\cos(\omega t')+{\frac {x-x_{0}\cos(\omega t)}{\sin(\omega t)}}\sin(\omega t')\Rightarrow {\frac {dx_{cl}(t')}{dt'}}=-\omega x_{0}\sin(\omega t')+\omega {\frac {x-x_{0}\cos(\omega t)}{\sin(\omega t)}}\cos(\omega t')$
$KE={\frac {1}{2}}m\left({\frac {dx_{cl}}{dt}}\right)^{2}={\frac {1}{2}}m\left[-\omega x_{0}\sin(\omega t')+\omega {\frac {x-x_{0}\cos(\omega t)}{\sin(\omega t)}}\cos(\omega t')\right]^{2}$
$PE={\frac {1}{2}}k(x_{cl}(t'))^{2}={\frac {1}{2}}k\left[x_{0}\cos(\omega t')+{\frac {x-x_{0}\cos(\omega t)}{\sin(\omega t)}}\sin(\omega t')\right]^{2}$
Substituting, integrating from time 0 to time $t\!$ and simplifying, we get:

$S=S(t,x,x_{0})={\frac {m\omega }{2\sin(\omega t)}}((x^{2}+x_{0}^{2})\cos(\omega t)-2xx_{0})$