Monthly Archives: September 2024

Problem: Before developing Fermi’s golden rule, it is necessary to first lay out the general framework of time-dependent perturbation theory, of which Fermi’s golden rule is a special case. To this effect, begin by considering the usual (Schrodinger picture) Hamiltonian … Continue reading →

The purpose of this post is to provide some examples of perturbation theory calculations in quantum mechanics by analyzing a specific perturbation called the Stark effect. For simplicity, we take as our quantum system a hydrogen atom with the usual … Continue reading →

Consider the three \(\ell=1\) spherical harmonics: \[Y_{1}^{-1}(\theta,\phi)=\sqrt{\frac{3}{8\pi}}\sin\theta e^{i\phi}\] \[Y_0^0(\theta)=\sqrt{\frac{3}{4\pi}}\cos\theta\] \[Y_{1}^{1}(\theta,\phi)=-\sqrt{\frac{3}{8\pi}}\sin\theta e^{-i\phi}\] Although the spherical harmonics are just functions of \(\theta,\phi\) on the sphere \(S^2\), one can introduce an \(r\)-dependence simply by multiplying each of them by \(r\). The resulting functions … Continue reading →

Problem: Working on the flat manifold of Minkowski spacetime \(X\cong\mathbf R^{1,3}\) equipped with the Lorentzian metric signature \((+,-,-,-)\), define what is meant by a Lorentz scalar, a Lorentz vector (also called a 4-vector), a Lorentz tensor, and a Lorentz spinor. … Continue reading →

The purpose of this post is to outline a derivation of the classical Maxwell distribution \(\rho_{\textbf V}(\textbf v|m,T)\), i.e. the probability density function for the continuous speed random vector \(\textbf V\) in a monatomic ideal gas given the atomic mass … Continue reading →

The purpose of this post is to lay out the basic theory of ensembles in statistical mechanics in addition to some examples. At the end, the goal will be to convincingly demonstrate ensemble equivalence. Any system (e.g. a gas, a … Continue reading →

Recall that if one confines a free quantum particle onto a circle \(S^1\) of radius \(R\), then the de Broglie wavelength of the \(n\)-th \(H=P^2/2m\)-eigenstate must be quantized in the obvious manner \(\lambda_n=2\pi R/n\), leading to the angular wavenumber \(k_n=n/R\), … Continue reading →

Consider a classical particle subject to Newton’s second law \(\dot{\textbf p}=\textbf F\). If one takes the dot product of both sides with the particle’s velocity \(\dot{\textbf x}\) so that \(\dot{\textbf p}\cdot\dot{\textbf x}=\textbf F\cdot\dot{\textbf x}\), the left hand side is an … Continue reading →

The purpose of this post is to contrast the classical treatment of Rutherford scattering with its quantum mechanical treatment. At the end, the surprise will be that for certain important quantities such as the differential cross-section, the classical and quantum … Continue reading →

Problem: Mobius transformations (also called fractional linear transformations) are maps \(\mathcal M:\textbf C\cup\{\infty\}\to\textbf C\cup\{\infty\}\) on the Riemann sphere \(\textbf C\cup\{\infty\}\) of the form \(\mathcal M(z):=\frac{az+b}{cz+d}\) for \(\det\begin{pmatrix}a&b\\c&d\end{pmatrix}\neq 0\). The purpose of this problem is to gain a deeper appreciation for … Continue reading →