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A particle of mass \(m\) moves in a potential well \(V(x)=m \omega^{2} x^{2} / 2\). Treating relativistic effects to order \(\beta^{2}=(p / m c)^{2}\), find the ground-state energy shift.

Short Answer

Expert verified
The ground-state energy shift is \(\Delta E_0 = \frac{3 \hbar^2 \omega^2}{16 m c^2}\).

Step by step solution

01

Express the Relativistic Energy

The relativistic energy of a particle is given by \[ E = rac{p^2}{2m} + rac{p^4}{8m^3 c^2} \]where the first term is the usual kinetic energy and the second term is the relativistic correction to order \(\beta^2\). This correction arises from expanding the energy in powers of \(p^2/2m^2 c^2\).
02

Hamiltonian for Quantum Harmonic Oscillator

The Hamiltonian of a harmonic oscillator is given by\[ H = rac{p^2}{2m} + rac{1}{2} m \omega^2 x^2 \]Incorporating the relativistic correction, the Hamiltonian becomes:\[ H = rac{p^2}{2m} + rac{1}{2} m \omega^2 x^2 + \frac{p^4}{8m^3 c^2} \]
03

Determine the Perturbation

The perturbation to the Hamiltonian due to relativistic effects is:\[ H' = \frac{p^4}{8m^3 c^2} \]This term will be used to determine the shift in the ground-state energy of the harmonic oscillator.
04

Calculate the Expectation Value

The ground state wave function of a harmonic oscillator is given by:\[ |0\rangle \]The expectation value of \( p^4 \) in the ground state can be found using:\[ \langle 0 | p^4 | 0 \rangle = 3 \left( \frac{\hbar^2}{2} m \omega \right)^2 \]Therefore, the first-order energy correction is:\[ \Delta E_0 = \frac{1}{8m^3 c^2} \times 3 \left( \frac{\hbar^2}{2} m \omega \right)^2 \]
05

Simplify to Find Energy Shift

Simplify the expression for the energy shift:\[ \Delta E_0 = \frac{3 \hbar^2 \omega^2}{16 m c^2} \]This expression gives the shift in the ground-state energy due to relativistic effects to order \(\beta^2\).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Relativistic Corrections
In quantum mechanics, relativistic corrections account for the effects predicted by the theory of relativity. As particles move, their energy isn't simply a matter of kinetic and potential energies. At high velocities, close to the speed of light, the energy includes relativistic effects. In this context, the correction is due to the momentum term expanded to account for particles moving fast enough that classical physics no longer accurately describes their behaviors. The correction in this exercise is derived from the expression:\[ E = \frac{p^2}{2m} + \frac{p^4}{8m^3 c^2} \]- The first term represents classical kinetic energy.- The term \(\frac{p^4}{8m^3 c^2}\) is the relativistic correction, accounting for velocities approaching light speed. These corrections are typically small and relevant primarily in quantum systems where velocities are significant, altering the energy landscape slightly for systems that are otherwise non-relativistic.
Ground-State Energy
The ground-state energy in a quantum harmonic oscillator is the lowest possible energy that a quantum mechanical system can possess. In a situation where no external forces are acting on the particle, this energy arises purely from the oscillatory nature of the quantum particle. The unperturbed ground-state energy for a simple harmonic oscillator can be represented by:\[ E_0 = \frac{1}{2} \hbar \omega \]- The harmonic oscillator's potential functions like a spring, with a mass and spring constant defining the system.- The energy state ensures that even at absolute zero, particles possess a minimum energy, attributed to the uncertainty principle.Introducing relativistic corrections affects the harmonic structure, shifting this base level or ground-state energy slightly. This shift is minor because the contributions from the high velocity effects to the composite energy are typically much smaller than the regular kinetic and potential energies.
Perturbation Theory
Perturbation theory is a mathematical tool used to find an approximate solution to a problem, by starting with a known solution of a similar, simpler problem. In quantum mechanics, it serves to account for small changes or 'perturbations' to a system's Hamiltonian.In this exercise, perturbation theory helps calculate the small shift in energy due to the relativistic correction term \( H' = \frac{p^4}{8m^3 c^2} \). This is added to the regular Hamiltonian of a harmonic oscillator.- First, identify the unperturbed Hamiltonian. Here, it's the usual energy expression without relativistic terms.- The additional \( H' \) term perturbs or slightly alters this Hamiltonian, changing the energy levels.- Using first-order perturbation theory, the shift in ground-state energy \( \Delta E_0 \) is calculated as: \[ \Delta E_0 = \frac{1}{8m^3 c^2} \cdot 3 \left( \frac{\hbar^2}{2} m \omega \right)^2 \] This approach allows physicists to predict small changes in systems that can be otherwise approximated classically, providing deeper insights into quantum behavior.

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Most popular questions from this chapter

A one-dimensional harmonic oscillator is in its ground state for \(t<0\). For \(t \geq 0\) it is subjected to a time-dependent but spatially uniform force (not potential!) in the \(x\)-direction, $$ F(t)=F_{0} e^{-t / \tau} $$ a. Using time-dependent perturbation theory to first order, obtain the probability of finding the oscillator in its first excited state for \(t>0\). Show that the \(t \rightarrow \infty\) ( \(\tau\) finite) limit of your expression is independent of time. Is this reasonable or surprising? b. Can we find higher excited states?

Compute the Stark effect for the \(2 s_{1 / 2}\) and \(2 p_{1 / 2}\) levels of hydrogen for a field \(\mathscr{E}\) sufficiently weak so that \(e \mathscr{E} a_{0}\) is small compared to the fine structure, but take the Lamb \(\operatorname{shift} \delta(\delta=1057 \mathrm{MHz})\) into account (that is, ignore \(2 p_{3 / 2}\) in this calculation). Show that for \(e \mathscr{E} a_{0} \ll \delta\), the energy shifts are quadratic in \(\mathscr{E}\), whereas for \(e \mathscr{E} a_{0} \gg \delta\) they are linear in \(\mathscr{E}\). Briefly discuss the consequences (if any) of time reversal for this problem. This problem is from Gottfried (1966), Problem 7-3.

A simple harmonic oscillator (in one dimension) is subjected to a perturbation $$ H_{1}=b x $$ where \(b\) is a real constant. a. Calculate the energy shift of the ground state to lowest nonvanishing order. b. Solve this problem exactly and compare with your result obtained in (a).

Consider a particle bound to a fixed center by a spherically symmetric potential \(V(r)\). a. Prove $$ |\psi(0)|^{2}=\left(\frac{m}{2 \pi \hbar^{2}}\right)\left\langle\frac{d V}{d r}\right\rangle $$ for all \(s\) states, ground and excited. b. Check this relation for the ground state of a three-dimensional isotropic oscillator, the hydrogen atom, and so on. (Note: This relation has actually been found to be useful in guessing the form of the potential between a quark and an antiquark. See Moxhay and Rosner, J. Math. Phys., 21 (1980) 1688.)

The unperturbed Hamiltonian of a two-state system is represented by $$ H_{0}=\left(\begin{array}{cc} E_{1}^{0} & 0 \\ 0 & E_{2}^{0} \end{array}\right) \text {. } $$ There is, in addition, a time-dependent perturbation $$ V(t)=\left(\begin{array}{cc} 0 & \lambda \cos \omega t \\ \lambda \cos \omega t & 0 \end{array}\right) \quad(\lambda \mathrm{real}) $$ a. At \(t=0\) the system is known to be in the first state, represented by $$ \left(\begin{array}{l} 1 \\ 0 \end{array}\right) $$ Using time-dependent perturbation theory and assuming that \(E_{1}^{0}-E_{2}^{0}\) is not close to \(\pm \hbar \omega\), derive an expression for the probability that the system be found in the second state represented by $$ \left(\begin{array}{l} 0 \\ 1 \end{array}\right) $$ as a function of \(t(t>0)\). b. Why is this procedure not valid when \(E_{1}^{0}-E_{2}^{0}\) is close to \(\pm \hbar \omega\) ?

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