Introduction to Quantum Mechanics · Problem 1.12

The Wave Function — David J. Griffiths

Physics — Quantum Mechanics · classical_momentum_distribution

Problem

What if we were interested in the distribution of momenta (p=mvp = mv), for the classical harmonic oscillator (Problem 1.11(b)). (a) Find the classical probability distribution ρ(p)\rho(p) (note that pp ranges from −2mE-\sqrt{2mE} to +2mE+\sqrt{2mE}). (b) Calculate ⟨p⟩\langle p \rangle, ⟨p2⟩\langle p^2 \rangle, and σp\sigma_p. (c) What's the classical uncertainty product, σxσp\sigma_x \sigma_p, for this system? Notice that this product can be as small as you like, classically, simply by sending E→0E \to 0. But in quantum mechanics, as we shall see in Chapter 2, the energy of a simple harmonic oscillator cannot be less than ℏω/2\hbar\omega/2, where ω=k/m\omega = \sqrt{k/m} is the classical frequency. In that case what can you say about the product σxσp\sigma_x \sigma_p?

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