CHAPTER 5 The Schrodinger Eqn. 5.1 The Schrödinger Wave Equation 5.2 Expectation Values 5.3 Infinite Square-Well Potential 5.4 Finite Square-Well Potential 5.5 Three-Dimensional Infinite- Potential Well 5.6 Simple Harmonic Oscillator 5.7 Barriers and Tunneling http://nobelprize.org/physics/laureates/1933/schrodinger-bio.html Erwin Schrödinger (1887-1961) A careful analysis of the process of observation in atomic physics has shown that the subatomic particles have no meaning as isolated entities, but can only be understood as interconnections between the preparation of an experiment and the subsequent measurement. - Erwin Schrödinger

Announcement: First exam will be on Wednesday Oct. 7th at 2:30pm in room 203 You will be given a formula sheet with Lorentz transformations, other basic equations The exam will probably consist of 3 problems which you must solve (not multiple choice) Focus will be relativity and elements of quantum mechanics

Opinions on quantum mechanics I think it is safe to say that no one understands quantum mechanics. Do not keep saying to yourself, if you can possibly avoid it, “But how can it be like that?” because you will get “down the drain” into a blind alley from which nobody has yet escaped. Nobody knows how it can be like that. - Richard Feynman http://www.bnl.gov/bnlweb/pubaf/pr/PR_display.asp?prID=05-37 Those who are not shocked when they first come across quantum mechanics cannot possibly have understood it. - Niels Bohr Richard Feynman (1918-1988)

Particle in a Box A particle (wave) of mass m is in a one-dimensional box of width ℓ. The box puts boundary conditions on the wave. The wave function must be zero at the walls of the box and on the outside. In order for the probability to vanish at the walls, we must have an integral number of half wavelengths in the box: The energy: The possible wavelengths are quantized and hence so are the energies:

Probability of the particle vs. position Note that E0 = 0 is not a possible energy level. The concept of energy levels has surfaced in a natural way by using waves. The probability of observing the particle between x and x + dx in each state is:

Properties of Valid Wave Functions Conditions on the wave function: 1. In order to avoid infinite probabilities, the wave function must be finite everywhere. 2. The wave function must be single valued. 3. The wave function must be twice differentiable. This means that it and its derivative must be continuous. (An exception to this rule occurs when V is infinite.) 4. In order to normalize a wave function, it must approach zero as x approaches infinity. Solutions that do not satisfy these properties do not generally correspond to physically realizable circumstances.

Normalization and Probability The probability P(x) dx of a particle being between x and x + dx is given in the equation The probability of the particle being between x1 and x2 is given by The wave function must also be normalized so that the probability of the particle being somewhere on the x axis is 1.

Expectation Values In quantum mechanics, we’ll compute expectation values. The expectation value, , is the weighted average of a given quantity. In general, the expected value of x is: If there are an infinite number of possibilities, and x is continuous: Quantum-mechanically: And the expectation of some function of x, g(x):

Bra-Ket Notation This expression is so important that physicists have a special notation for it. The entire expression is called a bracket. And is called the bra with the ket. The normalization condition is then:

The Schrödinger Wave Equation The Schrödinger Wave Equation for the wave function Y(x,t) for a particle in a potential V(x,t) in one dimension is: where The Schrodinger Equation is the fundamental equation of Quantum Mechanics. Note that it’s very different from the classical wave equation. But, except for its inherent complexity (the i), it will have similar solutions.

General Solution of the Schrödinger Wave Equation when V = 0 Try the usual solution: This works as long as: which says that the total energy is the kinetic energy.

General Solution of the Schrödinger Wave Equation when V = 0 In free space (with V = 0), the wave function is: which is a sine wave moving in the x direction. Notice that, unlike classical waves, we are not taking the real part of this function. Y is, in fact, complex. In general, the wave function is complex. But the physically measurable quantities must be real. These include the probability, position, momentum, and energy.

Time-Independent Schrödinger Wave Equation The potential in many cases will not depend explicitly on time: V = V(x). The Schrödinger equation’s dependence on time and position can then be separated. Let: And substitute into: which yields: Now divide by y(x) f(t): The left side depends only on t, and the right side depends only on x. So each side must be equal to a constant. The time-dependent side is:

Time-Independent Schrödinger Wave Equation Multiply both sides by f /iħ: which is an easy differential equation to solve: But recall our solution for the free particle: in which f(t) = exp(-iwt), so: w = B / ħ or B = ħw, which means that: B = E ! So multiplying by y(x), the spatial Schrödinger equation becomes:

Stationary States The wave function can now be written as: The probability density becomes: The probability distribution is constant in time. This is a standing-wave phenomenon and is called a stationary state. Most important quantum-mechanical problems will have stationary-state solutions. Always look for them first.

Operators The time-independent Schrödinger wave equation is as fundamental an equation in quantum mechanics as the time-dependent Schrödinger equation. So physicists often write simply: where: is an operator yielding the total energy (kinetic plus potential energies).

Operators Operators are important in quantum mechanics. All observables (e.g., energy, momentum, etc.) have corresponding operators. The kinetic energy operator is: Other operators are simpler, and some just involve multiplication. The potential energy operator is just multiplication by V(x).

Momentum Operator To find the operator for p, consider the derivative of the wave function of a free particle with respect to x: With k = p / ħ we have: This yields: This suggests we define the momentum operator as: . The expectation value of the momentum is: