# 4: Postulates and Principles of Quantum Mechanics

- Page ID
- 92372

It can help us to formulate the core postulates underlying our use of quantum mechanics. If chapter 1 is the *Declaration of Independence* (from classical mechanics), then this chapter is the *Constitution *of Quantum mechanics where all our discussions rely on.

- 4.1: The Wavefunction Specifies the State of a System
- Postulate 1: Every physically-realizable state of the system is described in quantum mechanics by a state function that contains all accessible physical information about the system in that state.

- 4.2: Quantum Operators Represent Classical Variables
- Every observable in quantum mechanics is represented by an operator which is used to obtain physical information about the observable from the state function. For an observable that is represented in classical physics by a function \(Q(x,p)\), the corresponding operator is \(Q(\hat{x},\hat{p})\).

- 4.3: Observable Quantities Must Be Eigenvalues of Quantum Mechanical Operators
- It is a general principle of Quantum Mechanics that there is an operator for every physical observable. A physical observable is anything that can be measured. If the wavefunction that describes a system is an eigenfunction of an operator, then the value of the associated observable is extracted from the eigenfunction by operating on the eigenfunction with the appropriate operator. The value of the observable for the system is the eigenvalue, and the system is said to be in an eigenstate.

- 4.4: The Time-Dependent Schrödinger Equation
- While the time-dependent Schrödinger equation predicts that wavefunctions can form standing waves (i.e., stationary states), that if classified and understood, becomes easier to solve the time-dependent Schrödinger equation for any state. Stationary states can also be described by the time-independent Schrödinger equation (used when the Hamiltonian is not explicitly time dependent). The solutions to the time-independent Schrödinger equation still have a time dependency.

- 4.5: The Eigenfunctions of Operators are Orthogonal
- The eigenvalues of operators associated with experimental measurements are all real; this is because the eigenfunctions of the Hamiltonian operator are orthogonal, and we also saw that the position and momentum of the particle could not be determined exactly. We now examine the generality of these insights by stating and proving some fundamental theorems. These theorems use the Hermitian property of quantum mechanical operators, which is described first.

- 4.6: Heisenburg Uncertainy Principle III - Commuting Operators
- If two operators commute then both quantities can be measured at the same time with infinite precision, if not then there is a tradeoff in the accuracy in the measurement for one quantity vs. the other. This is the mathematical representation of the Heisenberg Uncertainty principle.

- 4.E: Postulates and Principles of Quantum Mechanics (Exercises)
- These are homework exercises to accompany Chapter 4 of McQuarrie and Simon's "Physical Chemistry: A Molecular Approach" Textmap.