# Lecture 3 | Modern Physics: Quantum Mechanics (Stanford) | Summary and Q&A

## Summary

This video covers the mathematics of linear operators in quantum mechanics. The concepts of vector space, observable quantities, linear operators, and matrix elements are explained. The video introduces the notion of a basis of vectors and how matrix elements can be used to represent operators. The concept of hermitian operators and their significance in quantum mechanics is also discussed. The video concludes with the postulates of quantum mechanics, including the description of states by vectors, the correspondence between observables and hermitian operators, the eigenvalues and eigenvectors, and the probability of measurement outcomes.

## Questions & Answers

### Q: What is the role of linear algebra in quantum mechanics?

The mathematics of quantum mechanics is based on linear algebra concepts. Quantum mechanics involves the study of vectors in a vector space and how they transform under the action of linear operators. Linear algebra provides the mathematical framework to describe these transformations and derive various properties.

### Q: What do observable quantities represent in quantum mechanics?

Observable quantities in quantum mechanics are things that can be measured or observed in an experiment. These quantities, such as momentum or position, are represented by hermitian linear operators. These operators act on the states of a system and yield measurable results when applied to those states.

### Q: How are linear operators represented in quantum mechanics?

Linear operators in quantum mechanics are represented by matrices. The matrix elements of an operator describe the action of the operator on a pair of states. For example, the matrix element of an operator between vectors a and b is obtained by first applying the operator to vector a, taking the inner product with vector b, and obtaining a number that represents the matrix element.

### Q: What is a basis of vectors and why is it important?

A basis of vectors is a collection of vectors that form the building blocks for all other vectors in the vector space. In quantum mechanics, a basis of vectors is used to represent states and observable quantities. The vectors in a basis are orthogonal to each other and are of unit length. They provide a way to expand any vector in terms of their coefficients, which are obtained through the inner product operation.

### Q: How can the matrix elements of an operator be used to represent it?

By tabulating and recording all the matrix elements of an operator between basis vectors, a matrix representation of that operator can be constructed. The matrix is square and its size is equal to the dimensionality of the vector space. These matrix elements uniquely describe the operator and its action on the states of a system.

### Q: What are the properties of hermitian operators?

Hermitian operators in quantum mechanics are a special class of operators that have important properties. Their matrix elements are such that complex conjugating the bra and ket vectors does not change the value of the matrix element. Hermitian operators have real eigenvalues, meaning the values that can be obtained when measuring them are real numbers. Furthermore, these operators have a complete set of mutually orthogonal eigenvectors, which form a basis for the vector space.

### Q: What is the significance of eigenvalues and eigenvectors in quantum mechanics?

Eigenvalues and eigenvectors play a crucial role in quantum mechanics. The eigenvalues represent the possible values that can be obtained when measuring an observable corresponding to a hermitian operator. The eigenvectors are the states in which the observable has a definite value, and that value is given by the corresponding eigenvalue. They form a basis for the vector space and represent the states with well-defined values of observables.

### Q: How do the postulates of quantum mechanics describe the states and observables of a system?

The first postulate states that the states of a quantum mechanical system are described by vectors in a vector space. The second postulate states the correspondence between observables and hermitian operators. The third postulate states that the values of observables are given by the eigenvalues of the corresponding hermitian operators. The fourth postulate states that the eigenvectors of an observable represent the states with well-defined values of that observable. The fifth postulate gives the probability interpretation of measurement outcomes in terms of the inner product between the state vector and the eigenvector, squared or complex conjugated.

### Q: How are measurement outcomes related to the eigenvectors of an observable?

If the system is prepared in an eigenvector state of a particular observable, then the measurement of that observable will always yield the corresponding eigenvalue as the outcome. Conversely, if the system is not in an eigenvector state, the probability of obtaining a specific measurement outcome is given by the component of the state vector projected onto the eigenvector, squared or complex conjugated.

### Q: Can incompatible observables have simultaneous well-defined values in quantum mechanics?

No, incompatible observables in quantum mechanics cannot have simultaneous well-defined values. This is known as the uncertainty principle. The more precisely one observable is known, the less precisely the other incompatible observable can be known. This is a fundamental aspect of quantum mechanics and distinguishes it from classical mechanics. For example, position and momentum are incompatible observables, and their precise values cannot be simultaneously known.