Just a quick question about some basic terms in QM

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Discussion Overview

The discussion revolves around basic terminology in quantum mechanics (QM), specifically focusing on concepts such as eigenvalues, eigenvectors, eigenfunctions, and the distinction between different notations like Ψ and ψ. Participants explore how these terms relate to operator equations and the mathematical framework of QM.

Discussion Character

  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant expresses confusion about the definitions and applications of eigenvalues, eigenvectors, eigenfunctions, and eigenstates in QM.
  • Another participant explains that operators are typically denoted with hats, and distinguishes between big Psi (Ψ) for general state functions and small psi (ψ) for eigenfunctions or eigenstates.
  • A further contribution clarifies that the operator acting on small psi yields an eigenvalue, and discusses the linear combination of eigenstates represented by big Psi.
  • It is noted that eigenstates are fundamental, while eigenvectors are associated with discrete cases, and eigenfunctions with continuous variables.
  • One participant mentions that the difference between Ψ and ψ is primarily in their usage, with ψ for eigenstates and Ψ for more general states.
  • Another participant introduces the concept of Hilbert space and explains that eigenvectors of Hermitian operators form an orthonormal basis set for this space.

Areas of Agreement / Disagreement

Participants generally agree on the definitions of the terms discussed, but there are variations in how they are applied and understood, indicating that some aspects remain contested or unclear.

Contextual Notes

Some participants assume familiarity with vector spaces and linear algebra, which may limit the accessibility of certain explanations. The discussion also reflects varying interpretations of the terms and their applications in different contexts within QM.

oliver.smith8
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I'm trying to ge my head around some basic things but where ever I look I seem to get a different answer.

what is an eigenvalue, eigenvector, eigenfunction and eigen stat, all in the context of QM?
and how do these apply to the operator equation.

\hat{O} \psi = O \psi

what is the differnce between a \Psi and a \psi?

I hope somebody can help because I can't get my head round it!
 
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there are many different notations, but one usally denotes operators with hats, and big Psi for a general state-function and small psi's for eigenfunctions (bases).

now look at

<br /> \hat{O} \psi = O \psi<br />

The operator O is acting on small phi, and you get the eigenvalue O, thus small psi is an eigenstate (eigenfuction, eigenvector depending on what psi resembles).

Look at

<br /> \hat{O} \Psi = \hat{O} ( \psi_1 + \psi _2 + \psi _3) = O_1 \psi _1 O_2 + \psi _2 + O_3 \psi _3 <br />

The Big Psi is a linear combination of eigenstates to the operator O.

Eigenstate is the most fundamental, eigenvector you have in discrete cases, like spin. And eigenfunctions when you have continuous variables (momentum, position etc).

You will learn all of this by practice, don't panic

If you have done linear algebra, you should recognize these things.
 
Welcome to PF!

Hi oliver! Welcome to PF! :smile:

(have a psi: ψ and an Ô :wink:)

In Ô = O, Ô is the operator, O is the eigenvalue, and ψ is the eigenvector (same as eigenstate) …

so for example if Ô is the momentum operator, then O is a momentum value, and ψ is a wave function with that (pure) momentum.

I think the difference between Ψ and ψ is just that ψ tends to be used for eigenvectors (eigenstates),while Ψ is used for anything.
 
I'm assuming you know what a vector space is.

In quantum mechanics, states are members of a complex vector space \mathcal{H}, called Hilbert space. Observables are Hermitian operators on this vector space. An eigenvector of an operator \hat{O} is any vector v that satisfies the equaton \hat{O} v = \alpha v, where \alpha is a scalar, called the eigenvalue corresponding to v. The eigenvectors of observables (Hermitian operators) form an orthonormal basis set for \mathcal{H}, i.e. any state can be written as a linear combination of these eigenvectors. In the context of QM, the eigenvectors are usually called eigenstates. For example, if \hat{X} is the position operator, it's eigenvectors are called position eigenstates.
 

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