Help in showing that this inner product is zero

Click For Summary

Homework Help Overview

The discussion revolves around demonstrating that the inner product of two plane wave solutions, specifically ##(u_{\vec{k}},u^{*}_{\vec{k}'})##, is zero. The context involves concepts from quantum mechanics and functional analysis, particularly in relation to Hilbert spaces.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants discuss the direct calculation of the inner product and question the formulation of the problem. There is a suggestion to clarify the Hilbert space context, whether it is ##\mathrm{L}^2(\mathbb{R}^3)## or a finite region ##D##. Additionally, there is an exploration of the integral representation of the inner product and its relation to the Dirac-delta function.

Discussion Status

The discussion is ongoing, with participants seeking clarification on the problem setup and exploring different interpretations of the inner product. Some guidance has been offered regarding the mathematical representation, but no consensus has been reached on the approach or assumptions.

Contextual Notes

There is a noted ambiguity regarding the choice of Hilbert space, which may affect the interpretation of the inner product. The original poster's question is also considered to be inadequately posed, prompting further inquiry into the specifics of the problem.

user1139
Messages
71
Reaction score
8
Homework Statement
I want to show that the inner product between the plane wave solution and its conjugate is zero.
Relevant Equations
The inner product is defined as ##(u_{\vec{k}},u_{\vec{k}'})=-i\int u_{\vec{k}}\partial_{t}u^{*}_{\vec{k}'}-u^{*}_{\vec{k}'}\partial_{t} u_{\vec{k}}\,\mathrm{d}^3 x##
The unormalised plane wave solution is given as ##u_{\vec{k}}=e^{i\vec{k}\cdot\vec{x}-i\omega t}##. I want to show that ##(u_{\vec{k}},u^{*}_{\vec{k}'})=0##. However, I don't seem to be able to get the answer through direct calculation. Any hints on how to obtain the answer?
 
Physics news on Phys.org
Show your work please.
 
Your question is also not well posed. Which Hilbert space are we supposed to work in? Is it ##\mathrm{L}^2(\mathbb{R}^3)## or is it ##\mathrm{L}^2(D)## with some finite region ##D##?
 
It seems that the inner product is propotional to $$\int \exp [i(\vec{k}+\vec{k}')\cdot\vec{x}]\mathrm{d}^3 x.$$

Shouldn't it be a Dirac-delta function?
 
  • Like
Likes   Reactions: vanhees71

Similar threads

Replies
3
Views
2K
  • · Replies 12 ·
Replies
12
Views
3K
  • · Replies 5 ·
Replies
5
Views
3K
  • · Replies 2 ·
Replies
2
Views
2K
Replies
1
Views
1K
Replies
18
Views
2K
  • · Replies 25 ·
Replies
25
Views
2K
Replies
8
Views
2K
Replies
7
Views
2K
Replies
58
Views
7K