Why wave function should goto 0 f faster than 1/sqrt(x) at infinity

Click For Summary

Discussion Overview

The discussion revolves around the behavior of normalizable wave functions in quantum mechanics, specifically addressing why such functions should approach zero faster than 1/√x as x approaches infinity. Participants explore the implications of square integrability and reference various sources, including textbooks, to clarify their understanding.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant seeks clarification on why a normalizable wave function must decay faster than 1/√x as x approaches infinity, linking this to the concept of quadratic integrability.
  • Another participant argues that the claim is false and references an article that provides counterexamples, suggesting that the relationship is not as straightforward as implied.
  • A participant mentions that textbooks, such as Griffiths' "Introduction to Quantum Mechanics," provide insights but may differ in their claims based on editions, indicating a potential source of confusion.
  • One participant notes that Griffiths acknowledges the existence of pathological functions that are square integrable but do not conform to the 1/√x decay rule, suggesting a distinction between mathematical rigor and physical intuition.
  • Another participant elaborates that functions must decay faster than 1/x to ensure the integral does not diverge, reinforcing the idea that a wave function must decay faster than 1/√x.
  • A participant expresses gratitude for the discussion, indicating that it has helped them gain confidence in their understanding of quantum mechanics.

Areas of Agreement / Disagreement

Participants express differing views on the validity of the claim that normalizable wave functions must decay faster than 1/√x. While some reference authoritative texts to support their positions, others challenge the claim and provide counterexamples, indicating that the discussion remains unresolved.

Contextual Notes

There are references to specific examples and footnotes in textbooks that may vary by edition, which could affect the interpretation of the claims being discussed. The discussion also highlights the distinction between mathematical rigor and physical reasoning.

relativist
Messages
10
Reaction score
0
Can anybody please explain the reason why a normalizable wave function ψ(x) → 0 faster than 1/√x as x → ∞.

I can understand the reason why ∫ψψ*dx < ∞ But do not understand how quadratic integrability implies that.

I would be very thankful to anybody who can give me some idea.
 
Physics news on Phys.org
It doesn't. Check out example 2, starting on page 4 in this article. In particular, you should look at figure 2.1 (page 5).
 
I need a better explanation than that. Books like Introduction to Quiantum Mechanics by Griffiths state that see footnote 11 in Griffiths under section 1.4.

Can anybody please answer my question
 
Your question was answered, so I don't know why you're acting like it wasn't. The statement that you want to prove ("if \psi is square integrable, it goes to zero faster than 1/\sqrt x ") is false. The article I linked to explains why, and contains counterexamples.

I took a quick look at Griffiths. It's footnote 8 on page 11 in the copy I could get hold of quickly, so maybe the claim is different in your edition. In the text I'm looking at, he's talking about square integrable solutions of Schrödinger's equation. Maybe it's possible to show that there's no choice of the potential V that allows solutions whose first derivatives with respect to x aren't bounded. Since all the counterexamples I've seen have unbounded derivatives, I expect that it's possible to prove that square-integrability and bounded first derivatives implies that the function goes to zero.
 
If memory serves me right, Griffiths does explicitly mention that there are pathological functions which do not obey the 1/sqrt(x) rule but are square integrable over the whole real line. I believe he goes on to say that if this bothers you, you should become a mathematician. =]

Physicists are usually sloppy when it comes to math as compared with mathematicians. Suffice it to say that the 1/sqrt(x) rule is at the very least a good rule of thumb for square integrability.

1/x, when integrated over the part of the real line where x>1 (giving you a natural log function) diverges due to it not "decaying fast enough". That's why functions must decay FASTER than 1/x in order that this integral does not diverge. This corresponds to a wave-function which must decay faster than 1/sqrt(x) (since it will be squared).
 
Matterwave said:
If memory serves me right, Griffiths does explicitly mention that there are pathological functions which do not obey the 1/sqrt(x) rule but are square integrable over the whole real line. I believe he goes on to say that if this bothers you, you should become a mathematician. =]

Physicists are usually sloppy when it comes to math as compared with mathematicians. Suffice it to say that the 1/sqrt(x) rule is at the very least a good rule of thumb for square integrability.

1/x, when integrated over the part of the real line where x>1 (giving you a natural log function) diverges due to it not "decaying fast enough". That's why functions must decay FASTER than 1/x in order that this integral does not diverge. This corresponds to a wave-function which must decay faster than 1/sqrt(x) (since it will be squared).

Thanks for a nice answer. I fact I stumbled upon a problem in chapter 3 (problem 3.2) which got me thinking in the right direction after working on it. Your reply has helped to gain a good deal of confidence that I am making some progress in learning qm.

Thanks again,

Relativist.
 

Similar threads

  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 29 ·
Replies
29
Views
4K
  • · Replies 2 ·
Replies
2
Views
3K
  • · Replies 2 ·
Replies
2
Views
3K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 1 ·
Replies
1
Views
3K