Discrete vs continuous eigenvalues

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Discussion Overview

The discussion centers on the conditions that determine whether an operator has discrete or continuous eigenvalues, particularly in the context of quantum mechanics. Participants explore various examples such as energy, momentum, position, and spin, examining their eigenvalue characteristics and the implications of different physical systems.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants propose that the nature of eigenvalues is influenced by the range of the conjugate variable, suggesting that confinement (e.g., a box) leads to discrete momenta, while free systems yield continuous momenta.
  • Others argue that the energy spectrum is generally discrete for finite motions, where particles cannot be found infinitely far away, but continuous for certain conditions.
  • A participant introduces the idea that periodic systems may yield discrete energy values, while also noting that angular momentum is discrete due to its finite/periodic nature.
  • There is a challenge regarding the simple harmonic oscillator, which has discrete energy eigenvalues but a wave function that approaches zero at infinity without ever reaching it, raising questions about the probability of finding a particle at large distances.

Areas of Agreement / Disagreement

Participants express differing views on the conditions that lead to discrete versus continuous eigenvalues, with no consensus reached on the underlying principles or the implications of specific examples.

Contextual Notes

Some claims rely on specific definitions and assumptions about the systems being discussed, such as the nature of confinement and periodicity, which may not be universally applicable.

pellman
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What determines whether an operator has discrete or continuous eigenvalues?

Energy and momentum sometimes have discrete eigenvalues, sometimes continuous. Position is always continuous (isnt it?) Spin is always discrete (isn't it?) Why?
 
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Momentum always has continuous eigenvalues (unless you use the artificial box normalization, when it always has discrete eigenvalues). The time-independent Schroedinger equation (the eigenvalue problem for the Hamiltonian of the system) gives the eigenvalues of the energy of that particular system (each system has a different Hamiltonian).

It is a general rule that the energy spectrum is discrete for finite motions, i.e. motions in which the particle cannot be found infinitely far away. For such motions, the energy has a continuous spectrum. For motions in period structures, there is the added possibility that the energy can have any continuously varying value withing bands of finite bandwidth width separated by forbidden energy regions of finite gap.
 
I think I figured it out. It depends on the range of the conjugate variable. If our space is limited to a box, we get discrete momenta. Otherwise, continuous momenta. If our system is periodic in time, we get discrete energy. Angles are necessarily finite/periodic, so angular momentum is discrete.

I would wager that a deeper discussion would reveal that compactness is the key ingredient. That's just a guess.
 
Dickfore said:
It is a general rule that the energy spectrum is discrete for finite motions, i.e. motions in which the particle cannot be found infinitely far away.

I think this needs to be refined a bit. How about the simple harmonic oscillator? Discrete energy eigenvalues, but the wave function only approaches zero as x goes to infinity, never quite reaching it.
 
jtbell said:
I think this needs to be refined a bit. How about the simple harmonic oscillator? Discrete energy eigenvalues, but the wave function only approaches zero as x goes to infinity, never quite reaching it.

So, what is the probability for finding the particle at distances larger than a as [itex]a \rightarrow \infty[/itex]?
 

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