Discrete and continuous confusion

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Discussion Overview

The discussion centers on the apparent contradiction in quantum mechanics between discrete energy states for electron orbits and the continuous probability density function for the position of an electron. Participants explore the implications of these concepts and seek to understand how they coexist within the framework of quantum theory.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about how discrete energy states can coexist with a continuous probability density function for electron positions, suggesting this creates a paradox.
  • Another participant clarifies that definite energies are linked to wavefunctions known as energy eigenstates, which can have either discrete or continuous energy spectra depending on the situation.
  • A different viewpoint suggests that the confusion may stem from a semi-classical interpretation, referencing Bohr's model where only certain orbits are allowed.
  • Another participant challenges the notion of "exact" discrete energy states, arguing that real electrons experience energy broadening due to coupling with the vacuum, leading to smeared energy distributions.
  • This participant draws an analogy to resonances in electrical circuits, where damping causes a broadening of resonance frequencies, paralleling the concept of energy states in quantum mechanics.
  • They also mention the "mathematical uncertainty principle," indicating that achieving an exact energy state requires an infinite time in a single state, which is not feasible for systems with finite lifetimes.

Areas of Agreement / Disagreement

Participants do not reach a consensus, as multiple competing views are presented regarding the nature of energy states and their relationship to position probabilities in quantum mechanics.

Contextual Notes

There are limitations in the discussion, including assumptions about the nature of energy states, the impact of external factors like vacuum coupling, and the implications of the uncertainty principle, which remain unresolved.

qbslug
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I don't understand how in quantum mechanics we have discrete and exact energy states for electron orbits but then at the same time we have a continuous probability density function for the position of an electron.
This seems like a paradox (although I know it can't be) since considering a continuous position distribution you would expect the different positions of the electron to create different energies of the electron in a continuous way. How is this explained? How can you have one energy for different positions?
 
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Definite energies are not associated to each position state, but to certain wavefunctions called energy eigenstates (eigenvectors of the Hamiltonian operator). In certain situations, the energy spectrum associated with the energy eigenvectors is discrete. In other situations it may be continuous.
 
May be is becouse you're thinking semi-classicaly like in Bhor's rule where only certain orbits are allowed while others are forbidden.
 
qbslug said:
I don't understand how in quantum mechanics we have discrete and exact energy states for electron orbits but then at the same time we have a continuous probability density function for the position of an electron.

I don't think it is quite correct to say that we have "exact" discrete energy states for real electrons. A soon as you allow for coupling to the vacuum the energies are "smeared put" and the eigeneneriges are then just the centres of Lorentzian energy distributions.

It is pretty much analogues to resonances in an electrical LC circuit, in an ideal LC circuit there is a single discrete resonance frequency but as soon as you add some damping the resonance is broadened.

Note that this is just a consequences of the "mathematical uncertainty principle", in order to have an exact energy the electrons would have to stay in a single energy state for an infinite amount of time; a system with a finite lifetime can not have a truly discrete energy spectrum.
 

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