Energy Definition: Quantum Mechanics Explained

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Discussion Overview

The discussion centers around the definition of energy in quantum mechanics, exploring its relationship with the De-Broglie relations and the Hamiltonian operator. Participants examine whether these concepts are equivalent, their applicability under different conditions, and the implications of measurement in quantum systems.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants inquire whether energy in quantum mechanics is defined by the De-Broglie relations or the eigenvalue equation of the Hamiltonian operator, questioning the equivalence of these definitions.
  • Others argue that the energy observable is linked to the Hamiltonian operator, emphasizing that measurement yields values from the operator's spectrum, though the measurement problem remains complex and debated.
  • A participant questions the status of the De-Broglie relations, asking if it is a derived result or an axiom and whether it universally applies.
  • It is suggested that the De-Broglie relations are derived and only applicable in free space, as non-constant potentials complicate the assignment of a single wavelength to eigenstates.
  • One participant seeks clarification on whether the De-Broglie relation holds only for constant potentials and requests a definition of energy.
  • A participant shifts the discussion by asking for the definition of energy in classical mechanics, indicating a desire to draw parallels or contrasts between classical and quantum definitions.
  • Another participant provides a classical perspective, suggesting that energy is a conserved quantity due to the time-independence of the Lagrangian.
  • A request is made for an outline on how to derive the De-Broglie relation for energy, indicating an interest in the mathematical foundations of the topic.

Areas of Agreement / Disagreement

Participants express differing views on the applicability and derivation of the De-Broglie relations, with no consensus reached on the definition of energy in quantum mechanics or its relationship to classical mechanics.

Contextual Notes

The discussion highlights limitations in the applicability of the De-Broglie relations under varying potential conditions and the complexities surrounding the measurement problem in quantum mechanics.

xboy
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How is energy defined in quantum mechanics?is it defined by the De-Broglie relations or from the eigenvalue equation of H operator?Are the two somehow equivalent?
 
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Interesting question. Actually we have the energy observable (that is what is measured) and the Hamilton operator. If we measure the energy of a quantum system we should get a value from the operator's spectrum.

The measurement problem is really tricky and under debate, but energy is always associted to the hamiltonian.
 
What is the status of the De-Broglie relations then?Is it a derived result or an axiom?does it always hold?
 
It is derived. It doesn't always hold, because it really only applies to particles in free space. If there is a non-constant potential present, then you cannot necessarily even assign a single "wavelength" to the eigenstates (only a spectrum of them). For example, the Hydrogen atom's ground state is exponentially decaying with radius.
 
so the de-broglie relation holds only for constant potential,is that what you are saying?what is the definition of energy then?
 
Let me turn this around a bit -- what do you think the definition of energy in classical mechanics is?
 
ummm...a quantity that remains conserved because the lagrangian is time-independent?
can anyone please give me a sort of outline how i can derive the De-broglie relation for energy ?
 

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