Energy Conservation in Quantum Systems - Is it Possible?

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Discussion Overview

The discussion centers around the conservation of energy in quantum systems, particularly in the context of time-dependent Hamiltonians. Participants explore the implications of time-varying potentials on energy conservation, examining both theoretical and conceptual aspects.

Discussion Character

  • Debate/contested, Conceptual clarification, Technical explanation

Main Points Raised

  • Some participants propose that if the Hamiltonian of a quantum system evolves with time, energy is not conserved.
  • Others question where the energy goes if the Hamiltonian is time-dependent, noting that the Schrödinger equation can yield constant energy despite this.
  • It is suggested that viewing the Hamiltonian as a matrix implies that its eigenvalues change over time, which could affect energy conservation.
  • One participant introduces the concept of non-translationally invariant systems, arguing that breaking such invariance leads to non-conservation of momentum and, by extension, energy if time invariance is also broken.
  • Another participant discusses the implications of breaking time translational invariance, suggesting that energy conservation may not hold in such cases, and expresses confusion regarding stationary solutions in quantum mechanics.

Areas of Agreement / Disagreement

Participants express differing views on the implications of time-dependent Hamiltonians for energy conservation, with no consensus reached on whether energy can be conserved in such scenarios.

Contextual Notes

Participants reference various assumptions about invariance and the nature of Hamiltonians, but these assumptions are not universally agreed upon. The discussion includes unresolved mathematical considerations related to the time evolution of quantum states.

gonadas91
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Hi guys! one quick question, if in a quantum system the hamiltonian of a particle evolves with time (let's say, the potential is a function of t), the energy is not conserved right? I just want to be sure about this, thanks!
 
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gonadas91 said:
Hi guys! one quick question, if in a quantum system the hamiltonian of a particle evolves with time (let's say, the potential is a function of t), the energy is not conserved right? I just want to be sure about this, thanks!
Then, where the energies gone? Whether the Hamiltonian is time-dependent, Schrödinger equation gives constant energy.
 
But if you think of the hamiltonian as a matrix, that means that the matrix has different matrix elements for every different time, therefore its eigenvalues are not the same as time evolves.
 
gonadas91 said:
But if you think of the hamiltonian as a matrix, that means that the matrix has different matrix elements for every different time, therefore its eigenvalues are not the same as time evolves.
Then, the wave function have to vary depending on time to keep the energy as constant.
 
Ok, but think about a non translationally invariant system. Momentum is not conserved in such a system because translation invariance is broken. If you break rotation invariance, angular momentum is not conserved in such a system, and if you break time invariance, then energy shouldn't be conserved.
 
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If it breaks the time translational invariance, then yes. It may come from
[tex]\dfrac{\partial}{\partial t} \int dV \psi^*(x,t) \hat{H}(x,t) \psi(x,t) ,[/tex]
and it is not vanishing in general. Then, the energy can't conserved.

I confused with the stationary solution in quantum mechanics textbook. Sorry for that.
 

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