Is Probability in Quantum States Proportional to Energy Levels?

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Discussion Overview

The discussion revolves around the relationship between probability and energy levels in quantum mechanics, particularly in the context of a particle in a one-dimensional box. Participants explore whether the probability of finding a particle in a certain energy state can be expressed as a ratio of the state's energy to the total energy of all states. The conversation touches on foundational concepts in quantum mechanics, measurement, and the implications of preparing systems in specific states.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • One participant proposes that the probability of a particle being in a certain state can be calculated as the energy of that state divided by the total energy of all states.
  • Another participant challenges this view, stating that probabilities in quantum mechanics pertain to measurement outcomes rather than the states themselves, emphasizing the importance of how the system is prepared.
  • A further comment simplifies the proposed probability expression but questions its relevance to the original inquiry.
  • Participants discuss the implications of preparing a system in a specific energy state and the resulting probabilities of measuring different energy levels.
  • It is noted that if a system is prepared in a definite energy eigenstate, the probability of measuring any other energy level is zero.
  • One participant raises the question of whether probabilities can be assigned to energy states when the system's preparation is unknown, leading to a discussion about the limitations of such knowledge.
  • Another participant compares knowing the possible energy states to knowing the number of floors in a building, indicating that this knowledge alone does not provide information about the system's current state.
  • There is a mention of the Boltzmann Distribution in statistical mechanics, questioning why similar probabilistic relationships do not apply in this context.
  • Participants acknowledge the complexities of non-isolated systems and the need to consider interactions when discussing probabilities.

Areas of Agreement / Disagreement

Participants express differing views on the nature of probabilities in quantum mechanics, particularly regarding the implications of measurement and system preparation. There is no consensus on whether probabilities can be assigned to energy states without knowledge of the system's preparation.

Contextual Notes

Participants highlight the importance of considering whether a system is isolated or interacting with its environment, which affects the applicability of certain probabilistic models.

  • #31
bob012345 said:
I understand we can only measure eigenvalues. But we can certainly know the expectation value for the energy of the system.

If what you mean is "expectation value", then you need to say "expectation value". You can't just say "energy" and expect everyone to know that you mean "the expectation value of energy in some particular state that is not an eigenstate of energy".

bob012345 said:
I'm just trying to understand what that entails

You have been told two different ways what kind of preparation is necessary for a Boltzmann distribution to apply. I did in post #15, and @f95toli did in post #17. The two descriptions we gave are equivalent.

bob012345 said:
and how it would apply to a quantum system of a particle in a box?

It wouldn't. You have already been told why (@Nugatory did that in post #14).
 
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  • #32
The original question of this thread has been answered and I think I have synthesized in my mind all the answers given about that and the discussion of the Boltzmann distribution so I thank everyone for their answers.
 
  • #33
bob012345 said:
The original question of this thread has been answered and I think I have synthesized in my mind all the answers given about that and the discussion of the Boltzmann distribution so I thank everyone for their answers.

Thanks for the feedback! I'm glad we were able to answer your questions.
 
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