Expansion Coeff. & Probability of E in Schrodinger Eq.

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    Coefficients Expansion
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Discussion Overview

The discussion revolves around the concept of expansion coefficients in the context of the Schrödinger equation, specifically addressing the interpretation of these coefficients as probabilities related to energy values. Participants explore the nature of stationary and non-stationary states and the implications for probability distributions in quantum mechanics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant notes that the square of the expansion coefficient represents the probability density of an allowed energy value, questioning how probabilities apply to definite energy values in stationary states.
  • Another participant clarifies that the square of the coefficient indicates the probability of finding the particle in a specific state, not a probability density of energy values.
  • It is suggested that superpositions of states can lead to non-stationary states where the probability distribution varies over time, with an example involving the linear combination of stationary states in a particle in a box scenario.
  • Participants engage in light banter regarding the time taken to type responses and the challenges of using LaTeX for equations.
  • One participant confirms the current professor of quantum mechanics at Rensselaer Polytechnic Institute and shares their own experience with a different instructor.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of probabilities in relation to energy values in quantum mechanics, with no consensus reached on the implications of stationary versus non-stationary states.

Contextual Notes

The discussion includes assumptions about the definitions of stationary and non-stationary states, as well as the mathematical treatment of wave functions and probability distributions, which remain unresolved.

RPI_Quantum
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In dealing with solutions to the Schroedinger equation, I have come across the so-called expansion coefficients (c n ). I have read that the square of the coefficient is the probability density of an allowed value of E.

How is there a probability of any given value of E? I know that in a stationary state, there are definite values of E, so how is there any probability involved?
 
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RPI_Quantum said:
In dealing with solutions to the Schroedinger equation, I have come across the so-called expansion coefficients (c n ). I have read that the square of the coefficient is the probability density of an allowed value of E.

Not the probability density. |cn|2 is the probability that the particle will be found in state n.

How is there a probability of any given value of E? I know that in a stationary state, there are definite values of E, so how is there any probability involved?

States need not be stationary. You can have a superposition of states with different energies. Of course, if the state is of a single energy, then the probability that the system will be found with that one energy is 1.

BTW, am I to take it from your name that you're taking QM at Rensselaer Polytechnic Institute? If so, who's teaching it these days?
 
You can combine stationary states (in a linear combination) to get a non-stationary state in which the probability distribution "moves around". That is, the expectation value of x, <x>, is not constant, but varies with time.

A simple example is to take two stationary states of the one-dimensional "particle in a box" (infinite square well), say the two with lowest energy, and add them together:

[tex]\Psi (x, t) = \frac {1}{\sqrt {2}} ( \Psi_1 (x, t) + \Psi_2 (x, t))[/tex]

(the [tex]\sqrt {2}[/tex] is to make the sum normalized, provided the two original wave functions are normalized to begin with)

Calculate the probability distribution for this wave function and you'll see that it oscillates with frequency [tex](E_2 - E_1) / h[/tex].

Hmm, Tom can type faster than I can, apparently!
 
Don't tell me it took u 4hrs to type 15 lines... :-p

Daniel.
 
dextercioby said:
Don't tell me it took u 4hrs to type 15 lines... :-p

Hey, I sweated blood over that LaTeX! :eek:

(besides, at 1:30AM I can't read very well... :zzz: )
 
Thanks guys. And Tom, Professor Tim Hayes is the QM professor. He has been teaching it since before I got here (I am a sophomore). I take it you are an alumnus?
 
RPI_Quantum said:
Thanks guys. And Tom, Professor Tim Hayes is the QM professor. He has been teaching it since before I got here (I am a sophomore). I take it you are an alumnus?

Yep, I took it with Gwo-Ching Wang.
 

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