Expectation value of a dynamical variable problem.

In summary, the extra phase factor cancels out because it is multiplied with its conjugate. It doesn't matter because the extra phase factor is independent of x, so it doesn't change the final result.
  • #1
Normalization
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Homework Statement


Why does the extra phase factor cancel out? Is it because you are multiplying the wave-function with the extra phase factor by its conjugate and if so, why should it matter that the extra phase factor is independent of x?

All relevant information, the solution and equation referred to in the solution is given below:

Prob18.jpg


Homework Equations



Given in picture

The Attempt at a Solution



See problem statement
 
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  • #2
Normalization said:

Homework Statement


Why does the extra phase factor cancel out? Is it because you are multiplying the wave-function with the extra phase factor by its conjugate and if so, why should it matter that the extra phase factor is independent of x?

All relevant information, the solution and equation referred to in the solution is given below:

Prob18.jpg


Homework Equations



Given in picture

The Attempt at a Solution



See problem statement

Yes, the phase get multiplied by its conjugate. If the phase had had an x dependence then when the operator Q (which could contain d/dx) acts on ψ it could also change the phase factor.
 
  • #3
Oh I see :) Because the phase factor is independent of x it acts as a constant on ψ so taking the derivative of one of the ψ's wouldn't affect the value of the phase factor, but if it was dependent of x one would need to use the product rule to take the partial of one of the psi's.
 
  • #4
Normalization said:
Oh I see :) Because the phase factor is independent of x it acts as a constant on ψ so taking the derivative of one of the ψ's wouldn't affect the value of the phase factor, but if it was dependent of x one would need to use the product rule to take the partial of one of the psi's.

Right! The phase has to commute with the operator Q if it's not going to change expectation values.
 
  • #5
Cool, thanks :D
 

1. What is the expectation value of a dynamical variable?

The expectation value of a dynamical variable is a mathematical concept used in quantum mechanics to describe the average value of a physical property of a quantum mechanical system. It is often denoted as <A> and is calculated by taking the inner product of the state vector with the operator corresponding to the dynamical variable.

2. How is the expectation value of a dynamical variable related to uncertainty?

The expectation value of a dynamical variable represents the most probable outcome of a measurement of that variable. It is also related to the uncertainty in the system, as described by Heisenberg's uncertainty principle. The larger the uncertainty in a system, the larger the spread of possible expectation values for a given dynamical variable.

3. Can the expectation value of a dynamical variable be negative?

Yes, the expectation value of a dynamical variable can be negative. This can occur when the state vector of the system has a complex phase, which is a common occurrence in quantum mechanics. However, the magnitude of the expectation value is still a measure of the average value of the dynamical variable.

4. How does the expectation value of a dynamical variable change over time?

The expectation value of a dynamical variable is a time-dependent quantity and can change over time as the state of the system evolves. This is described by the Schrödinger equation, which governs the time evolution of quantum mechanical systems.

5. Can the expectation value of a dynamical variable be used to predict the exact outcome of a measurement?

No, the expectation value of a dynamical variable cannot be used to predict the exact outcome of a measurement. It represents the average value of the variable and does not take into account the quantum fluctuations that can occur during a measurement. These fluctuations are inherent to quantum mechanics and cannot be predicted with certainty.

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