Is This Equation for Expectation Values Correct?

In summary, the conversation discusses the equation \left\langle {\psi _i |A|\psi _j } \right\rangle = \left\langle {\psi _j |A^\dag |\psi _i } \right\rangle ^* and confirms that it is true. It also mentions that since the equation deals with scalars, the commutative property holds.
  • #1
Niles
1,866
0

Homework Statement


Hi

Say I have the following number:

[tex]
\left\langle {\psi _i |A|\psi _j } \right\rangle
[/tex]

1) First of all, am I correct when saying that

[tex]
\left\langle {\psi _i |A|\psi _j } \right\rangle = \left\langle {\psi _j |A^\dag |\psi _i } \right\rangle ^*
[/tex]

where the asterix denotes complex conjugation and the dagger means Hermitian conjugate?

2) Since what we are dealing with above is just scalars, then I am correct when I say the following is true, right?

[tex]
\left\langle {\psi _j |A^\dag |\psi _i } \right\rangle \left\langle {\psi _i |A|\psi _j } \right\rangle = \left\langle {\psi _i |A|\psi _j } \right\rangle \left\langle {\psi _j |A^\dag |\psi _i } \right\rangle
[/tex]


Niles.
 
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  • #2
Yup.
 
  • #3
Thanks,
 

FAQ: Is This Equation for Expectation Values Correct?

1. What is an expectation value in quantum mechanics?

An expectation value is the average value of a physical quantity in quantum mechanics. It is calculated by taking the sum of the possible outcomes of a measurement, each multiplied by its corresponding probability. It represents the average result that would be obtained if the measurement were repeated multiple times.

2. How is the expectation value calculated in quantum mechanics?

The expectation value is calculated using the equation E = Σ xi P(xi), where xi is the possible outcome of the measurement and P(xi) is the corresponding probability. This equation is based on the principles of quantum mechanics, which state that the measurement of a physical quantity will yield one of its possible eigenvalues with a probability determined by the wave function.

3. What is the significance of expectation values in quantum mechanics?

Expectation values play a crucial role in quantum mechanics as they provide a way to predict the outcome of a measurement. They also allow for the calculation of other important properties, such as uncertainty and variance, which can provide insight into the behavior of a quantum system.

4. How do expectation values differ from classical probabilities?

In classical mechanics, probabilities represent the likelihood of an event occurring, while in quantum mechanics, probabilities are associated with the possible outcomes of a measurement. Additionally, classical probabilities are always between 0 and 1, while quantum probabilities can take on complex values due to the wave nature of particles.

5. Can expectation values be measured directly in experiments?

No, expectation values cannot be measured directly in experiments. They are theoretical values calculated using mathematical equations based on the principles of quantum mechanics. However, they can provide predictions for the results of measurements, which can then be compared to actual experimental data.

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