# QM: Arbitrary operators and their eigenstates

• Niles
In summary, the conversation discusses the general solution to the Schrödinger equation and the probability of a wavefunction collapsing to an eigenstate upon measurement. The question is raised about finding the probability of collapsing to an eigenstate of an arbitrary operator. The calculation can be done in any basis, but both the operator and the wavefunction must be expressed in the same basis. The choice of using the Hamiltonian eigenstates as a basis is a matter of convenience, as it simplifies calculations in many problems. In particular, the Hamiltonian is the generator of time translation, making its eigenstates special in terms of time evolution.
Niles

## Homework Statement

Hi all.

When riding home today from school on my bike, I was thinking about some QM. The general solution to the Schrödinger equation for t=0 is given by:

$$\Psi(x,0)=\sum_n c_n\psi_n(x),$$

where $\psi_n(x)$ are the eigenfunctions of the Hamiltonian. We know that the probability of $\Psi(x)$ collapsing to one of the eigenstates is given by the absolute square in front of that particular eigenstate.

Now my question comes: Let's say we have an arbitrary operator Q: $Q\psi_n = q_n\psi_n$, where qn are the eigenvalues. Now what do I do if I want to find the probability of $\Psi(x)$ collapsing to one of the eigenstates of the operator Q upon measurement of the observable Q?

My own attempt is that we write $\Psi(x)$ as a new linear combination of the eigenstates of Q. But to me this seems very difficult.

I hope you can shed some light on this. Thanks in advance.

Niles.

Niles said:
My own attempt is that we write $\Psi(x)$ as a new linear combination of the eigenstates of Q.

That's exactly what you should do.

Great, that's really good.

I have a final question. Let's say we wish to find the expectation value of the operator Q, when our particle is in the state $\Psi$. The expectation value of Q is given by:

$$\left\langle Q \right\rangle = \left\langle \Psi \right|Q\left| \Psi \right\rangle.$$

But does this imply that $\Psi$ must be given in the same basis as the operator Q?

Niles said:
Great, that's really good.

I have a final question. Let's say we wish to find the expectation value of the operator Q, when our particle is in the state $\Psi$. The expectation value of Q is given by:

$$\left\langle Q \right\rangle = \left\langle \Psi \right|Q\left| \Psi \right\rangle.$$

But does this imply that $\Psi$ must be given in the same basis as the operator Q?

This calculation can be done in any basis one desires. The result is independent of the basis used. If the basis is not made of the eigenstates of Q then the representation of Q will not be diagonal, but that does not matter for the final result.

nrqed said:
This calculation can be done in any basis one desires.

But the only demand is that both $\Psi$ and Q are given in the same basis, right?

Niles said:
But the only demand is that both $\Psi$ and Q are given in the same basis, right?

Yes, to carry out the explicit calculation, Q and Psi must be written using the same basis. That's right.

Thanks for all your help, both of you.

I have one final question, which is related to what I wrote in my first post:

Niles said:
The general solution to the Schrödinger equation for t=0 is given by:

$$\Psi(x,0)=\sum_n c_n\psi_n(x),$$

where $\psi_n(x)$ are the eigenfunctions of the Hamiltonian.

Why is it that we choose to write it as a linear combination of the stationary states of the Hamiltonian, and not some other operator? Of course, as we talked about in this thread, we can express it as a linear combination of eigenfunctions of an arbitrary observable operator, but why have we chosen the Hamiltonian as the "default" operator?

Niles said:
I have one final question, which is related to what I wrote in my first post:

Why is it that we choose to write it as a linear combination of the stationary states of the Hamiltonian, and not some other operator? Of course, as we talked about in this thread, we can express it as a linear combination of eigenfunctions of an arbitrary observable operator, but why have we chosen the Hamiltonian as the "default" operator?

The choice of basis is a matter of convenience. For many problems (in atomic or nuclear physics) the use of Hamiltonian eigenfunctions makes calculations simpler than in other bases.

meopemuk said:
The choice of basis is a matter of convenience. For many problems (in atomic or nuclear physics) the use of Hamiltonian eigenfunctions makes calculations simpler than in other bases.

Ahh, I see.

In my book, the expectation value of momentum is defined as:

$$\left\langle p \right\rangle = \int {\psi ^* (x)\left( {\frac{\hbar }{i}\frac{d}{{dx}}} \right)} \,\psi (x)\,dx,$$

where * denotes the complex conjugate. I have written the wavefunction as a function of x to emphasize that it is the wavefunction expressed in the x-basis (i.e. the basis of the position operator).

This must imply (according to our above discussion) that the momentum-operator in the x-basis is given as:

$$\hat p = {\frac{\hbar }{i}\frac{d}{{dx}}}.$$

Questions:

1) Am I correct that it really is the momentum operator given in the x-basis?

2) If yes, then how does one find this result?

I hope I am not annoying you with my questions. Thanks in advance.

Niles.

Niles said:
I have one final question, which is related to what I wrote in my first post:

Why is it that we choose to write it as a linear combination of the stationary states of the Hamiltonian, and not some other operator? Of course, as we talked about in this thread, we can express it as a linear combination of eigenfunctions of an arbitrary observable operator, but why have we chosen the Hamiltonian as the "default" operator?

As meopemuk said, one can use any basis in principle. However, there is something special about using the eigenstates of the Hamiltonian. The hamiltonian is the generator of time translation. What this means is that it is the eigenstates of the Hamiltonian that have simpel time evolutions. They simply evolve as $\psi_n(x) exp(-iE_n t / \hbar)$. Eigenstates of other operators do not have simple time evolutions (unless they commute with the Hamiltonian and hence share common eigenstates). So if we want to know how a wavefunction evolves with time (let's say we are given it at t =0 and want to know what it wil be at a later time), we need to expand it over the eigenstates of the Hamiltonian in order to be able to write down the time dependence.

This is what makes the eigenstates of the Hamiltonian "special" .

xboy said:
1) Yes, absolutely!

2) In Quantum Mechanics, momentum is defined as the generator of translations. An infinitesimal translation shifts the system by a small amount. So we describe the action of an infinitesimal translation operator as : $$\hat T(dx) \psi (x) = \psi (x+dx)$$

Now the generator of any operator $$\hat T$$ can be written as

$$\hat T(dx) = 1-\i \hat K dx$$

Again by Taylor expansion to the first order,
$$\psi(x+dx) = \psi(x) + dx \partial_x \psi(x)$$

So you can see that K is just $$\frac{1 }{i} \frac{d}{{dx}}$$ . The $$\hbar$$ is multiplied to the generator of translator to get the dimensions right for the units we have chosen to work with.

I fixed the TeX

Thanks a lot, nrged [:

I should add that in the second equation, $$\hat K$$ is the generator.

Great, thanks for taking the time to answer all my question to all three of you. It is very nice to get answers and derivations to these questions. I really appreciate it.

## 1. What are arbitrary operators in quantum mechanics?

Arbitrary operators in quantum mechanics are mathematical representations of physical observables, such as position, momentum, energy, and spin. They are represented by linear operators that act on quantum states to produce new states.

## 2. How do you find the eigenstates of an arbitrary operator?

The eigenstates of an arbitrary operator can be found by solving the corresponding eigenvalue equation, where the operator acts on the eigenstate and returns the same state multiplied by a constant. This constant is known as the eigenvalue.

## 3. What is the significance of eigenstates in quantum mechanics?

Eigenstates are important in quantum mechanics because they represent the states in which physical observables have definite values. They also form a complete set of basis states, which means any state can be expressed as a linear combination of eigenstates.

## 4. Can arbitrary operators have degenerate eigenstates?

Yes, it is possible for arbitrary operators to have degenerate eigenstates, meaning multiple eigenstates with the same eigenvalue. This occurs when the operator has symmetries that result in different states having the same observable value.

## 5. How are arbitrary operators related to the uncertainty principle?

Arbitrary operators are related to the uncertainty principle through the commutator relationship. The uncertainty principle states that the more precisely one observable is known, the less precisely the other observable can be known. The commutator of two operators represents the uncertainty or lack of commutation between them.

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