# When does the exchange operator commute with the Hamiltonian

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• aliens123
In summary: H$$This is just a special case of the Euler-Lagrange equation.What about something like the following:Suppose that$$\Psi(r_1, r_2)$$are eigenstates of the Hamiltonian(s)$$H(r_1, r_2).$$Then$$H(r_1, r_2)\Psi(r_1, r_2) = E \Psi(r_1, r_2) $$for some scalar$$E.$$Furthermore, because the labeling is arbitrary at this point,$$H(r_2, r_1)\Psi(r_2
aliens123
I am attaching an image from David J. Griffith's "Introduction to Quantum Mechanics; Second Edition" page 205.

In the scenario described (the Hamiltonian treats the two particles identically) it follows that
$$PH = H, HP = H$$
and so $$HP=PH.$$

My question is: what are the necessary and sufficient conditions to have that $$[P,H]=0?$$ Clearly
$$PH = H, HP = H$$
is sufficient, but is it necessary?

Also, as a bonus question, because the exchange operator is an observable, what would it mean to "measure" this in a laboratory?

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aliens123 said:
In the scenario described (the Hamiltonian treats the two particles identically)

That's not the key point: the key point is that the two particles are identical. In other words, if we have a quantum system consisting of two electrons, for example, they are identical particles, so the Hamiltonian must treat them the same, so the exchange operator must commute with the Hamiltonian.

But if we have a quantum system consisting of an electron and a proton, for example, they are not identical particles so the Hamiltonian will not necessarily treat them the same, so the operator that exchanges them might not commute with the Hamiltonian.

PeterDonis said:
so the Hamiltonian must treat them the same, so the exchange operator must commute with the Hamiltonian.
Why is it the case that if the Hamiltonian treats them the same, then the exchange operator must commute with the Hamiltonian? Can you prove this?

aliens123 said:
Why is it the case that if the Hamiltonian treats them the same, then the exchange operator must commute with the Hamiltonian? Can you prove this?
Can you tell us what you think the Hamiltonian represents?

PeroK said:
Can you tell us what you think the Hamiltonian represents?
Energy

aliens123 said:
Energy
I would have said that it is the operator that represents the total energy of the system.

I was then going to ask how the total energy is affected when you exchange two identical particles?

PeroK said:
I would have said that it is the operator that represents the total energy of the system.

I was then going to ask how the total energy is affected when you exchange two identical particles?
Intuitively speaking I have no trouble with the concept that the energy shouldn't be "affected" by exchanging two identical particles. I guess what I am looking for is a more formal way of demonstrating that the operators must commute; i.e. how could you prove this to a stubborn person with no physical intuition whatsoever based solely on the definitions of the operators.

Furthermore, two operators which are Hermitian and therefore observables commute if and only if we can simultaneously measure one observable then measure the other without "affecting our knowledge" of the previously measured quantity. But what would it mean to make a measure of the exchange operator?

aliens123 said:
Intuitively speaking I have no trouble with the concept that the energy shouldn't be "affected" by exchanging two identical particles. I guess what I am looking for is a more formal way of demonstrating that the operators must commute; i.e. how could you prove this to a stubborn person with no physical intuition whatsoever based solely on the definitions of the operators.

Furthermore, two operators which are Hermitian and therefore observables commute if and only if we can simultaneously measure one observable then measure the other without "affecting our knowledge" of the previously measured quantity. But what would it mean to make a measure of the exchange operator?

An observable is represented by a Hermitian operator, but it's a moot point whether every Hermitian operator represents an observable. The identity operator is Hermitian, for example. In any case, whether the exchange operator represents an observable doesn't affect its properties.

If you have a system involving two identical particles, then it would be an elementary test of a proposed Hamiltonian that it commuted with the exchange operator. If you produced an operator that did not commute with the exchange operator, then that would disqualify it from being the valid Hamiltonian for the system.

That's how I would justify things.

PeroK said:
If you produced an operator that did not commute with the exchange operator, then that would disqualify it from being the valid Hamiltonian for the system.
What about something like the following:

Suppose that $$\Psi(r_1, r_2)$$ are eigenstates of the Hamiltonian(s) $$H(r_1, r_2).$$
Then
$$H(r_1, r_2)\Psi(r_1, r_2) = E \Psi(r_1, r_2)$$
for some scalar $$E.$$
Furthermore, because the labeling is arbitrary at this point,
$$H(r_2, r_1)\Psi(r_2, r_1) = E \Psi(r_2, r_1)$$
for the same scalar $$E.$$

Now suppose that $$H(r_1, r_2) = H(r_2, r_1).$$
Then
$$H(r_1, r_2)\Psi(r_1, r_2) = E \Psi(r_1, r_2)$$
and
$$H(r_2 r_1)\Psi(r_2, r_1) = H(r_1 r_2)\Psi(r_2, r_1) = E \Psi(r_2, r_1)$$
Then, because $$\Psi(r_1, r_2)$$ and $$\Psi(r_2, r_1)$$ have the same eigenvalue under the same Hermitian operator $$H(r_1 r_2)$$ it follows that they are the same vector (we assume they are normalized) up to a phase.

Now
$$PH\Psi = P(E \Psi) = EP\Psi = E\phi \Psi$$
$$HP\Psi = H(\phi \Psi) = \phi H\Psi = \phi E\Psi$$
So
$$[P,H]=0.$$

aliens123 said:
Why is it the case that if the Hamiltonian treats them the same, then the exchange operator must commute with the Hamiltonian?

aliens123 said:
In the scenario described (the Hamiltonian treats the two particles identically) it follows that
$$PH = H, HP = H$$
and so
$$HP = PH$$

PeterDonis said:
I think that what I wrote in the OP was wrong, shouldn't it instead be:
$$PH = \pm H, HP = \pm H$$

Regardless, my question in the OP was "the above is sufficient, but is it necessary?"

I am trying to find an argument which doesn't make reference to physical intuition, and instead relies only on the definitions of the operators. Of course, a system with two identical particles is a physical requirement, but how can we state this as some mathematical condition, and then use that mathematical condition to show what we desire?

aliens123 said:
I think that what I wrote in the OP was wrong, shouldn't it instead be:
$$PH = \pm H, HP = \pm H$$

Strictly speaking, yes. But ##PH = HP## is still valid.

aliens123 said:
Of course, a system with two identical particles is a physical requirement, but how can we state this as some mathematical condition

Griffiths states it, but not very explicitly. He is assuming the standard non-relativistic Hamiltonian for two particles:

$$H = \frac{p_1^2}{2m_1} + \frac{p_2^2}{2m_2} + V(r_1, r_2)$$

Then "identical particles" means, as he says, that ##m_1 = m_2## and ##V(r_1, r_2) = V(r_2, r_1)##, i.e., the potential is symmetric between the two particles. Then the Hamiltonian becomes

$$H = \frac{p_1^2 + p_2^2}{2m} + V(r_1, r_2)$$

It should be easy to see from the above that for any ##\psi##, ##H P \psi = P H \psi##. Or, to put it another way, it doesn't matter whether you switch the ##1## and ##2## subscripts on the ##p##'s and ##r##'s before or after you evaluate ##H##.

PeterDonis said:
Strictly speaking, yes. But ##PH = HP## is still valid.
Griffiths states it, but not very explicitly. He is assuming the standard non-relativistic Hamiltonian for two particles:

$$H = \frac{p_1^2}{2m_1} + \frac{p_2^2}{2m_2} + V(r_1, r_2)$$

Then "identical particles" means, as he says, that ##m_1 = m_2## and ##V(r_1, r_2) = V(r_2, r_1)##, i.e., the potential is symmetric between the two particles. Then the Hamiltonian becomes

$$H = \frac{p_1^2 + p_2^2}{2m} + V(r_1, r_2)$$

It should be easy to see from the above that for any ##\psi##, ##H P \psi = P H \psi##. Or, to put it another way, it doesn't matter whether you switch the ##1## and ##2## subscripts on the ##p##'s and ##r##'s before or after you evaluate ##H##.
Hi, thank you for your replies.

I can "see" intuitively in some sense that for any ##\psi##, ##H P \psi = P H \psi##, but how would you prove this? I already made an attempt in a previous post (which you may or may have not seen), but I am not sure if my argument is correct, or if there is a simpler, easier way. It seems like it should maybe be very easy and obvious.

aliens123 said:
how would you prove this?

By "easy to see" I meant "easy to prove". Try explicitly computing ##H (P \psi)## and ##P (H \psi)##, using the ##H## I previously posted. Doing it for an eigenstate of ##H##, as you did in your earlier post, is sufficient for a proof for all states (can you see why?), and makes the computation easier. (Also note that I wrote ##p## for momentum in my previous post, but ##p## is an operator so you need to use its position representation, which is ##p = - i \hbar \nabla##, so ##p^2 = \hbar^2 \nabla^2##.)

PeroK

## 1. What is the meaning of "commute" in this context?

In quantum mechanics, two operators commute if their order does not affect the outcome of an operation. In other words, the operators can be rearranged without changing the result.

## 2. Why is it important to know when the exchange operator commutes with the Hamiltonian?

The exchange operator represents the exchange of identical particles in a quantum system. Knowing when it commutes with the Hamiltonian is important because it affects the energy levels and properties of the system, which can have significant implications in various fields such as solid state physics and chemistry.

## 3. How can one determine when the exchange operator commutes with the Hamiltonian?

This can be determined by solving the Schrödinger equation for the system and analyzing the resulting wavefunction. If the exchange operator and Hamiltonian commute, the wavefunction will remain unchanged when the particles are exchanged. If they do not commute, the wavefunction will change, indicating that the energy levels and properties of the system are affected by the exchange.

## 4. What are some real-world applications of understanding when the exchange operator commutes with the Hamiltonian?

Some applications include understanding the behavior of electrons in metals and semiconductors, predicting the magnetic properties of materials, and studying the quantum states of atoms and molecules in chemistry.

## 5. Are there any known cases where the exchange operator does not commute with the Hamiltonian?

Yes, there are cases where the exchange operator does not commute with the Hamiltonian, such as in systems with strong interactions between particles or in systems with non-identical particles. In these cases, the exchange of particles can significantly affect the energy levels and properties of the system.

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