# what is it about position and momentum that forbids knowing both quantities at once?

by jeebs
Tags: forbids, knowing, momentum, position, quantities
P: 41
 Quote by CJames Is there something wrong with the link Fredrik posted earlier? http://www.kevinaylward.co.uk/qm/bal...ation_1970.pdf Page 365 I don't mean this to be sarcastic if it comes across that way.
Thanks! I was skimming this thread at work and didn't notice the link. That's exactly what I was looking for. I was actually amused to find this thread; I had a discussion with my room mate a couple weeks ago about HUP. Both of us are college undergrads, and our experience with quantum mechanics is limited to one semester of physical chemistry apiece (at different universities). I mentioned that I had been taught that HUP applied to sets of measurements of p and r and didn't apply to the position and momentum of a single particle. He replied that since the particle is itself a wave, and a wave has no defined absolute position, then HUP applies to a single particle. I don't have the physics background to respond to that, so I let it be. That paper is clearing up the picture for me. Forgive me for asking again for references, but I don't have the physics education to know off the bat... have there been any revisions to the interpretation of the statistical model in regard to a single particle measurement in the 40 years since this paper was published?
 P: 49 The most intuitive way, imho, to view the uncertainty principle is to recognize that the wavefunction of some system in momentum space is the Fourier transform of the wavefunction of that same system in position space. You can arrive at this by honoring the fact that the momentum operator is the propagator of translation, and that is how you can relate both wavefunctions to each other to arrive at the equation showing they are related by the Fourier transform. Now that we are convinced of this, we can look at the properties of the Fourier transform. If a function K is the Fourier transform of a function F, then the sharper F is, the broader is K. The uncertainty principle has nothing to do with observation. It does not arise due to observation. It is the nature of things that there are non-commuting observables (time and energy, momentum and position) that cannot be determined simultaneously to arbitrary precision. We find this unintuitive because in our universe, Planck's constant is way too small for quantum effects to have become intuitive to our senses, just like special relativisitic effects are also counter-intuitive to our senses since c is so big in our world. Imagine if we lived in a world where c is 30 mph ;)
 P: 3,015 @OP: The canonical commutation relation: $$[\hat{x}_{i}, \hat{p}_{k}] = i \, \hbar \, \delta_{i k}$$ EDIT: How to derive these relations? You need to identify momentum as the generator of translations in space (according to the correspondence principle and the fact that the physical quantity conserved due to homogeneity of space is linear momentum) and it acts on states that are eigenstates of position according to: $$\hat{T}(\mathbf{a}) \, \left|\mathbf{x}\right\rangle = \left|\mathbf{x} + \mathbf{a}\right\rangle, \; \hat{T}(\mathbf{a}) = \exp{\left(-\frac{i}{\hbar} \, \mathbf{a} \cdot \hat{\mathbf{p}}\right)}$$
 P: 343 I heard that these states of momentum and position are orthogonal in Hilbert space. Probably mathematics thinks that it is intuitive, but not us. Because in experiments that can be seen with naked eyes have no violation to classical mechanics which is described by definite phases...
 P: 3,015 $$\begin{array}{l} \hat{x}_{i} \, \hat{T}(\mathbf{a}) \, \left|\mathbf{x}\right\rangle = (x_{i} + a_{i}) \, \left|\mathbf{x} + \mathbf{a}\right\rangle \\ \hat{T}(\mathbf{a}) \, \hat{x}_{i} \, \left|\mathbf{x}\right\rangle = x_{i} \, \left|\mathbf{x} + \mathbf{a}\right\rangle \end{array} \Rightarrow \left[\hat{x}_{i}, \hat{T}(\mathbf{a})\right] = a_{i} \, \hat{T}(\mathbf{a})$$ Expanding to linear power in the translation vector $\mathbf{a}$, we get: $$-\frac{i}{\hbar} \, a_{k} \, \left[\hat{x}_{i}, \hat{p}_{k}\right] = a_{i} = \delta_{i k} \, a_{k}$$ Since this has to hold for arbitrary components $a_{k}$, we must have the commutation relations to hold. After you have a commutation relation between two operators, you can apply the same math that you mentioned in the OP (Cauchy-Schwartz ineqality) to derive an Uncertainty relation.
P: 3,015
 Quote by ZealScience I heard that these states of momentum and position are orthogonal in Hilbert space.
They are not. For example:
$$\langle x | p \rangle = \frac{1}{(2\pi \hbar)^{1/2}} \, \exp{\left(\frac{i}{\hbar} \, p \, x\right)} \neq 0$$
P: 343
 Quote by Dickfore They are not. For example: $$\langle x | p \rangle = \frac{1}{(2\pi \hbar)^{1/2}} \, \exp{\left(\frac{i}{\hbar} \, p \, x\right)} \neq 0$$
Can you specify the eigenvectors of position and momentum? I just started learning it and I am not familiar with it.
P: 3,015
 Quote by ZealScience Can you specify the eigenvectors of position and momentum? I just started learning it and I am not familiar with it.
FFFFFFFFFFFFF

I pressed the page back button on my keyboard and my whole response is gone. I hate this glitch on PF.
P: 343
 Quote by Dickfore FFFFFFFFFFFFF I pressed the page back button on my keyboard and my whole response is gone. I hate this glitch on PF.
Accidents can happen... it is not deterministic~~~
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P: 8,837
 Quote by Dickfore FFFFFFFFFFFFF I pressed the page back button on my keyboard and my whole response is gone. I hate this glitch on PF.
I have found that you can usually get it back if you press the page forward button and answer yes to the question if you want to send the information again.
P: 3,015
 Quote by Fredrik I have found that you can usually get it back if you press the page forward button and answer yes to the question if you want to send the information again.
For me the text gets erased.
P: 7,405
 Quote by Fredrik Good post nkadambi, but I must point out an inaccuracy in what you said. I didn't really understand this myself until recently. It is possible to measure position and momentum simultaneously. In fact, we often measure the momentum by measuring the position and interpreting the result as a momentum measurement. (Check out figure 3 in this pdf).
If I remember correctly, after measuring position, the particle collapses to an eigenstate of position, then spreads out due to time evolution. After measuring momentum, it collapses to an eigenstate of momentum, then remains in the same state since it is an eigenstate of the free particle Hamiltonian. What state does it collapse to after position and momentum are simultaneously measured?
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 Quote by atyy If I remember correctly, after measuring position, the particle collapses to an eigenstate of position, then spreads out due to time evolution. After measuring momentum, it collapses to an eigenstate of momentum, then remains in the same state since it is an eigenstate of the free particle Hamiltonian. What state does it collapse to after position and momentum are simultaneously measured?
In this example, the particle is absorbed by the detector, so no new state is prepared. If it had been the kind of detector that lets the particle pass through it, the new state would have been one with a sharply defined position (the wavefunction will be close to zero far from the detector).

 Quote by atyy After measuring momentum, it collapses to an eigenstate of momentum
This is what I was taught as well, but I wonder if it really makes sense. Even if we disregard all the difficulties associated with the fact that an eigenfunction of $-i\,\nabla$ isn't square-integrable, and interpret the statement as "after a momentum measurement, the wavefunction will have a sharply defined momentum", it still looks false to me. I don't know much about how things are measured, but it seems to me that momentum is always measured by measuring the position instead. It's possible that in most cases, the position measurement is inexact enough to give us something like exp(-x2) as the new wavefunction, so that both the wavefunction and its Fourier transform have a single peak, with widths of the same order of magnitude in units such that $\hbar=1$.
P: 7,405
 Quote by Fredrik This is what I was taught as well, but I wonder if it really makes sense. Even if we disregard all the difficulties associated with the fact that an eigenfunction of $-i\,\nabla$ isn't square-integrable, and interpret the statement as "after a momentum measurement, the wavefunction will have a sharply defined momentum", it still looks false to me. I don't know much about how things are measured, but it seems to me that momentum is always measured by measuring the position instead. It's possible that in most cases, the position measurement is inexact enough to give us something like exp(-x2) as the new wavefunction, so that both the wavefunction and its Fourier transform have a single peak, with widths of the same order of magnitude in units such that $\hbar=1$.
I haven't really thought this through myself, and am just going to ask questions as they come to mind (but OP please stop me if this is hijacking!). How about in the relativistic case, say like Compton scattering? Isn't momentum there measured without measuring position? OTOH, there is no "sensible" position operator in relativistic QFT, so maybe that's different?
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 Quote by ZealScience Can you specify the eigenvectors of position and momentum? I just started learning it and I am not familiar with it.
Neither of those operators have eigenvectors in the semi-inner product space of square-integrable functions from $\mathbb R$ into $\mathbb C$. If we view this space as a subspace of the vector space of all functions from $\mathbb R$ into $\mathbb C$, then the functions $u_p$ defined by $u_p(x)=e^{ipx}$ for all x are eigenfunctions (with eigenvalue p) of the momentum operator. Note that they are not square-integrable. The position operator doesn't have any eigenfunctions in this space either, but there's a trick. The "functions" $v_x$ such that $v_x(y)=\delta(y-x)$ for all y, where $\delta$ is the Dirac delta, can be thought of as "eigenfunctions" (with "eigenvalue" x) of the position operator, even though they aren't really functions.
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