mr. vodka said:
…it's smart to try and find thought experiments trying to disprove an established result, cause when you fail you will be able to establish where exactly your attempt went wrong, eventually giving more insight into the principle.
My questions are in the spirit of the comment above. I am in the '
early' stages of trying to better understand some of maths of QM/QFT, starting with Dirac's notation and linear vector spaces. As such, I am currently watching Professor V Balakrishnan video series of 31 lectures, although I am only at lecture-6. However, before diving into the abstraction of linear vector space, lecture 2-5, he starts with HUP in
lecture-1, so I had started to think about some of the surrounding issues. As a slight aside, I also found the following
Mathpages reference very helpful in understanding Dirac notation in terms of matrix vectors, which underpin both Heisenberg’s and Dirac’s formulation of QM and presumably led Heisenberg to the HUP. At the end of the Mathpages article there is a brief mention of HUP from which the following is taken by way of reference:
“From this it follows that quantum mechanics would entail no fundamental indeterminacy if it were possible to simultaneously diagonalize all the observables. In that case, all observables would commute, and there would be no “Heisenberg uncertainty”. However, we find that it is not possible to simultaneously diagonalize the operators corresponding to every pair of observables. Specifically, if we characterize a physical system in Hamiltonian terms by defining a set of configuration coordinates q1, q2, … and the corresponding momenta p1, p2, …, then we find the following commutation relations”
However, I am not sure how the mathematical premise of the non-commutative relationship between [q] and [p] is rationalised in physicality terms.
[1] q_mp_n – p_nq_m = i \hbar
For example, wouldn’t [q] and [p] be real observables of a system. If so, why does the result have a complex component. How is [1] reconciled with the HUP in [2].
[2] \Delta x \Delta p \geq \frac {\hbar}{2}
While I realize that HUP is often said to be a fundamental characteristic of the quantum realm and not a limitation of experimental measurement, I would like to try to clear up a couple of issues that have always confused me somewhat. It is sometimes stated that the description of the wave-particle duality (WPD) just reflects a limitation of classical language, when applied to the quantum realm. However, might it be argued that it is the classical concept a point-particle, which is the more conceptually difficult to reconcile with QM/QFT. For example, in QFT, is it not more ‘
logical’ to assume some form of wave process associated with the field in order to explain the transport of scalar energy in spacetime. If you follow this train of thought, wouldn’t a wave description have some uncertainty in its exact location [Δx] associated with its wavelength, such that [2] above might also be interpreted as:
[3] p= \hbar \kappa= \frac{h}{\lambda}
[4] \Delta p = \frac {h}{\Delta x}
The equating of \Delta x with \lambda when defining location is explained further below. However, we might first try to describe a photon in these terms, i.e.
[5] p= \frac{h}{\lambda}= \frac{hf}{c}
Now there would appear to be no ambiguity in terms of the frequency of a given photon from which the wavelength can be unambiguously calculated in vacuum using c=f \lambda, as presumably can the momentum [p]. So, in this case, is it is the location of a photon that might be said to be totally ambiguous, not the momentum itself, which we appear to be able to calculate independently of any exact knowledge of location. However, if we now try to describe a deBroglie matter-wave in terms of some localised wave packet, we appear to have no certainty of a single frequency/wavelength within a superposition wave model, such that the relationship below would appear to also contain some degree of uncertainty.
[6] phase velocity=\frac {\omega}{\kappa} = f \lambda
[7] group velocity=\frac {\partial \omega}{\partial \kappa} = \partial f* \partial \lambda
So in the case of an electron, for example, as opposed to a photon, it seems we cannot calculate the momentum with any absolute precision, as in [3], if both the exact frequency and wavelength are somewhat ambiguous within the description of a wave-packet, where the exact location of the particle would also appear to ambiguous, i.e. distributed within the length of the wave packet. Therefore, in this respect, \Delta x might be said to be analogous to the definition of deBroglie’s wavelength, i.e. not exactly certain.
While there may well be a flaw in this logic, it seems that HUP is primarily linked to the uncertainty of location. Therefore, I not sure why QM claims that \Delta x can be zero under any circumstances. Given that theory has to be verified by empirical measurements, how do practical experiments overcome these issues. Thanks