[david927]

That's a nice explanation but doesn't it give the impression that if we could find a better way to do that experiment, we could find a way around the problem, when instead it's a fundamental limit on what we can know about a quantum system?

[wodenokoto]

But isn't the reason why it is a fundamental problem, that fundamentally there is nothing smaller to throw?

[jacobmoe]

No, under most interpretations of QM, things literally behave differently at that scale. Under Copenhagen, the wave literally collapses into a fixed position/momentum. The pre measurement wave isn't a statement of our ignorance of the system but rather a description of reality. The many worlds is even more serious in its quantum literalism. Far from pushing around the subject of your experiment with a too-big measuring device, you're actually branching worlds where all predictions of the wave function occur.

[exoesquitur]

To me, many worlds + time (as an inviolate observed vector) being merely a consequence of our inability to observe without moving foreward in time based on our entropic process driven cociousness, seems by far the most comprehensive explanation of observable phenomenon.

That observational uncertainty increases as the probability of direct interaction decreases (distance, time) strongly supports the hypothesis that observable phenomena are dictated strongly by the presentation and characteristic relationship of the observer to the phenomenon.

We know on the micro scale that all possible states exist simultaneously.

It seems logical, even axiomatic then that on the macro scale the same applies, but that we can only observe the bandwidth of states in which it is possible for us to exist to make the observation.

To claim that this state uncertainty is magically resolved in all cases and coherently for all possible observers into a single set of states seems an extraordinary claim requiring extraordinary evidence.

[scotty79]

> The pre measurement wave isn't a statement of our ignorance of the system but rather a description of reality.

Post measurement particle is description of our ignorance not a description of reality.

It still evolves according to Schrödinger equation (which degrades to newtonian dynamics for sharp and narrow waves) but for historical reasons we choose to talk about it as it was little billiard ball, not still a wave just sharpened and narrowed down by intraction we call measurement.

[andreareina]

The problem is that fundamentally, there is a fixed amount of information that there is, that has to be distributed over two dimensions. Particles that are constrained to a small area (e.g. photons going through a slit, electrons bound to an atom) simply do not have a well-defined momentum. In fact the effect is something that you can experience with a sharp enough camera lens: as you close the aperture (therefore forcing the light going through it to be in a specific place) you slowly lose resolving power as the light stops behaving nicely and diffracts around/through the aperture.

[gus_massa]

I agree with both. This explanation is easier to understand, but it makes it look like a technological problem that can be solved, instead of a fundamental property of the universe.

[atomack]

I think david927's intuition is more correct here. The uncertainty in the position and momentum is intrinsic to quantum mechanics - it's built into the 'wave function'.

The suggestion that if one pushes away something by throwing something else builds on a purely classical intuition and wouldn't require quantum mechanics to explain if this was all we observed. The uncertainty in quantum mechanics is fundamental (to quantum mechanics) and emerges through a different, as yet unknown, mechanism.

[pdkl95]

3Blue1Brown has an extremely good explanation[1] of the intrinsic uncertainty, and why it's separate from measurement uncertainty. (the previous episode[2] is a recommended prerequisite for background on how the Fourier Transform works)

> emerges through a different, as yet unknown, mechanism.

In 3Blue1Bron's explanation[1], he shows how the intrinsic uncertainty is an inherent trade-off of trying to measure both position and frequency. A short wave packet only a few wavelengths long correlates with a narrow (precise) range of positions, but also correlates well with a very wide range of frequencies due. A Heisenberg-like uncertainty exists any time you are working with weave packets with length near the wavelength. 3Blue1Brown gives a very good example using Doppler radar.

[1] https://www.youtube.com/watch?v=MBnnXbOM5S4

[2] https://www.youtube.com/watch?v=spUNpyF58BY

[atomack]

Yes, I like these sources too. Good for building intuition. I would just add that Heisenberg-like here means that both systems share features of wave mechanics. Doppler type effects aren't quantum mechanical though.

When I suggest the mechanism is unknown, I mean that Heisenberg uncertainty is a postulate of quantum mechanics. In other words the fundamental reason that quantum mechanics should appeal to wave mechanics isn't really established - we don't really know yet the fundamental objects and interactions that lead to quantum mechanics (despite much effort).

[gus_massa]

I'm not sure about the historical part, but now the uncertainty principle is not an independent postulate. It's deduced form the non commutation of the operations to measure the position and the momentum of a particle. This can be done in the wave representation or in the matrix representation.

Moreover, similar calculations can be done with other measurements that don't conmute. One that is very important is the spin of a particle in the x, y, and z axis.

Another is the polarization of a photon in directions that are at 45°. For example, most of (all?) the experiments of the EPR paradox are done with polarization instead of position-momentum, because polarization is much easier to measure. https://en.wikipedia.org/wiki/EPR_paradox

[atomack]

It's a good point that uncertainty relations exist for all kinds of physical observables. But whether they're expressed as commutation relations or as in Heisenberg's original formulation, or whatever formulation you choose (wave mechanics, matrix mechanics, dirac representation, qft, or anything else one can think of) it's still asserted, rather than derived from an underlying set of fundamental physical objects and interactions.

[bonoboTP]

Why unknown? Heisenberg's uncertainty principle can be derived mathematically, using a property of the Fourier transform. It has nothing to do with disturbing the system during measurement.

[atomack]

I'd say that's more a mathematical statement than physical derivation. The effort of subjects like string theory is to lay down fundamental objects and interactions from which other theories (quantum mechanics, gravity) emerge. But I don't think there is a final word at the moment of what the fundamental theories than result in quantum mechanics should look like.

[gus_massa]

In string theory, they use quantum (super)strings, not classic strings that somehow simulate quantum particles. It's quantum all the way down.