Contextuality: a lesser-known but equally magical aspect of quantum mechanics | Hacker News Reader

Contextuality: a lesser-known but equally magical aspect of quantum mechanics

quantamagazine.org

50 pointspseudolus10 comments

When I was a grad student in the field, the institute I worked at had an "artist in residence" who made a pretty neat installation that lets you play with the notion of contextuality a bit more "hands on". You were supposed to walk around through an artistic rendition of a quantum computer and by passing near the "qubits" you performed measurements on them. Depending on your path through the installation you performed measurements that might not commute with each other, demonstrating the notion of contextuality. I helped with some simple computer vision necessary to run the interactive exhibit. It was a pretty great outreach event: https://blog.krastanov.org/2018/06/15/yqi-art-installation/

"Contextuality says that properties of particles, such as their position or polarization, exist only within the context of a measurement"

Does this "surprising" and "less known" fact about quantum mechanics follow from the well-known Heisenberg uncertainity principle, which states effectively that - the more precise the position of a particle is determined, the less precise the momentum can be?

They are related: The Heisenberg uncertainty principle is a consequence of a more fundamental fact that "measurement operators do not commute", which is the mathematical way of saying "measuring quantity A can result in resetting the expected result from measuring quantity B" (sorry, it is difficult to say something in normal English without it being imprecise). Contextuality is itself a more general way of thinking about non-commuting observables. If anything, I would say that the Heisenberg principle is a consequence of contextuality (but again, this is a vague handwavy statement).

Isn‘t it strange that all these aspects of non-relativistic quantum mechanics are always tested experimentally using photons, entities which are inherently relativistic?

Quantum mechanics is perfectly compatible with special relativity (quantum electrodynamics). It's general relativity, or rather a quantum theory of gravity, that lacks a complete solution.

The Schrödinger equation is not Lorentz invariant, so standard quantum mechanics is not compatible with relativity. Bells equations and all those stuff is all based on non-relativistic QM.

It is true that we have quantum electrodynamics as a relativistic version of QM. However in QED the number of particles are not fixed and a wave function can therefore not be defined.

What I'm criticizing is that most papers on Bells equations, contextuality, popescu-rohrlich boxes, spooky action on a distance, collapse of the wave function, decoherence simply ignore that QED exists and relativistic effects might play a role in describing the seemingly "magic" effects of QM.

Objective collapse interpretations create a false sense of non-relativistic behavior when there really isn't any. Only the correlations are non-local, and those get set up in advance.

I would say no, because the experiments are effectively independent of the mass and speed of the photons, and tend to focus on correlations between logical yes/no outcomes.

Photons happen to be convenient, but they work just as well with electrons or any other particle.

What's strange about that? It sounds like you're implying something but I don't know what

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