Does Heisenberg apply to a collapsed wavefunction?

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Discussion Overview

The discussion centers around the application of the Heisenberg Uncertainty Principle (HUP) to a collapsed wavefunction, exploring the nature of quantum states and measurements. Participants debate the implications of wave-particle duality, the interpretation of quantum mechanics, and the relationship between measurement and uncertainty in quantum systems.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants argue that HUP is fundamental and applies to wavefunctions, while others suggest it may not apply after a wavefunction collapses.
  • One participant presents an experiment involving a single-slit setup to illustrate their view that HUP can be violated, claiming they can determine both position and momentum more precisely than HUP allows.
  • Another participant counters that knowing when the screen lights up does not provide information about the photon's path through the slit, thus not localizing the photon.
  • Some participants emphasize that HUP is a constraint on the variances of observables and applies to all wavefunctions when considering averages.
  • There is a discussion about the nature of light and whether it should be considered a wave or a particle, with some arguing that quantum mechanics provides a single consistent formulation that does not rely on duality.
  • One participant suggests that confusion arises from traditional interpretations of HUP and recommends a modern statistical approach to understanding it.
  • Clarifications are made regarding the relationship between wavelength, position, and the width of the slit in the context of diffraction and uncertainty.

Areas of Agreement / Disagreement

Participants express differing views on whether HUP applies after wavefunction collapse, with no consensus reached. Some agree on the need for a modern interpretation of HUP, while others maintain traditional views.

Contextual Notes

Participants highlight potential misconceptions regarding the nature of collapsed wavefunctions and the relationship between position and momentum uncertainties. There are unresolved mathematical interpretations and dependencies on definitions related to wavefunctions and measurements.

  • #31
kith said:
"Measurement" is not a purely physical term but is related to how a person acquires knowledge. If you remove the pointer from your amperemeter, you don't have a measurement apparatus anymore although the physical interaction between your system of interest and the amperemeter doesn't change.

This is part of the difficulty with the whole measurement paradigm of what QM is and why its a difficult issue.

Basically in modern times a measurement is replaced with dechoherence which is independent of pointers in measurement apparatus etc. But this is an advanced issue best glossed over for HS students.

Thanks
Bill
 
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  • #32
Hi,

And 'collisions' and 'particle-like-behaviour', are these more or less synonyms?

-> is there particle-like-behaviour that does not involve collisions?

Paul, who is grateful for al the responses.
 
  • #33
T'Pau said:
Hi,

And 'collisions' and 'particle-like-behaviour', are these more or less synonyms?

-> is there particle-like-behaviour that does not involve collisions?

Paul, who is grateful for al the responses.

Tricky, tricky, tricky.

Yes, collisions are particle like behaviour.

However, in LHC type collisions, the rough idea is that one sends in free, non-interacting particles, the particles collide and interact, and then the outgoing particles from the collision are again free, non-interacting particles. The tricky part is that a free particle from a collision is the most unlocalized state, because it is a particle with a definite momentum. Since it has a definite momentum, it must have very undefinite position. But then what about all those particle tracks? Well, although the tracks looks small and localized to us, they are big from the point of view of the uncertainty principle.

http://books.google.com/books?id=CNCHDIobj0IC&vq=particle&source=gbs_navlinks_s
p117: To a particle the beam is the whole universe, and it is big!
 

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