Uncertainty principle violation?

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Discussion Overview

The discussion centers around the potential violation of Heisenberg's uncertainty principle (HUP) through a proposed experimental setup involving a velocity selector and a bubble chamber for measuring the position and momentum of electrons. The scope includes theoretical implications and conceptual clarifications regarding the HUP.

Discussion Character

  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant proposes a scenario where electrons are fired through a velocity selector and then into a bubble chamber, claiming to measure both position and momentum accurately, suggesting a violation of the HUP.
  • Another participant questions the measurement of electric and magnetic fields, arguing that any probe used to measure these fields would alter the fields themselves, thus affecting the measurements.
  • A third participant points out that the HUP is not concerned with single measurements of position and momentum but rather with statistical spreads in repeated measurements, indicating a common misconception about the principle.
  • Another participant emphasizes that the HUP is a quantitative statement and that the proposed scenario does not quantitatively violate it.
  • One participant notes that measuring momentum in one direction and position in others does not inherently violate the HUP, suggesting that the proposed measurements remain within acceptable limits.

Areas of Agreement / Disagreement

Participants express disagreement regarding the interpretation and implications of the HUP in the proposed scenario. There is no consensus on whether the scenario violates the principle, with multiple competing views presented.

Contextual Notes

The discussion highlights limitations in understanding the HUP, particularly regarding the nature of measurements and the statistical interpretation of position and momentum. The scenario's assumptions and the dependence on measurement techniques are also points of contention.

espen180
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I came up with a scenario in which I think I am violating Heisenbergs uncertainty principle.

Say I build a velocity selector, like the ones found in mass spectrometers, and I fire electrons from an electron gun through it. By measuring the Electric and magnetic fields, I can measure the velocities of the exiting electrons very accurately. The selector is in vacuum, so there is no friction slowdown of the electrons. I now let the electrons pass through a bubble chamber, leaving a trail. Assume the bubble chamber does not slow the electrons down. Now I know that at a certain time, an electron had a certain position (from the trail in the bubble chamber) and I know it had a certain velocity (from the velocity selector), so I know its position and its momentum.

Am I not violating Heisenbergs uncertainty principle now? If not, or if there are errors in my scenario, please explain.
 
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HOW do you measure the electric and magnetic fields? The only way I know to measure the field strength at a point is to put some kind of probe there and then the probe itself will alter the field.

That is the whole point of the uncertainty principle. In order to measure anything you must interact with it and that interaction will alter the thing being measured.
 
The more obvious problem here is that the HUP isn't about the measurement of a SINGLE position, and a single momentum. You will note that the expressions for \Delta(x) and \Delta(p) have a statistical spread in values. It means that you can either get a spread in a value after a repeated measurement, or your ability to predict the next value of a measurement.

I think I've illustrated this common misconception of the HUP somewhere in here using the single-slit diffraction.

Zz.
 
And to add yet a third objection, nowhere in this does it actually quantitatively violate the HUP. The HUP is a quantitative statement.
 
Even if you say you did measure the electron's momentum with the velocity selector and it's position with the bubble chamber, you haven't broken any rules yet.

You would have measured momentum in the x... Px
And the track tells you the position in the y and z directions.

Technically, you're still aloud to do that.
 

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