Why does measuring the position of a particle change its momentum?

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Discussion Overview

The discussion revolves around the question of why measuring the position of a particle affects its momentum, touching on concepts such as the Heisenberg Uncertainty Principle (HUP) and the implications of measurement in quantum mechanics. The scope includes theoretical interpretations and conceptual clarifications related to measurement effects in quantum physics.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants propose that measuring a particle involves interactions, such as shooting electromagnetic waves, which may change its momentum.
  • Others argue that the HUP is a fundamental limitation of nature and not merely a consequence of measurement techniques.
  • A participant references a Wikipedia article stating that measurement induces a disturbance, questioning whether this disturbance changes momentum.
  • Some participants assert that the notion of measurement clumsiness misleadingly suggests that particles have definite properties prior to measurement, which contradicts the deeper implications of the HUP.
  • One participant suggests that while the measurement effects are a valid interpretation of the HUP, they do not fully explain the inability to measure precise values for both position and momentum.
  • Another participant emphasizes that different interpretations of quantum phenomena exist, and it is challenging to determine which is more correct.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between measurement effects and the HUP, with some emphasizing the fundamental nature of the uncertainty principle and others advocating for the validity of measurement effects as a means of understanding the principle. No consensus is reached regarding the implications of measurement on the HUP.

Contextual Notes

Participants highlight the complexity of the relationship between measurement and quantum states, noting that assumptions about definite properties before measurement may not hold true. The discussion reflects ongoing debates about interpretations of quantum mechanics and the implications of the HUP.

k9b4
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Why does measuring the position of a particle change its momentum?

Is it because 'measuring' involves shooting EM waves or other particles at the particle, which changes its momentum?
 
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k9b4 said:
Why does measuring the position of a particle change its momentum?

Is it because 'measuring' involves shooting EM waves or other particles at the particle, which changes its momentum?
The HUP (Heisenberg Uncertainty Principle), which is what I assume you are asking about, has nothing to do with how things are measured, it is an expression of a fundamental limitation of nature.

You'll find lots of threads here on this forum about the HUP so I suggest a forum search.
 
In this wikipedia article http://en.wikipedia.org/wiki/Uncertainty_principle, it states:

"This ascribes the uncertainty in the measurable quantities to the jolt-like disturbance triggered by the act of observation."

Does that mean that by measuring the particle (ie shooting EM waves at it) changes its momentum?
 
k9b4 said:
In this wikipedia article http://en.wikipedia.org/wiki/Uncertainty_principle, it states:

"This ascribes the uncertainty in the measurable quantities to the jolt-like disturbance triggered by the act of observation."

Does that mean that by measuring the particle (ie shooting EM waves at it) changes its momentum?
It may, and it seems likely but I say again, this has NOTHING to do with the HUP. Did you bother to read the sentence directly after that one "Though widely repeated in textbooks, this physical argument is now known to be fundamentally misleading.[4][5]"
 
phinds said:
It may, and it seems likely but I say again, this has NOTHING to do with the HUP. Did you bother to read the sentence directly after that one "Though widely repeated in textbooks, this physical argument is now known to be fundamentally misleading.[4][5]"
Why is it fundamentally misleading?
 
k9b4 said:
Why is it fundamentally misleading?

It is misleading because it suggests that the particle has both a definite position and a definite momentum before we measure either, and it's just the unavoidable clumsiness of the measurement process that prevents us from discovering both.

In fact, the uncertainty principle is deeper than just unavoidable clumsiness of measurement. Any state in which the position is definite is necessarily a state in which the momentum is a superposition, and vice versa.
 
Nugatory said:
It is misleading because it suggests that the particle has both a definite position and a definite momentum before we measure either, and it's just the unavoidable clumsiness of the measurement process that prevents us from discovering both.

In fact, the uncertainty principle is deeper than just unavoidable clumsiness of measurement. Any state in which the position is definite is necessarily a state in which the momentum is a superposition, and vice versa.

Whilst this is undoubtably true, it is nice that a very understandable argument takes you to the same place. I would say that it's not a bad way into the topic of HUP, bearing in mind that it follows the historical pathway to it. Once the practical measurement idea has been accepted - and assuming that the newcomer can actually handle the more formal reasoning - then one can move forward. When you think of the number of times that people ask (nay, plead!) for a "physical interpretation" on PF, it seems reasonable to indulge those people a bit, along with the caveat that there's something more to it.

The result from the HUP is not unlike the result from the Pauli Exclusion Principle. You can work with both of them without losing sleep about the deeper meaning. (That's an Engineer speaking, of course!)
 
I don't know why it has become unfashionable to talk about measurement effects when talking about Heisenberg Uncertainty Principle. The measurement effects are a legitimate way of picturing HUP, and it's not wrong. There are different "pictures" of quantum mechanical phenomena, and it's hard to say one is more correct than the other. The HUP says that fundamentally the particle does not possesses a precise value for both noncommuting observables (eg momentum and position). But that doesn't explain why we can't measure the precise values.That's where the measurement effect comes in. This way we can answer both the why and the how.
 

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