Momentum of a stationary particle/wave?

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Discussion Overview

The discussion revolves around the implications of Heisenberg's uncertainty principle in the context of a stationary particle or wave, particularly focusing on the paradox of measuring position with high precision and its effect on momentum. Participants explore theoretical scenarios involving quantum mechanics, such as infinite potential wells and the behavior of particles within confined spaces.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that measuring a particle's position very precisely leads to an infinite uncertainty in momentum, raising questions about how a stationary particle can possess momentum.
  • Others argue that the uncertainty principle applies only at fixed moments in time, suggesting that the particle's position will be uncertain in subsequent measurements.
  • A participant challenges the assumption that a particle is stationary, emphasizing that measuring position does not imply the particle is not moving.
  • Another participant discusses the implications of trapping a particle in an infinite potential well, noting that as the well's size decreases, the energy density increases, complicating the relationship between position and momentum.
  • Some participants highlight that the state of the particle is likely a superposition of energy eigenstates rather than a single eigenstate, which affects momentum definitions.
  • Concerns are raised about the physicality of the infinite square well model, with some participants suggesting that it leads to unphysical scenarios that do not provide meaningful insights.
  • There is a discussion about the limitations of defining momentum observables in certain potential scenarios, with references to the harmonic oscillator as a more physically relevant model.
  • Participants express differing views on the interpretation of stationary states and their implications for particle localization, with some questioning the validity of the collapse interpretation in quantum mechanics.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the implications of the uncertainty principle, the nature of stationary states, and the physical relevance of theoretical models like the infinite square well. The discussion remains unresolved, with no consensus reached on the interpretations presented.

Contextual Notes

Limitations include the dependence on specific definitions of stationary states and the challenges in applying theoretical models to real-world scenarios. The discussion highlights the complexities involved in quantum mechanics and the need for careful consideration of assumptions in theoretical frameworks.

  • #31
bob012345 said:
If the particle has no momentum until it's measured then the question I've always had is what does that mean ?
It means that it has no momentum the same way that I have no lap when I'm standing up and no fist when my hand is open with fingers outstretched.
and must it involve a sentient being doing the measurement?
No. An inanimate device whose output no one ever looks at is quite sufficient.
 
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  • #32
Nugatory said:
It means that it has no momentum the same way that I have no lap when I'm standing up and no fist when my hand is open with fingers outstretched.No. An inanimate device whose output no one ever looks at is quite sufficient.
If an inanimate device is sufficient, then that device could conceivably be any other particle since any interaction might be considered a "measurement". Then, everything would be in a definite state between "measurements" which in the real world continuously happen.
 
  • #33
Nugatory said:
It means that it has no momentum the same way that I have no lap when I'm standing up and no fist when my hand is open with fingers outstretched.
What exactly do you mean by this? Do you mean that fundamentally quantum objects don't have an energy/momentum/position unless forced to adopt one by interaction with a classical object?
 
  • #34
bob012345 said:
?...since any interaction might be considered a "measurement"...
Not "any" interaction, but "any thermodynamically irreversible" interaction. Yes, and that's why Schrödinger's cat isn't in a superposition of dead and alive before we open the box - the cat is continuously interacting irreversibly with its environment, and this is generally the case for all the macroscopic objects around us. The phenomenon is called "decoherence", and it explains why the macroscopic world generally behaves classically. The particle in a box is a different situation altogether; it is interacting only with the square-well potential, reversibly.

You can google for "quantum decoherence", although the math might be a bit daunting. David Lindley's book "Where does the weirdness go" is a layman-friendly overview worth reading if you don't want to take on the math.
 
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  • #35
DarMM said:
What exactly do you mean by this? Do you mean that fundamentally quantum objects don't have an energy/momentum/position unless forced to adopt one by interaction with a classical object?
Not quite... but the context for the quote goes back to #22 of the thread.
 
  • #36
Nugatory said:
Not quite... but the context for the quote goes back to #22 of the thread.
I'm still not quite sure what you mean, is it simply that the particle has no position or momentum it just imprints one on the device?
 
  • #37
Nugatory said:
Not "any" interaction, but "any thermodynamically irreversible" interaction. Yes, and that's why Schrödinger's cat isn't in a superposition of dead and alive before we open the box - the cat is continuously interacting irreversibly with its environment, and this is generally the case for all the macroscopic objects around us. The phenomenon is called "decoherence", and it explains why the macroscopic world generally behaves classically. The particle in a box is a different situation altogether; it is interacting only with the square-well potential, reversibly.

You can google for "quantum decoherence", although the math might be a bit daunting. David Lindley's book "Where does the weirdness go" is a layman-friendly overview worth reading if you don't want to take on the math.
Thanks I've heard of it. It's what keeps my brain from already knowing everything to knowing almost nothing...:)
 

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