Laser Light & HUP: Understanding How Position & Momentum Affect Each Other

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Discussion Overview

The discussion revolves around the implications of the Heisenberg Uncertainty Principle (HUP) in the context of laser light passing through a small aperture. Participants explore how position and momentum are affected by this process, questioning the precision of momentum magnitude versus direction.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant asserts that a laser beam spreads after passing through a small hole due to the HUP, suggesting that measuring position accurately results in a loss of information about momentum, particularly its direction.
  • Another participant questions whether passing light through an aperture constitutes confinement as required by the HUP, proposing that "measuring" might be a more appropriate term.
  • It is noted that the HUP applies to each spatial dimension separately, raising questions about how this relates to the spreading of the beam after passing through the aperture.
  • A participant expresses a belief that while the direction of the momentum vector may be uncertain, the magnitude remains precise due to the conservation of energy, although they express uncertainty about this claim.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the implications of the HUP in this context. Multiple competing views remain regarding the effects of the aperture on the momentum of the laser light.

Contextual Notes

There are unresolved questions about the definitions of "confining" and "measuring" in relation to the HUP, as well as the specific conditions under which the momentum magnitude may or may not remain precise.

LarryS
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A beam of light from a laser projected on a screen with a very small hole in it will spread after it traverses the pin hole because of the HUP: Measuring position so accurately causes loss of information about the momentum. I believe that in the case of laser light, only information about the momentum vector’s direction is lost. The magnitude of the photon’s momentum vector is still precise because the photon’s energy is still precise.

If that is correct, then why is it correct? In general, there is nothing in the HUP that allows the momentum magnitude to remain precise.
 
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The Heisenberg uncertainty principle would apply if you really confined a system in the x and y directions. Passing it through an aperture is not really the same thing as confining it.
 
Bill_K said:
The Heisenberg uncertainty principle would apply if you really confined a system in the x and y directions. Passing it through an aperture is not really the same thing as confining it.

Maybe "measuring" is a better word than "confining".

It is my understanding that the HUP applies to each dimension, x, y and z, separately.

If passing it through a very small aperature is not confining or measuring it in the, say y dimension, then what accounts for the spreading of the beam after the aperature?
 
referframe said:
A beam of light from a laser projected on a screen with a very small hole in it will spread after it traverses the pin hole because of the HUP: Measuring position so accurately causes loss of information about the momentum. I believe that in the case of laser light, only information about the momentum vector’s direction is lost. The magnitude of the photon’s momentum vector is still precise because the photon’s energy is still precise.

If that is correct, then why is it correct? In general, there is nothing in the HUP that allows the momentum magnitude to remain precise.

I think that's correct. Energy is conserved. Momentum changes direction, both in the direction of the slit and perpendicular to the slit, but the magnitude of the momentum is the same. I'm not completely sure though.
 

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