Is the Heisenberg Uncertainty Principle Limited to Momentum?

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Discussion Overview

The discussion centers around the Heisenberg Uncertainty Principle (HUP) and whether it is limited to momentum or if it can also apply to other aspects, such as the velocity of light. Participants explore the implications of the principle in the context of light propagation and the predictability of position and speed.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions the application of the HUP to light, suggesting that both the speed and trajectory of light can be predicted simultaneously under certain conditions.
  • Another participant introduces the concept of diffraction, challenging the certainty of predicting a photon's position within a beam of light.
  • Several participants clarify that the HUP specifically addresses momentum rather than velocity, noting that the momentum of a photon is related to its wavelength, which introduces uncertainty.
  • It is mentioned that no light source is completely monochromatic, implying that there is always some uncertainty in the wavelength, and thus in momentum.
  • One participant elaborates that momentum has two components, with one being constant and known, while the other exhibits uncertainty as per the HUP.
  • There is a suggestion that despite various approaches to the topic, the HUP remains intact.

Areas of Agreement / Disagreement

Participants generally agree that the HUP pertains to momentum rather than velocity. However, there is disagreement regarding the implications of this principle on the predictability of light's behavior, particularly in relation to its trajectory and position.

Contextual Notes

Some assumptions about the behavior of light and the definitions of momentum and velocity may not be fully articulated, leading to potential misunderstandings in the discussion.

member 11137
Sorry for this first year level question but something is really not clear in my head concerning the Heisenberg's uncertainty principle. First remark: my question here is neither a critic of this principle nor a tentative to collapse it. No; I consider the propagation of the light in the air or in vacuum. I stay at the origin of an inertial frame. I know via experiments two things: 1) the speed of light with a height precision (c); 2) the trajectory of the light: it is automatically going straightforward as long as the beam does not encounter a mirror, a prism, a big concentration of matter, ... So: I can predict the position and the speed simultaneously ! What is wrong in this manner to present the reality ? Thanks for the help because I get some panic with this. Must I think that the trajectory only is an average one? I know: it must be unpleasant to always repeat the same things ...
 
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Do you know diffraction? If yes how can you predict with certainty the position of a photon within a beam of light?

Seratend.
 
The uncertainty principle deals with momentum, not velocity. The momentum of a photon is related to its wavelength by [itex]p = h / \lambda[/itex]. So, even though the speed of a light beam is exactly known (in vacuum), its momentum can still be uncertain. This corresponds to an uncertainty in the wavelength. No light source is completely monochromatic. Even a laser has a small but finite spread in wavelengths.
 
jtbell said:
The uncertainty principle deals with momentum, not velocity.
Of course ! Oh I am so sorry: I think I need holydays... Thanks for the rapid answer; to seratend too.
 
jtbell said:
The uncertainty principle deals with momentum, not velocity. The momentum of a photon is related to its wavelength by [itex]p = h / \lambda[/itex]. So, even though the speed of a light beam is exactly known (in vacuum), its momentum can still be uncertain. This corresponds to an uncertainty in the wavelength. No light source is completely monochromatic. Even a laser has a small but finite spread in wavelengths.

Just to add to this excellent answer...

Momentum commutes with position within the context of the HUP. Momentum, of course, having 2 components, one of which is the velocity which can be considered constant and known. The other component therefore exhibits the expected uncertainty as described above.

Funny that no matter how you approach it, the HUP alway comes out intact.
 

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