The inequality in the Heisenberg uncertainty relation

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Discussion Overview

The discussion revolves around the Heisenberg Uncertainty Principle (HUP) and the nature of its inequality. Participants explore the mathematical and conceptual foundations of the HUP, questioning why it is presented as an inequality rather than an equality, particularly in relation to wave packets and probability distributions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the reason for the HUP being an inequality, suggesting that the relationship between spatial frequency spectral width and spatial width might imply an equality.
  • Another participant notes that while each state has a variance for observables leading to an equality, the HUP specifies a minimum uncertainty across all states, with some states achieving this minimum, such as coherent states.
  • A participant later expresses a moment of confusion but then retracts their question, indicating a temporary mental block that has since resolved.
  • Some participants mention that wave packets can be calculated where the equality holds, introducing concepts like coherent and squeezed states.
  • There is a discussion about the necessity of Fock spaces for the calculations, with differing opinions on whether Schrödinger wave-mechanics alone is sufficient.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of Fock spaces versus Schrödinger wave-mechanics for discussing the HUP and its implications. There is no consensus on the reasons behind the inequality of the HUP, as various interpretations and explanations are presented.

Contextual Notes

Some assumptions regarding the definitions of states and the mathematical framework used in the discussion remain unresolved, particularly concerning the application of Fock spaces versus Schrödinger wave-mechanics.

Derek P
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I was musing about why the HUP is an inequality. If you analyse a wave packet the spatial frequency spectral width is inversely proportional to the spatial width. So there should be an equality such as Heisenberg's equation 3 in this paper. Has anyone got a simple explanation of where the inequality comes from? I initially thought it was the fact that we square everything to get probability distributions, but I seem to have a mental block so any help would be appreciated.

Should this be a B level question?
 
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Any state has a variance for each observable. So, there is an equality there which you can calculate for any specific state.

The HUP, however, specifies a minimum across all states. Some states, e.g. Coherent states of the harmonic oscillator have an uncertainty equal to the minimum. Most states have an uncertainty greater than this.
 
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Derek P said:
I was musing about why the HUP is an inequality. If you analyse a wave packet the spatial frequency spectral width is inversely proportional to the spatial width. So there should be an equality such as Heisenberg's equation 3 in this paper. Has anyone got a simple explanation of where the inequality comes from? I initially thought it was the fact that we square everything to get probability distributions, but I seem to have a mental block so any help would be appreciated.
Should this be a B level question?
One Indian meal later and mental block has gone. Forget it, thanks.
 
Last edited:
You can also calculate the wave packets, for which the equality sign holds. That's a way to introduce coherent and squeezed states!
 
vanhees71 said:
You can also calculate the wave packets, for which the equality sign holds. That's a way to introduce coherent and squeezed states!
Well, yes, once you have the infrastructure of Fock spaces in place :)
 
You don't need Fock spaces here. Just Schrödinger wave-mechanics is sufficient.
 
vanhees71 said:
You don't need Fock spaces here. Just Schrödinger wave-mechanics is sufficient.
Yes, you're right.
 

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