Why is h-bar rather than h used in Planck units?

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    Planck Units
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Discussion Overview

The discussion revolves around the choice of using h-bar (\hbar) instead of Planck's constant (h) in the context of Planck units. Participants explore the implications of this choice in theoretical frameworks and its relevance in various equations.

Discussion Character

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

Main Points Raised

  • Some participants note that the choice of using \hbar instead of h in Planck units may be seen as pragmatic, with various arbitrary choices made in defining units.
  • One participant suggests that \hbar is used more frequently in physics, particularly in contexts such as the momentum operator.
  • Another participant points out that \hbar is necessary for producing the dimensionless fine-structure constant, raising a question about the implications of using h instead.
  • It is mentioned that h/2pi appears in many equations, making \hbar a convenient constant, and that setting it to 1 simplifies expressions further.
  • One participant proposes that the fine-structure constant is one of several dimensionless numbers that can be derived from constants like e and h.
  • A participant introduces the idea of interpreting \hbar as the value of h per oscillation.

Areas of Agreement / Disagreement

Participants express varying viewpoints on the necessity and convenience of using \hbar over h, with no clear consensus reached on the implications of this choice or its broader significance in physics.

Contextual Notes

Some discussions touch on the arbitrary nature of unit choices and the dependence on specific contexts, such as the use of Atomic Units versus Planck units, but these aspects remain unresolved.

diagopod
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Learning a bit more about Planck units, it looks like a number of arguably arbitrary, or at least pragmatic, choices had to be made, regarding using G versus 4piG, using 1/4piEpsiolon0 versus Epsilon0, and so on, but in the reading I don't see any question that h/2pi, rather than h, is the best choice when it comes to h, and was just curious about that.
 
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Planck units are arbitrary, as are any set of units. The reason why \hbar is set to 1 rather than h is simply because you tend to use the former a lot more. (e.g. the momentum operator)

Personally i don't have any need for gravity, but do want a simpler expression for coulomb potentials, so I use Atomic Units instead.
 
alxm said:
The reason why \hbar is set to 1 rather than h is simply because you tend to use the former a lot more. (e.g. the momentum operator)

Thanks for the reply. I did notice that h-bar seems to be necessary to produce the dimensionless fine-structure constant. Without it, we would have to say 2 pi e^2 / hc (in cgs) right or am I missing something there?
 
It just turns out that h/2pi appears in so many equations that hbar is a convenient constant. Then it turns out that 1 is even more convenient.
 
clem said:
It just turns out that h/2pi appears in so many equations that hbar is a convenient constant. Then it turns out that 1 is even more convenient.

Thanks Clem. So in that sense, the fine structure constant is just one of several dimensionless numbers that could be produced by combining numbers lie e,h and so on?
 
Think of it as the value h per oscillation.
 

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