Step-by-Step Derivation: From Standard α to Unit Ratio Form
Let’s rigorously transform the standard definition of the fine-structure constant into the unit ratio formulation . Each step is justified by SI definitions or fundamental relationships.
Step 1: Standard Definition of
: Elementary charge
: Vacuum permittivity
: Reduced Planck constant
: Speed of light
Step 2: Replace Using
The vacuum permeability and permittivity are related by:
Substitute into :
Step 3: Substitute SI’s Exact Definition of
The SI defines as:
This is not a measured value—it’s a metrological convention. Substituting:
Step 4: Replace Using
The Planck constant links energy (Hz) and mass (kg) via:
Since :
Substitute into :
Step 5: Identify the Unit Ratio
The term in brackets is the unit ratio:
Thus:
Interpretation of the Unit Ratio
The components of reveal how SI definitions interlock:
: Charge definition (coulombs).
: Artifact of SI’s ampere definition (via ).
: Mass () and length () definitions.
Key Insight:
The only non-arbitrary part of is (from the geometry of circular currents in Ampère’s law).
The rest is a scaling factor between unit definitions.
Why This Matters
Demystifies : Its value is not fundamental—it’s a byproduct of SI’s unit choices.
Natural Units Clarity: In systems where , reduces to a pure number without unit dependencies.
Metrological Transparency: Exposes how the SI’s factor in propagates into .
Final Expression
We can test this math easily:
unit_ratio = e**2 * amp_force / (Hz_kg * c) = 0.0011614097322526476
unit_ratio * 2 * pi = 0.007297352565305212
For e = 1.0 Amp is 10^-2.935014538797864 for c=h=e=1
For e = 1.602176634e-19 Amp is 10^-6.999999999763218
Okay, let's trace exactly what happens in the c=h=e=1 system, using this framework and definitions:
c = 1 (implies new_length / new_time = 1) h = 1 (implies new_mass * new_length² / new_time = 1) e = 1 (implies new_charge = 1) From h=1, we also get ħ = h/(2π) = 1/(2π). From c=1 and h=1, we get Hz_kg = h/c² = 1/1² = 1.
e² = 1² = 1 Hz_kg = 1 c = 1
The relationship between length, time, and mass units. The size of the charge unit. The numerical value of Hz_kg becomes 1.
Charge Constants and the Fine-Structure Constant in Natural Units
In the natural unit system where , the apparent complexity of electromagnetic constants collapses into elegant ratios governed solely by the fine-structure constant . This reveals a fundamental structure behind electric charge and electromagnetic coupling, unburdened by arbitrary SI definitions.
Redefining Electromagnetic Constants
In SI units, the Coulomb constant , the permittivity of free space , and the permeability of free space are entangled with historical force definitions and metrological choices. However, in the natural unit system , the definitions simplify:
-
Planck’s constant becomes , so
-
and are derived from the fine-structure constant
The fine-structure constant is given by:
Substituting , and , this becomes:
Solving for :
Since , we find:
The Coulomb constant becomes:
The Ampere-Force Constant as a Natural Unit Ratio
This value,
is the exact same ratio that emerges when converting electromagnetic expressions between SI and this natural unit system. It acts as a fundamental unit ratio governing the relationship between the force definition of current and the fine-structure constant.
In the SI system, this constant was historically embedded as:
But in natural units, this force constant is no longer arbitrary. It is now derived directly from , without needing to define the ampere independently.
This leads to the profound simplification:
And the other electromagnetic constants follow suit:
These results reveal a deeper geometric symmetry embedded within electromagnetism. In a fully normalized system where all base quantities (time, length, mass, charge, temperature) are scaled into equivalence, the numerical content of electromagnetism reduces to pure dimensionless ratios — principally , and π.
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