Canonical nodes for the fundamental physical constants: c, G, ℏ, k_B, e, ε₀, μ₀, N_A. Other Layer-1 trees reference these via `instance_of` and `depends_on` edges instead of duplicating.
physical-constants
Speed of light in vacuum (c)
The speed at which electromagnetic radiation propagates in vacuum; defined exactly by the 2019 SI as 299 792 458 m/s.
Newtonian gravitational constant (G)
The coupling constant of Newtonian gravity: force between two masses is F = G·m1·m2/r². Currently the least precisely known fundamental…
Reduced Planck constant (ℏ)
Quantum of action (ℏ = h/2π). Defined exactly since the 2019 SI redefinition via h = 6.626 070 15 × 10⁻³⁴ J·s.
Boltzmann constant (k_B)
Bridges macroscopic temperature and microscopic energy: k_B = 1.380 649 × 10⁻²³ J/K (exact by SI definition).
Elementary charge (e)
Electric charge of a proton (and magnitude of electron charge). Defined exactly since 2019 as 1.602 176 634 × 10⁻¹⁹ C.
Vacuum electric permittivity (ε₀)
Constant relating electric field to charge in vacuum. Since 2019 it is a measured quantity derived from c, e, h, and the fine-structure…
Avogadro constant (N_A)
Number of constituent particles per mole. Defined exactly since 2019 as 6.022 140 76 × 10²³ mol⁻¹.
Universal gas constant (R)
The molar ideal-gas constant, equal by definition to N_A · k_B since the 2019 SI redefinition. Appears in the ideal gas law pV = nRT.
Standard gravity (g₀)
Conventional standard acceleration due to gravity at Earth's surface, adopted by the 3rd CGPM in 1901. Exactly 9.80665 m/s² by definition…
2019 SI redefinition (fixed constants)
7 SI units now defined via fixed h, e, k_B, N_A, c, Δν_Cs, K_cd; kilogram via Kibble balance.
Fine structure constant α measurement
α⁻¹ = 137.035999206(11) (Cs recoil 2018); g-2 measurement 137.035999084(21) gives ~2.5σ tension; fundamental constant.
Gravitational constant G
G = 6.67430(15)×10⁻¹¹ m³/(kg s²); 10⁻⁵ precision; torsion-balance and atom-interferometer measurements still show tensions.
21-cm hyperfine constant
ν_HF(H) = 1 420 405 751.768(1) Hz; used in radio astronomy to map neutral H across galaxies.
Planck units (length, mass, time)
ℓ_P = √(ℏG/c³)≈1.6×10⁻³⁵ m; t_P = ℓ_P/c; M_P = √(ℏc/G)≈2.2×10⁻⁸ kg; scale where quantum gravity expected.
Elementary charge via Josephson & QHE
e determined via 2e/h and h/e² metrological standards post-2019 SI; Josephson constant K_J = 483 597.848 GHz/V fixed.
Kibble balance (for kilogram)
Electromagnetic force balance with quantized Hall + Josephson; realizes mass via fixed h; <10⁻⁸ agreement across NMIs.
Cosmological parameter set (Planck)
Planck 2018: H₀=67.4 km/s/Mpc, Ω_m=0.315, Ω_Λ=0.685, σ_8=0.811, n_s=0.965, Ω_b h²=0.02237.
Natural units and conversions
ℏ=c=1 for HEP; ℏ=c=G=1 for gravity (Planck); ℏ=k_B=1 for thermodynamics; conversion factors ≈0.197 GeV·fm.
Time standards: TAI, UTC, TT
TAI from 400 atomic clocks worldwide; UTC = TAI - leap seconds; TT relativistic-corrected for Earth surface.