Merge #811

811: Update to new SI definition and CODATA 2018 r=hgrecco a=Jellby The SI has been redefined, by giving exact values to the Planck, Boltzmann and Avogadro constants, and the elementary charge. There are also new recommended values for other constants (CODATA 2018). I've rewritten the `constants-en.txt` and `default-en.txt` according to these changes and also, I believe, for a better structure. The most significant changes: * All units and constants defined from a few "experimental" values. * For as much as I could find them, all factors are exact, except when noted. Floating point representation can introduce inaccuracies, though. * US and imperial units may have changed their "reference" (to what I considered is most reasonable or authoritative), in practice values stay the same within floating point errors. * Added some units and quantities. * Fixed some units like Nm or au and parsec. * Set the default Btu to the "ISO" Btu, and the default year to the Julian year (as recommended by the IAU) * Defined units of the Gaussian system consistently, which makes them have different dimensionality from the corresponding SI units. Added a Gaussian context to allow conversions (with some caveats). * Removed some units for which I could find no definition or reasonable use: chemical_faraday, physical_faraday, sidereal_hour, sidereal_minute, sidereal_second, work_year, work_month (they can of course be added back, hopefully with a better formal definition). * Added atomic and Planck systems. * Rearranged the order of units to "importance" (kind of) or field, rather than alphabetic. I'm happy to undo some of the changes if they are considered undesirable. Note if/when #809 is accepted, I'd like to apply some minor fixes here. Had to modify a couple of tests: * For some reason (floating point errors, I guess), 4 mM -> mole/liter is no longer equal to 4e-3 M, only "almost equal". * Updated the value of the Boltzmann constant in a test, a difference of 1.45e-5 % seems to be too much. Co-authored-by: Jellby <[email protected]>
hgrecco · May 25, 2019 · b5127a2 · b5127a2
2 parents 7fb8391 + 620e5d1
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diff --git a/AUTHORS b/AUTHORS
@@ -19,6 +19,7 @@ Other contributors, listed alphabetically, are:
 * Felix Hummel <[email protected]>
 * Francisco Couzo <[email protected]>
 * Giel van Schijndel <[email protected]>
+* Ignacio Fdez. Galván <[email protected]>
 * James Rowe <[email protected]>
 * Jim Turner <[email protected]>
 * Joel B. Mohler <[email protected]>

diff --git a/pint/constants_en.txt b/pint/constants_en.txt
@@ -1,53 +1,73 @@
 # Default Pint constants definition file
 # Based on the International System of Units
 # Language: english
-# Source: http://physics.nist.gov/cuu/Constants/Table/allascii.txt
-# :copyright: 2013 by Pint Authors, see AUTHORS for more details.
+# Source: https://physics.nist.gov/cuu/Constants/
+#         https://physics.nist.gov/PhysRefData/XrayTrans/Html/search.html
+# :copyright: 2013,2019 by Pint Authors, see AUTHORS for more details.
 
-ln10 = 2.302585092994046
+#### MATHEMATICAL CONSTANTS ####
+# As computed by Maxima with fpprec:50
 
-speed_of_light = 299792458 * meter / second = c
-standard_gravity = 9.806650 * meter / second ** 2 = g_0 = g_n = gravity
-vacuum_permeability = 4 * pi * 1e-7 * newton / ampere ** 2 = mu_0 = magnetic_constant
-vacuum_permittivity = 1 / (mu_0 * c **2 ) = epsilon_0 = electric_constant
-impedance_of_free_space = mu_0 * c = Z_0 = characteristic_impedance_of_vacuum
+#pi    = 3.1415926535897932384626433832795028841971693993751 = π  # pi
+tansec = 4.8481368111333441675396429478852851658848753880815e-6   # tangent of 1 arc-second ~ arc_second/radian
+ln10   = 2.3025850929940456840179914546843642076011014886288      # natural logarithm of 10
+wien_x = 4.9651142317442763036987591313228939440555849867973      # solution to (x-5)*exp(x)+5 = 0 => x = W(5/exp(5))+5
+wien_u = 2.8214393721220788934031913302944851953458817440731      # solution to (u-3)*exp(u)+3 = 0 => u = W(3/exp(3))+3
 
-# 0.000 000 29 e-34
-planck_constant = 6.62606957e-34 J s = h
-hbar = planck_constant / (2 * pi) = ħ
+#### DEFINED EXACT CONSTANTS ####
 
-# 0.000 80 e-11
-newtonian_constant_of_gravitation = 6.67384e-11 m^3 kg^-1 s^-2
+speed_of_light = 299792458 m/s = c = c_0                      # since 1983
+planck_constant = 6.62607015e-34 J s = h                      # since May 2019
+elementary_charge = 1.602176634e-19 C = e                     # since May 2019
+avogadro_number = 6.02214076e23                               # since May 2019
+boltzmann_constant = 1.380649e-23 J K^-1 = k = k_B            # since May 2019
+standard_gravity = 9.80665 m/s^2 = g_0 = g0 = g_n = gravity   # since 1901
+standard_atmosphere = 1.01325e5 Pa = atm = atmosphere         # since 1954
+conventional_josephson_constant = 4.835979e14 Hz / V = K_J90  # since Jan 1990
+conventional_von_klitzing_constant = 2.5812807e4 ohm = R_K90  # since Jan 1990
 
-# 0.000 000 035 e-19
-# elementary_charge = 1.602176565e-19 C = e
+#### DERIVED EXACT CONSTANTS ####
+# Floating-point conversion may introduce inaccuracies
 
-# 0.000 0075
-molar_gas_constant = 8.3144621 J mol^-1 K^-1 = R
+zeta = c / (cm/s) = ζ
+dirac_constant = h / (2 * π) = ħ = hbar = atomic_unit_of_action = a_u_action
+avogadro_constant = avogadro_number * mol^-1 = N_A
+molar_gas_constant = k * N_A = R
+faraday_constant = e * N_A
+conductance_quantum = 2 * e ** 2 / h = G_0
+magnetic_flux_quantum = h / (2 * e) = Φ_0 = Phi_0
+josephson_constant = 2 * e / h = K_J
+von_klitzing_constant = h / e ** 2 = R_K
+stefan_boltzmann_constant = 2 / 15 * π ** 5 * k ** 4 / (h ** 3 * c ** 2) = σ = sigma
+first_radiation_constant = 2 * π * h * c ** 2 = c_1
+second_radiation_constant = h * c / k = c_2
+wien_wavelength_displacement_law_constant = h * c / (k * wien_x)
+wien_frequency_displacement_law_constant = wien_u * k / h
 
-# 0.000 000 0024 e-3
-fine_structure_constant = 7.2973525698e-3
+#### MEASURED CONSTANTS ####
+# Recommended CODATA-2018 values
+# To some extent, what is measured and what is derived is a bit arbitrary.
+# The choice of measured constants is based on convenience and on available uncertainty.
+# The uncertainty in the last significant digits is given in parentheses as a comment.
 
-# 0.000 000 27 e23
-avogadro_number = 6.02214129e23 mol^-1 = N_A
+newtonian_constant_of_gravitation = 6.67408e-11 m^3/(kg s^2) = _ = gravitational_constant  # (15)
+rydberg_constant = 1.0973731568160e7 * m^-1 = R_∞ = R_inf                                  # (21)
+electron_g_factor = -2.00231930436256 = g_e                                                # (35)
+atomic_mass_constant = 1.66053906660e-27 kg = m_u                                          # (50)
+electron_mass = 9.1093837015e-31 kg = m_e = atomic_unit_of_mass = a_u_mass                 # (28)
+proton_mass = 1.67262192369e-27 kg = m_p                                                   # (51)
+neutron_mass = 1.67492749804e-27 kg = m_n                                                  # (95)
+lattice_spacing_of_Si = 1.920155716e-10 m = d_220                                          # (32)
+K_alpha_Cu_d_220 = 0.80232719                                                              # (22)
+K_alpha_Mo_d_220 = 0.36940604                                                              # (19)
+K_alpha_W_d_220 = 0.108852175                                                              # (98)
 
-# 0.000 0013 e-23
-boltzmann_constant = 1.3806488e-23 J K^-1 = k
+#### DERIVED CONSTANTS ####
 
-# 0.000 021 e-8
-stefan_boltzmann_constant = 5.670373e-8 W m^-2 K^-4 = σ
-
-# 0.000 0053 e10
-wien_frequency_displacement_law_constant = 5.8789254e10 Hz K^-1
-
-# 0.000 055
-rydberg_constant = 10973731.568539 m^-1
-
-# 0.000 000 40 e-31
-electron_mass = 9.10938291e-31 kg = m_e
-
-# 0.000 000 074 e-27
-neutron_mass = 1.674927351e-27 kg = m_n
-
-# 0.000 000 074 e-27
-proton_mass = 1.672621777e-27 kg = m_p
+fine_structure_constant = (2 * h * R_inf / (m_e * c)) ** 0.5 = α = alpha
+vacuum_permeability = 2 * α * h / (e ** 2 * c) = µ_0 = mu_0 = mu0 = magnetic_constant
+vacuum_permittivity = e ** 2 / (2 * α * h * c) = ε_0 = epsilon_0 = eps_0 = eps0 = electric_constant
+impedance_of_free_space = 2 * α * h / e ** 2 = Z_0 = characteristic_impedance_of_vacuum
+coulomb_constant = α * hbar * c / e ** 2 = k_C
+classical_electron_radius = α * hbar / (m_e * c) = r_e
+thomson_cross_section = 8 / 3 * π * r_e ** 2 = σ_e = sigma_e