# HEP System **Namespace:** `mp_units::hep` **Module:** `mp_units.systems` **Header:** `` **Secondary Headers:** - `` - `` - `` ## Dimensions | Name | Symbol | |------|:------:| | `dim_amount_of_substance` | N | | `dim_angle` | α | | `dim_electric_charge` | Q | | `dim_energy` | E | | `dim_length` | L | | `dim_luminous_intensity` | I | | `dim_temperature` | Θ | | `dim_time` | T | ## Quantities | Quantity | Character | Dimension | is_kind | non_negative | Kind of | Parent | Equation | Hierarchy | |----------|:---------:|:---------:|:-------:|:------------:|:-------:|:------:|----------|:---------:| | Q_value | Real | E | — | ✓ | [energy](#energy) | [energy](#energy) | — | [view](../hierarchies/energy_hep.md) | | absorbed_dose | Real | L²T⁻² | ✓ | ✓ | [absorbed_dose](#absorbed_dose) | — | pow<2>([length](#length)) / pow<2>([duration](#duration)) | [view](../hierarchies/absorbed_dose_hep.md) | | activity | Real | T⁻¹ | ✓ | ✓ | [activity](#activity) | — | inverse([duration](#duration)) | [view](../hierarchies/activity_hep.md) | | amount_of_substance | Real | N | ✓ | ✓ | [amount_of_substance](#amount_of_substance) | — | — | [view](../hierarchies/amount_of_substance_hep.md) | | angle | Real | — | — | — | — | — | alias to [angular::angle](#angle) | — | | area | Real | L² | ✓ | ✓ | [area](#area) | — | pow<2>([length](#length)) | [view](../hierarchies/area_hep.md) | | azimuthal_angle | Real | α | — | — | [angular::angle](#angle) | [angular::angle](#angle) | — | [view](../hierarchies/angle.md) | | binding_energy | Real | E | — | ✓ | [energy](#energy) | [energy](#energy) | — | [view](../hierarchies/energy_hep.md) | | center_of_mass_energy | Real | E | — | ✓ | [energy](#energy) | [energy](#energy) | — | [view](../hierarchies/energy_hep.md) | | coordinate_time | Real | T | ✓ | ✓ | [coordinate_time](#coordinate_time) | [duration](#duration) | — | [view](../hierarchies/duration_hep.md) | | cross_section | Real | L² | — | ✓ | [area](#area) | [area](#area) | — | [view](../hierarchies/area_hep.md) | | decay_constant | Real | T⁻¹ | — | ✓ | [frequency](#frequency) | [frequency](#frequency) | — | [view](../hierarchies/frequency_hep.md) | | decay_length | Real | L | — | ✓ | [length](#length) | [length](#length) | — | [view](../hierarchies/length_hep.md) | | dimensionless | Real | 1 | ✓ | — | [dimensionless](core.md#dimensionless) | — | — | — | | displacement | Vector | L | — | — | [length](#length) | [length](#length) | — | [view](../hierarchies/length_hep.md) | | duration | Real | T | ✓ | ✓ | [duration](#duration) | — | — | [view](../hierarchies/duration_hep.md) | | effective_mass | Real | ET²L⁻² | — | ✓ | [mass](#mass) | [mass](#mass) | — | [view](../hierarchies/mass_hep.md) | | electric_capacitance | Real | Q²E⁻¹ | ✓ | ✓ | [electric_capacitance](#electric_capacitance) | — | pow<2>([electric_charge](#electric_charge)) / [energy](#energy) | [view](../hierarchies/electric_capacitance.md) | | electric_charge | Real | Q | ✓ | — | [electric_charge](#electric_charge) | — | — | [view](../hierarchies/electric_charge_hep.md) | | electric_current | Real | QT⁻¹ | ✓ | — | [electric_current](#electric_current) | — | [electric_charge](#electric_charge) / [duration](#duration) | [view](../hierarchies/electric_current_hep.md) | | electric_potential | Real | EQ⁻¹ | ✓ | — | [electric_potential](#electric_potential) | — | [energy](#energy) / [electric_charge](#electric_charge) | [view](../hierarchies/electric_potential_hep.md) | | electric_resistance | Real | ETQ⁻² | ✓ | ✓ | [electric_resistance](#electric_resistance) | — | [energy](#energy) * [duration](#duration) / pow<2>([electric_charge](#electric_charge)) | [view](../hierarchies/electric_resistance.md) | | energy | Real | E | ✓ | ✓ | [energy](#energy) | — | — | [view](../hierarchies/energy_hep.md) | | excitation_energy | Real | E | — | ✓ | [energy](#energy) | [energy](#energy) | — | [view](../hierarchies/energy_hep.md) | | force | Real | EL⁻¹ | ✓ | ✓ | [force](#force) | — | [energy](#energy) / [length](#length) | [view](../hierarchies/force_hep.md) | | frequency | Real | T⁻¹ | ✓ | ✓ | [frequency](#frequency) | — | inverse([duration](#duration)) | [view](../hierarchies/frequency_hep.md) | | half_life | Real | T | — | ✓ | [duration](#duration) | [lifetime](#lifetime) | — | [view](../hierarchies/duration_hep.md) | | height | Real | L | — | ✓ | [length](#length) | [length](#length) | — | [view](../hierarchies/length_hep.md) | | illuminance | Real | α²IL⁻² | ✓ | ✓ | [illuminance](#illuminance) | — | [luminous_flux](#luminous_flux) / pow<2>([length](#length)) | [view](../hierarchies/illuminance.md) | | impact_parameter | Real | L | — | ✓ | [length](#length) | [length](#length) | — | [view](../hierarchies/length_hep.md) | | inductance | Real | ET²Q⁻² | ✓ | ✓ | [inductance](#inductance) | — | pow<2>([duration](#duration)) * [energy](#energy) / pow<2>([electric_charge](#electric_charge)) | [view](../hierarchies/inductance_hep.md) | | interaction_length | Real | L | ✓ | ✓ | [interaction_length](#interaction_length) | [length](#length) | — | [view](../hierarchies/length_hep.md) | | invariant_mass | Real | ET²L⁻² | — | ✓ | [mass](#mass) | [mass](#mass) | — | [view](../hierarchies/mass_hep.md) | | ionization_energy | Real | E | — | ✓ | [energy](#energy) | [energy](#energy) | — | [view](../hierarchies/energy_hep.md) | | kinetic_energy | Real | E | — | ✓ | [energy](#energy) | [total_energy](#total_energy) | — | [view](../hierarchies/energy_hep.md) | | length | Real | L | ✓ | ✓ | [length](#length) | — | — | [view](../hierarchies/length_hep.md) | | lifetime | Real | T | — | ✓ | [duration](#duration) | [duration](#duration) | — | [view](../hierarchies/duration_hep.md) | | lorentz_factor | Real | — | ✓ | — | [lorentz_factor](#lorentz_factor) | [dimensionless](core.md#dimensionless) | — | [view](../hierarchies/dimensionless.md) | | luminous_flux | Real | α²I | ✓ | ✓ | [luminous_flux](#luminous_flux) | — | [luminous_intensity](#luminous_intensity) * [solid_angle](#solid_angle) | [view](../hierarchies/luminous_flux.md) | | luminous_intensity | Real | I | ✓ | ✓ | [luminous_intensity](#luminous_intensity) | — | — | [view](../hierarchies/luminous_intensity_hep.md) | | magnetic_field | Real | ETQ⁻¹L⁻² | ✓ | — | [magnetic_field](#magnetic_field) | — | [duration](#duration) * [energy](#energy) / [electric_charge](#electric_charge) / pow<2>([length](#length)) | [view](../hierarchies/magnetic_field.md) | | magnetic_flux | Real | ETQ⁻¹ | ✓ | — | [magnetic_flux](#magnetic_flux) | — | [duration](#duration) * [energy](#energy) / [electric_charge](#electric_charge) | [view](../hierarchies/magnetic_flux_hep.md) | | mass | Real | ET²L⁻² | ✓ | ✓ | [mass](#mass) | — | [energy](#energy) * pow<2>([duration](#duration)) / pow<2>([length](#length)) | [view](../hierarchies/mass_hep.md) | | mean_free_path | Real | L | — | ✓ | [length](#length) | [length](#length) | — | [view](../hierarchies/length_hep.md) | | mean_lifetime | Real | T | — | ✓ | [duration](#duration) | [lifetime](#lifetime) | — | [view](../hierarchies/duration_hep.md) | | missing_energy | Real | E | — | ✓ | [energy](#energy) | [energy](#energy) | — | [view](../hierarchies/energy_hep.md) | | momentum | Real | ETL⁻¹ | ✓ | ✓ | [momentum](#momentum) | — | [energy](#energy) * [duration](#duration) / [length](#length) | [view](../hierarchies/momentum_hep.md) | | nuclear_interaction_length | Real | L | — | ✓ | [interaction_length](#interaction_length) | [interaction_length](#interaction_length) | — | [view](../hierarchies/length_hep.md) | | number_density | Real | L⁻³ | ✓ | ✓ | [number_density](#number_density) | — | inverse([volume](#volume)) | [view](../hierarchies/number_density.md) | | opening_angle | Real | α | — | — | [angular::angle](#angle) | [angular::angle](#angle) | — | [view](../hierarchies/angle.md) | | path_length | Real | L | — | ✓ | [length](#length) | [length](#length) | — | [view](../hierarchies/length_hep.md) | | phase | Real | — | ✓ | — | [phase](#phase) | [dimensionless](core.md#dimensionless) | — | [view](../hierarchies/dimensionless.md) | | polar_angle | Real | α | — | — | [angular::angle](#angle) | [angular::angle](#angle) | — | [view](../hierarchies/angle.md) | | position_vector | Vector | L | — | — | [length](#length) | [displacement](#displacement) | — | [view](../hierarchies/length_hep.md) | | power | Real | ET⁻¹ | ✓ | ✓ | [power](#power) | — | [energy](#energy) / [duration](#duration) | [view](../hierarchies/power_hep.md) | | pressure | Real | EL⁻³ | ✓ | ✓ | [pressure](#pressure) | — | [energy](#energy) / pow<3>([length](#length)) | [view](../hierarchies/pressure_hep.md) | | proper_time | Real | T | ✓ | ✓ | [proper_time](#proper_time) | [duration](#duration) | — | [view](../hierarchies/duration_hep.md) | | proper_velocity | Vector | LT⁻¹ | ✓ | — | [proper_velocity](#proper_velocity) | — | [displacement](#displacement) / [proper_time](#proper_time) | [view](../hierarchies/proper_velocity.md) | | radiation_length | Real | L | ✓ | ✓ | [radiation_length](#radiation_length) | [length](#length) | — | [view](../hierarchies/length_hep.md) | | radius | Real | L | — | ✓ | [length](#length) | [length](#length) | — | [view](../hierarchies/length_hep.md) | | range | Real | L | — | ✓ | [length](#length) | [length](#length) | — | [view](../hierarchies/length_hep.md) | | reduced_mass | Real | ET²L⁻² | — | ✓ | [mass](#mass) | [mass](#mass) | — | [view](../hierarchies/mass_hep.md) | | relativistic_beta | Real | — | ✓ | — | [relativistic_beta](#relativistic_beta) | [dimensionless](core.md#dimensionless) | — | [view](../hierarchies/dimensionless.md) | | rest_mass | Real | ET²L⁻² | — | ✓ | [mass](#mass) | [mass](#mass) | — | [view](../hierarchies/mass_hep.md) | | rest_mass_energy | Real | E | — | ✓ | [energy](#energy) | [total_energy](#total_energy) | — | [view](../hierarchies/energy_hep.md) | | scattering_angle | Real | α | — | — | [angular::angle](#angle) | [angular::angle](#angle) | — | [view](../hierarchies/angle.md) | | separation_energy | Real | E | — | ✓ | [energy](#energy) | [binding_energy](#binding_energy) | — | [view](../hierarchies/energy_hep.md) | | solid_angle | Real | — | — | — | — | — | alias to [angular::solid_angle](#solid_angle) | — | | speed | Real | LT⁻¹ | ✓ | ✓ | [speed](#speed) | — | [path_length](#path_length) / [duration](#duration) | [view](../hierarchies/speed_hep.md) | | temperature | Real | Θ | ✓ | ✓ | [temperature](#temperature) | — | — | [view](../hierarchies/temperature.md) | | threshold_energy | Real | E | — | ✓ | [energy](#energy) | [energy](#energy) | — | [view](../hierarchies/energy_hep.md) | | time_of_flight | Real | T | — | ✓ | [duration](#duration) | [duration](#duration) | — | [view](../hierarchies/duration_hep.md) | | total_energy | Real | E | — | ✓ | [energy](#energy) | [energy](#energy) | — | [view](../hierarchies/energy_hep.md) | | transverse_energy | Real | E | — | ✓ | [energy](#energy) | [energy](#energy) | — | [view](../hierarchies/energy_hep.md) | | transverse_momentum | Real | ETL⁻¹ | — | ✓ | [momentum](#momentum) | [momentum](#momentum) | — | [view](../hierarchies/momentum_hep.md) | | velocity | Vector | LT⁻¹ | ✓ | — | [velocity](#velocity) | — | [displacement](#displacement) / [duration](#duration) | [view](../hierarchies/velocity.md) | | vertex_position | Real | L | — | ✓ | [length](#length) | [length](#length) | — | [view](../hierarchies/length_hep.md) | | volume | Real | L³ | ✓ | ✓ | [volume](#volume) | — | pow<3>([length](#length)) | [view](../hierarchies/volume_hep.md) | | wavelength | Real | L | — | ✓ | [length](#length) | [length](#length) | — | [view](../hierarchies/length_hep.md) | | width | Real | L | — | ✓ | [length](#length) | [length](#length) | — | [view](../hierarchies/length_hep.md) | ## Units | Unit Name | Symbol | unit_symbol | Definition | |-----------|:------:|:-----------:|------------| | ampere | A | — | [coulomb](#coulomb) / [second](#second) | | angstrom | Å (A) | — | mag_power<10, -10> * [meter](#meter) | | astronomical_unit | au | — | mag<149'597'870'700> * [meter](#meter) | | atmosphere | atm | — | mag<101'325> * [pascal](#pascal) | | bar | bar | — | mag_power<10, 5> * [pascal](#pascal) | | barn | b | `b` | mag_power<10, -28> * square([meter](#meter)) | | becquerel | Bq | `Bq` | [one](core.md#one) / [second](#second)
kind: [activity](#activity) | | candela | cd | — | kind: [luminous_intensity](#luminous_intensity) | | coulomb | C | — | mag<6'241'509'074> * mag_power<10, 9> * [eplus](#eplus) | | curie | Ci | `Ci` | mag_ratio<37, 10> * mag_power<10, 10> * [becquerel](#becquerel) | | degree | — | — | alias to [angular::degree](angular.md#degree) | | electronvolt | eV | `eV` | kind: [energy](#energy) | | eplus | e⁺ (e+) | — | kind: [electric_charge](#electric_charge) | | farad | F | — | [coulomb](#coulomb) / [volt](#volt) | | fermi | — | — | [si::femto](si.md#femto)<[meter](#meter)> | | gauss | G | — | mag_power<10, -4> * [tesla](#tesla) | | gradian | — | — | alias to [angular::gradian](angular.md#gradian) | | gram | g | `g` | mag_ratio<1, 1000> * [joule](#joule) * square([second](#second)) / square([meter](#meter)) | | gray | Gy | — | [joule](#joule) / [si::kilo](si.md#kilo)<[gram](#gram)>
kind: [absorbed_dose](#absorbed_dose) | | halfpi | — | — | mag_ratio<1, 2> * [pi](#pi) | | henry | H | — | [weber](#weber) / [ampere](#ampere) | | hertz | Hz | — | [one](core.md#one) / [second](#second)
kind: [frequency](#frequency) | | joule | J | — | [electronvolt](#electronvolt) * [coulomb](#coulomb) / [eplus](#eplus) | | kelvin | K | — | kind: [temperature](#temperature) | | liter | L | `L` | cubic([si::deci](si.md#deci)<[meter](#meter)>) | | lumen | lm | — | [candela](#candela) * [steradian](#steradian) | | lux | lx | — | [lumen](#lumen) / square([meter](#meter)) | | meter | m | `m` | kind: [length](#length) | | mole | mol | — | kind: [amount_of_substance](#amount_of_substance) | | newton | N | — | [joule](#joule) / [meter](#meter) | | ohm | Ω (ohm) | — | [volt](#volt) / [ampere](#ampere) | | parsec | pc | `pc` | [astronomical_unit](#astronomical_unit) / (mag_ratio<1, 3600> * [degree](#degree)) | | pascal | Pa | — | [newton](#newton) / square([meter](#meter)) | | perCent | — | — | alias to [mp_units::percent](core.md#percent) | | perMillion | — | — | alias to [mp_units::parts_per_million](core.md#parts_per_million) | | perThousand | — | — | alias to [mp_units::per_mille](core.md#per_mille) | | radian | — | — | alias to [angular::radian](angular.md#radian) | | revolution | — | — | alias to [angular::revolution](angular.md#revolution) | | second | s | `s` | kind: [duration](#duration) | | steradian | — | — | alias to [angular::steradian](angular.md#steradian) | | tesla | T | — | [weber](#weber) / square([meter](#meter)) | | twopi | — | — | mag<2> * [pi](#pi) | | volt | V | — | [electronvolt](#electronvolt) / [eplus](#eplus) | | watt | W | — | [joule](#joule) / [second](#second) | | weber | Wb | — | [volt](#volt) * [second](#second) | ## Constants | Name | Symbol | unit_symbol | Definition | |------|:------:|:------------:|------------| | avogadro_constant | N_A | `N_A` | mag_ratio<602'214'076, 100'000'000> * mag_power<10, 23> / [mole](#mole) | | codata2014::atomic_mass_unit | u | — | mag_ratio<9'314'940'954, 10'000'000> * [si::mega](si.md#mega)<[electronvolt](#electronvolt)> / square([speed_of_light_in_vacuum](#speed_of_light_in_vacuum)) | | codata2014::bohr_magneton | μ_B (mu_B) | — | mag_ratio<9'274'009'994, 1'000'000'000> * mag_power<10, -24> * [joule](#joule) / [tesla](#tesla) | | codata2014::bohr_radius | a_0 | — | mag_ratio<52'917'721'067, 10'000'000'000> * mag_power<10, -11> * [meter](#meter) | | codata2014::boltzmann_constant | k_B | — | mag_ratio<86'173'303, 10'000'000> * mag_power<10, -11> * [si::mega](si.md#mega)<[electronvolt](#electronvolt)> / [kelvin](#kelvin) | | codata2014::classical_electron_radius | r_e | — | mag_ratio<28'179'403'227, 10'000'000'000> * mag_power<10, -15> * [meter](#meter) | | codata2014::electron_compton_wavelength | λ_C (lambda_C) | — | mag_ratio<24'263'102'367, 10'000'000'000> * mag_power<10, -12> * [meter](#meter) | | codata2014::electron_mass | m_e | — | mag_ratio<5'109'989'461, 10'000'000'000> * [si::mega](si.md#mega)<[electronvolt](#electronvolt)> / square([speed_of_light_in_vacuum](#speed_of_light_in_vacuum)) | | codata2014::fine_structure_constant | α (alpha) | — | mag_ratio<72'973'525'664, 10'000'000'000> * mag_power<10, -3> * [one](core.md#one) | | codata2014::neutron_mass | m_n | — | mag_ratio<9'395'654'133, 10'000'000> * [si::mega](si.md#mega)<[electronvolt](#electronvolt)> / square([speed_of_light_in_vacuum](#speed_of_light_in_vacuum)) | | codata2014::nuclear_magneton | μ_N (mu_N) | — | mag_ratio<5'050'783'699, 1'000'000'000> * mag_power<10, -27> * [joule](#joule) / [tesla](#tesla) | | codata2014::proton_mass | m_p | — | mag_ratio<9'382'720'813, 10'000'000> * [si::mega](si.md#mega)<[electronvolt](#electronvolt)> / square([speed_of_light_in_vacuum](#speed_of_light_in_vacuum)) | | codata2018::atomic_mass_unit | u | `u` | mag_ratio<93'149'410'242, 100'000'000> * [si::mega](si.md#mega)<[electronvolt](#electronvolt)> / square([speed_of_light_in_vacuum](#speed_of_light_in_vacuum)) | | codata2018::bohr_magneton | μ_B (mu_B) | `mu_B` | mag_ratio<92'740'100'783, 10'000'000'000> * mag_power<10, -24> * [joule](#joule) / [tesla](#tesla) | | codata2018::bohr_radius | a_0 | `a_0` | mag_ratio<529'177'210'903, 100'000'000'000> * mag_power<10, -11> * [meter](#meter) | | codata2018::boltzmann_constant | k_B | `k_B` | mag_ratio<8'617'333'262, 1'000'000'000> * mag_power<10, -11> * [si::mega](si.md#mega)<[electronvolt](#electronvolt)> / [kelvin](#kelvin) | | codata2018::classical_electron_radius | r_e | `r_e` | mag_ratio<28'179'403'262, 10'000'000'000> * mag_power<10, -15> * [meter](#meter) | | codata2018::electron_compton_wavelength | λ_C (lambda_C) | `lambda_C` | mag_ratio<242'631'023'867, 100'000'000'000> * mag_power<10, -12> * [meter](#meter) | | codata2018::electron_mass | m_e | `m_e` | mag_ratio<51'099'895, 100'000'000> * [si::mega](si.md#mega)<[electronvolt](#electronvolt)> / square([speed_of_light_in_vacuum](#speed_of_light_in_vacuum)) | | codata2018::fine_structure_constant | α (alpha) | `alpha` | mag_ratio<72'973'525'693, 10'000'000'000> * mag_power<10, -3> * [one](core.md#one) | | codata2018::neutron_mass | m_n | `m_n` | mag_ratio<93'956'542'052, 100'000'000> * [si::mega](si.md#mega)<[electronvolt](#electronvolt)> / square([speed_of_light_in_vacuum](#speed_of_light_in_vacuum)) | | codata2018::nuclear_magneton | μ_N (mu_N) | `mu_N` | mag_ratio<50'507'837'461, 10'000'000'000> * mag_power<10, -27> * [joule](#joule) / [tesla](#tesla) | | codata2018::proton_mass | m_p | `m_p` | mag_ratio<938'272'08816, 100'000'000> * [si::mega](si.md#mega)<[electronvolt](#electronvolt)> / square([speed_of_light_in_vacuum](#speed_of_light_in_vacuum)) | | codata2022::atomic_mass_unit | u | — | mag_ratio<93'149'410'372, 100'000'000> * [si::mega](si.md#mega)<[electronvolt](#electronvolt)> / square([speed_of_light_in_vacuum](#speed_of_light_in_vacuum)) | | codata2022::bohr_magneton | μ_B (mu_B) | — | mag_ratio<92'740'100'657, 10'000'000'000> * mag_power<10, -24> * [joule](#joule) / [tesla](#tesla) | | codata2022::bohr_radius | a_0 | — | mag_ratio<529'177'210'544, 100'000'000'000> * mag_power<10, -11> * [meter](#meter) | | codata2022::boltzmann_constant | — | — | alias to [codata2018::boltzmann_constant](#codata2018-boltzmann_constant) | | codata2022::classical_electron_radius | r_e | — | mag_ratio<28'179'403'205, 10'000'000'000> * mag_power<10, -15> * [meter](#meter) | | codata2022::electron_compton_wavelength | — | — | alias to [codata2018::electron_compton_wavelength](#codata2018-electron_compton_wavelength) | | codata2022::electron_mass | m_e | — | mag_ratio<51'099'895'069, 100'000'000'000> * [si::mega](si.md#mega)<[electronvolt](#electronvolt)> / square([speed_of_light_in_vacuum](#speed_of_light_in_vacuum)) | | codata2022::fine_structure_constant | α (alpha) | — | mag_ratio<72'973'525'643, 10'000'000'000> * mag_power<10, -3> * [one](core.md#one) | | codata2022::neutron_mass | m_n | — | mag_ratio<93'956'542'194, 100'000'000> * [si::mega](si.md#mega)<[electronvolt](#electronvolt)> / square([speed_of_light_in_vacuum](#speed_of_light_in_vacuum)) | | codata2022::nuclear_magneton | μ_N (mu_N) | — | mag_ratio<50'507'837'393, 10'000'000'000> * mag_power<10, -27> * [joule](#joule) / [tesla](#tesla) | | codata2022::proton_mass | m_p | — | mag_ratio<93'827'208'943, 100'000'000> * [si::mega](si.md#mega)<[electronvolt](#electronvolt)> / square([speed_of_light_in_vacuum](#speed_of_light_in_vacuum)) | | elementary_charge | e | `e` | [eplus](#eplus) | | gas_threshold | gas_threshold | — | mag<10> * [si::milli](si.md#milli)<[gram](#gram)> / cubic([si::centi](si.md#centi)<[meter](#meter)>) | | permeability_of_vacuum | μ₀ (mu_0) | `mu_0` | mag<4> * mag_power<10, -7> * [π](core.md#π) * [henry](#henry) / [meter](#meter) | | pi | — | — | alias to [mp_units::pi](#pi) | | planck_constant | h | `h` | mag_ratio<662'607'015, 100'000'000> * mag_power<10, -34> * [joule](#joule) * [second](#second) | | speed_of_light_in_vacuum | c | `c` | mag<299'792'458> * [meter](#meter) / [second](#second) | | standard_pressure | P_STP | — | [atmosphere](#atmosphere) | | standard_temperature | T_STP | — | mag_ratio<27'315, 100> * [kelvin](#kelvin) | | universe_mean_density | ρ_universe (rho_universe) | — | mag_power<10, -25> * [gram](#gram) / cubic([si::centi](si.md#centi)<[meter](#meter)>) | !!! note "Inline Namespaces" The `mp_units::hep::codata2018` namespace is inline in `mp_units::hep`, making its members directly accessible from the parent namespace.