hydro::units::cgs Namespace Reference

Entities for converting between geometric units where \(G=c=M_{\odot}=1\) and CGS units. More...

Variables
constexpr double	speed_of_light = 29979245800.0
	The speed of light in cm/s (This is exact)
constexpr double	G_Newton = 6.67430e-8
	Newton's gravitational constant as given by https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.93.025010. G is likely to vary between sources, even beyond the reported uncertainty.
constexpr double	mass_unit = G_Newton_times_m_sun / G_Newton
	mass_cgs = mass_geometric * mass_unit Heuristically the mass of the sun in grams
constexpr double	length_unit = G_Newton_times_m_sun / square(speed_of_light)
	length_cgs = length_geometric * length_unit Heuristically, half the schwarzschild radius of the sun in cm
constexpr double	time_unit = length_unit / speed_of_light
	time_cgs = time_geometric * time_unit Heuristically, half the light crossing time of a solar schwarzschild radius in seconds
constexpr double	rest_mass_density_unit
	rho_cgs = rho_geometric * rho_unit Heuristically, the density in g/cm^3 of matter ~2200 times the density of atomic nuclei. Note this is much larger than any density realized in the universe. More...
constexpr double	pressure_unit
	pressure_cgs = pressure_geometric * pressure_unit Heuristically, the quantity above but times the speed of light squared in cgs More...
constexpr double	atomic_mass_unit = 1.66053906660e-24
	The accepted value for the atomic mass unit in g, uncertainty +-30e-10.
constexpr double	neutron_mass = 1.67492749804e-24
	The neutron mass, given in grams, uncertainty at 5.7 x 10^-10 level.
constexpr double	proton_mass = 1.672621898e-24
	The proton mass, given in grams. Uncertainty at 2e-8 level.
constexpr double	electron_volt = 1.602176634e-12
	The electron-volt (eV) given in ergs, which is known exactly in SI/cgs.
constexpr double	gauss_unit = 2.3558985e+19
	CGS unit of Gauss. More...

Detailed Description

Entities for converting between geometric units where \(G=c=M_{\odot}=1\) and CGS units.

Variable Documentation

gauss_unit

constexpr double hydro::units::cgs::gauss_unit = 2.3558985e+19

constexpr

CGS unit of Gauss.

Equals mass_unit^(1/2) * length_unit^(-1/2) * time_unit^(-1).

Conversion rule for electromagnetic fields is

magnetic_field_cgs = magnetic_field_geometric * gauss_unit

or

electric_field_cgs = electric_field_geometric * gauss_unit

Warning

Before changing units using this value, make sure the unit systems you are converting values between are using the same unit convention for electromagnetic variables as well. Gaussian and Heaviside-Lorentz unit convention (in electromagnetism) have a factor of \(\sqrt{4\pi}\) difference in variables. See this Wikipedia page for how to convert between different unit systems in electromagnetism.

Note that ForceFree and grmhd::ValenciaDivClean evolution systems are adopting the geometrized Heaviside-Lorentz unit for magnetic fields.

e.g. Suppose magnetic field has value \(10^{-5}\) with the code unit in the ValenciaDivClean evolution system. This corresponds to

10^(-5) * gauss_unit = 2.3558985e+14 Gauss

in the CGS Heaviside-Lorentz unit.

If one wants to convert it to the usual CGS Gaussian unit, extra factor of \(\sqrt{4\pi}\) needs to be multiplied:

10^(-5) * gauss_unit * sqrt(4pi) = 8.35144274e+14 Gauss

pressure_unit

constexpr double hydro::units::cgs::pressure_unit

constexpr

Initial value:

=
    mass_unit / (length_unit * time_unit * time_unit)

pressure_cgs = pressure_geometric * pressure_unit Heuristically, the quantity above but times the speed of light squared in cgs

rest_mass_density_unit

constexpr double hydro::units::cgs::rest_mass_density_unit

constexpr

Initial value:

=
    mass_unit / (length_unit * length_unit * length_unit)

rho_cgs = rho_geometric * rho_unit Heuristically, the density in g/cm^3 of matter ~2200 times the density of atomic nuclei. Note this is much larger than any density realized in the universe.