units.hpp File Reference

The unit system every quantity in the project is expressed in. More...

#include "orrery/core/types.hpp"

Go to the source code of this file.

Variables
constexpr Real	orrery::core::kGravitationalConstant = 1
	The gravitational constant, one by the definition of the unit system.

Detailed Description

The unit system every quantity in the project is expressed in.

Orrery works in N-body units: the gravitational constant is one, and mass, length and time are whatever the caller's configuration makes them. This is the convention the stellar dynamics literature states its results in, so a virial ratio or a Plummer energy computed here can be compared against a published figure without a conversion standing between them.

The alternative, carrying SI or astronomical values, would put a factor of 6.674e-11 or 4pi^2 into the innermost multiplication of every force evaluation, and would make the dynamic range of the intermediate products far wider than it needs to be. In single precision, which is the configuration the largest runs use, that range is not free.

A caller with a system in physical units scales it on the way in. That conversion belongs to the configuration layer that reads the run description, not to the kernels, which should see numbers of order one. ADR-0007 records the alternatives.

Variable Documentation

kGravitationalConstant

Real orrery::core::kGravitationalConstant = 1

inlineconstexpr

The gravitational constant, one by the definition of the unit system.

Named rather than left implicit so that the places where it belongs are visible in the source. An expression for a potential energy that shows no G at all reads as though the physics were forgotten, and a reader cannot tell the difference between a deliberate choice of units and an omission.

It is a constant rather than a parameter for the same reason the softening is shared rather than passed independently to each caller: the unit system is a property of the whole simulation, and letting a force kernel and an energy diagnostic each take their own value is how the two come to disagree.