Conversion of units

[Former user]

Dear Einstein toolkit users, There is a saying in Theoretical Physics that developing a new piece of theory usually takes 3 years. But it takes - at least - some more years for converting results from "natural" units to those which can be used by experimental physicists.

On this backgroup I would like to opt to add to std-output files a conversion table from units used by ET to SI Units, for example. But I leave it to commutiy to convince me that this was done severtal times before.

best wishes and have a nice week-end Peter from Kiel, Germany

btw: Some may be interested about my personal background. I'm doing CFD for some time, esp, applied to wind turbine aerodynamics. But I have a strong interest in Theoretical Physics and its history. I remember the 1979 Einstein centennial meeting where Ehlers, Penrose and others made some impressive presentations. Wheeler then was on the other side of the "iron curtain" (at the Humbold-University) meeting Klaus Fuchs whom he knowed from the Manhattan Project.

[Roland Haas]

Since the part in the ET that typically usually has “scale” is the fluid code, we document the conventional units in HydroBase’s interface.ccl: https://bitbucket.org/einsteintoolkit/einsteinbase/src/master/HydroBase/interface.ccl

Outputting this (static) conversion each run seems overkill. Some thorns (eg the TOVSolver or LORENE reader) will output some overview information about the fluid body they constructed in SI units as well as “Cactus” units.

Note that it is perfectly possible to run simulations with different units, eg a mass unit of 1e6Msun when simulating accretion onto supermassive black holes. One may have to adjust some numerical parameters here and there (eg “epsilon” for WENO schemes).

The other assumption that is present is that, eg TwoPunctures makes the implicit assumption that the mass of the black holes is ~1 when it constructs its grid. Very different black hole masses may lead to sub-optimal grids.