[Users] units

[Erik Schnetter]

Francisco

Without hydrodynamics, nothing fundamental sets the mass scale. If you
assume that c = G = 1, then there is a rescaling freedom for M, the black
hole mass. Apart from this, it is up to the designer of the parameter file
what they call M. I think that the ADM mass is a good choice, but others
might use a different convention. In practice, one examines the black hole
masses and the ADM mass in a simulation, which is then returned in terms of
the grid spacing, which is related to the time step size, thus you see what
M is used in this particular case.

For example, in <http://einsteintoolkit.org/gallery/bbh/index.html> I
assume that M_ADM = 1, but I don't know whether this is taken from the
initial conditions or whether this is measured after the initial junk
radiation has left.

I hope my answer isn't too trivial.

-erik


On Wed, Apr 12, 2017 at 3:30 PM, Francisco Guzman <guzman at ifm.umich.mx>
wrote:

>
> Hi all,
>
> Could you guys please point me to the reference where I can see how units
> are defined for a binary black hole simulation?
>
> (Or possibly simply answer):
>
> I just want to know what the units of space and time (M) are.
> What is M? is it the ADM mass at initial time? the addition of the two
> black hole masses?... or so...
>
> I apologize in advance in case my question is terribly dummy.
>
> Thanks,
> Francisco
>
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>


-- 
Erik Schnetter <schnetter at cct.lsu.edu>
http://www.perimeterinstitute.ca/personal/eschnetter/
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