[Users] units

[Ian Hinder]

On 12 Apr 2017, at 21:30, 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. 

Hi Francisco,

I made a document a long time ago with my understanding of the situation.  I always refer back to it, so I attach it here in case it is useful.  I might not explain some of the aspects in quite the same way now though.

The upshot is that there is a single unit in the simulation, which we can call M.  Lengths, times and masses can be measured in this unit.  It is *conventional* to arrange for the sum of the masses of the BHs to sum to 1 in these units, so that M = m1+m2.  This makes it easy to quickly compare two BBH simulations without having to rescale the units of each.  Note that this could be the masses of the punctures, the initial horizon masses, or the horizon masses after a short amount of evolution, to account for absorption of junk energy.

If you want to rigorously compare different simulations, it's best to compare dimensionless numbers from each simulation, e.g. by dividing e.g. a time by the *measured* masses of the BHs, e.g. t_merger / (m1+m2).  Then it doesn't matter what the original unit of the simulation was.

-- 
Ian Hinder
http://members.aei.mpg.de/ianhin
-------------- next part --------------
A non-text attachment was scrubbed...
Name: units.pdf
Type: application/pdf
Size: 62442 bytes
Desc: not available
Url : http://lists.einsteintoolkit.org/pipermail/users/attachments/20170412/2b4369ca/attachment-0001.pdf