[Users] units

[Francisco Guzman]

Thanks for the help Barry, Ian and Erik. 

Cheers, 
Francisco 


From: "Barry Wardell" <barry.wardell at gmail.com> 
To: "Ian Hinder" <ian.hinder at aei.mpg.de> 
Cc: "Einstein Toolkit Users" <users at einsteintoolkit.org> 
Sent: Wednesday, April 12, 2017 3:58:07 PM 
Subject: Re: [Users] units 

Hi Francisco, 
To add a little more detail to what Ian and Erik already said, you might find it helpful to look at the Waveforms SimulationTools tutorial [ http://simulationtools.org/examples/Waveforms.cdf ] from the BBH gallery example page. This gives a short explanation of the units in that simulation along with demonstrations of how to interpret the data from the simulation in terms of those units. 

Regards, 
Barry 

On 12 April 2017 at 21:21, Ian Hinder < ian.hinder at aei.mpg.de > wrote: 




On 12 Apr 2017, at 22:12, Erik Schnetter < schnetter at cct.lsu.edu > wrote: 


BQ_BEGIN

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. 



Hi Erik, 

If you want the units of the simulation to be the ADM mass, then you need a mechanism to iterate the bare mass parameters of the punctures to make this the case. We don't have that in TwoPunctures; we iterate until we get the puncture masses equal to the targets. I think it is useful to compare two BBH systems where the BHs have the same total mass. This would be natural also if you were comparing with PN. I'm not sure it is so useful to compare systems with the same ADM mass; the ADM mass will consist of the BH masses, plus the energy content in the junk radiation, and the orbital energy. The junk radiation isn't very interesting, which is why I am rarely interested in M_ADM. 


BQ_BEGIN

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. 

BQ_END

No, M_ADM is not 1 for that parameter file. It's 

initial-ADM-energy = 0.9899366929086094169 

In the parameter file, we have 
q = 36.0/29.0               # Mass ratio: q = mp/mm >= 1
M = 1.0                     # Total mass 
mp = M * q/(1+q)           # Heavier, larger BH, AH1, SS 0
mm = M * 1/(1+q)           # Lighter, smaller BH, AH2, SS 1 
TwoPunctures::target_M_plus             = $mp
TwoPunctures::target_M_minus            = $mm 

so the sum of the puncture masses is 1 in the units of the simulation, so the units of the simulation are the sum of the puncture masses. 

-- 
Ian Hinder 
http://members.aei.mpg.de/ianhin 


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BQ_END



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