Re: How important is GR in order to calc the precession of Mercury
From: Nicolaas Vroom (nicolaas.vroom_at_pandora.be)
Date: 01/27/05
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Date: Thu, 27 Jan 2005 21:02:12 +0000 (UTC)
This responds is partly written as a reply of the thread:
"non-GR theories of Gravity" initiated by Phillip Helbig.
"Nicolaas Vroom" <nicolaas.vroom@pandora.be> schreef in bericht
news:Nfjzd.10044$_C2.478443@phobos.telenet-ops.be...
> If you want to simulate the movement of galaxies
> IMO there are two possibilities.
> In both cases you start by selecting a reference point
> (or origin) and a time t0 (or a now)
>
> The first possibility is based on what you see.
> That means you place yourself at the origin and you
> observe the positions of the planets. Those positions
> are the starting point of your simulation.
> The second possibility is identical as the first, but the
> starting position of the simulation is not the observed
> position but the predicted position at the time t0 (now)
This second possibilty is the prefered strategy
but things are not that simple.
In order to calculate the predicted positions at time t0
you need a model.
One model can be Newton's Law.
The most important parameter of Newton's Law are
the masses of all the objects included in your simulation.
In order to calculate those masses you need as many
as possible observations of all your objects.
You need a set of estimated values for all your masses
and a set of initial positions (*) for your simulation.
With those two sets of information and with Newton's law
you can calculate the observed positions at the time
of those observations.
And you can calculate an overall error factor.
Next you can do the same for a different set of estimated
values for all your masses and you can again
calculate an overall error factor.
And again.
The set of estimated masses with the smallest overall
error factor is the best.
(*) Your simulation starts with a set of initial positions
all at the same moment. Very often that set is not available
because most probably all you observations are
at a different moment. That means you have to calculate
those initial positions with the masses based on your
best estimate.
This makes this whole exercise very complex.
A different model can be MOND.
MOND stands for (Milgrom's?) Modified Newton Dynamics.
As the name explains MOND is based on Newton's Law
slightly modified with at least one additional parameter.
If you want to use MOND you have to estimate this parameter
and a set of masses for all the objects included.
Again you have to calculate an overall error factor
and the smallest overall error factor gives the best estimates.
MOND is a better theory than Newton's Law if the final
overall error factor using MOND is the smallest of the two.
In principle you can also use a different model.
For example you can use Newton's Law modified
with a parameter which takes into account that
Newton's Law does not act instantaneous.
Again you have to calculate an overall
error factor and the story repeats it self.
(It should be mentioned that each of those
3 theories gives different mass estimates)
For example you can use GR
Hopes this helps.
> Nicolaas Vroom
> http://users.pandora.be/nicvroom/
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