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Buzz Bloom
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The motivation for this post comes from a discussion on another thread
It seems worthwhile to me to try a simplification in this post to hopefully avoid the complications raised in the cited thread.
Assume an otherwise empty universe with a very thin spherical shell of radius R0 at time t = 0, with a uniform density of dust particles. There is zero radiation, zero pressure, and zero temperature.
If it is reasonable to do so, make additional assumptions:
There is a test particle of negligible mass in a circular orbit around the spherical shell at a radius of
I would much appreciate some help in understanding how to calculate this additional mass-energy (hopefully without having to do any tensor calculus). This is the main intended focus for starting this thread.
The following is my naive attempt to make this calculation. I expect that it is probably wrong, but I am hopeful that I can be educated to understand why it is wrong.
If we ignore the needed added mass Mg, then what effect will result from the fact that all the particles in the shell for time t > 0 will be falling towards the center of the sphere? As the particles fall they acquire kinetic energy. Will the added kinetic energy has a gravitational effect on the orbit of the test particle?
I am guessing that the falling of the particles with increasing kinetic energy will not actually effect the orbit. If I am mistaken I hope someone will correct this guess. My reasoning is that the sum of kinetic and potential energy is a constant as the particles fall. The stationary shell at t=0 has potential energy and the addition of kinetic energy equals the reduction of potential energy.
With Newtonian gravity, the shell as a whole exerts an attraction on each particle as if the shell were replaced by a mass at the center equal to N m / 2. Therefore,
First, the Mg mass based on the potential energy will also have a gravitation effect on the particles, so there will be second order potential energy effects, etc., adding to the above Mg.
Second, The calculation of Mg due to the potential energy may require a GR correction to the Newtonian calculation.
All comments are most welcome.
Regards,
Buzz
It seems worthwhile to me to try a simplification in this post to hopefully avoid the complications raised in the cited thread.
Assume an otherwise empty universe with a very thin spherical shell of radius R0 at time t = 0, with a uniform density of dust particles. There is zero radiation, zero pressure, and zero temperature.
If it is reasonable to do so, make additional assumptions:
each dust particle is a very tiny sphere of one milligram mass;
there are N>>1 of these particle;
they are approximately arranged uniformly on the surface of the sphere; and
the fact that it is impossible to make the arrangement completely uniform (and therefore the density is not exactly uniform) is not relevant to the reasoning presented later in this post.
If it is not reasonable to make these assumptions, I would appreciate a post explaining why so that I can try a restatement in a different and acceptably reasonable way.there are N>>1 of these particle;
they are approximately arranged uniformly on the surface of the sphere; and
the fact that it is impossible to make the arrangement completely uniform (and therefore the density is not exactly uniform) is not relevant to the reasoning presented later in this post.
There is a test particle of negligible mass in a circular orbit around the spherical shell at a radius of
Rt > R0.
Using Newtonian gravity, the orbital velocity isv = √(N m G / Rt)
GR gives an approximate modified velocityv ~= √(N m G / (Rt - Rs))
where Rs is the Swartzchild radiusRs = 2 G N m / c2.
The source of the non-exactness for v is that the total gravitational mass of the shell is not exactly = N m. I understand that the dust particles in the shell interact with each other gravitationally, and this interaction creates an additional gravitational mass-energy Mg.I would much appreciate some help in understanding how to calculate this additional mass-energy (hopefully without having to do any tensor calculus). This is the main intended focus for starting this thread.
The following is my naive attempt to make this calculation. I expect that it is probably wrong, but I am hopeful that I can be educated to understand why it is wrong.
If we ignore the needed added mass Mg, then what effect will result from the fact that all the particles in the shell for time t > 0 will be falling towards the center of the sphere? As the particles fall they acquire kinetic energy. Will the added kinetic energy has a gravitational effect on the orbit of the test particle?
I am guessing that the falling of the particles with increasing kinetic energy will not actually effect the orbit. If I am mistaken I hope someone will correct this guess. My reasoning is that the sum of kinetic and potential energy is a constant as the particles fall. The stationary shell at t=0 has potential energy and the addition of kinetic energy equals the reduction of potential energy.
With Newtonian gravity, the shell as a whole exerts an attraction on each particle as if the shell were replaced by a mass at the center equal to N m / 2. Therefore,
Mg = G N m / (2 c2 R0).
Some corrections may be needed.
First, the Mg mass based on the potential energy will also have a gravitation effect on the particles, so there will be second order potential energy effects, etc., adding to the above Mg.
Second, The calculation of Mg due to the potential energy may require a GR correction to the Newtonian calculation.
All comments are most welcome.
Regards,
Buzz
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