Re: Dyson Sphere Repulsive Gravity Part 2 Galactic Motion...

Johnny1a wrote:

A couple of months ago I postulated a Dyson Sphere scenario (meaning
the solid-shell/habitable inner surface version) in which the
equivalent of an inner gravity is produced by a super-advanced
techology: a _repulsive_ force emitted from a surface outside the
photosphere but within the main shell, obeying the inverse-square law,
that drops off to a repulsive force of 1G at the inner surface of the
shell.

As I indicated earlier, this means that any object at distance r experiences an acceleration away from the star with magnitude

a = K/r^2,

where K = 2.21 x 10^23 m^3/s^2.

Anyway, I got to thinking, _where_ is this Dyson Sphere going to end
up? It's going to be exchanging momentum with other stars via its
repulsive field, which will vastly overwhelm the positive attraction
of the system's natural mass. It provides superb protection against
impactors of any sort, but it will also tend to cause the Sphere to
repel other stars (or repel itself from them, same thing).

If we assume this thing starts out in a Sol-like trajectory and in the
general part of the Galaxy as Sol, will it tend to migrate out of the
Galaxy? (That's what my intuition suggests, but that can be
misleading in this sort of situation.)

As you say, the repulsion overwhelms any attraction due to normal gravity for either the Dyson-shelled star or other stars, so we can ignore it entirely. We can only assume that Newton's second law applies and that any repulsive force that this magical field applies to nearby objects will result in equal but opposite forces on the star itself, which means that the forces repulsing an object and the star are equal and opposite. So the acceleration A experienced by the star due to this is

m a = M A,

where m is the mass of the object being repulsed and M is the mass of the star (well, Dyson-shelled star system). Ignoring the Dyson shell itself and just assuming the star is the mass of the Sun, M = 2.0 x 10^30 kg, then the acceleration experienced by repulsing objects around it is

A = (m/M) K/r^2.

This repulsive acceleration is, of course, additive. So consider the repulsion by the core of the Galaxy. It comprises perhaps 10% of the mass of the Galaxy, or about 4 x 10^40 kg, and lies at a distance of about 8 kpc = 2 x 10^20 m. That means the repulsive acceleration on the star from the galactic core alone would be 1 x 10^-7 m/s^2. That doesn't seem like much, but it's enormous compared to the actual centripetal (yes, centripetal) acceleration of the Sun (in its place) around the galactic center.

The Sun is at a distance of 2 x 10^20 m and orbits the Galaxy at a speed of 220 km/s, so its centripetal acceleration is 2 x 10^-10 m/s^2, about a thousand times less than the repulsive, centrifugal acceleration.

Really, even doing any computations here was unnecessary. The star repulses other objects with an inverse square law that vastly outmatches normal gravitational forces. The star is bound to the Galaxy by these gravitational forces, so the property that it repulses all other objects with much greater magnitude than it is attracted to them must necessarily mean that it will be ejected from the Galaxy forthwith.

--
Erik Max Francis && max@xxxxxxxxxxx && http://www.alcyone.com/max/
San Jose, CA, USA && 37 20 N 121 53 W && AIM, Y!M erikmaxfrancis
You are in the music / In the man's car next to me
-- Sade
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Relevant Pages

  • Re: Dyson Sphere Repulsive Gravity Part 2 Galactic Motion...
    ... the repulsion overwhelms any attraction due to normal ... objects will result in equal but opposite forces on the star itself, ... So the acceleration A experienced by the star due to this is ... repulsion by the core of the Galaxy. ...
    (rec.arts.sf.science)
  • Re: Is the Sun mostly dark matter ?
    ... This is in contrast to ordinary matter, where the sun represents the overwhelming majority of the mass of the solar system. ... First you have to explain that somehow DM doesn't interact with either itself or with Regular Matter other than through gravity. ... What is different about the gravity well of a galaxy than the gravity well of a star or planet that allows DM to slow down and enter into orbit around it? ...
    (sci.physics)
  • Re: Gravity is repulsive
    ... >>I have found no proof that gravity is an attractive force of matter. ... The complex concepts of dark energy and matter had to be made ... more space between objects then the greater the force of repulsion. ...
    (sci.astro.amateur)
  • Re: The Repulsiveness of Dark Energy
    ... rather that empty space repelled matter omnidirectionaly and, ... to be in freespace would have you experienceing the repulsion ... equaly from all directions and, hence, feeling no "gravity". ...
    (rec.arts.sf.fandom)
  • Re: Lorentz Considerations on Gravitation
    ... particles is slightly stronger than the repulsion of equal charged ... Experiment showed gravity is modelled closely using the metric tensor - EM ... Pound-Rebka experiment. ...
    (sci.physics.relativity)