What percentage of stars would need to become Shkadov thrusters to move the whole galaxy?
The advanced civilization in my story is trying to reach as many galaxies as possible in the next few billions years before the expansion of space has sent those galaxies permanently out of reach without some form of FTL travel which my story does not have.
I plan to have von Neumann probe-like self replicating machines powered by laser relays from Nicoll-Dyson beams, possibly a double beam from opposite sides keeping the star in its location whilst sending probes in opposite directions.
Once in the new galaxies the self replicating machines will gather materials and turn the majority of stars into Shkadov thrusters, the collective thrust of the stars and the gravitational hold of the galaxy will move the whole galaxy towards the civilizations home galaxy. This assumption that whole galaxies can be moved was taken from the following videos: https://www.gregschool.org/gregschoollessons/2018/6/12/shkadov-thrusters https://www.youtube.com/watch?v=_VetAm7fCS0
My question is what percentage of the stars need to become Shkadov thrusters to move the whole galaxy? Would there become a point where converting more stars is taking more time than it is worth for the additional speed? The civilization isn't worried about the time the journeys will take, as long as the galaxies eventually arrive to their area of space.
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1 answer
This wouldn't work.
The big issue is that stars make up only a few percent of the Milky Way's total mass - estimates vary by a bit, but I've heard numbers in the 3-5% range. For instance, McMillan 2011 claims about 4.98%. Dark matter makes up a substantial amount, as do interstellar clouds - i.e. non-rigid bodies.
The point is, even if you attached a thruster to every single star in the galaxy, it wouldn't produce any significant gravitational effects on the rest of the galaxy, and it certainly wouldn't be enough to move the Milky Way.
You could try and find a galaxy that 1) has an extremely low amount of dark matter, like NGC 1052-DF2 or NGC 1052-DF4 and 2) is old and gas-poor, meaning most of its star formation is over and much of its mass is locked in stars and stellar remnants, not interstellar clouds. I'd have to learn more about stellar populations of these ultra diffuse galaxies to say anything intelligent on the subject, though.
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