I wanted to comment that creating quasars may be difficult, but found that it may be done relatively simple. Let’s assume that aliens don’t have any magical technology to move stars or convert energy in matter.
In that case, they could create a quasar by directing many stars to the center of the galaxy: falling stars will increase accretion rate in the central black hole and thus its luminosity (note that too heavy black holes may be not luminous, as they will eat stars without destroying them), and by regulating the rate and types of falling stars the quasar spectrum could be manipulated.
But how to move stars? One idea is that if aliens could change a trajectory of a star slightly, it will eventually pass near another star, make a “gravitational manoeuvre” and fall to the center to the galaxy. Falling to the center of the galaxy would probably require tens of millions years (based on Sun’s rotation period of 250 mln years). Finding an appropriate star and changing the star’s trajectory to pass near it will require probably also at least millions years.
But how to change the trajectory of a star? One idea is to organise impacts of the star with large comets. It is not difficult, as remote Oort cloud objects (or better wandering small planets, as they are not part of already established orbital movement of the star) need only small perturbations to start falling down on the central star, which could be done via nuclear explosions or even smaller impacts.
The impacts with comets will have very small effects on the star’s trajectory. For example, Pluto’s mass is 100 million times less than Sun’s mass and impact with Pluto-size object will probably change Sun’s trajectory only on 1 mm/sec, but it will be like 1 billion km difference in 20 million years. Close flyby by stars are very rare, so may take tens of million of years of very complex space billiard to organise need flyby.
All this suggests that creating an artificial quasar is possible, but may take up to 100 million years in a typical galaxy; changing the galaxy’s luminosity by tiling it with Dyson Spheres could be probably done much quicker, in a less than 1 million years. Thus, creating artificial quasars as beacons make sense only if the difference in 100 mln years is not substantial, that is on a few billions years distances.
I wanted to comment that creating quasars may be difficult, but found that it may be done relatively simple. Let’s assume that aliens don’t have any magical technology to move stars or convert energy in matter.
In that case, they could create a quasar by directing many stars to the center of the galaxy: falling stars will increase accretion rate in the central black hole and thus its luminosity (note that too heavy black holes may be not luminous, as they will eat stars without destroying them), and by regulating the rate and types of falling stars the quasar spectrum could be manipulated.
But how to move stars? One idea is that if aliens could change a trajectory of a star slightly, it will eventually pass near another star, make a “gravitational manoeuvre” and fall to the center to the galaxy. Falling to the center of the galaxy would probably require tens of millions years (based on Sun’s rotation period of 250 mln years). Finding an appropriate star and changing the star’s trajectory to pass near it will require probably also at least millions years.
But how to change the trajectory of a star? One idea is to organise impacts of the star with large comets. It is not difficult, as remote Oort cloud objects (or better wandering small planets, as they are not part of already established orbital movement of the star) need only small perturbations to start falling down on the central star, which could be done via nuclear explosions or even smaller impacts.
The impacts with comets will have very small effects on the star’s trajectory. For example, Pluto’s mass is 100 million times less than Sun’s mass and impact with Pluto-size object will probably change Sun’s trajectory only on 1 mm/sec, but it will be like 1 billion km difference in 20 million years. Close flyby by stars are very rare, so may take tens of million of years of very complex space billiard to organise need flyby.
All this suggests that creating an artificial quasar is possible, but may take up to 100 million years in a typical galaxy; changing the galaxy’s luminosity by tiling it with Dyson Spheres could be probably done much quicker, in a less than 1 million years. Thus, creating artificial quasars as beacons make sense only if the difference in 100 mln years is not substantial, that is on a few billions years distances.