It was suggested that alien AI, if it wants to be visible, could create an artificial quasar, and a groups of such quasars could be visible as clearly artificial object on the distances of at least several billion light years. In this comment I will look on the ways how such quasar could be created with surprisingly small efforts and estimate need time for it.
Let’s assume that aliens don’t have any magical technology to move stars or to convert energy in matter.
In that case, they could create a quasar by directing many normal stars to the center of the galaxy: falling stars will increase the accretion rate in the central black hole and thus its luminosity, and the aliens could regulate the rate and types of falling stars to make changes in the quasar luminosity.
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 will fall to the center of the galaxy. Falling to the center of the galaxy would probably require tens of millions years (based on Sun’s orbital period of 250 mln years).
But how make small changes in the trajectory of a star? One idea is to impact the star with large comets. It is not difficult, as remote Oort cloud objects (or 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 smaller impacts by smaller astreoids.
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, which will turn into 1 billion km difference in 20 million years. Close flyby by stars are very rare, so it 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 it 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 makes sense only if the difference in 100 mln years is not substantial.
But how make small changes in the trajectory of a star? One idea is to impact the star with large comets. It is not difficult, as remote Oort cloud objects (or 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 smaller impacts by smaller astreoids.
I don’t think this works; conservation of momentum means that the impact is almost fully counteracted by the gravitational pull that accelerated the comet to such speed (so that, in the end, the delta-v imparted to the star is precisely what you imparted with your nuclear explosions or smaller asteroids).
Maybe a Shkadov thruster is what you want? (It’s slow going, though; this article suggests 60ly/200My, accounting for acceleration.)
To solve the “moment problem”, free wandering comets or planets could be used for impacts with a star. There are probably many of them, and they have significant initial radial speeds relative to the star.
It was suggested that alien AI, if it wants to be visible, could create an artificial quasar, and a groups of such quasars could be visible as clearly artificial object on the distances of at least several billion light years. In this comment I will look on the ways how such quasar could be created with surprisingly small efforts and estimate need time for it.
Let’s assume that aliens don’t have any magical technology to move stars or to convert energy in matter.
In that case, they could create a quasar by directing many normal stars to the center of the galaxy: falling stars will increase the accretion rate in the central black hole and thus its luminosity, and the aliens could regulate the rate and types of falling stars to make changes in the quasar luminosity.
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 will fall to the center of the galaxy. Falling to the center of the galaxy would probably require tens of millions years (based on Sun’s orbital period of 250 mln years).
But how make small changes in the trajectory of a star? One idea is to impact the star with large comets. It is not difficult, as remote Oort cloud objects (or 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 smaller impacts by smaller astreoids.
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, which will turn into 1 billion km difference in 20 million years. Close flyby by stars are very rare, so it 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 it 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 makes sense only if the difference in 100 mln years is not substantial.
I don’t think this works; conservation of momentum means that the impact is almost fully counteracted by the gravitational pull that accelerated the comet to such speed (so that, in the end, the delta-v imparted to the star is precisely what you imparted with your nuclear explosions or smaller asteroids).
Maybe a Shkadov thruster is what you want? (It’s slow going, though; this article suggests 60ly/200My, accounting for acceleration.)
To solve the “moment problem”, free wandering comets or planets could be used for impacts with a star. There are probably many of them, and they have significant initial radial speeds relative to the star.