Some more thoughts pertaining to limits of detection:
The Milky Way weighs 5.8e11 times M*, which itself is 2e30kg. Total mass of the galaxy = 1.2e42kg.
If all that mass were converted to energy with perfect efficiency, say via black hole evaporation, or annihilation with antimatter, then that’s a total of 1.0e59 joules.
That many joules over 5 billion years (1.5e17 s) is a power of 7e41 watts. At a radius of 7 billion light years (6.6e25m), that’s an energy flux of 1.3e-11 W/(m*m).
The sun puts out about 1400 W/(m*m)at our distance. So the sun would be about 1e14 times brighter than this distant galaxy trying to get our attention. Move the sun 1e7 x farther away to about 158 light years to match this brightness, and you get a ~8.5 magnitude star, never visible without aid. (Note: If using 1000x as much energy it becomes a clearly visible star and among our top 20 or so.)
So, if a type III civilization were using the entire mass-energy of 1 galaxy with 100% efficiency and used this resource to signal continuously for 5 billion years, they would not be bright enough to see unaided. We would still probably notice the light as a third-rate star if it wasn’t blocked by dust.
How could they make it unusual enough to be noticed as a signal? Perhaps the signal has a complete blackbody spectrum, but they surround the galaxy with an unusual spectral absorption signature. Example: Surrounding the galaxy they could have concentric clouds of He, Li, B, N, Na, Al, etc. The elements with a prime atomic number.
That’s unusual enough to draw attention. Maybe they could even encode a message in the degree of absorption.
Some more thoughts pertaining to limits of detection:
The Milky Way weighs 5.8e11 times M*, which itself is 2e30kg. Total mass of the galaxy = 1.2e42kg.
If all that mass were converted to energy with perfect efficiency, say via black hole evaporation, or annihilation with antimatter, then that’s a total of 1.0e59 joules.
That many joules over 5 billion years (1.5e17 s) is a power of 7e41 watts. At a radius of 7 billion light years (6.6e25m), that’s an energy flux of 1.3e-11 W/(m*m).
The sun puts out about 1400 W/(m*m)at our distance. So the sun would be about 1e14 times brighter than this distant galaxy trying to get our attention. Move the sun 1e7 x farther away to about 158 light years to match this brightness, and you get a ~8.5 magnitude star, never visible without aid. (Note: If using 1000x as much energy it becomes a clearly visible star and among our top 20 or so.)
So, if a type III civilization were using the entire mass-energy of 1 galaxy with 100% efficiency and used this resource to signal continuously for 5 billion years, they would not be bright enough to see unaided. We would still probably notice the light as a third-rate star if it wasn’t blocked by dust.
How could they make it unusual enough to be noticed as a signal? Perhaps the signal has a complete blackbody spectrum, but they surround the galaxy with an unusual spectral absorption signature. Example: Surrounding the galaxy they could have concentric clouds of He, Li, B, N, Na, Al, etc. The elements with a prime atomic number.
That’s unusual enough to draw attention. Maybe they could even encode a message in the degree of absorption.