Lets suppose that the milky way has 1010 (5% of all stars) stars suitable for life. (because some stars are too small or close to the galactic core.
Scaling up by mass gives around 1021 stars of interest.
As planetary location is not known, they must fill the entire habitable zone with energy. Radius of earths orbit 1011m so area around 1022m2
This gives 1043m2 total that it must illuminate to hit us.
If they want to broadcast the galaxies mass-energy over 3billion years (1017s) then flux is
1059/1043/1017=0.1W/m2, 100X a full moons illumination.
If they choose to send a 1 second pulse of energy every 30 years, That is 1:109 time ratio so the flux for that second can be 0.1×109=108W/m2, This is comparable to the beam of current laser weapons or being within 1km of the trinity test, and would likely set fire to most exposed organics and heat the surface of black rocks to a red glow. As we can see, the aliens can reduce power usage by orders of magnitude and still be highly noticeable.
I may be misunderstanding: Are you suggesting a targeted beam to the habitable zone of every star they can see?
If so, I don’t see how that could work, considering that most stars visible at time of transmission would be dead by the time the transmission reaches them. Also the fact that they have orthogonal velocity that would be difficult or impossible to measure and account for.
That was what I was considering. I was hoping the aliens had telescopes that could see the collapsing cloud of gas and work out where the star would end up.
Milky way 1042kg=1059J
Mass of observable universe =1053kg
Radius of observable universe 46Gly
Lets suppose that the milky way has 1010 (5% of all stars) stars suitable for life. (because some stars are too small or close to the galactic core.
Scaling up by mass gives around 1021 stars of interest.
As planetary location is not known, they must fill the entire habitable zone with energy. Radius of earths orbit 1011m so area around 1022m2
This gives 1043m2 total that it must illuminate to hit us.
If they want to broadcast the galaxies mass-energy over 3billion years (1017s) then flux is
1059/1043/1017=0.1W/m2, 100X a full moons illumination.
If they choose to send a 1 second pulse of energy every 30 years, That is 1:109 time ratio so the flux for that second can be 0.1×109=108W/m2, This is comparable to the beam of current laser weapons or being within 1km of the trinity test, and would likely set fire to most exposed organics and heat the surface of black rocks to a red glow. As we can see, the aliens can reduce power usage by orders of magnitude and still be highly noticeable.
I may be misunderstanding: Are you suggesting a targeted beam to the habitable zone of every star they can see?
If so, I don’t see how that could work, considering that most stars visible at time of transmission would be dead by the time the transmission reaches them. Also the fact that they have orthogonal velocity that would be difficult or impossible to measure and account for.
My apologies if I have misunderstood.
That was what I was considering. I was hoping the aliens had telescopes that could see the collapsing cloud of gas and work out where the star would end up.