This example discusses how a type III civilization could signal its existence to a technological civilization halfway across the visible universe (~7 billion light years) over a time span of 5 billion years. Constraints: It should use a relatively small percent of its available resources, and the methods should not rely on unproven physics.
In the nearest 100 star systems (which include ~150 stars), there are 8 white dwarfs (5% of the stars). There is a distribution of masses, but most white dwarfs are between 0.5 and 0.7 (average ~ 0.6) times the mass of the sun (M*). A white dwarf cannot be more than ~1.44 M* because the self gravity becomes too strong to be supported by electron degeneracy pressure.
Type 1A supernovae occur when a white dwarf reaches ~1.44 M* via accretion from a companion star that is expanding. The white dwarf collapses, a large percent of the mass undergoes fusion, and it releases 1e44 to 2e44 joules of energy. Type 1A supernovae have a characteristic brightness profile and spectrum, and are readily identified. They occur naturally at a rate of approximately 1-2 per century (1-2 per ~3e9 seconds) in the Milky Way. They have been detected from as far away as 10 billion light years.
The Milky Way contains 1e11 to 4e11 stars. Therefore, there are up to 2e10 white dwarfs in the Milky Way.
An advanced civilization that wants to send an omnidirectional signal could intentionally induce type 1A supernovae by coalescing white dwarfs or crashing other stars into them. If using only white dwarfs (5% of the stars in the galaxy, maybe ~2-3% of its mass), then 1.44/0.6 = 2.4 average white dwarfs per supernova explosion would be required. This would allow 2e10 / 2.4 = 8e9 type 1A supernovae total in the galaxy.
This could be done by calculated, relatively small shifts in velocity that cause interstellar collisions many years in the future. For example: Two stars are calculated to pass within a light year of each other (1e16 m) in 10 million years (3e14 s). A shift in velocity on the order of 1e16m/3e14s = 33 m/s will instead cause a collision. Acting over 3e13 s (1 million years), this would require a constant acceleration of ~1e-12 m/(s^2). This could be accomplished by light pressure with mirror satellites or other low acceleration means that are a small fraction of a star system’s mass. In this example it would require ~10 million years to get signalling started, but that is 0.2% of the timescale under discussion (5 billion years).
If the signal needs to be maintained for 5 billion years (1.5e17 seconds), then the civilization could on average initiate a type 1A supernova every 1.5e17 / 8e9 = 1.9e7 seconds = 217 days, which would be ~50-100x the natural rate. If visible to us, we would notice a galaxy with so many supernovae.
Same question as to Wei Dai: do we notice all type 1A supernovaea that occur, or just some of them? The fact that we’ve only noticed out to 10 billion light years suggests we probably can’t see all of them?
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.
This example discusses how a type III civilization could signal its existence to a technological civilization halfway across the visible universe (~7 billion light years) over a time span of 5 billion years. Constraints: It should use a relatively small percent of its available resources, and the methods should not rely on unproven physics.
In the nearest 100 star systems (which include ~150 stars), there are 8 white dwarfs (5% of the stars). There is a distribution of masses, but most white dwarfs are between 0.5 and 0.7 (average ~ 0.6) times the mass of the sun (M*). A white dwarf cannot be more than ~1.44 M* because the self gravity becomes too strong to be supported by electron degeneracy pressure.
Type 1A supernovae occur when a white dwarf reaches ~1.44 M* via accretion from a companion star that is expanding. The white dwarf collapses, a large percent of the mass undergoes fusion, and it releases 1e44 to 2e44 joules of energy. Type 1A supernovae have a characteristic brightness profile and spectrum, and are readily identified. They occur naturally at a rate of approximately 1-2 per century (1-2 per ~3e9 seconds) in the Milky Way. They have been detected from as far away as 10 billion light years.
The Milky Way contains 1e11 to 4e11 stars. Therefore, there are up to 2e10 white dwarfs in the Milky Way.
An advanced civilization that wants to send an omnidirectional signal could intentionally induce type 1A supernovae by coalescing white dwarfs or crashing other stars into them. If using only white dwarfs (5% of the stars in the galaxy, maybe ~2-3% of its mass), then 1.44/0.6 = 2.4 average white dwarfs per supernova explosion would be required. This would allow 2e10 / 2.4 = 8e9 type 1A supernovae total in the galaxy.
This could be done by calculated, relatively small shifts in velocity that cause interstellar collisions many years in the future. For example: Two stars are calculated to pass within a light year of each other (1e16 m) in 10 million years (3e14 s). A shift in velocity on the order of 1e16m/3e14s = 33 m/s will instead cause a collision. Acting over 3e13 s (1 million years), this would require a constant acceleration of ~1e-12 m/(s^2). This could be accomplished by light pressure with mirror satellites or other low acceleration means that are a small fraction of a star system’s mass. In this example it would require ~10 million years to get signalling started, but that is 0.2% of the timescale under discussion (5 billion years).
If the signal needs to be maintained for 5 billion years (1.5e17 seconds), then the civilization could on average initiate a type 1A supernova every 1.5e17 / 8e9 = 1.9e7 seconds = 217 days, which would be ~50-100x the natural rate. If visible to us, we would notice a galaxy with so many supernovae.
https://en.wikipedia.org/wiki/White_dwarf
http://www.astro.gsu.edu/RECONS/TOP100.posted.htm
https://en.wikipedia.org/wiki/Supernova
https://en.wikipedia.org/wiki/Milky_Way
Same question as to Wei Dai: do we notice all type 1A supernovaea that occur, or just some of them? The fact that we’ve only noticed out to 10 billion light years suggests we probably can’t see all of them?
I expect we don’t notice most of them. We may notice a lot more the next few decades though. Some would still probably be hidden behind dust.
If we only notice 10% (say), then that seems to increase the cost of being noticed by 10x, so wouldn’t yet be above the bar.
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.