My first idea is to make two really big black holes and then make them merge. We observed gravitational waves from two black holes with solar masses of around 25 solar masses each located 1.8 billion light years away. Presumably this force decreases as an inverse square times exponential decay; ignoring the exponential decay this suggests to me that we need 100 times as much mass to be as prominent from 18 billion light years. A galaxy mass is around 10^12 solar masses. So if we spent 2500 solar masses on this each year, it would be at least as prominent as the gravitational wave that we detected, and we could do this a billion times with a galaxy. To be safe, I’d 10x the strength of the waves, so that we could do it 100 million times with a galaxy.
Currently our instruments aren’t sensitive enough to detect which galaxy was emitting these bizarrely strong gravitational waves. So I’d combine this with Wei Dai’s suggestion of making an extremely bright beacon using the accretion disks resulting from the creation of these black holes.
The first merger event that Ligo detected was 1 billion ly away and turned 1 solar mass into gravitational waves. 1030kg=1047J at a distance of 109×1016=1025m so energy flux received is approx 1047×(1025)−2=10−3J/m2 The main peak power output from the merging black holes lasted around one second. A full moon illuminates earth with around 10−3W. So even if the aliens are great at making gravitational waves, they aren’t a good way to communicate. If they send a gravitational wave signal just powerful enough for us to detect with our most sensitive instruments, with the same power as light they could outshine the moon. Light is just more easily detected.
My main concern with this is the same as the problem listed on Wei Dai’s answer: whether a star near us is likely to block out this light. The sun is about 10^9m across. A star that’s 10 thousand light years away (this is 10% of the diameter of the Milky Way) occupies about (1e9m / (10000 lightyears * 2 * pi))**2 = 10^-24 of the night sky. A galaxy that’s 20 billion light years away occupies something like (100000 lightyears / 20 billion lightyears) ** 2 ~= 2.5e-11. So galaxies occupy more space than stars. So it would be weird if individual stars blocked out a whole galaxy.
Another piece of idea: If you’re extremely techno-optimistic, then I think it would be better to emit light at weird wavelengths than to just emit a lot of light. Eg emitting light at two wavelengths with ratio pi or something. This seems much more unmistakably intelligence-caused than an extremely bright light.
Same question as Michael: if there were a point source with weird spectrum outside of any galaxy, about as bright as the average galaxy, would we reliably notice it?
I’m a bit confused if you already read my comment.
If you fake extremely high red-shift, it would likely be noticed. Radio galaxies were systematically used when looking for distant objects so if someone at the same time created very bright radio source & optical counterpart with impossibly high redshift, it would grab attention to the source, and you can signal intelligence using spectra later.
My first idea is to make two really big black holes and then make them merge. We observed gravitational waves from two black holes with solar masses of around 25 solar masses each located 1.8 billion light years away. Presumably this force decreases as an inverse square times exponential decay; ignoring the exponential decay this suggests to me that we need 100 times as much mass to be as prominent from 18 billion light years. A galaxy mass is around 10^12 solar masses. So if we spent 2500 solar masses on this each year, it would be at least as prominent as the gravitational wave that we detected, and we could do this a billion times with a galaxy. To be safe, I’d 10x the strength of the waves, so that we could do it 100 million times with a galaxy.
Currently our instruments aren’t sensitive enough to detect which galaxy was emitting these bizarrely strong gravitational waves. So I’d combine this with Wei Dai’s suggestion of making an extremely bright beacon using the accretion disks resulting from the creation of these black holes.
The first merger event that Ligo detected was 1 billion ly away and turned 1 solar mass into gravitational waves. 1030kg=1047J at a distance of 109×1016=1025m so energy flux received is approx 1047×(1025)−2=10−3J/m2 The main peak power output from the merging black holes lasted around one second. A full moon illuminates earth with around 10−3W. So even if the aliens are great at making gravitational waves, they aren’t a good way to communicate. If they send a gravitational wave signal just powerful enough for us to detect with our most sensitive instruments, with the same power as light they could outshine the moon. Light is just more easily detected.
My main concern with this is the same as the problem listed on Wei Dai’s answer: whether a star near us is likely to block out this light. The sun is about 10^9m across. A star that’s 10 thousand light years away (this is 10% of the diameter of the Milky Way) occupies about (1e9m / (10000 lightyears * 2 * pi))**2 = 10^-24 of the night sky. A galaxy that’s 20 billion light years away occupies something like (100000 lightyears / 20 billion lightyears) ** 2 ~= 2.5e-11. So galaxies occupy more space than stars. So it would be weird if individual stars blocked out a whole galaxy.
Another piece of idea: If you’re extremely techno-optimistic, then I think it would be better to emit light at weird wavelengths than to just emit a lot of light. Eg emitting light at two wavelengths with ratio pi or something. This seems much more unmistakably intelligence-caused than an extremely bright light.
Same question as Michael: if there were a point source with weird spectrum outside of any galaxy, about as bright as the average galaxy, would we reliably notice it?
I’m a bit confused if you already read my comment.
If you fake extremely high red-shift, it would likely be noticed. Radio galaxies were systematically used when looking for distant objects so if someone at the same time created very bright radio source & optical counterpart with impossibly high redshift, it would grab attention to the source, and you can signal intelligence using spectra later.