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.
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.