If they are a really old and hostile civ, then they already would have found and destroyed us. Since that is not what we observe, they are either not hostile or are relatively young.
Assume for a moment the worst case—they are hostile and young. Due to observation selection effects, most observers detect either no alien civs or alien civs around their same age.
Say it takes 1000 years to go from early space civ (us today), to megastructure civ (them), and say 100 years from early space to get large space telescopes/sensors sufficient to detect most earth size planets within a few 1,000 lyrs. So they are only about 1000 years ahead of us, but 1480 years away.
They would build interstellar sensors around 1100 AD, at which point they would image us from 380 BC. They’d see a biosphere, but hopefully not yet any evidence for civilization, as that shouldn’t really be detectable until the industrial era—about 2200 years later.
To detect earth civ circa 380 BC or earlier they’d probably need images from a probe near the planet or at least near the sun, which implies a minimum of ~3000 year round trip time.
In general, I think one can construct an argument that we should expect to have roughly order ~D time until any contact/invasion, where D is the distance in lyrs between us and the alien civ.
Except that a SETI-type attack such as the one described in Hoyle’s A for Andromeda (mentioned in the link provided by turchin above) would not necessarily target us specifically; it could be launched by aliens with no knowledge of our existence.
Us personally? That depends on when they started their attack.
If they are a really old and hostile civ, then they already would have found and destroyed us. Since that is not what we observe, they are either not hostile or are relatively young.
Assume for a moment the worst case—they are hostile and young. Due to observation selection effects, most observers detect either no alien civs or alien civs around their same age.
Say it takes 1000 years to go from early space civ (us today), to megastructure civ (them), and say 100 years from early space to get large space telescopes/sensors sufficient to detect most earth size planets within a few 1,000 lyrs. So they are only about 1000 years ahead of us, but 1480 years away.
They would build interstellar sensors around 1100 AD, at which point they would image us from 380 BC. They’d see a biosphere, but hopefully not yet any evidence for civilization, as that shouldn’t really be detectable until the industrial era—about 2200 years later.
To detect earth civ circa 380 BC or earlier they’d probably need images from a probe near the planet or at least near the sun, which implies a minimum of ~3000 year round trip time.
In general, I think one can construct an argument that we should expect to have roughly order ~D time until any contact/invasion, where D is the distance in lyrs between us and the alien civ.
Except that a SETI-type attack such as the one described in Hoyle’s A for Andromeda (mentioned in the link provided by turchin above) would not necessarily target us specifically; it could be launched by aliens with no knowledge of our existence.