> Liu assumes that it is somehow a lot easier to wipe out a civilization than communicate with it...
This may be true, though. We struggle to effectively communicate with creatures on our own planet available in real-time, like dolphins and elephants, who appear to think and have language of sorts.
Launching some relativistic sand at another planet may well be a lot easier.
We seem to communicate pretty well with dolphins, elephants, and other animals. Consider trainers who work with these animals on a daily basis and have rich and productive relationships with them.
We can explain "go press that button" or "lift your leg" fairly well. We can't, as yet, put a dolphin through something like an elementary school level education.
This may be true, though. We struggle to effectively communicate with creatures on our own planet available in real-time, like dolphins and elephants, who appear to think and have language of sorts.
Launching some relativistic sand at another planet may well be a lot easier.