&gt; <i>It&#x27;s not like a fence is required; the area simply has to be avoided.</i><p>That might not be enough. It depends on your criteria for isolation. At the very least you need to prevent us from passively detecting you. How hard that is can be anyone&#x27;s guess, but most people would agree it gets harder with increasing energy output and proximity. When I say that cloaking outside EM sources might no be enough, I&#x27;m talking about what to do with probes exiting the solar system, and the consequences of dealing with them. To be effective, they would also need to take precautions of blocking technologies they are not sure we possess in the first place. It&#x27;s a tricky endeavor for them.<p>I would say there is a very practical difference between just avoiding us and keeping us contained.<p>&gt; <i>And FTL travel? How about near light speed instead?</i><p>There is a risk of repetition, I refer you to the discussion betwen Grishnak and myself below.