What they're looking for:

We look for the technology. ... And what does that tell us? It means that anything that we can find ... is probably not going to be at the microbial scale. Because you have to build things.

Why aliens won't attack: If aliens were able to visit Earth, that would mean they would have technological capabilities sophisticated enough not to need slaves, food or other planets. ... If aliens were to come here, it would be simply to explore. Considering the age of the universe, we probably wouldn't be their first extraterrestrial encounter, either.