There are two kinds of alien civilizations. “Quiet” aliens don’t expand or change much, and then they die. We have little data on them, and so must mostly speculate, via methods like the Drake equation.
“Loud” aliens, in contrast, visibly change the volumes they control, and just keep expanding fast until they meet each other. They should be easy to see as they will be reorganizing the stars and galaxies they are in.
If grabby aliens exist and start expanding at over one-third the speed of light then we would not see their changes until the aliens get very close. Robin Hanson models that there is intelligent grabby aliens once in every million galaxies.
There is more to this and it is discussed between Lex and Robin and in animated videos.
Brian Wang is a Futurist Thought Leader and a popular Science blogger with 1 million readers per month. His blog Nextbigfuture.com is ranked #1 Science News Blog. It covers many disruptive technology and trends including Space, Robotics, Artificial Intelligence, Medicine, Anti-aging Biotechnology, and Nanotechnology.
Known for identifying cutting edge technologies, he is currently a Co-Founder of a startup and fundraiser for high potential early-stage companies. He is the Head of Research for Allocations for deep technology investments and an Angel Investor at Space Angels.
A frequent speaker at corporations, he has been a TEDx speaker, a Singularity University speaker and guest at numerous interviews for radio and podcasts. He is open to public speaking and advising engagements.