SETI uses the tools of astronomy to try and find evidence of someone else’s technology out there. Our own technologies are visible over interstellar distances, and theirs might be as well. It might be that a massive network of communications, or some shield against asteroid impact, or some huge astro-engineering project that we can’t even begin to conceive of, could generate signals at radio or optical frequencies that a determined program of searching might detect. For millennia, we’ve turned to philosophers for guidance on this question of whether there’s intelligent life out there. Now, we are using the tools of the 21st century to try and observe what is, rather than ask what should be, believed. Join Jill Tarter, Martin Rees, Jonathan Knowles and Graham Mackintosh as we explore the state of, and importance of the search for evidence of our cosmic neighbors.