Mr Stevens postulates that there is a 50/50 chance that robotic, or postbiological, sentient aliens do exist. On Earth we have supercomputers that are faster than human brains. In a few decades, some scientists predict, the Technological Singularity event will be reached where computers will not only achieve sentience, but will be smarter than their human creators (Skynet anyone?).
Thus, some feel that it may not be long (just a few decades) before the once science fiction concept of downloading one's brain into a computer becomes an actuality. There are limits to biological systems (which could be thought of as biological machines) that advanced robots could overcome. Biological sentient races may want to evolve themselves into postbiological beings - robots. There will be people vying for this - a chance to live forever (or at least a very long time). As the robotic body gets too worn out to be effectively repaired, one just uploads to a new (and maybe improved) model. Or one could switch bodies as situations warrant:
In a few decades, shall some of us become Cylons?
"Jane, did you forget?
"You're still in your party body. You're working on the space platform today."
"Oh! Right! I need to download to my astrobody! I guess I just love being in my party body too much; I overlooked -"
"- that I was still in it."
"Yeah, yeah, we know where your "heart" really is!"
Jane, laughing, goes to her closet to switch bodies...
Blog in Space III: The Beatles' "Across the Universe", robots make a very pragmatic choice for traveling long interstellar distances (if no faster than light speed travel exists), and for exploring planets that may be extremely hostile to our biological systems due to attacks by alien viruses and predators, as well as surviving a n alien atmosphere hostile to human life. They don't need space suits, or to carry large stores of food. Thus, it is probably easier and safer for sentient robots to explore alien planets.
Some argue, though, that are advantages to an organic brain that trumps speed (intuitive leaps, imaginative creativity). However, a few days ago (13 Aug. 2008) it was reported by the BBC News that researchers had created a robot that was largely controlled by a group of 300,000 living rat brain cells. It is a project to study how memory is laid down as the robot learns how to navigate around objects. Maybe the new human will be a blend of biological and postbiological systems. In a few decades, maybe just call us the Borg...
"Could Robot Aliens Exist?" Popular Science. Sept. 2008: 83. Print.
Kurzweil, Ray. The Singularity Is Near: When Humans Transcend Biology. New York: Penguin, 2006. Print.
"Rat-brain robot aids memory study." BBC News. 13 Aug. 2008. Web 16 Aug. 2008. <http://news.bbc.co.uk/2/hi/technology/7559150.stm>.
Vinge, Vernor. "The Coming Technological Singularity: How to Survive in the Post-Human Era." San Diego State University. 1993. Web. 16 Aug. 2008. <http://www-rohan.sdsu.edu/faculty/vinge/misc/singularity.html>.