For ET-believers like Carl Sagan, the sheer number of stars suggests that there are many civilizations out there, some of them more intelligent than we, and some of them much older and farther along (technologically) than we. It is these assumptions that for such believers justify scouring space for their communications (as in the SETI Project).
But realistically there are fatal problems to any hypothetical advanced civilization in physically contacting us. And these problems have nothing to do with level of technology (as is generally assumed), and everything to do with the physical laws of the universe.
As is discussed in much more depth in the book Lights in the Sky and Little Green Men, there are physical limits to space travel speeds that make crossing the distances between stars in our universe physically impossible (or, at least, an endeavor necessarily spanning the lifetimes of many generations of the ETs in question). I know this is hard to hear for those of us raised on Star Trek and Star Wars, but I was asked here for scientific thoughts, not fictional stories.
Moreover, as astronomers continue to look for potential life-support planets outside our solar system, they are systematically eliminating all systems within any remotely reasonable distance from Earth.
So, the latest scientific discoveries have demonstrated the extreme unlikelihood of life anywhere else in the universe. In addition, the physics of the universe are such that travel across space is prohibitive for even a hypothetical advanced being. All of this, then, begs the question
What of all the sightings--of UFOs and ETs--and reports of actual encounters with and abductions by aliens?I'll focus on that question in the next post.
*If God made other life-support planets and then created life on them, He has chosen not to reveal that to us. Part of C.S. Lewis' motivation in writing his Space Trilogy was to show that extra-terrestrial life would not be inconsistent with Christianity.