I don't believe in flying saucers... The energy requirements of interstellar travel are so great that it is inconceivable to me that any creatures piloting their ships across the vast depths of space would do so only in order to play games with us over a period of decades. Isaac Asimov
If interstellar travel is as time- or energy- demanding as the above figures indicate, it is far from obvious what the motive for colonization might be. Bernard M. Oliver
Human uploads have such a natural advantage over present-day people in the environment of space, it's exceedingly unlikely flesh-and-blood beings will ever engage in interstellar travel. Frank J. Tipler