Once again I have to defend sci-fi and fantasy to my fellows, and as i do with many of my arguments I repeat my point here to make myself feel better.  The problem with arguing about believability is that much of the believability is work the comes from the readers.  People opposed to SF and Fant. aren't willing to do this work.  Yes, the good stuff must suspend you're disbeliefe but you also must allow it to be suspended.  I like to think that they are trying to make a connection with you, saying "Please come on this journey with me.  Have faith my friend."  It's not how much can they trick me (special effects if it's a movie), but am I willing to believe in them. That's why cult followings happen.  We believe so hard.  That's why fandoms get huge and fanficcers go nuts.
But outsiders never do understand... they call US outsiders... the theme of outsiders being the heros in science fiction and fantasy stories is another thing to explore, it creates alot of the appeal.  But that's for another day... another post.