Handwavium. What is it and why do reviewers excoriate writers over it? Well, handwavium in science fiction is when an author writes about a scientifically impossible idea without attempting to create a basis for it; essentially, the author says, “It is because it is.” This seems to come up mostly regarding starflight or FTL (faster than light) travel when the author doesn’t try to create some basis for its existence. Instead, the characters just jump into their spaceship and fly around the galaxy. Of course, the problem with giving a plausible form of starflight is that we can’t come close to doing it in reality. The Bussard ramjet, first published back in 1960, provided a concept for reaching relativistic speeds without impossible amounts of propellant and was the foundation for Poul Anderson’s famous Tau Zero. Even though the original proton-proton reaction won’t do the trick, the nuclear catalytic ramjet idea published in the 1970’s might. But you still have to go fast enough to light the ramjet. I’ve seen that published as 6% the speed of light in texts on propulsion. Figuring a means of propulsion to get to that speed is also a problem (it’s the amount of propellant that is astronomical). There are concepts for propellant-less drives, so maybe that’s one way to go. Trying to go FTL is even more speculative. There are publications about wormholes but, unless there are pre-existing ones we could find (and haven’t so far), making them seems to imply a need for negative mass. The Aclubierre “warp drive” concept has been published by physicist Miguel Aclubierre but I think this also needs negative mass (this is based on what I’ve read, I cannot handle the math involved). End of the day, using any of the approaches that have some grounding in science as the basis for a star drive is going to leave the author waving his hands at some point and saying, “You just gotta believe it!” (I suppose you could wave a wand just as easily, but then we’d call it fantasy.) What I find interesting is that people will come down hard on books that don’t provide some background for starflight but don’t do so on books that have characters, for example, who have their nervous systems re-wired to provide lightning fast reflexes or other super-human traits. One is every bit as much handwavium as the other. Where I am going to go with this is that I think we, as readers, need to decide what is our personal comfort zone for suspending disbelief. If we need some sort of rationale that at least starts from known physics or medicine or biology then those are the books we will enjoy and we should avoid ones that don’t. If we are willing to simply take it as a given that we can fly to Tau Ceti in a week as long as the characters and the plot are good, then that should be fine, too, because no matter how much science we try to build in, at some point we have to suspend disbelief. You can’t have your cake and eat it, too (that would violate conservation of matter …)