The Probability Trilogy Ranks with the Best of Hard Sci-Fi

When it comes to hard science fiction, few authors come to my mind more readily than Nancy Kress. Within her extensive body of work, much of which explores the genre with as close an adherence to real science as possible (while still allowing for, you know, the fiction part), is a trilogy that I return to time and again, if only to marvel at the sheer number of ideas she crams into it alongside a fantastic outer space adventure: the Probability trilogy—Probability Moon, Probability Sun, and Probability Space.

In these three books, humanity has expanded out into the universe with the help of a network of ancient alien star gates that allows for instantaneous transport between points in space. Our colonization efforts have brought us into contact with many other alien species, all of which seem to share many of our physical and genetic traits, suggesting that, millennia ago, some great progenitor race—perhaps the same one that constructed the transport network—seeded the galaxy with the building blocks of life.

Unsurprisingly, being human, we’ve also come into conflict with another species, known as the Fallers, who are slowly winning the battle. And the funny thing about the Fallers: they appear to be of different stock than just about all other life in the universe, and they don’t appear to be interested in a peaceful coexistence. Humanity’s days are numbered.

However, there’s some hope: in our explorations, we discover World, a planet inhabited a primitive humanoid species with a rather unusual trait: a form of telepathy that allows them all agree on a universal truth about any given situation, to an individual (any attempt to hold a differing opinion causes physical pain). And this strange species isn’t even the real purpose of the mission to World; the true target is an alien artifact that has crash-landed there, an artificial construct that could be our best hope of victory against the Fallers.

Kress splits her time between the humans and aliens of World, building a fantastic story of first contact and exploring how alien cultures might meet and inspect one another. It’s a long-standing trope for science fiction, but she has assembled a take on it that feels fresh, even years after the books were first published.

There’s a great sense of scientific knowledge that informs the character’s actions and motivations, and in the second book, Kress continues the exploration of the artifact as humanity pushes back against the ever-advancing Fallers. Though we’ve found the object and have strong reason to think it will be a critical weapon in the war, nobody is quite sure what it does, how it should be used—or what the consequences will be.

In the third novel, humanity takes the fight directly to the Faller’s home system. The battle grows desperate, and the use of the artifact will prove to have dire consequences for the entire galaxy. This final installment earned Kress the John W. Campbell Memorial Award for Best Novel, and wraps up the story in grand fashion.

One of the things that helps the Probability trilogy stand out on the bookshelf is Kress’ willingness to writes across subgenres: she dabbles with hard science, military SF, and first contact tropes, all against the broad canvas of a space opera. While some of the tools she’s playing with have been honed by generations of SF writers—there are similarities to books like Orson Scott Card’s Ender’s Game, for example—Kress’s trilogy is an innovative, and more importantly, interesting adventure. Even her take on wormhole-assisted travel (something we’ve seen time and time again) is given a fascinating new wrinkle: Ship A can travel from gate to gate, but if Ship B enters the transportation network before the Ship A exits, Ship A will emerge where Ship B went in. It’s the kind of brain-fuzzing SF logic that’s great fun to contemplate, one of those cool ideas that makes the story seem larger than it really is.

It’s difficult for any book to remain top-of-mind years after publication, but the Probability trilogy is one of those back-shelf series that deserves rediscovery, especially for hard sci-fi fans. They’re among the best you’ll find in the subgenre.