A team investigates the alien artifact on the planet World, while interacting with the natives and interrogating an enemy.

I like this kind of character drama, where the people are also involved in doing physics.

The author intertwines physics and characters really well. In true fashion of hard science fiction, the science is presented in detail, and it is very convincing. Who knows whether the theories will be very wrong in the future, or perhaps right? The author has taken a very real phenomenon, probability, and given it some basis. As she has her characters say at one point, certain things occur with a certain probability. Take a large enough statistical sample, and it will always occur with the same probability. Why? Nobody knows. So one of the characters postulates the "probon", analogous to the graviton (still unproven in our time) and other bases that transmit the fundamental forces through the universe. 

The main characters here are Lyle Kaufman, an Army Colonel overseeing the project, the physicist Tom Capelo, and the special genetically modified Sensitive, who can read other people's emotional state by observing their body language. The other characters are incidental, including the main ones from Probability Moon.

Back in Probability Moon, we were introduced to World, and their native population, which were socially restrained because of Shared Reality, such that nauseating headaches would occur if people did not agree on what was happening. They declared humans Unreal because they did not share reality, but because one character gave his life for the natives, to protect them against the blast at the end of the book, humans were declared real again. Of course, we discover early in this book that the blast didn't affect World in the slightest, even though the rest of the bodies in its solar system were turned radioactive. They speculate that the artifact that Gruber found in the tunnels in the mountains protected World, and through several tests, they are proven correct.

Most of the plot concerns how the people go about testing the artefact, and whether it responds correctly. Everybody does their jobs to the best of their abilities, and the conflict comes about not because they are at each others' throats or from an outside threat, but simply because their jobs conflict. Being part of a military expedition means that they know the hierarchy, especially since they are at a state of war. When they go too far with their demands or their work, they pay the price, with barely a protest.

My favorite character was the gruff Capelo. I thought he would become a character that I loved to hate, but I was wrong. He was gruff with everybody, mainly because he had trouble explaining himself to laymen -it was wasted effort, better spent thinking up solutions to physics problems. He and the other scientists dig up the artefact, test it on World, take it into space to test it some more, return it to World for more tests, and finally take it away to help protect Earth's solar system.

The protection settings raise serious questions. If firing a nuclear blast onto the opposite side of the planet does nothing when the artefact is activated, does that mean nobody can fire a weapon on the planet at all? Or is it only external weapons? Will the military have trouble keeping order, not being able to use their weapons in the Sol system?

The artefact is also responsible for the Worlders' shared reality, an inadvertent by-product. By taking some of them off the planet, Ann (a main character from the last book) shows how quickly corruption sets in. Voratur was ready to steal some of the profits from their bargain as soon as he realized that shared reality was gone, relenting only because he knew it would return back on World. When the artefact is taken off World for good, chaos begins to set in. As Ann said, they are about to enter their Dark Ages, where cities isolate themselves, and criminals (who didn't exist before) become more bold. We only see a little of what happens, but it is a sad state of affairs. Protecting Earth from the Fallers requires destroying the culture of World. Is it justified? That should be debated for centuries. But it can be argued, and the military obviously takes this view, that because humans are more technologically advanced, and more spread out, that they are of more value. Of course, World is unique, doesn't have colonies that they could hide away on, and they have a selfless culture -they had one, that is. Leaving Ann and Gruber on the surface with Enli and some others hopefully means that we will get to see more of the evolution of World in the third book, Probability Space.

It is the presentation of the good conflicts, and situations with no good answers, that make this trilogy so exciting.

The third plot, after the artefact and shared reality's demise, is the interrogation of a Faller prisoner by Marbet. There is a small love story, completely unrequited until the end, between Lyle and Marbet, which was quite touching, especially since his emotions conflicted so much with his duties -and he had to put her in the brig for treason! I didn't understand why a Faller would be so interested in a human child, so the capture was less interesting for me. The interrogation took place necessarily behind the scenes, too, because only a Sensitive could distinguish body features so alien.

The most interesting parts of this take place as interactions between Lyle and Marbet. After he gets used to her being nude as part of her work, he begins to trust her, granting her more and more leeway in her work. I wondered at the possibility of the Faller knowing so much about their new shield technology. For a layman it seemed that he knew too much. But they managed to capture a physicist! This guy must have worked on an artefact found by their people. Maybe we were too lucky to find this guy, and keep him alive. Apparently not all Fallers are quick to suicide.

Marbet gets thrown in jail after showing the Faller that humans have found an artefact of their own. Lyle was too quick at that, apparently, because he needed to know some major details before he stopped Marbet from ever seeing the Faller again. Stuck, I wondered if that was the end of that project. But another minor point cleared it up for us, getting Lyle out of his jam, at least temporarily.

I had hoped from the very beginning that Capelo's children were along in the book for a good reason. They didn't intrude too much into a dangerous plot where, as the other characters state, they don't belong. But they get along with Marbet, and one inadvertently hears Marbet talking about the Faller with another of the team. Capelo's wife was killed in a Faller attack, so he becomes livid when he finds out. After becoming subdued, he breaks Marbet out of the brig and has her lead him to the Faller, which he intends to kill. Lyle predicts all of this wonderfully, so he is waiting, to foil Capelo's plan and to allow Marbet to continue her work for a short while until the Navy figures out about the jailbreak. Capelo becomes more interested in the physics than killing the Faller, and right before they are all thrown into custody, they learn that activating two devices on the highest setting in the same system could destroy the fabric of space-time!

With that knowledge, Capelo, when he is subdued and locked in his quarters again, works out some of the fundamental equations for the physics involved. When they get back to Mars, their results outweigh their actions, and nobody goes to jail. As contrived as this sounds, it is actually pulled off quite reasonably. The leader of the military is quite scary, and has way too much power. I wonder how that will play out, if at all, in the next book.

A lot of useful technology has come into being by the time of this novel, and I love the way it is taken for granted. Humanity has developed nanotechnology to a level where nanobots can manipulate individual atoms, level out caves, create mono-filament wire one atom thick, and disintegrate the immobilizing spray. String theory is the central physics concept which is taken as a given, and from which everything else is derived. A grand unified theory of forces is in the past!

And then there are the space tunnels, which nobody knows how to operate, and which can be rather confusing to explain. But they work, so nobody really cares.

This book has most of everything, from a real science plot, interaction with aliens, and good interactions between characters, who all act professionally. I'm not sure why I waited so long between the first two books, but I don't plan to wait long before finishing the trilogy.