Schild’s Ladder

It’s been a while since I’ve read a Greg Egan book. I often love the ideas he explores, particularly in Diaspora. But I sometimes find his stories difficult to get through. That was definitely true of a previous book I read, Incandescence, which takes place in the setting of an interesting interstellar civilization. But the story seems to slide into a thinly veiled tutorial on general relativity.

I had heard that Schild’s Ladder was one of his more accessible works, but held off reading it until now. I’m glad I did. A decade ago a substantial portion would have been unintelligible. Today I know enough about quantum mechanics to follow many of the discussions and tactics used in the book. But I’m not sure the average reader would find this book that accessible. It doesn’t get into the math (although there are a few diagrams), but it helps to have a conceptual understanding of how superposition, decoherence, entanglement, and similar concepts work.

The setting is twenty thousand years in the future in a posthuman interstellar civilization. People can backup their minds and transfer to new bodies, and are generally immortal. This is hard sci-fi so no faster than light travel. In most cases traveling interstellar distances, typically by transmission, means separation from friends and loved ones for decades or centuries.

In some cases when someone travels, their entire home planet population will go into “slowdown” until they return in order to avoid having them become too out of sync. Slowdown is a protocol that it’s possible to cheat on and exist in real time while everyone else is moving and thinking at a miniscule fraction of the normal pace.

There are also a group of humans known as anachronauts, who left Earth thousands of years earlier, before mind copying had become viable. They travel in sleeper ships, occasionally stopping at a world to check in on how humanity is developing. As the name implies, they’re largely throwbacks, and have difficulty accepting many of the changes that have taken place.

One of those changes is that sexual dimorphism no longer exists among humans. The “he” and “she” pronouns only continue as linguistic conventions relative to types of names. Sex requires people’s bodies to react against each other’s pheromones and gradually become compatible, maximizing the chances that it’s monogamous and consensual. Egan doesn’t describe his characters in physical terms, which likely downplays just how strange we would find them. (This conception of sexuality also reminds me of Ursula Le Guin’s Gethens in The Left Hand of Darkness.)

People’s minds operate on qusps, quantum “singleton” processors, which ensure that any decision is worked out in an isolated quantum superposition and then promoted prior to interaction with the environment, so that a person only ever makes one decision. In other words, they don’t branch (many-worlds style) into multiple versions based on their decisions. (Although it’s accepted that they can and do branch due to other quantum outcomes.) So the premise assumes wave function realism, which becomes important at various points in the story.

Quantum mechanics and general relativity have been reconciled into a framework known as the Sarumpaet rules, presented as a descendent of loop quantum gravity theory. The rules are thousands of years old and heavily tested and validated. At the beginning of the story, it’s hard for the characters to take seriously the idea they could be wrong. But it becomes clear they are when an experiment goes horribly wrong, leading to the accidental creation of a “novo-vacuum” that begins expanding out at half the speed of light, consuming everything it comes in contact with.

As the centuries pass, the novo-vacuum consumes hundreds of star systems necessitating large scale evacuations and migrations. A space station, named the Rindler, is built just outside the boundary of the novo-vacuum with its speed matched to the expansion rate, and deploying scientific instruments to probe the “far side,” a nickname for the novo-vacuum, as opposed to the “near side” for regular space.

Initially built by scientists wanting to study and understand the far side, the Rindler‘s population has swelled as additional people have arrived to participate in the studies. But a couple of camps have formed: the Preservationists, who want to stop the novo-vacuum and destroy it to preserve as many of the existing planets as possible, and the Yielders, who want to stop the far side’s growth but then study it. Relations between the two sides have become bitter.

Tchicaya, the protagonist, is a Yielder who has just arrived on the Rindler. His childhood friend, Mariama, arrives shortly after him. He has not seen her in centuries. She is a Preservationist. Which is ironic because as children, she was the more adventurous one while Tchicaya the one most inclined to preserve the existing status-quo.

This book has a moderate amount of conflict in it, even some violent conflict, something often missing in Egan’s stories. Although as in his other posthuman books, the society envisioned is pretty utopian. The violence comes from the anachronauts, who are from outside the utopia. And the story eventually converges on a typical story frame for Egan, two people together on an odyssey of discovery.

I don’t think I’m spoiling much by noting the final portions of the story explore the changed physics inside the far side. This is a long standing fascination for Egan. He loves exploring alien physics, something that, based on the descriptions of many of his more recent books, has only increased over the years.

I enjoyed this book, but I’m not sure I would have enjoyed it as much without being familiar with the science. Egan’s work is pretty much the hardest of hard sci-fi, which means quantum physics is central to the plot. Often in sci-fi the story can be enjoyed by people who aren’t necessarily into the scientific speculation. I’d say that’s less true here. There is some character drama, but relatively limited. If the idea of characters working on an intractable problem in a quantum superposition so that some version of them finds the right answer sounds like your jam, then it’s probably worth checking out.

Have you read it? If so, any ideas from it that particularly resonated with you?