Artificial aliens?

 kw: book reviews, nonfiction, science, biology, physics, cosmology, genetics, first contact

To put Sara Imari Walker's last question first, will first contact with a true alien occur in a laboratory? Dr. Walker's book Life as No One Knows It: The Physics of Life's Emergence is a heady brew of ideas. She has a wonderful kind of mind that looks at things from angles others would never imagine existed, and she has managed to find a number of kindred souls with a similar talent.

Dr. Walker and her colleagues propose Assembly Theory, a new view of evolution in which technology has become an agent of evolutionary change, and "selection" takes on a meaning that would mightily bemuse Darwin. Her explanations make sense. I don't yet understand enough about assembly theory to attempt an explanation. I must content myself with a few bullet points gleaned from the third chapter ("Life is what?"; intended to be pronounced with a distinct upward lilt, as, "Life is WHAT??"). These items do not describe life, but rather objects. Objects as the new theory envisions them:

• Objects are finite and distinguishable
• Objects are breakable (I would have said, "can be disassembled")
• Objects exist more than once
• Objects are lineages
• Objects form via selection

Some objects are living things. Some are not. A wooden table of the kind I built long ago to hold my mouse pad next to my desk is an example. I constructed it using nine pieces of wood, eight large wood screws, 24 small nails, and some glue. The top is plywood, which itself was fabricated by someone else before I bought it from a lumberyard and cut it to size. We can inventory the statements. 

• My little mouse-pad table is finite (less than a meter tall) and is distinct from other items of furniture in this room, some purchased (and thus built by someone else) and others built here, by me.
• It can be taken apart, by removing the fasteners and breaking the glue joints (no doubt tearing wood in the process). I don't plan to do so.
• I have made other similar tables, which I suppose qualifies at "exists more than once." Worldwide, there are many tables of many designs, but all related in general shape and function.
• Is it a lineage? The idea of "table", generalized from "supporting platform" is certainly a lineage, going back to the first table or table-like platform built by a human or prehuman creature.
• "Selection" in such a case means that the I selected to make a table rather than a cantilevered shelf hung off my desk, or other possible means of supporting my mouse pad.

Consider a screwdriver. The author writes, "An evolutionary chain of objects is necessary to assemble screwdrivers into existence." In other words, the forebears of screwdrivers include machine shops, mining and metals productions facilities, and creatures like us to think them up. In the case of living things, the "lineage" of everything alive today goes back about 3.9 billion years. All known life comes from life. What came before?

This is the crux of the matter for assembly theory. The theorists use these concepts to imagine life as we don't know it, life as no one knows it.

Side question: Can objects be produced by other objects that are not living? They can, which is apparently where all nonliving and never-previously-living things arose. For example, a planet such as Earth was assembled from dust, rocks, etc., under the influence of gravity and electromagnetism and sundry other possible forces. Also, natural, nonliving actions produce raindrops and snowflakes in clouds, sand and gravel from rocks, and so forth. 

However, the production of objects by nonliving processes yields objects with very few unique parts. Living objects tend to be much more complex, as do many of their products. Thus this conjecture, stated at least a couple of times in the book, "Life is the only thing in the universe that can make objects with many unique parts." The author notes that many kinds of minerals don't form in the absence of life. I recall taking a class in minerology, in which the professor stated, "There are about 1,000 mineral species known in the earliest rocks, before there was much life on Earth. There are now more than 4,000 minerals, and most of them can only form because there is life on Earth." Today the number of catalogued minerals is almost 7,000, and that may grow to more than 10,000 as busy geologists keep finding new stuff.

This principle yields the basis of the author's "assembly index", a measure of the minimum number of steps needed to produce an object. This gets us away from looking for life that is too chemically similar to "Earthlings" (from microbes and viruses to whales and forests, with us in the mid-range). If the assembly index of numerous objects collected on an exoplanet is large enough, we could conclude that life most likely produced them. The critical number is, according to the author, fifteen.

Frankly, I don't know how assembly index is calculated. I read that the assembly index of the molecule ATP is fifteen. This molecule has 47 atoms. Perhaps the calculation allows the synthesis to begin with smaller molecules that natural processes have already produced: water, carbon dioxide, ammonia, phosphorus oxide, and perhaps even small hydrocarbons such as ethane (ethane can result from abiotic, or nonliving, processes, but in the presence of a living biosphere we never observe it).

Therefore, the number 15 seems to be a good "filter" to discern objects produced by life.

A second prong in the approach by proponents of assembly theory is the development of a "chemputer", a semi-automated way to sort of "3d print" molecules, designed to have certain properties, in an attempt to produce a chemical system that takes on the attributes of life: reproduction, ingestion of supplies, elimination of wastes, relationships (very broadly construed), for example. Were such an effort to succeed, using large numbers of chemputers to explore the "assembly space" of small-to-medium sized molecules, there might indeed be alien life produced in the laboratory. It would be as alien as anything we might find on a planet far away, perhaps even more so. 

The key to grasping assembly theory is the claim that all things life can build are historically contingent. We can see this in the visible relatedness between the wide range of animals known as "tetrapods". They all have four limbs. The mythical flying horse Pegasus cannot evolve from a horse, because to do so would require adding two new limbs (to become wings) to a body that already works well with four limbs. The intermediate steps required don't make sense (and let's ignore the physics of wings long enough to support a half-ton animal). In fact, mammalian hexapods in general aren't likely to arise, because the competition from extremely well-adapted tetrapods has already pretty much filled all available ecological niches for critters bigger than a cockroach…on land, at least.

Historical contingency is a key concept. Finger-and-toenails didn't evolve all at once. They descended from claws. Claws came from something else. All living things trace back to a single-celled creature called LUCA, the Last Universal Common Ancestor. LUCA may not be the first cell. At one time other living things could have existed alongside LUCA that may have had quite different chemistry and cellular mechanisms. But only LUCA has descendants: us, and every living thing in the biosphere of Earth.

Our chain of imagined forebears stops with LUCA. Our knowledge is further constrained, because it is extremely unlikely that any creature now living is descended without change from LUCA. When I say "extremely unlikely", that middle E needs to be drawn out, "Extreeeeeeeeeeeemely!" Meaning, utterly impossible unless the universe is truly infinite, with an infinite number, not just of planets, but of biospheres. Even if we somehow track back the chain of biochemical contingency to "show" us a robust model of LUCA, the chain stops there.

I like the idea of the chemputer. But I don't hold out much hope. The assembly space of "small molecules" is too big. For example, if we "restrict" ourselves to the twenty most common elements, all of which are found in known life, and all of which are likely to be in any possible kind of life, and further, we call a molecule "small" if it has fifty or fewer atoms (just a tad bigger than ATP), the number of possible chemical species (most of them quite chemically unstable) is close to ten-to-the-power-of-65. A 66-digit integer. How large is that number? The number of stars in the visible universe is thought to be a 22- or 23-digit number. Could the average number of planets be as great as ten? Even if that were so, the number of planets in all the galaxies in the visible universe is no larger than the largest 24-digit number. It is hard to think about 66-digit numbers in any useful way. 

Dr. Walker dreams of being the researcher in the cartoon above, "meeting" the first true alien in the laboratory. I hope her dream comes true. But I think it more likely that we'll come across something, somewhere, soon, that proves life as we don't yet know it does in fact exist.

Sing for your supper - er - survival

kw: book reviews, science fiction, space aliens, first contact, singing, competitions

Well, after a few serious books in a row, I was ready for some mindless escapism. Boy, did I find that! Space Opera, by Catherynne M. Valente is about the silliest bit of SF I've come across, yet based on an idea worth thinking about.

Scenario: When the aliens arrive, they arrive to everyone at once. By some kind of projection, a bizarre-looking alien being visits every single human on Earth, wherever they happen to be at the time, and converses with everyone individually (only much later in the book is there just a hint about translation technology). After various amounts of time for the human involved to get over the shock, the discussion gets serious (as serious as anything gets in this book): One human person or group is to be entered into a Galactic singing contest, to determine if humans are sentient. It turns out that "sentience" is not so self-evident as we'd like to think.

According to the Esca, the species chosen to "recruit" a human singing group, all entrants in the song festival are ranked, including any newcomers (in this case, humans). If a newcomer winds up ranked dead last, the species is indeed "Dead Last": the entire species is to be exterminated and the planetary biosphere will be given another few million years to evolve a new, purportedly sentient, species. Rinse, Dry, Repeat as necessary. The Esca were the most recent species to "enter" the contest, and are quite proud that they came in Eighth, rather than something like Seventy-Fourth, or whatever Dead Last would be. The proven sentient beings of the Galaxy have decided that the extinction of an occasional species is the price to be paid for the peace-promoting songfest. It succeeded "Galactic War X", after all.

To give you an idea of the hyper-enthusiasm of language Ms Valente, here is the description of the leading member of the Absolute Zeroes, the group that is taken a few thousand light-years to the competition venue, a certain Mr. Decibel Jones (né Danesh Jalo):

…a leggy psychedelic ambidextrous omnisexual gendersplat glitterpunk financially punch-drunk ethnically ambiguous glamrock messiah…

The prose reminds me of teachers of the art of composition, who advised us, "Use more adjectives!" This book is about 2/3 adjectives, frequently in sentences that run 100-200 words. Sentences full of immense enthusiasm. Also full of neologisms. Amazingly, newly coined words seem to fit into the text around them and practically define themselves. Other than the occasional jarring bit of potty-mouth, prose like that just washes over me like perfumed olive oil.

The concept is actually based on a real bit of history, though "last place" in a certain global competition does not result in anyone's being exterminated.

I don't know if I could call this escapism at its best, but it is certainly escapism of a most compelling sort.

First Contact - bugs versus gods?

kw: book reviews, science fiction, first contact, china, cultural revolution

What could induce someone to such deep hatred for humanity, as to wish our total destruction? A good place to begin might be the Chinese "Great Leap Forward", known in the West as the Cultural Revolution (hereafter, CR). This great leap backward set China back about a century, and it was only because of a massive shift in political attitudes—facilitated by Mao's untimely death in 1976—that the nation has been able to (nearly) grow into a 21^st Century superpower. Make no mistake, China's leaders are still committed Communists, but their social mind-control is only a whisker of what existed prior to the 1970's.

In The Three-Body Problem author Liu Cixin, using evocative and shocking prose, begins by setting the stage upon the background of the CR and a handful of persons so traumatized by it that they abandon hope that humanity can become upright and beneficent. One in particular, astrophysicist Ye Wenjie, determines a possible way to send a powerful signal to nearby stars using the Sun as an amplifier. When she gets a response more than 8 years later, a key element is in place. She devises a plan and eventually becomes titular leader of a group devoted to inviting the aliens to invade, and promising to collaborate with them in either reforming or destroying human society and perhaps the human race. As that sentence hints, there are factions within her organization, upon which much drama later in the book depends.

The star system thus contacted is commonly called Alpha Centauri, a 3-star system that is the "nearest star" to our solar system. Any planets of such a system are likely to have chaotic orbits, or at best alternating between stable and unstable orbits and thus climates. Thus the book's title. These aliens have strong motivation to move to a planet with a billions-of-years history of climatic stability, at least relative to theirs. They are called Trisolarians, for their three suns, and in late chapters are seen to have a considerable technological advantage over humans. I find that paradoxical; few of their hundreds of civilizations lasted longer than several generations, so how could they advance so far?

The book's translator, Ken Liu, writes in an afterword of the responsibility to provide not just a word-by-word translation (our different grammars don't really allow that anyway), but one that evokes emotions and illustrates concepts in such a way that the reader can partake of the author's thinking. (The author and translator are probably not closely related; Liu is the fourth most common Chinese surname.) For me, the writing is more straightforward than most modern English prose, which I found refreshing. It harked back to the fiction of my youth, when at least science fiction writers had fewer literary pretensions.

This is book one of a trilogy. All three have been translated, so I intend to track down the other two so I can see where the author is going, after closing with the foolishly arrogant message from the Trisolarians, "You're bugs!". However, their fleet will take four centuries to arrive. Can humanity achieve enough progress in that time to avert species-wide disaster? That is no settled matter, and curious Trisolarian AI's called Sophons may make it moot. I guess we must stay tuned.

For a Chinese writer to produce such a novel, in China, strongly indicates just how far that nation's leaders have moved from Maoist super-totalitarianism.

Letting ourselves be found, revisited

kw: book reviews, science fiction, first contact, spacecraft, SETI

I reviewed James Gunn's The Listeners just a month back (enter Gunn in the search box above). He takes another whack at the subject in Gift from the Stars.

The prior novel concerned exchange of radio messages, which occurred over a ninety-year span. I wondered at the time how anyone could predict "answer day" with any accuracy, given the several-light-year uncertainty in the distance to the star. The message received was hidden within a replay of very old radio transmissions, so I supposed that the time lag could be calculated.

This novel has the message arriving by gravity waves, bearing plans for a working starcraft and antimatter energy conversion. An engineer has decoded the message from the first gravity wave detector, written a book containing the plans, hidden within an apparently crackpot UFO scenario, then gone insane with worry...he was unstable to begin with (a stereotype of the highly creative). Another engineer, someone who knows rocket science, finds the book.

The long and short of it is, the improbable happens; the engineer and a few friends republish the plans via the Internet, and later manage to wangle agreement to build a starship from the new planetary authority (the old was swept away by an economic revolution based on antimatter energy).

This story is more satisfying than the former, though a few things are infinitely more fantastic. However, I know the Corollary to Finagle's Law: everything is going to cost ten times as much and take twice as long as you thought. Constructing a ship to carry 200+ people for a few years, regardless of propulsion method, would bankrupt the entire planet. A pity.

How would having neighbors change us?

kw: book reviews, science fiction, first contact, SETI

I read only a portion or two of James Gunn's The Listeners in the 1970s, when it appeared piecemeal. It was refreshing to find the whole volume and devour this 35-year-old classic.

If a civilization on another planet were to send a signal we could receive, what would be its content? Some years ago, some folks devised a message in several parts, and put it, part by part, weeks between them, on a web site for all to decipher. It was clever, and many clever people deciphered it at least in part. As it turned out, not some parts of it were amenable to more than one interpretation. Making a completely unambiguous communication of any complexity is hard, and perhaps impossible (remember the old story "To Serve Man", which turned out to be about a cook book).

Jim Gunn has come up with perhaps the best compromise. The message, once the signal is detected, is composed primarily of a hash of early radio transmissions. The content of those transmissions yields a good estimate of the distance to the alien star: 45 light years. But there is more. Each short segment of the hash is followed by a staticky blip. Each blip contains simply several pulses. Once the project workers realize they can be considered an on-off raster pattern, it doesn't take long to produce an image that gives a few clues to the aliens' appearance and number system, plus a few words.

Much of the dramatic tension in the book surrounds first, the apparent futility of the Project before a signal is detected, and the meaning of the message it conveys. Of course, for the novel to go anywhere after this point, the message must be answered with one of our own; and it is, a similar image. Ninety years later, one would expect an answer, and the denouement follows from that answer.

I found myself wondering, would the world really develop such an extended peaceful period after receiving the first alien message? War and chaos are equally likely, particularly once people realize knowledge coming from an alien source will be a valuable resource like any other: something worth fighting for. We are unlikely to unite in the face of The Other until we physically face The Other.

I think the religious response in the novel is too pat. That a religion could be formed around the "Solitarian" perspective seems incredible. More likely, a "Christian" sect would arise who found the idea of aliens to be some kind of attack on the uniqueness of God.

These are minor flaws. As a classic in the field, The Listeners sets the standard others must still meet.

Defense of the giant grasshoppers

kw: book reviews, science fiction, historical fiction, medieval period, aliens, first contact

Years ago, I read a novella that placed a time-traveling scientist in a medieval village during the 14th century plague. It devolved to a psychological study of the young woman, herself immunized against Yersinia pestis and a host of other ills, becoming the last person alive in the village, finally burying the village priest.

Eifelheim by Michael Flynn reminded me of it. I know that this book is an expansion of an earlier novella by Flynn, but I haven't determined if it was the one I read or not. There is no time traveler here, though the cosmological theory developed by one character hints at a way to travel in time.

The basic story, 80% narrative of 14th-Century life and 20% vignettes of the lives of the historians studying the earlier events, is of a starcraft that barely escapes crashing in the Black Forest, but lands damaged. Its insectile inhabitants are, rather astonishingly, welcomed by first one, then two or three of the first humans to locate their landing site. Gradually they become accepted by many, but not all, of the villagers. At the end, the ship is repaired, at least enough to take off, though a successful return to home is not assured. A few aliens remain behind, preferring certain, but slow, death in the German village to the rather significant chance of an infinitely-prolonged dying if the ship gets "stuck" in a space warp. Should their fellows succeed, they plan to return for them.

The aliens cannot gain full nourishment from Earth foods, because their biochemistry requires 21 amino acids, while nearly all Earth life lives with 20. The missing ingredient is never found, and the ship does not return. Rather, everyone dies of plague, except the aliens who die of malnutrition, and the village gains the name Teufelheim (Devil's home), later elided to Eifelheim.

Back in the present, a historian is trying to determine why this location alone, now with the name Eifelheim, was never resettled after being depopulated by plague in 1349. His lover, a physicist, is trying to develop a better model of cosmology than 11-dimensioned string theory. About the time she succeeds, she happens to see a strangely-configured illuminated capital letter in a manuscript the historian is studying, and recognizes that it is a circuit diagram similar to one she had designed to detect signs of her theory's special particles. The manuscript and others were located by a librarian who is trying to seduce the historian; at the end, it seems she might have done so, but that strand is left hanging.

In fact, a number of strands are left hanging, in the rather obvious way that is becoming common—it almost harks back to the cliffhanger endings of Perils of Pauline episodes. While the historian and librarian, with the help of a narrator-colleague, do confirm that the "demons" of certain documents really were aliens, no other subplot is completed. It's no surprise that Flynn wishes to write more books. Ending a novel with no more than a hope of the solution is an unsatisfying way to prompt for a series of sequelae.

As different an alien as he could imagine

kw: book reviews, science fiction, space travel, aliens, first contact

Peter Watts is a very accommodating writer. Following an increasingly popular practice, he has appended to his novel Blindsight a section explaining and discussing some of his ideas. Blindsight is the first book by Watts that I've read, so I don't know what he did with his earlier Rifter series (four books). In this book, he plunks us down in a society that is taking genetic and cyborganic tinkering pretty far, to the point of restoring recently lost species such as the Vampire, a Human subspecies that, being a predator on the next-most-efficient predator (us), has mental skills needed to lead increasingly complex projects, such as the mission to an object in the Oort cloud that seems to harbor an alien intelligence.

The narrator is a Synthesist, Siri Keaton. In this case, that means he's someone who had half his brain removed to eliminate debilitating epileptic seizures (complete hemispherectomy is now rare; functional hemispherectomy, which disables one hemisphere but leaves most of it in place, is performed on a few hundred people yearly). While they were at it, the installed some equipment in its place. Now, he is able to read body language well enough to just about read your mind. He is one of a team of four heavily altered and enhanced people sent to study the object they call Ben, and contact any aliens they might find. Their team leader is a Vampire.

The novel's title comes from the ability of the aliens, once encountered, to hide in plain sight. In the presence of one person, they can take advantage of the saccades, the tiny movements our eyes make constantly, to trick the brain into ignoring them. These aliens are really something. Watts has posited an alien physiology for which magnetic fields (really strong ones) and ionizing radiation interact with super-resistant proteins and superconducting polymers to produce creatures that read the electronic signals in a human brain at a distance, and the myoelectric signals from muscles, to read the people much better than Siri could hope to. All this is discovered over much time, and for much of the book, things keep going on in one mysterious way after another.

The tensions of studying the alien Scramblers, and an intelligence—artificial or otherwise, we don't find out—that controls their city-size orbiting habitat, pale by comparison to the tensions between the four very different human/cyborg crew, and the most-different member, the Vampire. In the end, we find that selective attention, which enables conscious beings to function in a very stimulus-rich world, is a fatal detriment to us in the presence of creatures that seem not to be conscious as we know it, but can process all stimuli in real time; their attention is not selective, so they learn really, really fast.

The concept of a Chinese Room is used to great effect to explain the difference. This thought experiment by John Searle is used to question the meaning and usefulness of the Turing Test. One the one hand, Turing asked, if you communicate with a computer by teletype, and cannot determine whether it truly is a computer, is it intelligent? On the other, Searle proposed a person in a room who receives written or printed messages that he cannot understand, but that he can recognize and find in a large (very large, I'd think!) rule book. He doesn't even know they are Chinese ideographs. He just looks up in the rule book how to respond, and writes the response on a fresh sheet of paper and passes it outside the room to an observer. Is the Chinese Room intelligent?

Some think consciousness is an illusion. Descartes offered the best reply (I think, thus I am). I know I am conscious, and you know that you are conscious. That poor soul over there doesn't think either one of us is conscious, but perhaps he (she?) just suffers from impaired self-consciousness...

Space Opera for the eXtreme generation

kw: book reviews, science fiction, space opera, aliens, first contact

Here is everyone's blackest nightmare "first contact" story. Mysterious aliens have been contacted over a number of years by robotic and AI spacecraft of The Polity, an AI-ruled space empire. No image of the aliens' likeness, or of their spacecraft, has so far been permitted to return, only a few sparse records and distant views of a technological civilization on some of their outlying worlds.

Finally, a meeting is arranged. The docking bay specified by the Prador is unnervingly huge, big enough for them to be animate locomotives. They are, instead, big crabs. Very big crabs with very tough shells, nearly irresistible weapons, unthinkable hostility, and a taste for human flesh. They are looking for slaves and food, and we seem to fit the bill.

In Prador Moon, Neal Asher pulls out every stop on this old nightmare. Of course, humans just have to win, even at a terrible cost. It takes the confluence of an entirely fanatical warrior who has seen his lover wantonly destroyed and macerated, a couple of AIs with their own mysterious ways—using human or humanoid bodies—, and a woman with a mysteriously enhanced computer implant, a few million times more capable than the implants most folks wear; all these combine to drive off the alien attack. But there's plenty of grist for sequels...and The Polity has already been the subject of a few of Asher's books.

Who you are depends on what World you've learned.

kw: book reviews, science fiction, interstellar colonization, first contact, posthumanism

About five years ago, walking in the park with my wife, I saw two little girls—third grade age or younger—running and skipping along, while talking on cell phones. I said to my wife, "In ten years or so, we won't buy cell phones. They'll be installed just behind the ear from birth. You'll tell them who to call." It will probably be quite a bit more than another five years, but I believe people will have built-in communications available in my lifetime. We just need to get speech recognition into a chip the size of a fingernail, powered by body heat (the 5° difference between core and skin).

My father's pacemaker, just a bit bigger than a man's wristwatch case, has some of the enabling technologies. It communicates by low-power radio with the doctor's computer (only within the office at present), can be reprogrammed easily, and has a five-year battery life. Newer models' batteries can be recharged by an induction loop (you need to do slow, overnight charging, to avoid excessive heating).

Fast-forward fourteen thousand years, at least. The humans of Earthly origin in Learning the World by Ken MacLeod, have Virtualities and other services, delivered by a synergy between circuits in their heads and public carrier services. The cell phone has morphed into an experience as immersive as you'd like. That is but one good idea from this book.

Another is fully translating the words and lives of two mutually alien species. Consider this: How would you translate, «Cette phrase est écrite en français», from French to English? There are valid arguments for translating it either "This sentence is written in French" or "This sentence is written in English." A certain novel, with dramatic tension based on the historic animosity between French and German people, and written in French, has been translated into several languages (sorry, I don't recall the title, just the story). The action takes place years into one of the occupations of France by Germany. It contains a number of short German phrases, and some phonetic dialog in French with a German accent. One wise translator, rendering the book into Polish, substituted Russians and Russian for the Germans and their language, and used phonetic dialog of Polish spoken with a Russian accent! It is by all accounts the most effective of the translations, because a Polish person reading it feels the same feelings a French person feels when reading the original French novel.

I have read many science fiction novels written from the viewpoint of interstellar aliens. Even in those most sympathetic to the nonhuman protagonists, there is a tendency to have them call themselves something like "Gvorch", and speak sentimentally about "gvorchkind". Often, the English language is distorted, and maintains thus a consciousness in the reader that the "alien" is really alien.

Ken MacLeod will have none of that. Both species' words and thoughts are presented in clear, straightforward English (except when the person is being deceptive). Both call themselves "human", "man", "woman". From the first transmission of images and sound until the two kinds of people meet, each thinks of the other in quite a xenophobic way, mixing fascination with disgust.

Actually, the batlike residents of the planet Ground are more cohesive and uniform than the interstellar travelers from the vicinity of Earth. The ship has three populations: Founders, parents of the Ship generation, which is the second, and Crew. The founders have been aboard for four centuries, as have the crew. But the crew live in free-fall, in a section of the habitat having an ecology adapted to microgravity, while the founders live in the rotating section of the ship. The ship generation is trained to colonize and settle an asteroid belt, and perhaps the occasional planet, though they are more comfortable in enclosed spaces. Once a system is colonized, the ship will go onward, with some of each population switching allegiance so that some ship generation become founders to the next colony trip, a few founders and crew remain behind, and so forth. There are more details, of course, but this gives a bit of a flavor.

Another good idea: the stock market. The ship is full of futures traders, and their fortunes wax and wane based on perceptions of which colony segments will do better or worse.

And another: that the spread of a planetary race into its home system gradually changes the color of its star, perceived from afar. Freeman Dyson posited that a really advanced society would gradually englobe its star with solar-powered orbiting habitats, changing it to an infrared-emitting body. MacLeod instead expects the plant life of largely transparent orbiting habitats will make the star greener. This has implications in the progress of the story.

A word on morals: While the story is a morality play, the characters are amoral as regards sex. This is the most consistent characteristic of science fiction since the 1960s, so is unremarkable, but must be borne in mind.

I read because of ideas. Ideas are our most powerful tools and weapons. This novel is packed with both new ideas older ideas further developed. Hands down, it is the best-thought-out story of interstellar colonization I've read.