Songs of Distant Earth uses a few main scientific devices, but the foremost is the effects of relativistic time dilation. Remember high school physics class? The faster an object moves, the closer it comes to the speed of light, the slower time passes for that object. (At the speed of light time does not pass at all, incidentally) So, while 50 years may pass for somebody who is stationary on the surface of a planet, for instance, a person aboard a sufficiently fast-moving spaceship will only experience the passage of a few.
In the earliest 21st Century, we are told, Earth's scientists discover a way to harness the power of neutrinos to explore the inner workings of the sun. To everyone's horror, they find that the Sun is a time bomb: set to explode into a nova sometime around the year 3600. Humanity may survive, but not on Earth. The nations of the world band together and construct colossal "seedships" capable of traveling at vast sub-light speeds and containing enough genetic material (of people, animals, plants, insects, etc) to populate distant worlds from scratch. Many are sent, and the dwindling population of Earth prepares for the coming cataclysm.
The narrative opens on Thalassa, an oceanic paradise world 160 light-years from Earth consisting of a huge sea and a habitable archipelago of islands. It is about 3800. Thalassa is one of the planets conquered by a seedship, and a happy, carefree human society has existed there for about 700 years. They have no radio contact with Earth, as their antenna has been in disrepair for years due to their blissful negligence. One day, however, they receive visitors: the last starship to leave Earth, making a stop on its way to Sagan II, its final destination.
Unlike the seedships, this is a lifeboat of sorts, carrying a million refugees from Earth frozen in stasis, and a small live crew. These are Terran natives, who were born on Earth, and who witnessed its terrible destruction. I feel like the interaction between the Terrans and the Thalassans is almost Biblical in nature: the Thalassans are innocent occupiers of a new Eden, and the Terrans come from the sky with the knowledge of good and evil. Indeed, a recurring theme is the reluctance of the Terran visitors to "corrupt" the Thalassans' harmonious society.
Among the novel's many themes is the loneliness of a humanity divorced from Earth. There is real, unexpected poignancy in the way Clarke describes Earth's final days, and the after-effects its destruction has on the survivors. I think a lot about how most of our brains are so removed from the natural environment they evolved in, so this concept really hit home with me. It is moving the way the characters struggle with their memories of loved ones from their home planet: all dead 200 years ago, but to the survivors awaking from their frozen sleep aboard the starship, it all happened yesterday.
Clarke has such an easygoing and engaging style; it's easy to blow through 100 pages of one of his books without even realizing it. This book in particular, though, has a more poetic and lyrical style than his other work.
This is not, like Clarke's greatest novel (Rendezvous with Rama), a pure work of hard sci-fi. There is human drama, mostly dealing with the effects of time dilation on personal relationships and the schism that grows between those that want to continue on to Sagan II and those that want to remain on Thalassa. There is some small cultural conflict between the native Thalassans and the Terrans, primarily over sex (the Thalassans live in a mostly possessiveness-free society). This is not the main thrust of the book but it does nudge it from the extreme of "hard sci-fi" into "social sci-fi" a bit. This is not an area Clarke was strongest in, at least compared to the political sci-fi subgenre's greats (like Ursula LeGuin), but I'm very glad he added those dynamics anyway, because the effects of the book's scientific concepts on its society are important considerations.
Clarke makes a theme of lost cultural memory - not all of Earth's books, music, and art was able to be sent to the stars with the seedships. In fact much of it was deliberately purged: anything relating to religion, for instance. As a character explains:
"It is possible to build a rational and humane culture completely free from the threat of supernatural restraints. Though in principle I don't approve of censorship, it seems that those who prepared the archives for the Thalassan colony succeeded in an almost-impossible task. They purged the history and literature of ten thousand years, and the result has justified their efforts. We must be very cautious before replacing anything that was lost--however beautiful, however moving a work of art.A recurring motif in the story is that a situation will remind a character of some great saying. The reader will know the source, the character does not.
The Thalassans were never poisoned by the decay products of dead religions, and in seven hundred years no prophet has arisen here to preach a new faith. The very word "God" has almost vanished from their language, and they're quite surprised--or amused--when we happen to use it."
Even though, as a work of literature, this book is slightly diminished compared to Rama or 2001 because of its broad (too broad?) political focus, it is stronger in a way, because Clarke lets himself ruminate on human emotions in a way he didn't with those other books. This is a sad book, and it is a resonant one too. Human attachments can never survive the passage of time, and Clarke is able to show that quite literally here. The characters are treated sensitively, whereas in many of Clarke's other novels they simply provide a point of view. Like I said, that is not a fault of those novels, but the emotional depth of this one is one of its great strengths.
That being said, if this book has a fault, it suffers from an embarrassment of riches of sorts. There are a few too many avenues that are introduced that are never fully explored. Maybe this was Clarke's intention; the scientific curiosities of the universe are so vast that to fully explain those that the characters encounter would be disingenuous. Be that as it may, the story's most interesting diversion (the possible sentience of some life-forms discovered on Thalassa) would probably have made a compelling novel in itself, and it feels glossed-over and unfinished here. There are so many concepts introduced in Songs of Distant Earth, just as casual asides, it really makes me marvel at how incredible Clarke's mind was. When the Terrans first land on Thalassa, for instance, they are able to understand each other perfectly because the invention of sound recording in the 20th century has frozen language in place, much the same way that the development of human society freezes human biology in place, by compensating for evolutionary selection factors.
Even though I found myself wishing the book had been a bit longer due to this multitude of partially-explored concepts, I suppose that is just a side effect of one of Clarke's strengths: he always left the reader wanting more.