The sophons
The aliens in this novel use a sub-atomic technology called "sophons" to help them to monitor and inhibit the humans. This is a symbolic comment on the possibilities of machine technology. The human race is barely able to fathom a standard model of the sub-atomic world, but the aliens are already able to manipulate matter at a subatomic level. For the humans, the technology inhibits their own progress, so that sophons are kind of like a technological disease on the planet, and the sophons represent the looming threat of the unseen, because they are too small to be perceived.
The UN
In this mythic future, the UN is more than it is today. For instance, in the book, the UN abducts the scientist's family as hostages to for the man to work for the government. Luo Ji encounters the UN as an absolute power because he is literally unable to escape their supervision. Plus, there is the actual threat which is considerable; if the aliens are not stopped, it could mean a terrifying and brutal end to human life. The UN seems to represent the conglomerate authority of the planet's governments, which is sort of what it does already, but this novel is more globalized than real life, so far.
Technology and progress
The novel plays around with the idea of change and time. Because the sophons interrupt human electronics, they represent the aliens' sovereignty over human life. If the humans cannot figure out some counterattack, they will never make progress again. This removes technological process from the flow of time, which is bad news—especially when the Trisolarians represent technological sophistication beyond the human imagination.
The leap in time
For Luo Ji, time skips two centuries. In his experience, two centuries have passed instantly. In real life, his body was preserved in a state of hibernation for two hundred years through cryogenic freezing. This drastic jump in time represents the novel's most important symbol. Time feels in the moment like it is not changing, sometimes. But it is always changing, forward in time. Because he does not see what fills the gaps between his experiences, Luo Ji's sense of time is literally a juxtaposition of two different generations. The idea is a fascinating thought experiment.
The probe
The Trisolarians seem to be gaining the upper hand when a probe arrives from their planet. Luo Ji is able to learn enough during that encounter that he is able to broker a treaty with an alien society unlike anything the world could think of. That means that the probe is a kind of karmic catalyst. The Trisolarians, because of their own strategies, ended up providing Luo with exactly the time and information he needed to defeat them. The probe represents a kind of ironic counterpoint to the climax of the novel.
