>and then use the entangled pair of particles (one on earth, its mate on Trisolaris) to monitor events on earth in real-time.

This, of course, breaks the known laws of physics, since lightspeed is a hard limit on the speed of causality. You can't use entanglement that way in the real world (if QM is anywhere close to correct)

The sophons themselves were a piece of magic science fiction. Which I think is fine because the author really doesn’t ask you to suspend your disbelief all that much throughout the books. The star plucking is another example, as far as we know you can’t use a star to do that.

But accelerating them towards earth “at the speed of light” isn’t exactly a problem. The LHC accelerates protons to about 3 m/s less than the speed of light, and as far as the plot is concerned the sophons travelling here at the speed of light, or some tiny fraction of a percent less than the speed of light doesn’t make any difference.

Right, that was the huge issue I had with the story. In most respects it seemed to be trying hard to be speculative hard SF in the Arthur C Clarke vein (i.e. fine to introduce exotic new physics, but only very carefully and consistently) so it was very surprising to have that gaping hole at the centre.

The aliens had instantaneous communications, and could directly influence events on earth, but still had to travel at sublight speeds? It wanted at least an acknowledgement of the inconsistency, and a token effort at explanation. As others have noted, it’s not at all clear why they couldn’t simply have killed off the humans remotely.

I can't reply to roywiggins for some reason, but it's possible that the solar systems are closer in other dimensions or something like that. Probably not though, because the higher dimensions are so small. I assume the author didn't think about it until it was too late, or they couldn't fix it.