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"I don’t see how anyone could call Banks hard sci-fi."You know what? You're right. I'm not even going to defend it. In writing a response, I came across this section on The Culture Wikipedia page, and: Yes, not hard sci-fi. More like Star Trek.
https://en.wikipedia.org/wiki/The_Culture#Science_and_techno...
> Can I ask what you like about Egan’s work? I’m not familiar.
Read this description of Diaspora: https://www.gregegan.net/DIASPORA/DIASPORA.html
If that sounds intriguing, you might like his work. He starts with a what-if physics theory informed by his mathematics background. It's heady, positing the universe is a 4-dimensional sphere embedded in a 12-dimensional torus, which allows wormholes to exist and for the protagonists to move through them. From that he extrapolates about what that would mean.
So, maybe not hard sci-fi. Is there such a thing as "math" sci-fi?
There's a scene at the end of the novel. It's short, and not narratively important, so I'm ok with telling you, but it might be a spoiler. If you read the description above and you plan to read it, skip the next paragraph, but it's what really hooked me.
The protagonists discover an artifact that changes slightly as they move from universe to universe. They decide to 'follow' it across the universes, hoping to find the civilization that built it. Tens, hundreds, thousands, billions of universes pass by, and in so doing, they lose connection to their home: "more than seven million broken links, and over ninety billion years of identified [time] slippage". And so they decide to continue, because even if they find their way home again, their home (our, Earth) Universe would be unrecognizable, perhaps dying. At last, when analyzed, the artifact turns out to be (among other things) a massive sculpture "spanning more than a quadrillion dimensions". Collapsing it down to 5 dimensions, so they can get an approximation of what it looks like, "it was a four-legged, four-armed creature, with one arm stretched high above its head... reaching up. To the infinite number of levels above. To all the worlds it would never see, never touch, never understand." That precis does not do it justice, but. Just, astonishing.