From “Revelation Space” by Alistair Reynolds, describing what was once a neutron star but had become part black hole, part time portal, and part massive geological computer:
These ripples of causal shock met the incoming particles and established a grid of causal interference, a standing wave extending symmetrically into the past and the future.
Enmeshed in this grid, the collapsed object was no longer sure that it was meant to be a black hole. The initial conditions had always been borderline, and perhaps these entanglements could be avoided if it remained poised above its Schwarzschild radius; if it collapsed down to a stable configuration of strange quarks and degenerate neutrons instead.
It flickered indeterminately between the two states. The indeterminacy crystallized, and what remained behind was something unique in the universe – except that elsewhere, similar transformations were being wrought on other black holes, similar causal paradoxes coming into being.
The object settled on a stable configuration whereby its paradoxical nature was not immediately obvious to the outside universe. Externally, it resembled a neutron star – for the first few centimeters of its crust, at least. Below, the nuclear matter had been catalysed into intricate forms capable of lightning-swift computation, a self-organisation which had emerged spontaneously from the resolution of its two opposed states. The crust seethed and processed, containing information at the theoretical maximum density of storage of matter, anywhere in the universe.
And it thought.
Below, the crust blended seamlessly with a flickering storm of unresolved possibility, as the interior of the collapsed object danced to the music of acausality. While the crust ran endless simulations endless computations, the core bridged the future and the past, allowing information to channel effortlessly between them. The crust, in effect, had become one of a massive parallel-processor, except that the other elements in its array were the future and past versions of itself.
From page 562. Above image of Caseopeia A, a neutron star; image via NASA.