Theory

Krasnikov tubes were theorized in 1995 by Russian scientist Sergey Krasnikov. They are lines of flat spacetime trailed by a ship travelling at a significant fraction of the speed of light, which allow superluminal passage in the direction opposite to the initial displacement. They can be understood as "one-way wormholes", permitting nigh-instantaneous one-directional travel by connecting two points in time and space. Contrary to wormholes, however, Krasnikov tubes are physical structures in space, as the sequence of flat spacetime manifests itself in three-dimensional space. In the image of Alcubierre drives, Krasnikov tubes require exotic matter with negative energy density to be deployed and maintained, and are thus well beyond the technological scope of humankind. They are also greatly inferior to the (still ill-understood) capabilities of the geometry drive.

In the search for other FTL-capable species in the Milky Way, Krasnikov tubes were often considered as an interesting technosignature to hunt for due to their concrete presence in real space, as opposed to wormholes whose mouths are nigh-impossible to find at a distance (and may indeed be found on planetary surfaces, such as most hidden entrances to the Pale Path). However, if ancient wormhole remnants were discovered as early as forty years after the advent of the geometry drive, Krasnikov tubes were only found after the discovery of the Southern Approaches and its network of K-tubes known as the Ropes, built by a mysterious spacefaring civilisation dubbed the Yxi.

The Study of the Ropes

The seminal study of Morgan Goldweather of the Starmoth Initiative (Shrouds of the Skies: A Survey of the Southern Approaches, Tsiolkovski Institute Publishing, 0.89) showed that the tubes of the Ropes are strikingly similar to the megastructure envisioned by Sergey Krasnikov. Averaging five lightyears in length and a kilometer in width, the fifty K-tubes of the Ropes present themselves as transparent tunnels in space, whose mouth is reminiscent of a mine shaft and unlike both a black hole and a wormhole mouth. Their immaterial structures make them incredibly resilient -- if the few Yxi structures found around the K-tubes are any indicator, their civilisation ceased to exist a million years ago, yet the Krasnikov tubes remain in place and usable. Goldweather speculated that, contrary to wormholes, the Krasnikov tubes of the Ropes were "one-off" structures, requiring the expenditure of negative mass matter during establishment, but with little to no maintenance cost afterwards. The behaviour of light inside a Krasnikov tube implies that any ship sent throughout one would appear infinitely distorted to an outside observer. However, it is complex to test that hypothesis.

Of Causality and Geometry Drives

A single Krasnikov tube is not violating causality, because with a one-way displacement, the traveller cannot return before they left, and thus they do not intersect the past of their own light cone. The tube is a time machine, but it is only future-oriented. However, two tubes, allowing for round trips, would work as a  past-oriented time machine and create a closed timelike curve (CTC), where causality breaks and effects can precede causes. This is why the Ropes form such a convoluted network, were round trips are strictly impossible with sublight ships.

The geometry drive is also capable of producing such curves, and indeed past-oriented time travel has already been observed with geometry drives, like the one thousand year time travel of the fleet that founded Station Zero, the Asuja incident or the temporal seeding of Azur. Thus, when Goldweather first approached the Ropes, she assumed there was no basic incompatibility between the K-tubes and the geometry drive. However, automated ships sent through the tubes, be they FTL-capable or not, systematically disappeared without a trace upon approaching the mouth of the tubes. In Goldweather's eyes, it made little sense -- indeed, some of her trajectories would have created closed timelike curves, presumably triggering some defence mechanism from the K-tube, but not all of them. How come all the ships had vanished, then?

A convincing answer was proposed by Aramanae Talasea, of Azur, in her open-source study Limited Omniscience: Implications of the Geometry Drive Behaviour in the Ropes. First, she noticed, both the Station Zero translation and the Azur seeding were not "dangerous" closed timelike curves from the point of view of the ships, as the travel times involved did not actually allow them to actually meet their past selves before they would perform the translation, nor to send information at lightspeed. And indeed, she also noticed, the drive had been shown capable of preventing actual closed timelike curves from forming (in spite of presumed "temporal unshackling" by overenthusiastic navigators), meaning it had, somehow, a way to evaluate these situations and react accordingly (potentially, notes Talasea in a subsequent article, as a result of its very existence -- the only reason why the geometry drive can exist in the first place is if the universe is coherent, thus true CTCs cannot exist). Thus, speculated Talasea, the reason why the geometry drive reacts so strongly to K-tubes is because it finds itself unable to evaluate whether or not the new trajectory is at risk of creating a CTC, and applies a strict precautionary approach by shutting itself down and, presumably, destroying the ship. She calls this the Limited Omniscience Safeguard. Though not accepted by the entire scientific community, Talasea's elegant theory has been supported by Goldweather themselves. It does not, however, explain why geometry drives can be used around and inside the Pale Path, albeit it has been speculated that the wormhole network possesses a means to "balance out" the timeline by displacing ships forwards in time at exit.

The Starmoth Initiative has been trying to circumvent the problem by sending a sublight probe towards the end of the shortest Rope, mimicking the presumed movement of Yxi ships ; launched five years ago, the drone spaceship still has to reach the destination.

Illustration from Krasnikov's original article: https://arxiv.org/abs/gr-qc/06...