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...

The Well of Finistelle is the first element of the Pale Path discovered by human explorers. This paracausal megastructure was found five lightyears away from Finistelle Station, seven thousand lightyears above the galactic plane. It was presumably built by the same civilisation that assembled the Pale Path.

The Well is a megastructure that assumes the shape of a perfectly immobile, hollow needle, three hundred thousand kilometres long and five hundred meters wide. It is made of a single compound that has yet to be identified, but seems to straddle the line between organic material and metal, a common property of transbiological technology. Originally white, this compound turns to grey, then black as it degrades with time and, presumably, exposure to cosmic radiation. The core structure of the Well is assumed to be about five to fifteen billion years old. Only one third of the megastructure exists in our own three-dimensional universe, while the rest seems to be physically present in different, self-contained dimensions. This makes the Well the largest example of trans-dimensional architecture known in the galaxy. Though we can only speculate about its purpose, it is assumed that the Finistelle Well serves two main functions: anchoring the first dimensional bubbles of the Pale Path and connecting them with our own universe.

As is the case with most technological remnants of the Pale Path's builders, the Finistelle Well is in an advanced state of disrepair. It is unknown why the structure of the Well is weakened to such a dramatic extent, but the latest Starmoth Initiative research suggests that the entire structure had structural faults to begin with, owing to a miscalculation in assessing the actual strain a trans-dimensional pin is submitted to. While surprising for such a technologically advanced civilisation, this error can be understood if one takes into account the fact that the Pale Path was probably an experimental concept, perhaps even a hobby project, that may not have received all the care it should have. While the Well isn't expected to fall apart before another billion years, the slow dereliction of its structure results in a thick cloud of dust accumulating at the Pale Path-facing entrance of the pin under dimensional forces. This greatly complicates operations in and around the Finistelle Well.

Humankind is far from the first species to discover the Well, and the Milky Way-facing part of the megastructure is littered with shipwrecks, ancient research stations and space debris. Most of them haven't fared much better than the Well itself and are but empty, featureless hulks littered with micrometeorites impacts. Almost a hundred thousand Sequence wrecks have been identified inside and around the Well, the result of a massive battle that happened roughly ten million years ago, when the interstellar empire tried to breach the Pale Path in order to conduct a crusade against its creators. It is unknown whether this is how the Sequence gained entry to the Path, or if they found another gate, but they appeared to have used concentrated UREB strikes against the outer armour of the Well. While it eradicated whatever defence systems the megastructure might have possessed, it also greatly compromised its structural elements, shortening its life by a good billion years.

Navigating inside the Well is rather difficult due to the sheer density of debris, dust and wrecks alongside the spine of the pin. Geometry drive usage is strictly forbidden inside the Well, where dimensional interference greatly reduces accuracy and makes translations inflict grievous damage on the weakened megastructure. While Azur Effect drives are not affected in the same way, their effects on the Well are even more critical -- it is expected that repeated Azur translations near the base of the megastructure could outright shatter it. In order to travel up and down the Finistelle Well, the Starmoth Initiative has deployed two space elevator-like cables running from one end of the Well to the other. Alongside these cables have been installed two repurposed Mansa Mussa cargo ships, Occident Express and Trans-Europe Express. Stripped of their engines, these vessels are used as trains, running along the cables on their own nuclear power to ferry passengers and cargo from Finistelle Station, at the Milky Way-facing end of the Well and Finistelle Terminus, near the Pale Path's entrance.

At the very bottom of the Well, the second waystation enables access to a smaller tunnel that itself gives way to a section of liminal space, the buffer area between the dimensional bubbles of the Pale Path. From there, and with a bit of luck and madness, most of the contained dimensions are accessible.

//Opening high-level log.

//Accessing mainframe -- Starmoth Initiative Vessel "Axial Tilt Zero".

Current location: Przybylski's Star (HD101065), 355 lightyears from reference point.

Elapsed time on location: 78 days, 11 hours, 34 minutes (estimated -- local timeline deviation in effect).

//Beginning of log.

Subspace anomalies detected upon translation in system: geometry drive reports false alarm (class 01 - unknown cause) from FTL collision detection system. Full reboot of navigation system required to bypass warning, despite no significant concentration of matter detected at arrival point (local dust density at standard baseline for F-type star). No planets detected in system, though suspected presence of asteroid rings confirmed. Main star follows characteristics commonly attributed to Ap and Bp type stars, with a very low rotation period of over two baseline centuries. Other phenomena inside the star appeared to be slowed down to an extreme degree as well.

Spectral pings reveal low amounts of iron and nickel, as well as high amounts of strontium, holmium, niobium, scandium, yttrium, caesium, uranium and other elements belonging to lanthanides. Other exotic elements are present, including actinides such as neptunium. Spectral characteristics are only partially coherent with Ap and Bp type stars. Comparison to surveyed stars did not return any perfect matches, however several partial spectral similarities have been noted with Sequence-occupied stars, as well as Finistelle's veil-stars. Closer inspection of the star revealed local variations in magnetic field and convection cycles beneath the star surface, hinting at the presence of a nascent sylph.

Hypothesis: the presence of elements with short half-lives can be explained if they are the decayed forms of ultra-heavy elements, following the model first established during the spectral study of IS-1761 (see also: Calavera's Star, Traverse Archives).

During a closer orbital pass (within 300,000 kilometers of the star), on-board equipment started reporting temporal anomalies, with both atomic and crystalline clocks signalling chronological deviations way beyond expected parameters. Furthermore, external ablative armor sensors reported numerous nanometric impacts during the low pass. Both concentration and speed did not match the primary hypothesis of planetary or asteroid debris. Two probes sent in a lower orbit reported even more extreme temporal deviation, with on-board clocks ending up entirely unsynchronized after fifty-five seconds. Complete loss of communications following critical OS corruption occurred after one hundred and twenty-two minutes. Ultimate value for temporal dilation was -375% and falling. Strangely enough, local temporal dilation did not seem to trigger any significant redshift effect, which is characteristic of a paracausal event. 

Conclusion: very low rotation period is due to extreme local time dilation.

Analysis of collected debris revealed an unusual concentration of rare or even unknown ultra-heavy elements at the very edges of the periodic table. Orbital dispersion appeared compatible with slow orbital decay of a vast amount of shattered debris. Complete mass of material is estimated to be anywhere between one to three Earth masses, with temporal deviations ensuring a way lower decay rate than expected. Prospective reconstruction of debris field hints at the presence of at least two ring-like structures circling the star. Interestingly enough, micrometeorite impact analysis on high orbit debris showed that the initial structure was built between 100 and 500 million years ago, i.e before the star appeared. Overall structure is coherent with the pre-decay architecture of veil stars A and B in Finistelle.

Prospective conclusion: Przybylski's Star is a collapsed gate to the Pale Path.

[Reading: Pale Path entry in the Starmoth Initiative Yearly Newsletter]

The world is porous. Reality has holes and gaps running through it like tunnels in karst. We have been aware of this ever since the discovery of the geometry drive. It cuts passages through reality then sews them back in its wake.

It is not, however, the only example of dimensional engineering in the galaxy.

The Pale Path is ancient, probably more than the Sequence, though it is hard to assess the age of such a feat of dimensional engineering. The oldest references to its existence have been found on Kapteyn which, as far as we can tell, would mean the Pale Path is at least a billion years old. Of course, the perception and effects of time can be less than intuitive when dimensional works are involved.

Interestingly enough, we heard of the Pale Path way before encountering it in the flesh. In retrospect, it was obvious that the inscriptions on Kapteyn talking about a slithering trail through the void referred to the Pale Path, but at the time we did not have any point of comparison. More explicit were the Sequence writings identifying a webway pattern circling the galaxy as a potential target for a genocidal crusade, though they led us to believe the Pale Path was just some kind of derelict wormhole network similar to what the Sequence itself seemingly tried (and failed) to build aeons ago.

The discovery of the Finistelle Veil, the first entrance to the Pale Path ever found, showed how wrong we were. Granted, on a surface level, the Pale Path can be considered like a wormhole-adjacent FTL system, connecting a network of ancient gates together through the halo of stars and dead galaxies circling the Milky Way. If it was only an FTL network, it would already be an impressive feat of generation engineering, the only means of FTL travel other than the geometry drive, surpassing even the achievements of the Sequence. However, the Pale Path's nature as a wormhole network seems almost accidental, a by-product of the real nature of this dimensional architecture.

In theory, the inside of a wormhole should be empty. That is what wormholes are, tunnels through time and space, and the inside of a tunnel is only used for transit. This is not the case for the Pale Path. The inside of its wormholes grows and expands into localized subspace phenomena we baptized Metrics. A Metric is a vast, albeit strangely finite, pocket universe contained within the Pale Path, that often has its own set of physics. It is unknown how many Metrics reside in the Pale Path, as the vast network has been only partially explored and some of the intermediary Metrics are downright hostile not only to human life but to the very existence of regular matter. Worse even, the Pale Path is in a state of advanced disrepair, with only a fraction of its extent having been spared by time, architectural failures, design flaws and the wanton destruction carried out by the Sequence. Our preliminary studies revealed that only a fraction of the Pale Path might be still intact, with 5 to 6% of its original Metrics being accessible by humans.

In very simple terms, the Pale Path is a five hundred thousand lightyears long string of parallel worlds.

Illustration: Public Domain NASA/Caltech.