The Corridors: faster than light, along roads someone built

Faster-than-light signals arrived with a test tube inside the solar system in 2410 and interstellar threads from the 2480s (the first thread probe left in 2440); faster-than-light travel between stars does not yet exist. Both obey one rule: you can only go faster than light along a path something has already travelled slower than light — and only back towards where that slow journey started. A route that is fast in both directions needs a slow voyage from each end. In 2497 one interstellar thread works, Proxima's, and it runs one way, from Proxima to Earth. No ship corridor has been built anywhere.

The physics this universe runs on

The universe has a real preferred frame — cosmic rest — and faster-than-light motion is possible only forward in cosmic time.

• Cosmic rest. It is the frame in which the cosmic microwave background looks the same in every direction. Earth moves through it at about 370 km/s, and cosmic time is kept by clocks at rest in it.
• The frame is hidden. Its effects on ordinary matter are far too small for ordinary experiments to see, which is why twenty-first-century physics found no trace of it. They become significant only in extreme, engineered curvature: the walls of a tube. The Aether Result of the 2260s, from laboratories founded during the Embodied Wave, was the first measurement of it, and it overturned no earlier one.
• Why there is no time travel. Every superluminal path is ordered by cosmic time, so arriving before you left is impossible. A tube that would carry anything backwards in cosmic time cannot form, and attempts to force one collapse.
• Why the roads run one way. The outbound trip itself cannot be shortened. Crew inside a tube cannot create or steer it, so a route must be prepared by someone who has already travelled it.
• Tubes strain. Stars move relative to one another, and a tube's anchors move with them. A tube tolerates strain (stretch, compression or sideways shift) of up to about 2% of its length; past that, it fails and must be re-laid by a new slow voyage.

How it works in practice

What a tube is. A tube has no material wall along its length. A slow vessel crossing unprepared space imprints its path on the hidden frame, and the anchors at each end hold that imprint open by pouring energy into it. All the ordinary matter is at the ends: the anchors, and the imprinting gear the builder carried. A tube costs one slow voyage, two anchors, and the power to run them for as long as anyone wants it.

Wide or fast. For a given anchor power, a tube can be wide or fast, not both. The narrower its bore, the more steeply its wall can tilt the way ahead into cosmic time, and anchor power grows with the square of the bore. Two standard kinds result; in 2497 only the first has been built between stars.

• Nothing with mass rides a thread. Matter entering a thread's mouth breaks its coherence and the thread collapses; so does any beam above a few kilowatts. Threads therefore cannot carry probes, seeds, nanomachines, weapons or power. They carry text, data and compressed images; signals arrive faint and noisy, and transmission time is metered and politically allocated.
• A ship can trail a thread. From the 2440s, any interstellar ship fitted with a thread-layer can lay a thread behind it at modest extra cost, so a new colony can speak to home on arrival — but home cannot answer faster than light until the colony sends a probe back.
• A corridor anchor is a national project. A colony of tens of millions could run a thread anchor almost for free, but a corridor anchor draws as much power as a large nation, which no colony in 2497 has: Proxima's ten thousand people could not run one. A track ship would take about 28 years to Proxima.
• Every road has two halves. Earth's track ship to a star would open the fast route from that star to Earth. The fast route from Earth to the star needs a slow voyage back from the star, whether by the same ship returning or the star's own.
• Riding a corridor would take about a day per light-year in cosmic time. A ship enters and leaves only at anchors, cannot stop or steer mid-route, and reaches the far anchor at the speed it entered, so a corridor cannot be used to hurl a ship at a planet. It is more train than starship.
• Cosmic time. Threads keep cosmic time as their shared calendar. Earth's clocks drift from it by about 24 seconds a year (see Time dilation), corrected routinely.

Tubes inside the solar system

A tube inside a star system needs anchors that do not orbit, or it must be temporary. Planetary orbits barely register over interstellar distances, but inside a star system they tear apart a tube between two orbiting anchors within days or weeks. Even two anchors sharing an orbit fail, because the line between them turns. Three patterns result.

• Hover-tubes. A hovering anchor is held still against the Sun's pull by continuous thrust. Far out this is cheap: at Saturn's distance a 100,000-tonne station needs about 80 GW of fusion thrust and about nine tonnes of propellant a year. Near Earth the pull is about ninety times stronger, so permanent hover-tubes could link only outer-system stations. Ships must cancel their orbital speed to board, which costs fuel and would keep hover-tubes trunk lines, not local transport. None runs in 2497. Each end would need about 180 GW, anchor and thrust together, for as long as the tube is wanted, and the outer-system stations worth linking belong to different blocs; Saturn Anchor, the first planned, lies beyond the end date.
• Flash tubes. A tube between orbiting anchors lasts days to weeks. Flash tubes could serve evacuations and megaprojects, when moving a great deal in a short window would justify a fresh slow pass. Their stores would be filled over months or years, then spent in days. The 2410 test tube was the first: a Mirror Race prestige project whose two 100 GW anchors ran for eleven days on stores filled by an early array's beam over about six years. Its anchors were not on Earth and Saturn but at the two ends of the probe's path, placed there in advance and switched on only once the probe arrived, so its eleven days ran from switch-on.
• The inner system. Between Earth, Mars and the Belt, ships remain the way to travel: fusion ships take months, and beam-pushed ships take weeks where a bloc's beams reach. Leaving Earth itself is cheap: the elevators lift freight and people to geostationary height, where the ships begin.

Tube energy. No array of 2410 could beam the test tube's 200 GW, so it ran on stored power (see Flash tubes, above, and Tube energy, in The Mirror Race). Energy alone would not keep permanent tubes out: two hover anchors need about 360 GW between them, well under 1% of off-Earth fusion. What keeps them out before 2497 is everything else (see Hover-tubes, above). The stations worth linking belong to different blocs, every ship must shed its orbital speed to board, and no route yet carries enough traffic to pay for two anchors run for decades.

Rules of the road

• Only a slow pass through unprepared space lays a new route. A ship riding an existing tube is carried inside it and leaves nothing behind. A track ship therefore cannot ride home on a corridor while laying the reverse half; it must fly the return slowly.
• The fast direction is fixed by construction. A tube's wall is laid in cosmic-time order as the builder passes, so nothing can reverse a finished tube. A mouth admits traffic only in the fast direction; a ship going the other way flies outside the tube, slowly.
• Tubes join only at anchors. A ship can leave at a corridor's anchor and fly on slowly, but any tube that onward voyage lays joins the network only at that anchor; there is no branching mid-tube.
• Relays work, but only through real links. Messages can hop through any chain of correctly directed threads, so a missing link can be bypassed only if such a chain exists.
• Anchors keep tubes alive. Each tube needs a powered anchor station at both ends for energy and station-keeping; shut one down and the tube decays within months. Whoever holds the anchors controls the traffic.
• Who controls a connection. One ship making a round trip can lay both halves of a route; so can ships from each end. What makes every connection to an inhabited system a negotiation is sovereignty, not physics: each anchor sits in someone's star system, draws that system's power and falls under its law. An uninhabited system can be wired both ways by whoever reaches it first. A colony that later governs itself inherits whatever anchors were built before it did, and that inheritance is often contested.

Timeline to 2497

Time dilation: when it matters

The Lorentz factor (γ) is how much slower a moving clock runs; the traveller ages by the trip time divided by γ.

Time dilation is real but modest throughout. The big dislocations in this universe come from light-lag, slow ships and one-way threads, not relativity.

Why anyone still travels slowly

In 2497 slow travel is the only kind between stars, and corridors, when they come, will not end it. A probe can lay a thread, but not a working corridor, and five rules of this universe keep colonists flying at a fraction of light speed.

1. A corridor needs a power station at the far end. A corridor anchor draws about 100 GW, a large nation's grid, and it must sit at the destination. A thread anchor runs on a probe's own reactor; a corridor anchor has to be built and run by someone who lives there.
2. Machines may not build it alone. Building a 100 GW station at an empty star means decades of self-replicating industry running unsupervised, exactly what the Swarm Years taught everyone to fear. Since the Partition, autonomous factories beyond the oversight of persons have been illegal, and the Caloris Runaway of c. 2430 made that the most quoted rule of all. Anchors will be built by persons on site, human or Raised. A probe that could build one would, in law and in practice, be a colony.
3. Going is faster than waiting. A route runs fast only back towards where its slow journey began, so a fast route from home to a new star needs a voyage out and another back before anyone builds the far anchor. To Tau Ceti at 9% of light speed, that is about 135 years out and 135 back. Colonists who simply go arrive in 135.
4. The first to arrive owns the anchors. An uninhabited system can be wired by whoever reaches it first, and every anchor falls under the law of the system it sits in. A probe cannot hold a claim; people on the ground can. That is one reason the Mandate sent people to Barnard's Star rather than a probe.
5. History. The Lifeboat (2230) and Proxima's ship (c. 2310s) left before tubes existed; threads date from the 2440s, and no corridor has yet been built.

The pattern everyone expects is this: a thread probe goes ahead to survey and report; a crewed ship follows, trailing a thread; the colony grows for generations until it can afford a corridor anchor; only then does a track ship fly the return leg and open the corridor. It gives the Raised a lasting trade: persons who can stay awake for centuries are the natural builders of far anchors.