The Causeway

Nine hundred and seventy-five years ago, a civilisation with one star spent everything it had on a thing it knew it would never see finish.

It built robot ships and threw them at the nearest stars at a twentieth of the speed of light. Each ship carried one half of an anchor pair; the other half stayed home. Where a ship arrived and woke, a door opened — and the two halves of a pair, however far apart, behave as one object. Step onto the plate at this end and you are at the other end before you have finished stepping.

Twenty years per light year of flight. The first ships went out, and the people who launched them died, and their grandchildren died, and eighty-five years after the launch the first door opened at Alpha Centauri, and somebody who had been told about it by a grandparent walked through it.

That network is the Causeway. Twenty-eight systems hang off it. It was not built by anybody alive. It was inherited, and the parts of it that can be reached are maintained.

The one rule everything else comes from

Only matter crosses, and only through a door.

Not light. Not radio. Not a signal, not a word, not a photograph, not a gram of cargo that nobody is carrying. A person, and whatever that person has in their hands or on their back. That is the entire list.

So:

News travels at the speed of a courier getting round to it. A letter from Sirius to Earth takes as long as a fifteen-year-old takes to walk eight light years’ worth of doors, which is about four minutes if nobody stops for lunch, and about nine days if everybody does.
Inside a system, radio works perfectly well, and everybody forgets this, because everybody only ever thinks about doors. Two stations round the same star talk all day. It is only between stars that nothing crosses but a person.
The moment you step through, nobody can reach you. Your handler briefs you on this side and then cannot follow, cannot call, cannot send help and cannot change the plan. Whatever you find on the other side, you are the only one who knows about it until you walk back.

A door is not a place

This is the part that catches people out, and it is worth getting straight now, because everything frightening in this setting comes out of it.

A door is not tied to a spot on the ground. It is tied to a depth — to the sum of all the gravity in the neighbourhood, and it sits wherever that particular value currently is. Think of a water level rather than an address.

Mostly that value drifts a few metres in a century and nobody writes anything down. Occasionally the sum has more than one answer, and the far mouth jumps to a different one. That is called straying, and there is a word for it because it has happened often enough to need one.

Which is why the most important object in human space is a board somebody writes on by hand.

The Standing

Four metres of board on the gallery wall above the anchor hall at the Nine, with every door anybody knows about written up on it by hand, and where it currently goes, and who last stood it, and which watch they stood it in.

Four things on a line. People read the first two.

The fourth is the only one that matters, because a door tells you where it went when somebody last looked, and a door can stray in the time it takes you to walk down the stairs. A line is not true. A line is when.

There is a line on that board reading Kapteyn’s — no answer this watch. It was written in the spring of year six hundred and two by somebody who expected to rub it out before supper. Eleven thousand people live behind it. The star is still there. You can see it from the upper gallery.

And nobody will ever build another one

Not because the knowledge is lost. Any engineer at the Yards can tell you how a pair is made. Because the thing that would pay for a new door cannot be gathered through the doors there already are — no bulk, no fleet, no treasury, only bodies and boxes. Twenty-eight systems can never pool what twenty-eight systems have.

They attempted four thousand, one hundred and nineteen pairs. Three thousand and eleven worked. A thousand and eight did not, and each of those was a decade of somebody’s whole civilisation turned into scrap, and they kept going.

Every first-through works this out in their first year. You may as well have it now, sitting down.