The Four Sentences

There are three sieve-houses on Traherne Station, and by now my sister has spoken in all of them.

The trade is older than it pretends to be. When someone dies, their effects are emptied of everything legible — messages, pictures, accounts — and what remains is entropy: the hardware noise their terminals harvested for encryption, the timing jitter of thirty years of keystrokes, thermal hiss off every sensor they ever owned. On station the body’s mass is reclaimed, so this is what a family keeps instead. The ash, people call it. A sieve-house will read it for you.

The readers are not frauds, exactly. They show their working. They take the ash and run it through decodings — every seventh bit, every ninth; the stream folded against itself; runs mapped to letters under this alphabet or that — until the noise says something. It always says something. Over the door of the Halkyn house there is a line in brass: COMPLETE DISORDER IS IMPOSSIBLE. That is a real theorem, more or less. Make anything large enough and it must contain pattern; there is no such thing as a long sequence with nothing in it. The sieve-readers quote it as scripture. I know it as the reason their scripture can’t be tested.

I assay randomness for the exchange. Lottery draws, arbitration coins, the entropy pools the banks lease: my stamp certifies that a source has no structure anyone can use. It is a strange trade to share a corridor with mediums. We work from the same theorem. We stand on opposite sides of it.

Ceri was second comms on the Gwennol, an ore shuttle on the Talcen run. Eleven days out, inbound, a thruster mount failed and put her into a tumble the crew never arrested. The ship broke up short of the shielding limit, four aboard, no recoveries.

Through the tumble, the station kept recording. That is procedure: the ground side archives raw samples of the downlink whether or not there is a link worth the name. When the Gwennol’s high-gain swung off us, the signal dropped below threshold, and what the receiver digitised from then on was mostly its own front end — thermal noise in our amplifier, here, in our own rack. Forty-one seconds of it, until the transponder died.

Forty-one seconds of us, listening to ourselves. That is the recording the sieve-houses read. The archive calls it GWENNOL/TERMINAL. My mother calls it Ceri’s last words.

She went to Talfan, at the Halkyn house. They begin with an interview, and Mam gave him two hours of it: what Ceri laughed at, what she called our father, how she signed off a call. Then the machine. Stride 7,919, alphabet C, third folding, and there it was in the read-out, whole: DONT WAIT UP.

It was what Ceri said every time she shipped out, from the age of nineteen. Mam bought the framed print. The decoding parameters run along the bottom margin like a provenance.

I did what anyone in my trade would do. The Halkyn method, as their own pamphlet gives it, admits five hundred and twelve strides, forty alphabets, six foldings, any starting offset, and a liberal way with spelling — dropped letters are charged as a blemish, not a miss. And Talfan had left that interview with dozens of phrases that would have satisfied my mother, any one of them, spelt anyhow. Priced fairly, the expected number of finds isn’t small. The method cannot come back empty; emptiness is not among its possible outputs.

The controls finished before my arithmetic did. The Halkyn method, applied faithfully, finds DONT WAIT UP in the scrubber telemetry from C-ring. It finds it in calibration noise the observatory logged six years before Ceri was born. It finds it in a pool in my own certification archive — one I had stamped structureless, and was right to.

I put the three printouts on Mam’s table, next to Talfan’s frame.

“So you’d find it anywhere,” she said.

“Anywhere. Yes.”

“Then it being here doesn’t prove she sent it. And it doesn’t prove she didn’t. Your test comes out the same both ways. That’s what you’ve shown me.”

I had come to show her the reading was worthless. What I had shown her — she was right — was that it was untouchable. A test that returns the same verdict on truth and on falsehood has no power at all; I would fail a junior who brought me one. She poured the tea. The sieve, I understood then, was never in the machine. Talfan had spent two hours sieving my mother, and the noise obliged, as noise will.

But she had moved me off my own ground, so I went to different ground. I pulled GWENNOL/TERMINAL myself and did the one thing the sieve-houses never do, which is ask the channel what it could carry.

The tumble had a period of about nine seconds. Twice in the forty-one, the high-gain’s boresight swept across the station, and a real carrier came up out of the floor: three-tenths of a second, then six-tenths. The demodulator caught two sync preambles in those windows — known bits, which is a gift, because known bits can be scored. Three hundred and thirty-one symbols arrived in all, and against the preambles they were wrong forty-five times in the hundred.

Forty-five in the hundred. Fifty would be no link at all — a coin. At forty-five, each symbol carries about a hundred-and-thirtieth part of a bit, and three hundred and thirty-one of them come to a little over two bits. That is the whole of what crossed from the Gwennol to us after the mount failed. Not two bits we failed to recover — two bits anyone could ever recover, by any method, with any machine, from now until the archive is dust. The theorem does not care how clever you are. It says what the channel held, and the channel held two bits.

And it held them. That is the other thing I found. The frame flags in the second window show the low-rate emergency channel keyed, live, mid-message. Someone aboard was sending while the ship came apart. For six-tenths of a second the recording is of the Gwennol and not of ourselves — and whatever they keyed is in those symbols the way one face is in a crowd of two-to-the-three-hundred-and-thirty-first faces. At forty-five errors in the hundred, every sentence of that length fits the data very nearly equally well. DONT WAIT UP fits. So does its opposite. So does every sentence Ceri never said in her life.

Two bits is not nothing. Two bits distinguishes four messages — if the four are agreed beforehand. One sentence out of four known ones, spread thin across three hundred symbols, survives forty-five errors in the hundred; I ran it; it crosses, just. The channel would have carried a farewell. What it could not carry was a farewell and the means of reading it, both. The code has to be on the far side already. The time to agree the sentences is before.

I told Mam all of it, flags and preambles and the arithmetic of the two bits.

“So she did say something.”

“Someone was keying the emergency channel. It’s in the flags. It would have been her seat.”

“And you can’t read it.”

“No one can. Ever. That part is provable.”

She was quiet for a while. “Good,” she said at last. “Then it’s hers. Not Talfan’s.”

She kept the frame. It is a true thing Ceri used to say, she told me, whoever said it this time.

We wrote our codebook that night, at the kitchen table, and it took until the small hours, because a sentence you may only send when everything else has failed is the hardest kind of writing there is. All well; go on. That one was mine. It was quick. That was hers, and I let it stand, because she is sixty-eight and I fly four times a year, and we both knew which of us it was for. Don’t wait up — she would not be moved on that, and I did not try very hard.

The fourth we left empty for a long time. Near two in the morning Mam said that one of the four things a person might need to say is the thing you could never think of beforehand, and the code ought to own up to it. So the fourth sentence is: It’s none of the others.

We registered the book with the exchange, the way you would a will. Two copies, one key. I think about the fourth sentence more than the rest. If it ever comes, it will tell whoever is left almost nothing: that something happened past all our imagining. And one thing more, which two bits will always carry, whatever the errors, whichever the sentence — that in the middle of it, someone was sending.