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The printer had put a black stripe through the middle of the word EVACUATE. Hannah pulled the sheet free, held it towards the ceiling light and decided that the first global experiment in sending information into the past was probably going to require another printer. Ruth Calder was balancing a tray of coffee in one hand and her telephone against her shoulder with the other. ‘Again?’ she asked, watching Hannah turn the page to catch the light. A man on the phone had been explaining for some minutes why his delegation deserved an extra forty minutes at the following week's hearings. ‘It's one sheet of paper,’ Hannah said. ‘We've asked it to do very little.’ The stripe ran cleanly through the most important word in the proposed warning. She could probably work it out from the surviving letters, which was precisely the sort of indulgence a recipient twenty years earlier shouldn't be expected to make. ‘It's working in the same building as us. That takes a toll.’ Ruth put the cups down without ending her call, listened to the man for another few seconds and assured him that she was not disputing the importance of his proposal. ‘I'm disputing the existence of forty minutes.’ The office had been borrowed from a United Nations agency whose name still appeared on several glass partitions. Beyond the windows a tram moved along the avenue, full of people on their way to work. Hannah had grown used to noticing those ordinary journeys whenever the language inside the building became grandiose. On the pavement, a woman with a folded pushchair was attempting to explain something to a tram driver who had already closed his doors. She lowered the sheet. The message proposed that a coastal town should be evacuated before a storm whose date was given in the American format. Apart from the stripe, the wording contained two abbreviations its author had refused to explain, the name of a town shared by three places and a link to a website that had not existed at the destination date. Hannah held it up once more. ‘There's no point in printing these if the toner is going to eat them.’ Ruth had ended her call and was writing the printer's asset number on the back of an envelope. ‘Can I reject this without a second opinion?’ Hannah asked. ‘The author says the website address will make sense after the event. He hasn't explained how the recipient is meant to know which town he wants evacuated.’ ‘You've got the authority,’ Ruth said. When Hannah showed her the link, she turned the page sideways as though a different angle might help. ‘Tell him to send the address after the event, then.’ ‘He can't.’ ‘Yes, I see. He seems to have discovered the problem himself.’ Ruth took the ruined sheet with her on the way to find the facilities manager, having first reminded Hannah that they still needed a paper record of every screening decision. Their electronic records were apparently inadequate proof that they took their work seriously. Her title was director of operations, although most of her day seemed to consist of preventing fourteen nations from falling out over room bookings. Before Geneva she had managed disaster-relief logistics, and eminent scientists treating practical arrangements as a failure of imagination did very little to impress her. Hannah took a sip of coffee, found it had been made with milk despite her asking for it black, and opened the next submission. Her formal title was director of message integrity. Until two years earlier she had been a professor who studied communication failures in emergencies: evacuation warnings whose place names could be mistaken for one another, public-health notices that instructed people to take actions they could not physically perform, the little gaps between what an expert believed they had said and what somebody frightened and tired actually heard. She had expected the international project to consult her for a week. Then they had asked her to stay for six months, and after that nobody had bothered pretending there was an obvious end to the arrangement. The machine was being built elsewhere. Hannah had seen most of it, though some components could only be visited in a clean suit after surrendering anything with a battery. Her responsibility was the sentence that would be placed inside it. She often thought the division of labour was absurd. Several thousand people had spent years producing a technology of almost unimaginable sophistication, and when they had finished, all that would remain was an instruction that might be misunderstood by a stranger. On Hannah's screen, the next proposal read: BUY GOLD APRIL 2008. SELL SEPTEMBER 2011. USE PROFITS TO BUILD HOSPITALS AND BUY VACCINES. It passed the character-count test without difficulty. Almost everything else would require a much longer explanation than the author had provided. The form asked the proposer to identify who, twenty years earlier, would have received the message and how that person could have invested on the scale proposed. In the answer box the man had written, THE GOVERNMENT WILL FIGURE IT OUT. Hannah had seen that answer in one form or another several hundred times. It usually preceded a calculation of lives saved that assumed nobody would object, steal the money, mismanage the hospitals or alter their plans after hearing that the government possessed information from the future. She marked it for financial and historical review. It deserved a hearing on the merits even if she thought the premise flimsy. Their screening rules did not permit her to discard a proposal simply because she disliked it. Ruth returned with a maintenance ticket and a fresh sheet of paper. ‘One minute before the technical briefing,’ she said, placing the sheet beside Hannah's keyboard. ‘Possibly less if they start without you.’ ‘I've been here since eight.’ Hannah saved her notes and checked the opening slide on the meeting-room display. Two points twenty years apart were joined by a curved line, with an arrow at one end. ‘I asked them to take that off.’ Ruth followed her finger to the screen. ‘Because it's pointing at the wrong point?’ ‘Because it says we've demonstrated the direction of transfer. We haven't demonstrated transfer at all.’ Hannah found the edit controls, although whoever had made the deck had protected parts of the layout. She ungrouped the drawing and selected the arrowhead. ‘You could speak to Sana,’ Ruth suggested. She had a shipment missing somewhere between two Swiss loading bays and was evidently trying to leave. ‘I did.’ Hannah replaced the arrow with a plain line. ‘It'll cause an argument in the briefing if we leave it there, and we'll spend six weeks correcting a picture.’ ‘Then speak to Sana again. She rather likes arguing about pictures.’ Ruth waited until Hannah had saved the slide, took one of the untouched coffees and headed for the corridor. There were sixty-three days until the predicted transmission window. The figure occupied a corner of every project screen, an engineering deadline converted into a public object of fascination. Outside the security perimeter, news crews periodically positioned themselves beside a large digital display installed by a broadcaster. Hannah disliked the clock. At zero the display would announce the end of a window, whether or not the transmitter had accomplished anything. The known historical target was not a person or a place chosen because either possessed some special significance. Twenty years earlier a British experimental detector, during an apparently routine calibration, had registered a physical state that modern scientists believed could be matched by the transmitter under construction. The accessible laboratory record contained no message. Over the past decade those measurements had become the foundation of an unlikely branch of physics, and then of a machine being built with enough international money that nobody could pretend the work was merely speculative. Nobody had shown that information could travel backwards. That was still the experiment. The scientific team had, however, convinced a large number of careful people that the historical event could support exactly one connection. It was a short one, with a fixed capacity of one hundred and forty characters in the detector's original restricted alphabet. The modern team could reproduce aspects of the signal in ordinary laboratory tests. No one could test the backwards part without using the only opportunity they expected ever to have. Every time Hannah explained this to somebody new, the first question was why they couldn't send a test message and try again if it failed. She had asked it herself when she joined. At the briefing room entrance a security officer scanned her badge. Inside, the delegates were taking their accustomed places. A woman from a Pacific island state had brought printed tide maps; a European minister's representative had a folder heavy enough to bend in his hands. An aide stood at the side wall, trying to work out whether the microphones were meant to be live. Someone had moved the water jug so that it concealed a translator's view of the screen. The room was neither more nor less impressive than other conference rooms Hannah had sat in. There was no satisfactory way to arrange chairs for a discussion about changing the past. Dr Sana Qureshi was already at the front of the room, bending over a laptop while she tried to clip back her dark hair. The clip snapped, leaving her holding two useless pieces. She put them in her bag, glanced at the projected slide and said, ‘You've removed my arrow.’ ‘It's a connecting line now.’ Hannah placed her papers beside the lectern, taking care not to cover the printed alphabet reference. Sana had spent three days getting the symbols on it approved by every participating laboratory. ‘It's a perfectly good arrow. It points from one thing to another.’ Sana moved the projector cable so that the connection at the wall was no longer stretched tight. ‘Although I suppose you'd prefer to say one thing might possibly communicate with another.’ ‘That would be more accurate.’ Hannah could hear the delegates collecting headphones behind them. ‘Did you change the phase-stability slide?’ ‘I took it out. The replacement gets us through the uncertainty without the word tensor, which is how I intend to preserve the afternoon.’ Sana reopened her diagrams. ‘You are exhausting to work with.’ ‘I've heard,’ Hannah said, and made room for her beside the screen. Sana was among the physicists who had helped establish the capacity limit. She spoke about error bounds with an ease that made other scientists seem apologetic about mathematics, and the previous afternoon she had complained when a television interviewer rounded a number for convenience. Hannah had liked her almost from their first disagreement. It helped that Sana could recognise when an argument had been won and, eventually, stop making it. The briefing began twelve minutes late. The chair apologised, a translator requested a slower pace and the first delegate asked which punctuation marks the original detector would recognise. Sana brought up the approved alphabet table, enlarging it until the last row was legible from the rear of the room. ‘Selected conventional marks,’ she said. ‘Sixty-four symbols, six recoverable bits each. That gives us eight hundred and forty bits for the message. An emoji doesn't become free information just because somebody calls it one character.’ The delegate representing a European ministry was already writing. ‘But one could encode it. A numerical sequence, perhaps. Coordinates or a reference to a database.’ He turned a page in his folder to find the example he'd brought. ‘Certainly. Then the laboratory twenty years ago has to know how to interpret it.’ Sana let the alphabet table remain on screen. ‘If you send them a code they can't decode, you've used the channel very efficiently to tell them nothing useful.’ Hannah wrote a note to ask Sana not to repeat the last line for a television interviewer. They'd make her deliver it until even she sounded as though she was quoting herself. A representative from a large Asian government wanted to know the prospects of finding a second receiving event. Sana gave the answer she had been giving for months. A chance, yes; a known candidate, no. The equipment could not be directed arbitrarily into the past. The old detector provided the only state they knew how to address, and their predicted opportunity to match it was approaching. When it was Hannah's turn, she spoke about interpretation, practical instructions and the trouble caused by packing a short warning with information that seemed authoritative but could not help the person receiving it. Her example concerned a fictional chemical incident and directed people to a shelter that would not be built for another six years. ‘In the future the shelter is familiar,’ she said, pointing at the date. ‘At the point of reception it doesn't exist.’ ‘Were those laboratory staff under any obligation to tell the British government?’ a delegate asked. Hannah explained that nobody could impose a duty retrospectively on people who had already lived through the relevant day. The team could reconstruct the laboratory's procedures and contacts, then assess the likely route by which an extraordinary result might reach someone with authority to act. ‘And if they do nothing?’ the delegate asked, his pen poised above the assessment form. ‘That has to affect the benefits we claim for any message.’ Hannah had heard versions of the question throughout the consultation. ‘We can examine how a recipient might understand an instruction. We can talk to people who worked in the institutions concerned. We cannot make the recipient act.’ He wanted a probability. She told him there would be cases in which an honest assessment could offer no numerical answer, and he went back to his notes without disguising his dissatisfaction. She wasn't especially pleased with the answer either. When the briefing ended, Ruth intercepted Hannah with a call from London. A member of the independent ethics panel wanted a personal discussion about whether their screening criteria disadvantaged countries with incomplete historical records. Ruth was trying to fit the call into a day already divided into fifteen-minute boxes. Hannah agreed with the criticism and said so, but she would not pretend evidence existed where it didn't. Ruth warned her that the caller would want a fuller answer. ‘So do I,’ Hannah said, taking the briefing papers from her. They returned along a side corridor where the old agency's evacuation plan was still pinned beside a fire door. The diagram showed a staircase that now led to a storeroom stacked with locked equipment cases. Hannah photographed it and forwarded the picture to the building manager. Ruth waited while she attached the location. ‘They'll hate you if you keep finding things like this.’ Hannah thought they ought to move the boxes; Ruth, who had been trying for two weeks to get the boxes moved, could hardly disagree. The messages from home had accumulated during the meeting. Hannah's mother had sent a photograph of a leaking kitchen tap with a message: ‘Do plumbers charge just to come and look now?’ Hannah had no idea. She replied that they probably did, and that she would find someone on Sunday when she called. Below the photograph was an older message from her mother, sent a month earlier, that Hannah had never deleted. It concerned the anniversary of Hartmann and whether Hannah wanted her to come down for the small gathering the families held each year. Hannah had replied that she might be abroad. Her mother had written back: WE'LL SEE. Her brother Dominic had been thirty when he died. The number sometimes startled Hannah when she thought about it, not because thirty was particularly young to die, though of course it was, but because he had remained thirty while she kept moving into the parts of life he had never reached. Her mother still talked about him as though he might come through the back door having bought the wrong ingredients for dinner. Their arguments, the small ones, had survived more faithfully than some of the good memories. Dominic once took the car for a service without asking and left her stranded with groceries outside a railway station. He would have argued to this day that he had done her a favour. Hannah put her phone away. Through the open office door Ruth was telling a contractor that a two-week delay was not acceptable simply because a shipment had been delivered to the wrong side of Switzerland. On the desk, Hannah's rejection queue had grown by another forty-six submissions. She tried the printer feed again and opened the next proposal.