subconscious is an odd thing. There doesn’t have to be a perfectly obvious rationale for anything she said.’ ‘That’s precisely what we thought, but Sukhoi was adamant. As she recovered, her memories of working with Mercier only sharpened. She recalled the minutest details about him — what he had looked like, what he had liked to eat and drink, his sense of humour, even his background; what he had done before he came to the Chateau. The more we tried to convince her that Mercier had not been real, the more hysterical she became.’ ‘She was deranged, then.’ ‘Every other test said she wasn’t, Mr Clavain. If she had a delusional system, it was focused solely on the prior existence of Mercier. And so I began to wonder.’ Clavain looked at H and nodded for him to continue. ‘I did some research,’ H added. ‘It was easy enough to dig into Rust Belt records — those that had survived the plague, anyway. And I found that certain aspects of Sukhoi’s story checked out with alarming accuracy.’ ‘Such as?’ ‘There had been someone named Yves Mercier, born in the same carousel that Sukhoi claimed.’ ‘It can’t be that unusual a name amongst Demarchists.’ ‘No, probably not. But in fact there was only one. And his date of birth accorded precisely with Sukhoi’s recollections. The only difference was that this Mercier — the real one — had died many years earlier. He had been killed shortly after the Melding Plague destroyed the Glitter Band.’ Clavain forced a shrug, but with less conviction that he would have wished. ‘A coincidence, then.’ ‘Perhaps. But you see, this particular Yves Mercier was already a student at the time. He was well advanced on studies into exactly the same quantum-vacuum phenomena that would, according to Sukhoi, eventually bring him into my orbit.’ Clavain no longer wanted to be in the room. He stepped up, back into the blue-lanterned corridor. ‘You’re saying her Mercier really existed?’ ‘Yes, I am. At which point I found myself faced with two possibilities. Either Sukhoi was somehow aware of the dead Mercier’s life story, and for one reason or another chose to believe that he had not in fact died, or that she was actually telling the truth.’ ‘But that isn’t possible.’ ‘I rather think it may be, Mr Clavain. I think everything Pauline Sukhoi told me may have been the literal truth; that in some way we can’t quite comprehend, Yves Mercier never died for her. That she worked with him, here in the room you have just left, and that Merrier was present when the accident happened.’ ‘But Merrier did die. You’ve seen the records for yourself.’ ‘But suppose he didn’t. Suppose that he survived the Melding Plague, went on to work on general quantum-vacuum theory, and eventually attracted my attention. Suppose also that he ended up working with Sukhoi, together on the same experiment, exploring the less stable state transitions. And suppose then that there was an accident, one that involved a shift to a very dangerous state indeed. According to Sukhoi, Merrier was much closer to the field generator than she was when it happened.’ ‘It killed him.’ ‘More than that, Mr Clavain. It made him cease to have existed.’ H watched Clavain and nodded with tutorly patience. ‘It was as if his entire life story, his entire world-line, had been unstitched from our reality, right back to the point when he was killed during the Melding Plague. That, I suppose, was the most logical point at which he could have died in our mutual world-line, the one you and I share.’ ‘But not for Sukhoi,’ Clavain said. ‘No, not for her. She remembered how things had been before. I suppose she was close enough to the focus that her memories were entangled, knotted-up with the prior version of events. When Merrier was erased, she nonetheless ’ retained her memories of him. So she was not mad at all, not remotely delusional. She was merely the witness to an event so horrific that it transcends all understanding. Does it chill you, Mr Clavain, to think that an experiment could have this outcome?‘ ‘You already told me it was dangerous.’ ‘More than we ever realised at the time. I wonder how many world-lines were wrenched out of existence before there was ever a witness close enough to feel the change?’ Clavain said, ‘What exactly was it that these experiments were related to, if you don’t mind my asking?’ ‘That’s the interesting part. State transitions, as I have said — exploring the more exotic quantum-vacuum manifolds. We can suck some of the inertia out of matter, and depending on the field state we can keep sucking it out until the matter’s inertial mass becomes asymptotic with zero. According to Einstein, matter with no mass has no choice but to travel at the speed of light. It will have become photonic, light-like.’ ‘Is that what happened to Merrier?’ ‘No — not quite. In so far as I understood Sukhoi’s work, it appeared that the zero-mass state would be very difficult to realise physically. As it neared the zero-mass state, the vacuum would be inclined to flip to the other side. Sukhoi called it a tunnelling phenomenon.’ Clavain raised an eyebrow. ‘The other side?’ ‘The quantum-vacuum state in which matter has imaginary inertial mass. By imaginary I mean in the purely mathematical sense, in the sense that the square root of minus one is an imaginary number. Of course, you immediately see what that would imply.’ ‘You’re talking about tachyonic matter,’ Clavain said. ‘Matter travelling faster than light.’ ‘Yes.’ Clavain’s host seemed pleased. ‘It appears that Merrier and Sukhoi’s final experiment concerned the transition between tardyonic — the matter we are familiar with — and tachyonic matter states. They were exploring the vacuum states that would allow the construction of a faster-than-light propulsion system.’ ‘That’s simply not possible,’ Clavain said. H put a hand on his shoulder. ‘Actually, I don’t think that is quite the right way to think about it. The grubs knew, of course. This technology had been theirs, and yet they chose to crawl between the stars. That should have told us all we needed to know. It is not that it is impossible, merely that it is very, very inadvisable.’ For a long time they stood in silence, on the threshold of the bleak room where Merrier had been unthreaded from existence. ‘Has anyone attempted those experiments again?’ Clavain asked. ‘No, not after what happened to Merrier. Quite frankly, no one was very keen to do any further work on the grub machinery. We’d learned enough as it was. The basement was evacuated. Almost no one ever comes down here these days. Those who do sometimes say they see ghosts; perhaps they’re the residual shadows of all those who suffered the same fate as Merrier. I’ve never seen the ghosts myself, I have to say, and people’s minds do play tricks on them.’ He forced false cheer into his voice, an effort that had the opposite effect to that intended. ‘One mustn’t credit such things. You don’t believe in ghosts, do you, Mr Clavain?’ ‘I never used to,’ he said, wishing devoutly to be
