They had arranged to make twenty trials, so Bahram merely nodded while Khalid checked the clock by a shielded second lantern, and had Paxtakor mark down the time, which was two beats and a third.

They tried it again, and again the light appeared from Iwang the same moment Bahram opened their lantern. Once Khalid became used to the speed of the exchange, the trials all took less than a beat. For Bahram it was as if he was opening the door on the lantern across the valley; it was shocking how fast Iwang was, not to mention the light. Once he even pretended to open the door, pushing lightly then stopping, to see if the Tibetan was perhaps reading his mind.

'All right,' Khalid said after the twentieth trial. 'It's a good thing we're only doing twenty. We would get so good we would begin to see theirs before we opened ours.'

Everyone laughed. Khalid had become snappish during the trials themselves, but now he seemed content, and they were relieved. They made their way down the hill to town talking loudly and drinking from the wine bag, even Khalid, who very seldom drank any more, though it had once been one of his chief pleasures. They had tested their reflexes back in the compound, and so knew that most of their trials had been timed at that very same speed, or faster. 'If we throw out the first trial, and average the rest, it's going to be about the same speed as our procedure itself.'

Bahram said, 'Light must be instantaneous.'

'Instantaneous motion? Infinite speed? I don't think Iwang will ever agree to that notion, certainly not as a result of this demonstration alone.'

'What do you think?'

'Me? I think we need to be farther apart. But we have demonstrated that light is fast, no doubt of that.'

They traversed the empty ruins of Afrasiab by taking the ancient city's main north-south road to the bridge. The servants began to hurry ahead, leaving Khalid and Bahram behind.

Khalid was humming urimusically, and hearing it, remembering the full pages of the old man's notebooks, Bahram said, 'How is it you are so happy these days, Father?'

Khalid looked at him, surprised. 'Me? I'm not happy.'

'But you are!'

Khalid laughed. 'My Bahram, you are a simple soul.'

Suddenly he waved his right wrist with its stump under Bahram's nose. 'Look at this, boy. Look at this! How could I be happy with this? Of course I couldn't. It's dishonour, it's all my stupidity and greed, right there for everyone to see and remember, every day. Allah is wise, even in his punishments. I am dishonoured for ever in this life, and will never be able to recover from it. Never eat cleanly, never clean myself cleanly, never stroke Fedwa's hair at night. That life is over. And all because of fear, and pride. Of course I'm ashamed, of course I'm angry – at Nadir, the Khan, at myself, at Allah, yes Him too! At all of you! I'll never stop being angry, never!'

'Ah,' Bahram said, shocked.

They walked along a while in silence, through the starlit ruins.

Khalid sighed. 'But look you, youth – given all that – what am I supposed to do? I'm only fifty years old, I have some time left before Allah takes me, and I have to fill that time. And I have my pride, despite all. And people are watching me, of course. I was a prominent man, and people enjoyed watching my fall, of course they did, and they watch still! So what kind of story am I going to give them next? Because that's what we are to other people, boy, we are their gossip. That's all civilization is, a giant mill grinding out gossip. And so I could be the story of the man who rode high and fell hard, and had his spirit broken and crawled off into a hole like a dog, to die as soon as he could manage it. Or I could be the story of a man who rode high and fell hard, and then got up defiant, and walked away in a new direction. Someone who never looked back, someone who never gave the mob any satisfaction. And that's the story I'm going to make them all eat. They can fuck themselves if they want any other kind of a story out of me. I'm a tiger, boy, I was a tiger in a previous existence, I must have been, I dream about it all the time, stalking through trees and hunting things. Now I have my tiger hitched to my chariot, and off we go!' He skimmed his left hand off towards the city ahead of them. 'This is the key, youth, you must learn to hitch your tiger to your chariot.'

Bahram nodded. 'Demonstrations to make.'

'Yes! Yes!' Khalid stopped and gestured up at the spangle of stars. 'And this is the best part, boy, the most marvellous thing, because it is all so very damned interesting! It isn't just something to while away the time, or to get away from this,' waving his stump again, 'it's the only thing that matters! I mean, why are we here, youth? Why are we here?'

'To make more love.'

'All right, fair enough. But how do we best love this world Allah gave us? We do it by learning it! It's here, all of a piece, beautiful every morning, and we go and rub it in the dust, making our khans and our caliphates and such. It's absurd. But if you try to understand things, if you look at the world and say why does that happen, why do things fall, why does the sun come up every morning and shine on us, and warm the air and fill the leaves with green how does all this happen? What rules has Allah used to make this beautiful world? Then it is all transformed. God sees that you appreciate it. And even if He doesn't, even if you never know anything in the end, even if it's impossible to know, you can still try.'

'And you're learning a lot,' Bahram said.

'Not really. Not at all. But with a mathematician like Iwang on hand, we can maybe work out a few simple things, or make little beginnings to pass on to others. This is God's real work, Bahram. God didn't give us this world for us to stand around in it chewing our food like camels. Mohammed himself said, Pursue learning even if it take you to China! And now with Iwang, we have brought China to us. It makes it all the more interesting.'

'So you are happy, you see? just as I said.'

'Happy and angry. Happily angry. Everything, all at once. That's life, boy. You just keep getting fuller, until you burst and Allah takes you and casts your soul into another life later on. And so everything just keeps getting fuller.'

An early cock crowed on the edge of the town. In the sky to the east the stars were winking out. The servants reached Khalid's compound ahead of them and opened up, but Khalid stopped outside among the great piles of charcoal, looking around with evident satisfaction. 'There's Iwang now,' he said quietly.

The big Tibetan slouched up to them like a bear, body weary but a grin on his face.

'Well?' he said.

'Too fast to measure,' Khalid admitted.

Iwang grunted.

Khalid handed him the wine bag, and he took a long swig.

'Light,' he said. 'What can you say?'

The eastern sky was filling with this mysterious substance or quality. Iwang swayed side to side like a bear dancing to music, as obviously happy as Bahram had ever seen him. The two old men had enjoyed their night's work. Iwang's party had had a night of mishaps, drinking wine, getting lost, falling in ditches, singing songs, mistaking other lights for Khalid's lantern, and then, during the tests, having no idea what kind of times were being registered back on Afrasiab Hill, an ignorance which had struck them as funny. They had become silly.

But these adventures were not the source of Iwang's good humour rather it was some train of thought of his own, which had put him under a description, as the sufis said, murmuring things in his own language, hummed deep in his chest. The servants were singing a song for the coming of dawn.

He said to Khalid and Bahram, 'Coming down the ridge I was falling asleep on my feet, and thinking about your demonstration cast me into a vision. Thinking of your light, winking in the darkness across the valley, I thought, if I could see all moments at once, each distinct and alone as the world sailed through the stars, each that little bit different… if I moved through each moment as if through different rooms in space, I could map the world's own travel. Every step I took down the ridge was as it were a separate world, a slice of infinity made up of that step's world. So I stepped from world to world, step by step, never seeing the ground in the dark, and it seemed to me that if there was a number that would bespeak the location of each footfall, then the whole ridge would be revealed thereby, by drawing a line from one footfall to the next. Our blind feet do it instinctively in the dark, and we are equally blind to the ultimate reality, but we could nevertheless grasp the whole by regular touches. Then we could say this is what is there, or that, trusting that there were no great boulders or potholes between steps, and so the whole shape of the ridge would be known. With every step I walked from world to world.'