Fireball by John Christopher

andJews believed in a single god and regarded it as the worst possible sin to worship or even acknowledge any other. But the Roman emperor was supposed to be a god, and everyone had to swear an oath to him. The Christians and Jews refused. It didn’t matter so much about the Jews because after the Romans destroyed Jerusalem, they were scattered and survived only in isolated groups. But early Christianity was more widespread and dynamic. It got right to the heart of the empire; the chief bishop was the bishop of Rome. Christians were very noisy about refusing to bend a knee to the god-emperor. The ones who refused got martyred, and the martyrdoms converted more people to Christianity. As far as the Romans were concerned, it was a vicious circle.
    â€œIn this world, Julian fixed things neatly. He decreed that no living man could be deified, including the emperor. Including himself, in fact. So the oath of allegiance became just that, an acknowledgement to a ruler, not to a god. The Christians were tolerated instead of being persecuted—they’re barred from public office, but that’s not something to die for—and the movement ran out of steam. Julian was deified after his death—the last emperorwho was, incidentally—but that didn’t matter. It didn’t bother the Christians, as long as they were left in peace to worship their own god.”
    Brad poured more of the lemon drink. There were plants in pots round the pool, some quite tall. A bird flew down through the open roof and settled on one. A sparrow; birds hadn’t changed.
    Simon said: “Thanks.” The drink was very pleasant. The whole setup scored high for comfort, for luxury, in fact. “What was the fireball, do you think? Nothing to do with ball lightning anyway.”
    Brad shook his head. “No. You’d need to be an Einstein even to understand how to set about trying to work it out. These parallel worlds exist side by side, occupying the same space and time, yet separate. That seems to require some basic underlying medium, like the old concept of aether. Maybe it can fray or warp in places, allowing two worlds to come into contact, and the fireball was that sort of fraying.”
    You’d need to be brighter, Simon thought, than he was to understand what Brad was talking about. He said: “So, do we wait for another one to come along and take us home?”
    â€œMight be a long wait. And could we be sure we’dmake it back to where we started? If there’s one parallel world, I’d guess there’s an infinite number of them. We might wind up where Hitler won or the bubonic plague wiped out the human race.”
    â€œSo we’re stuck?” Brad nodded. “But what do we do ? We can’t just stay on as permanent guests of Quintus Cornelius surely.”
    â€œIt’s not so bad here.” Brad stretched. “Plenty of activities. And Quintus Cornelius wants us to talk with his bishop—the Bishop of London. He’s at some conference in Rome, or rather on his way back from it. He’s due in London—Londinium, that is—quite soon.”
    â€œAnd we’re to tell him we come from a parallel world? Are you sure he won’t decide to have us burned as witches, or warlocks, or whatever?”
    â€œThat’s another way these Christians are different from ours. They’ve never got around to burning people. Quintus Cornelius doesn’t think there’ll be any theological hassle anyway. Multiple worlds isn’t like multiple gods.”
    There was a sound of footsteps approaching, light on the tiled floor. Brad got to his feet, and Simon followed suit.
    As he had guessed from the footsteps, it was a girl who came towards them. She was in her early teens and wore a tunic of what looked like white silk, the top gathered in folds on her shoulders and secured with a gold brooch. She had a thin gold chain round her waist and gold-painted sandals.
    Brad