Petra Mayer had spent her mid-twenties in prison for hammering nails into a Montana redwood. Few states send people to prison for harming a tree, Montana included. Only the girl hadn't been trying to damage the tree, she'd been there to halt a lumber firm busy taking down five-hundred-year-old redwoods to sell as specialist timber.

The nail caught the chain of a power saw and stripped off its links, exploding the cutting belt like chain shot and taking off the fingers of a foreman overseeing the lumberjack.

The court case made headline news across America and Petra Mayer used her time inside to write a best-selling polemic on ecology, biodiversity and sustainable growth. Simplicity of style was one of the things which helped Tao and the Way of Global Maintenance to become a New York Times bestseller. The other was the fact she stuck all the biology, physics and economics at the back in an appendix called "Other Stuff," where it could be safely ignored by the bulk of her readers.

The President was one of the few who actually bothered to check her sums. So when his old Professor finally dialled the direct number, Gene Newman opened his cell phone, saw who it was and promptly stood up, setting off a chain reaction that had chairs scraping the floor all around the Situation Room.

"Thank you. That will be all."

Around the table half a dozen advisors, department heads and military chiefs nodded. "Thank you, Mr. President."

"Petra," he said, watching the door close.

"Mr. President?"

"You just saved me from my Defense Secretary."

The slightly disbelieving snort from the other end made it clear that Petra Mayer found this unlikely.

"You wanted to talk to me?"

"That's why I called," said the Professor. Only her age and the weight of her news excused the impatience in her voice. "You've got a problem."

"I've got dozens," said the President. "Where do you want me to start?"

"No," said the voice on the other end of his phone. "I mean you've got a problem."

"Tell me..."

"So you see, what you and I think of as physical laws our friend considers the cosmological equivalent of local weather conditions..."

At the end of the conversation Gene Newman was not entirely sure he'd understood everything his old tutor had told him. The President imagined Petra Mayer knew this and it was only respect for Gene's office that made her refrain from firing questions at him as if they were both still in a seminar.

"It's important?"

"Gene... If this is for real then it changes everything. Think about it. We're surrounded by a high-energy field... No," the Professor corrected herself, "we exist inside a high-energy field, one we barely notice because it runs through us and through everything else and suddenly there's a chance we can tap into this field."

"That is good, right?"

"It's terrifying," said Petra Mayer. "How do you think China will react if her oilfields become worthless overnight? What's a bankrupt Saudi Arabia going to do to the balance of power in the Gulf? Shit, imagine al Qaeda with this technology. Gene, I wouldn't dream of telling you your job but--"

"Okay," said the President, "I can see the problem."

Silence came from the other end.

"What?" Gene Newman said.

"That's not it... Natalia Aziz thinks that one of the equations might relate to time. You're not going to like this." Gene Newman was Catholic, although not so Catholic that he had more than two children.

"Tell me."

"You need to know that Natalia Aziz believes in God."

"Of course she does. The woman's a Moslem."

"No, I mean as a scientist Aziz thinks God exists. It looks like Prisoner Zero thinks the same. Only one of his theorems seems to suggest that God will die."

"When?"

"At the end of time. When the universe comes to an end."

"What happens then?"

"God's born again."

"Along with a new universe?"

"You've got it, but he's probably not talking about God in the sense you're talking about God."

"You know," Gene Newman said, "we're not even going to go there."

"Okay," said Professor Mayer. "You've got your own people examining the photographs, right?"

Here the President was on firmer ground. "Of course I've got people looking," he said. "The best brains in the CIA and the Pentagon are examining them as we talk."

"That should be impressive." Petra Mayer's view on both was well known. As was the thickness of a file she'd eventually prized out of the CIA under section 552 of the Freedom of Information Act, having argued successfully that her file could no longer be regarded as operational.

There was an age when most Western scientists distinguished between space and time. This period lasted roughly from the Dark Ages to the beginning of quantum physics. A period during which the West caught up with and finally overtook the laws of science the Arab world had taken for granted for centuries.

Although few people thought of it in these terms, the shape given to time always had been a religious choice. For those within the Judeo-Christian tradition time was subservient to God and ran like a river. Other faiths saw time differently, as an illusion or a great circle that always came back to where it began and in which history endlessly repeated itself.

It was only with quantum physics that the idea of time as space and the universe as a series of endless illusions really entered Western thought. Before this, time had a beginning and an end over which God would preside. Space was by turn the home of heaven, a starry mantel and a clockwork suburb operated by angels.

As it became possible for astronomers to go further, some suggested that our solar system was not necessarily the centre of the known universe. This destruction of the heliocentric view happened at about the point God was taken out of the time equation.

And then in 1904 a minor clerk at the Swiss Patent Office in Berne wrote a brief pamphlet, a side effect of which was that time and space became, like mass and energy, so inextricably linked they turned into variants of the same thing.

Petra Mayer was not a believer in unalloyed Einstein, any more than she believed in the angelic host, time running in only one direction or the universe as an ever-expanding balloon of mostly dark matter.

"You've got more than just the one photograph, right?" Petra Mayer said. She was having trouble keeping the impatience out of her voice. Old age and cancer were not treating her kindly.

"Yes," said the President, reopening a file. "They're one of the things we've just been discussing. We got copies this morning." Gene Newman leant over the table of the Situation Room to take another look.

There were five photographs in all. Three close-ups of the prisoner's ripped hands and two shots of his shit-smeared cage, with Prisoner Zero cropped at the waist in the foreground. Over the man's naked shoulder could be seen sketches and what looked like one half of a mirror-image equation.

"Do any of them give us more?"

Comparing the best of Pier Angelo's originals with the shot used on the front of that day's Washington Post told President Newman what he already suspected. "Afraid not," he said. There was no difference. The picture desk had used the best shot and used the whole thing.

"You can't execute him," said Petra Mayer. "We need the rest of that equation."

President Newman sighed. "You think I don't know that?"

Inside every adult was a child, or so it is said. Professor Petra Mayer was different. Inside Petra Mayer was an impossibly beautiful, barefoot adolescent who wouldn't have been seen dead giving her inner child the time of day.

In fact, that child had been left so far behind it no longer even haunted the edges of the adult's unconscious, its banishment an act of will so extreme that even Petra Mayer's husband had no idea of the sorry foundling his wife had once been. All he remembered was the ghost of the adolescent who had still, but only just, been visible behind her eyes when they first met.