How could that bereaved father not watch this? How could he not relive those terrible moments over and over? How could he turn away from this child, trapped within the machine, as vivid as life and yet unable to grow a second older or to do anything the slightest bit different, ever again?

And how could that father bear to live in a world in which the terrible clarity of the incident was available for him to replay any time he wished, from any angle he chose — and yet knowing he would never be able to change a single detail?

How indulgent he had been — David himself — to sit and watch gruesome episodes from the history of the Church, incidents centuries removed from his own reality. After all, Columbus’ crimes hurt nobody now — save perhaps the man himself, David thought grimly. How much greater had been the courage of Mary, a lonely, flawed child, as, alone, she faced the moment that had shaped her life, for good or ill.

For this, he realized, is the core of the WormCam experience: not timid spying or voyeurism, not the viewing of some impossibly remote period of history, but the chance to review the glowing incidents that make up my life.

But my eyes have not evolved to see such sights. My heart has not evolved to cope with such repeated revelations. Once, time was called the great healer; now the healing balm of distance has been torn away.

We have been granted the eyes of God, he thought, eyes which can see the immutable, bloodstained past as if it were today. But we are not God, and the burning light of that history may destroy us.

Anger coalesced. Immutability. Why should he accept such unfairness? Maybe there was something he could do about that.

But first he would have to figure out what to say to Heather.

The next time he called, when more weeks had gone by, Bobby was shocked by David’s deterioration.

David was wearing a baggy jumpsuit that looked as if it hadn’t been changed for days. His hair was mussed, and he had shaved only carelessly. The apartment was even more of a mess now, the furniture littered with SoftScreens, opened-out books and journals, yellow pads, abandoned pens. On the floor, stacked around an overflowing garbage pail, there were soiled paper plates and pizza boxes and microwave junk-food cartons.

But David seemed defensive, perhaps apologetic. “It’s not what you’re thinking. WormCam addiction, yes? I may be an obsessive, Bobby, but I think I pulled myself back from that.”

“Then what.”

“I have been working.”

A whiteboard had been set up against one wall; it was covered with scarlet scrawl, equations, scraps of phrases in English and French, connected by swirling arrows and loops.

Bobby said carefully, “Heather told me you dropped out of the 12,000 Days project. The Christ TrueBio.”

“Yes, I dropped out. Surely you understand why.”

“Then what have you been doing here, David?”

David sighed. “I tried to touch the past, Bobby. I tried, and I failed.”

“…Whoa,” said Bobby. “Did I understand that right? You tried to use a wormhole to affect the past? Is that what you’re saying? But your theory says that’s impossible. Doesn’t it?”

“Yes. I tried anyway. I ran some tests in the Wormworks. I tried to send a signal back in time, through a small wormhole, to myself. Just across a few milliseconds, but enough to prove the principle.”

“And?”

David smiled wryly. “Signals can travel forward in time through a wormhole. That’s how we view the past. But when I tried to send a signal back in time, there was feedback. Imagine a photon leaving my wormhole mouth a few seconds in the past. It can fly to the future mouth, travel back in time, and emerge from the past mouth at the precise moment it started its trip. It overlies its earlier self.”

“- and doubles the energy.”

“Actually more than that, because of Doppler effects. It’s a positive feedback loop. The bit of radiation can travel through the wormhole over and over, piling up energy extracted from the wormhole itself. Eventually it becomes so strong it destroys the wormhole — a fraction of a second before it operates as a full time machine.”

“And so your test wormhole went bang.”

David said dryly, “With more vigor than I’d anticipated. It looks as if dear old Hawking was right about chronology protection. The laws of physics do not allow backwards-operating time machines. The past is a relativistic block universe, the future is quantum uncertainty, and the two are joined at the present — which, I suppose, is a quantum gravity interface… I am sorry. The technicalities do not matter. The past, you see, is like an advancing ice sheet, encroaching on the fluid future; each event is frozen into its place in the crystal structure, fixed forever.

“What is important is that I know, better than anyone on the planet, that the past is immutable, unchangeable — open to us to observe, through the wormholes, but fixed. Do you understand how this feels?”

Bobby walked through the apartment, stepping over mounds of paper and books. “Fine. You’re suffering. You use abstruse physics as therapy. What about your family? Do you ever spare a thought for us?”

David closed his eyes. “Tell me. Please.”

Bobby took a breath. “Well, Hiram’s gone into deeper hiding. But he’s planning to make even more money from weather forecasting — vastly better predictions, based on precise data centuries deep, thanks to the WormCam. He thinks it may even be possible to develop climate control systems, given the new understanding we have of long-term climate shifts.”

“Hiram is -” David sought the right word. “ — a phenomenon. Is there no limit to his capitalistic imagination? And the news of Kate?”

“The jury’s out.”

“I thought the evidence was circumstantial.”

“It is. But to actually see her at her terminal at the time the crime was committed, to see that she had the opportunity — I think that swayed a lot of the jurors.”

“What will you do if she’s convicted?”

“I haven’t decided.” That was true. The end of the trial was a black hole, waiting to consume Bobby’s future, as unavoidable and as unwelcome as death. So he did his best not to think about it.

“I saw Heather,” he said. “She’s well, in spite of everything. She’s published her Lincoln TrueBio.”

“Good piece of work. And her pieces on the Aral Sea war were remarkable.” David eyed Bobby. “You must be proud of her — of your mother.”

Bobby thought that over. “I suppose I should be. But I’m not sure how I’m supposed to feel about her. You know, I watched her with Mary. For all their friction, there’s a bond there. It’s like a steel rope that connects them. I don’t feel anything like that. It’s probably my fault.”

“You said you watched them? Past tense?”

Bobby faced him. “I guess you haven’t heard, Mary left home.”

“…Ah. How disappointing.”

“They had one final fight about the way Mary was using the WormCam. Heather is frantic with worry.”

“Why doesn’t she trace Mary?”

“She’s tried.”

David snorted. “Ridiculous. How can any of us hide from the WormCam?”

“Evidently there are ways. Look, David, isn’t it time you rejoined the human race?”

David caged his hands, a big man, deeply distressed. “But it is so unbearable,” he said. “This is surely why Mary fled. I tried, remember. I tried to find a way to fix things — to fix the broken past. And I found that none of us has a choice about history. Not even God. I have experimental proof. Don’t you see? Watching all that blood, that rapine and plunder and murder… If I could deflect one Crusader’s sword, save the life of one Arawak child.”

“And so you’re escaping into arid physics.”