"Dear, no," said Sarah. "Don’t do that."

He turned around and started pacing toward them. "It’s the only thing that makes sense. I never wanted this in the first place, and I sure as hell don’t want it if you’re not getting it, too."

"But it’s a blessing," said Sarah. "It’s everything we talked about: seeing our grandchildren grow up; seeing their children. I can’t — I won’t — let you give that up."

He shook his head. "No. I don’t want it. Not anymore." He stopped walking, and looked directly at Petra. "Undo it."

Petra’s brown eyes were wide. "I can’t. We can’t."

"What do you mean, you can’t?" said Don.

"Your treatment has been done," Petra said. "Your telomeres are lengthened, your free radicals are flushed, your DNA has been repaired, and on and on. There’s no way to undo it."

"There must be," he said.

Petra lifted her shoulders philosophically. "There hasn’t been a lot of research funding for finding ways to shorten the human lifespan."

"But you must be able to arrest the rejuvenation, no? I mean, right, I understand that I can’t go back to being eighty-seven physically. Okay, fine. I’m — what? — I suppose I look about seventy now, right? Just stop the rollback here." He pointed his index finger straight down, as if marking a spot. Seventy he could live with; that wouldn’t be so bad, wouldn’t be an insurmountable gulf. Why, old Ivan Krehmer, he was married to a woman fifteen years younger than himself. Offhand, Don couldn’t think of a case in their social circle where the woman was a decade and a half older than the man, but surely these days that was common, too.

"There’s no way to stop it early," said Petra. "We hard-coded into the gene therapy how far back the rollback will go. It’s inexorable once begun. Each time your cells divide, you’ll get physically younger and more robust until the target is reached."

"Do another round of gene therapy, then," Don said. "You know, to countermand—"

"We’ve tried that with lab animals," Petra said, "just to see what happens."

"And?"

She shrugged her shoulders. "It kills them. Cell division comes to a complete halt.

No, you have to let the rollback play out. Oh, we could cancel the planned follow-up surgeries — fixing your teeth, your knee joints, getting you that new kidney once you’re strong enough to stand going under the knife. But what would be the point of that?"

Don felt his pulse racing. "So I’m still going to end up physically twenty-five?"

Petra nodded. "It’ll take a couple of months for the rejuvenation to finish, but when it does, that’ll be your biological age, and then you’ll start aging forward again from that point, at the normal rate."

"Jesus," he said. Twenty-five. With Sarah staying eighty-seven. "Good Jesus Christ."

Petra was looking shell-shocked, and she was slowly, almost imperceptibly, shaking her head back and forth. "What?" demanded Don.

The doctor looked up, and it seemed to take her eyes a moment to focus. "Sorry," she said. "I just — well, I just never thought I’d end up having to apologize for giving someone another sixty or seventy years of life."

Don crouched down next to his seated wife. How excruciating doing that would have been just a short time ago — and yet it gave him no pleasure now to be able to do it with ease. "I am sorry, honey," he said. "I am so sorry."

But Sarah was shaking her head. "Don’t be. It’s going to be all right. You’ll see."

How could it be all right? he wondered. They’d spent their lives in synch, born the same year, growing up with the same events in the background. Both remembered precisely where they were when Neil Armstrong set foot upon the moon during the year they’d each turned nine. Both had been teenagers when Watergate happened; in their twenties when the Berlin Wall fell; in their thirties when the Soviet Union collapsed; in their forties for the first detection of alien life. Even before they’d met, they’d been marching through the stages of life together, jointly aging, and improving, like two bottles of wine of the same vintage.

Don’s head was swimming, and so, it seemed, was his vision. Sarah’s face appeared blurred, the tears in his eyes doing what Rejuvenex’s sorcery couldn’t, erasing her wrinkles, smoothing out her features.

Like most SETI researchers, Sarah had worked late many nights after that first alien transmission had been received back in 2009. Don had come to see her in her office at the University of Toronto on one of those evenings, after he’d finished his work at the CBC.

"Anybody home?" he’d called out.

Sarah had swung around, smiling, as he came through the door carrying a red-and-white Pizza Hut box. "You’re an angel!" she crowed. "Thank you!"

"Oh," he said. "Did you want something, as well?"

"Pig! What did you get?"

"A large Pepperoni Lover’s… ’cause, um, I like pepperoni, and we’re lovers…"

"Awww," said Sarah. She actually preferred mushrooms, but he couldn’t stand them.

Coupling that with his dislike for fish had given rise to the little speech she’d listened politely to him give on numerous occasions, a pseudo-justification that he thought was witty for his eating choices: "You should only eat food that’s as evolved as you are. Only warm-blooded animals — mammals and birds — and only photosynthesizing plants."

"Thanks for coming by," she said, "but what about the kids?"

"I called Carl, told him to order a pizza for him and Emily. Said he could take some money out of my nightstand."

"When Donald Halifax parties, everybody parties," she said, smiling.

He was looking around for somewhere to set the pizza box. She leapt to her feet and moved a globe of the celestial sphere off the top of a filing cabinet, setting it on the floor. He placed the box where the globe had been and opened its lid. She was pleased to see some steam rising. Not too surprising; the Hut was just up on Bloor Street.

"So, how’s it going?" he asked. This wasn’t the first time he’d brought food to her office. He kept a plate, knife, and fork in one of the office cupboards, and he got them now. Sarah, meanwhile, pulled out a piece of pizza, severing the cheesy filaments with her fingers.

"It’s a race," she said, sitting down in the chair in front of her workstation. "I’m making progress, but who knows how it compares to what everyone else is achieving? I mean, sure, there’s a lot of sharing of notes going on online, but I doubt anyone is revealing everything yet."

He found the other office chair — a beat-up folding one — and sat next to her. She was used to the way her husband ate pizza, but couldn’t actually say she liked it. The crust wasn’t part of his diet — of course, the greasy Pizza Hut deep-dish crust probably shouldn’t be part of anyone’s diet, although she found it impossible to resist. He got the toppings off with a fork, swirling it in the molten cheese almost as though he were eating spaghetti. He also ate sandwiches a similar way, digging out the fillings with cutlery while leaving the bread behind.

"Anyway, we’d always expected that math would be the universal language," Sarah continued, "and I guess it is. But the aliens have managed something with it that I wouldn’t have thought possible."

"Show me," Don said, moving his chair closer to her workstation.

"First, they establish a pair of symbols that everybody working on this agrees serve as brackets, containing other things. See that sequence there?" She pointed at a series of blocks on her computer screen. "That’s the open bracket, and that one there" — pointing at another place on the screen — "is the closing bracket. Well, I’ve been doing a rough-and-ready transliteration of everything as I go along — you know, rendering it in symbols we use. So, here’s what the first part of the message says." She flipped to another window. It was displaying this: