[An object at rest will remain at rest unless acted upon by an external and unbalanced force . An object in motion will remain in motion unless acted upon by an external and unbalanced force.]

"The simple fact is, you didn't care enough."

The Doctor (not Jack's Doctor, but he'll do) raises an eyebrow. "How do you come to that conclusion?"

"You left me on an empty space station. You didn't come back. You and Rose just went off on your merry way and left me. You'd never have done that to Rose. You'd have loved her too much."

"You lot, you're all the same. 'Tell me how much you love me,' 'Tell me you don't love anyone else,' 'Show me how much you care.' Don't know why I put with it."

They're not drunk, because neither one's prepared to let their guard down and drop the enhanced metabolism. But they're willing to pretend.

"I wasn't really at my best," says the Doctor, almost apologetic. "And then... well, then you were gone. Knew you'd be alright. You're tough, you're capable. You'd have stayed anyway," he adds, opening another bottle of beer with something Torchwood have never been able to figure out. Jack's pretty sure it's not supposed to open bottles.

"I wouldn't have stayed."

The Doctor looks at him, ages a couple of centuries. "I know what people leave for. You're a fighter who doesn't want to fight all the time. You'd have stayed to rebuild."

"What about Rose?"

"She," says the Doctor, "would have lasted another year, another six months. Then she'd have gone home, gone to university, might have built houses in Africa. I always thought she'd have made a good social worker. But no," he tells the bottle, "the universe never lets things turn out like they should."

Jack picks at a label with short, practical nails. "What about you? What are you going to be when you grow up?"

The Doctor looks at him and throws him something like a smile. "Peter never grows up."

"Peter was looking for his mother. Someone to love him."

"Do you?" asks the Doctor. "Love me?" He looks scared and hopeful and hurt and happy all at once.

Jack thinks for a moment. "I don't think you've ever met anyone who didn't."

"That's the worst part," says the Doctor. "No, second worst. Worst is-"

"That you love them right back."

"Two hearts, can't help it."

Like gods, thinks Jack. They hid themselves away in case they pushed too hard, did nothing in case they loved all the mortals to death.

Jack's not mortal anymore. The last machine of a dead civilisation fixed him the only way it knew how. Except that now he wonders if it meant him as a gift. It (she) loves the Doctor too, in its (her) silent, biomechanical way. He looks at the old, old alien and hopes he didn't ask it (her) for a consort.

[The rate of change of momentum of a body is proportional to the resultant force acting on the body and is in the same direction.]

"Men don't have bras," says the Doctor. "That's one advantage."

Jack tugs his own shirt over his head and kisses the Doctor again. It won't shut him up, but Jack likes the taste of something from another world, teeth and tongue and lips all distance and imagined stardust. Everything is not-quite, and Jack could close his eyes and be anywhere, anywhere that isn't Cardiff, Earth, orbiting a middle-aged sun.

"Bras are an improvement, though," says the alien when their lips part. "Few centuries earlier and by the time you've got her out of her clothes you've forgotten what you wanted in the first place." He helps Jack with tie and shirt buttons, calm and unhurried. Jack would mistake it for disinterest if the insistent pressure against his abdomen didn't tell him that the Doctor is just as hard as he is.

Jack loves people who want him. It used to be all he ever asked for.

This new Doctor is more passive than the old one, and Jack is left to tug at clothing as the Doctor complains about impractical underwear and "the lumpy bits on mammals."

"Do you want me?" asks Jack, when he's negotiated his way to expanses of skin.

The Doctor stretches lazily on the bed, sprawled carelessly and unselfconsciously. "Just don't get me pregnant," he says, deadpanning it convincingly enough that Jack has to take a moment to shake the idea from his head. It's not an answer, either, not one with the words he wanted.

"It's not funny," he says, feeling hearts speeding under alien skin and human touch.

"I'm extinct," says the Doctor. "Gallows humour's the only thing I've got left."

"You've got me."

"Yeah. Yeah, I have."

[All forces occur in pairs, and these two forces are equal in magnitude and opposite in direction.]

Jack fucks the Doctor like he'd fuck anyone else. He's had aliens before, and he's always thought of sex as a true equaliser, reducing anything to gasps and grunts and groans.

Not Time Lords, it seems.

"You're beautiful, Jack," says the Doctor, arching at Jack's answering thrust. His eyes are always open, watching and studying, a deep brown sparked with something at the edge of Jack's visual range. It twitches away when he tries to place it, and maybe there's no one else alive who could name that suggestion of colour.

Heat draws them together, and the alien stretched out beneath him is cool enough to steal Jack's warmth. He pushes deeper, harder, drives the Doctor's eyelids to flutter closed for an instant and forces a word from his lips that floats like summer air. He wants to learn that language, erase death from its sounds.

Jack moves faster and the Doctor comes, sticky and wet and faintly yellow. He swears in English this time, but the accent underneath it is from somewhere else entirely. The Doctor looks at him, sated and calm and the light in his eyes is love.

Jack is wanted, and he lets go, climaxing inside an alien for the first time in far too long. Tension flows from his muscles and he falls forwards and sideways, collapsing into infinity and sweat-soaked sheets.

He drifts until he feels the Doctor's fingers stroking his skin and hot breath in his ear.

"Let's go fight some pirates," says the boy who never quite grew up.

Jack knows he shouldn't say yes, but he's never been able to resist feeling wanted. And he's got plenty of time to learn.