Title: Tempus Fugit
Author's LJ/DWJ: kerravonsen
Categories: Crossover
Warnings: None
Author's Webpage/Fic list: AO3 (Not complete), Fic tag,
Why this should be read:

The Doctor and Sam discuss temporal mechanics. Scientific accuracy meets "wibbly-wobbly timey-wimey stuff," handled by an experienced and talented author.  What's not to love?

"Suppose I were to write myself a note," Sam said, "and send it back in time-"

"You'd have to have a very good reason," the Doctor said.

"I remembered getting the note in the past and knew I'd have to send it to myself." It had been General Hammond who'd written the note to himself, of course, but she couldn't talk about it at all unless she pretended it was hypothetical. And maybe Dr. Smith would have some insights that would shake up her own tired circle of thoughts on the subject.

"Okay, that might be a good reason," he said.

"But it's an ontological paradox," Sam protested.

"That's the safest kind," said the Doctor.

"Safest?"

"A closed loop is stable," the Doctor said. "Of course, from a non-linear, non-subjective viewpoint, it isn't actually a loop. More like a spiral. Well, not a spiral, exactly..." He picked up a squeeze-bottle of mustard, and drew a circle with it on his bread-plate. "You have a loop..." Then he picked up the bottle of ketchup and squeezed another circle, right on top of the mustard.

"And then another loop," Sam said, leaning forward. "But there was a first loop," she said. "The one where the information came from."

"Exactly," the Doctor said. "Once the loop is closed, the origin is lost, but that doesn't mean that there wasn't one."

"But doesn't that defy entropy?" Sam said.

"Objects are subject to entropy," the Doctor said. "Information isn't quite the same." He cut a piece off his steak and dipped it in the mustard-ketchup mixture on his bread plate. "Hmmm, American mustard is... hardly there, isn't it?" He chewed some more. "Say I'm trapped in 1969..."

She tried to look unconcerned. It was probably nothing. A coincidence. "Why 1969?"

"Dunno, I haven't made a study of the variable time displacement of Weeping Angels. Bit hard to collect the data safely. But not a bad year to be stuck in, really. I mean, the Moon Landing! Brilliant, that. Beans on toast is not so brilliant, but if you've got tea, well, everything is better with tea." He demonstrated this adage by taking a sip from his cup.

Sam took another bite of her salad, hardly tasting it. "So you're stuck in 1969..." She was right about him shaking up her thoughts; he might be a little crazy, but he had her complete attention.

Tempus Fugit