Michael Swanwick's Periodic Table of Science Fiction

Iridium

192.22

Don't Look to the Skies!

I stared out into the cloudless blue Cretaceous sky. "I don't see

anything coming."

"I wanted to get here early," Rachel said. "There'll be more time

travelers soon, though. The last day of the Age of Dinosaurs has got to

be a hot-ticket destination."

"No, I mean I don't see any sign of the Chicxulub impactor?the comet or

meteor or whatever it is that's supposed to be about to smash into the

Earth. Shouldn't we be able to see it from here?"

Rachel squinted into the sky. "You'd think. It certainly is a mystery."

"So how do we know we came to the right time?"

"Stratigraphy, old son. Way back in the twentieth century Alvarez pÈre et

fils realized that a thin boundary of clay marking the boundary between

the Cretaceous and the Tertiary was unnaturally rich in iridium, no

matter where in the world you examined it. Iridium is a lot more common

in comets and asteroids than it is on Earth. So they postulated something

big hit the planet, burned the forests, and killed the dinos?probably in

the long nuclear winter afterwards."

"Sounds dangerous."

"No need to worry. We'll only be here long enough to snap a few photos.

We'll cut out before the destruction actually reaches us."

"Good. Hey! Isn't iridium the same stuff that's used to power our time

machines? You don't suppose??"

"No, no, of course not," Rachel said dismissively. "Oh, yes, certainly,

there's a chance of chronodestablization. But it's very slight. Only one

in ten thousand jaunts destabilizes. That's certainly a risk worth

taking."

"Yeah, but if it did, it would go off with the force of an atom bomb.

That's what they told me in orientation, anyway."

"Yes, but a single bomb wouldn't destroy the whole ecosystem! It would

take?"

Her eyes bugged out. Time travelers were popping into existence

everywhere. On the hilltops, by the banks of the rivers, in the mangrove

swamps?everywhere. From every age possessing time travel they came in the

hundreds, the thousands, the millions.? God knows how many there were in

the entire world. More, possibly, than there were dinosaurs.

To every side of us light flashed into the sky and flashed and flashed

and flashed, as one out of every ten thousand time machines destabilized

and expressed itself as a thermonuclear explosion.