The cavern, huge and warehouse-like with gleaming metal walls, littered with miscellaneous heavy electronics, hung with gantries like spiderwebs, usually had more than two people in it.

That was because it had gone five, and the regular technicians had knocked off for the night. Remaining were their boss, and his boss.

"Time travel is possible. And you can go back as well as forwards. And there is only one timeline."

"That's something of a contradiction, 'Doc'."

"Quit calling me that. I don't have a doctorate, nor do I have a car with doors which open upwards. I'm an upstart kid with wild but workable ideas. My name is James."

"Well, James, this theory is very interesting, but what stops you from going back in time and changing history?"

"Exactly! Exactly! You've got it!"

"What?"

"What, indeed!"

"Not following you."

"If we tried to go back in time and kill Hitler, something would stop us. Somehow. The mathematics proves this. The question is, what? Would it be luck, or coincidence or some metaphysical force or a sentient being who protects timelines? What prevents paradoxes from forming? That's what I want to find out."

"You want to actually go back in time and try to kill Hitler?"

"Couldn't if we wanted to. Practical issues. Check this chart out."

One of the enormous cavern's enormous walls, the bare one, switched on and an image silently appeared. It was pink, and had a red and yellow wave of some kind, increasing in amplitude as it moved from left to right.

James gestured along the horizontal axis with a lab-coated arm. "This is time. The wave measures something which can best be described as the permeability of the timestream - how easy it is to exit or re-enter the timestream at that point in time. Most of the time it's all but impossible to travel through time - most of these eras can never be visited. But it goes in waves. Every few billion years, for just a few years, you can travel through time. By the most astounding coincidence we've just started one right now. It began in 1997 and goes on until the end of next year."

"So Hitler is out," said Frank. James was employed by the military to turn his dribbling mad ideas into wild, thrashing, country-conquering reality. As well as handling the money, Frank acted like the sprinkler attachment on the hose of James's imagination, directing his attention in profitable directions.

"Wouldn't be my style in any case. No, what I want to do is go back to here - the previous oscillation. Four billion, five hundred and sixty million years ago, just before the Earth was formed."

"And what? Change the course of evolution? Wipe out life as we know it?"

"No. That can't happen. It's mathematically impossible, any of the techs will tell you. I want to go back and try. And see what happens to stop us. Mind your ears."

"Mind my ears what what?"

"Your ears are about to pop. Want some gum?"

"We're a quarter of a mile under a mountain, why are my ears going to pop? Aaargh! Yow!"

The picture above them changed.

"What just happened?"

"We ran hull material into the cavern walls and installed the time travel conduits around it. We've made the entire cavern into a time machine. It's also spaceworthy - there's a weird field effect going on keeping us from explosively decompressing in space, although you still get the pressure change thing. Don't worry, we already tried this a bundle of times. It's perfectly safe. Although I personally don't get the numbers, Adrian assures me it works."

"Where are we? What's that?" The screen had become an open starfield, and, stretching from top to bottom and side to side of the gigantic cavern wall, standing here near the "edge" was more than unnerving. You could take a step out into space.

"By mucking about with the figures we found we can alter our re-entry point so we don't wind up down on the planet itself. We're in space, four and a half billion years ago. That reddish glowing blob in the centre of the rough accretion cloud in the middle of the window - which is what this screen is currently serving as - is the nascent Earth. The planet will be fully formed in a few tens of millions of years."

"So now you're going to change history?"

"I'm going to try. History can't be changed. Relax."

"How? This planet is unrecognisable. What can you possibly... try to do to this Earth that we would still see four and a half billion years later?"

"Well, I can destroy it," said James, holding up a terrifyingly small button on the end of a long cable. The cable wound around their feet and snaked off into the darkest corners of the back of the room, behind lurking, humming machinery.

Frank boggled. "You have a doomsday device? You have a doomsday device and you didn't tell me?"

"Frank! Frank, Frank. Boss. Of course I have a doomsday device. What do I look like, a sane person? You see the places on the chart where the wave hits zero? Those are the permeable bits of the timestream. One in the 'present day'. One, here, four and a half billion years BCE, just before the creation of Earth. One another four and a half billion years before this. And finally, one at the very beginning of time. The Big Bang. It would be suicidal to visit the Big Bang, but we can open a wormhole leading to it, releasing some of its original power here and now. The wormhole is unstable and slams shut after about a ten-millionth of a second, which is just enough time for it to release about enough energy to destroy an Earth-like planet ten times over. It's not quite Death Star level, but it's still a pretty impressive explosion." James caught Frank's eye. "Or... so... I... am... told," he added. "Look, there is absolutely nothing to worry about. History cannot be changed. That is Earth. Earth was not destroyed before it was created. Something, somehow, is going to stop me from pressing this button and blowing it up."

"You should have told me you had this technology."

"Well, I was going to, the very next time it turned out to be strategically advantageous for the United States to blow up a planet. Remember this weapon only works for a few more years anyway. It's nothing. Three-two-one-blastoff!"

James pressed the button.

The Earth blew up.

"OH, DEARY ME," he exclaimed.


Continued!

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