"Have you ever built a time machine, Ed?" I ask as we are driven to the airfield where a high-speed jet is apparently waiting for us.

"Thought about it a bundle of times. Even drew up a sketch of the prototype once, but tore it up and set fire to the pieces. The consequences could have been catastrophic and the risk was too great."

"Freeow," I remark. This is Ed talking.


One supersonic flight courtesy of the American air force and ten hours later, an elevator door slides open to reveal that the interior of Mount Kerrig in Nevada is air-conditioned, surprisingly well-lit, and wholly un-claustrophobic. Something like a hundred metres below ground, we are shown to a large, white-painted warehouse-like cavern in which what looks like a fully-assembled, operational particle accelerator has been surrounded with enough miscellaneous heavy electronics to build another one. Cordoned off at the rim of the device - most of which runs through the rest of the mountain's interior - is a cuboidal opening which appears to have been made out of an elevator car. Around the place are a handful of white-coated scientists, and - apparently - me and Ed.

Meeting them is the strangest experience. Here is another me - someone who might well know everything about me. I can't help feeling there's a fair bit about me that I'm unhappy with the thought of other people knowing. Ed seems pretty cool with his alternate self, though.

They claim to be time travellers. A series of tests verify that they share our facial features, fingerprints and DNA. They stake a good claim to being us.

Are they? And if so, what do we do about it?

Alternate Ed is apparently as smart as regular Ed, with a few extra bits of knowledge into the bargain - for, as one would expect from one of the world's first time travellers and a dabbler in what I have grown to term "practical science fiction", he has a working knowledge of time travel and time machines. He arranges a meeting so that he can explain exactly what happened at midnight two days ago.


Alternate Ed - from now on, I'm calling him "Ed-A" and alternate Sam "Sam-A" - has the attention of quite a lot of Army officers, a dozen scientists, me, Ed and Sam-A. He also has a whiteboard on which he draws expressive - and irreproducible - diagrams while he talks. "Here's the story. The models of time travel that we have all seen presented in the movies Back To The Future, Tomb Raider or even The Terminator are inherently paradoxical and hence flawed. While travelling forwards in time is perfectly legitimate, akin to simply disappearing for a thousand years or whatever, it is impossible to go backwards in time and kill your own grandfather, since your grandfather was not killed at that time. Temporal paradoxes, by definition, cannot come into existence. In short, you can't change the past.

"You can, however, go backwards in time.

"We resolve this by invoking a variation on many-worlds theory." He draws a horizontal line, names it "TIMELINE A" and, with two crosses, marks the year 1900 and the year 2000. "Suppose I want to go back in time a hundred years from the year 2000. I set up my machine, climb in and turn it on. I appear in 1900. Now, as we all know, there were no time machines in the year 1900, and I can't have changed that. What I've done instead is create a new universe. The act of appearing in 1900 "nudges" the universe from timeline A, onto a different track, B." He draws a second line, parallel to the first, names it "TIMELINE B" and marks the years 2000 and 1900 on that line as well. He then draws a dotted line, leading from 2000 in timeline A to 1900 in timeline B.

"In 1900-A, nothing happens. In 2000-A, I build a time machine and go back a hundred years. And I'm never seen again. In 1900-B, something different happens, namely the appearance of my time machine. Now both universes share the same past, but due to my entry in 1900, they now have different futures. How different they are depends on what I do at that point. I could immediately return forwards in time, resulting in only very minor changes to the year 1900, though as we all know, chaos theory would conspire to inflate these. Or I could, for example, find and kill my great-grandfather. Basically, in 2000-B, anything can happen. This can include me-B never inventing a time machine in the first place.

"Every time somebody jumps backwards in time, the universe splits. Every piece of fruit that was sent back in that time machine - which works perfectly, by the way, we brought some blueprints you can check against - did indeed arrive a few minutes before the experiment began... each in a different universe. Whether, in the name of causality, the alternate scientists in those alternate universes decided to proceed with the experiment or not, is up to chance, and there's no way we can find out, but if they didn't the worst that could have happened was that the extra piece of fruit they had lying around went rotten."

Ed-A wipes off the numbers of the years and replaces 1900 with "midnight two days ago", 2000 with "in a week's time" and puts an extra cross between the two in timeline B, marking it "NOW". "Here's where it starts to get interesting. In our timeline, the one which we'll call timeline A, in about a week's time, all of the above theory will be discovered by scientists working at the Kerrig Facility - you guys. Feeling their debt to me, that's me-A, they invite - where's Dr. Dan Streetmentioner's book of 1001 Tense Formations when you need it? - they invite me-A and Sam-A to come here and become the first time travellers. We then travel back in time approximately one week, disappearing from their timeline entirely.

"And reappearing in this one.

"This is timeline B. This timeline split off from timeline A around two days ago at the point when me-A and Sam-A did or did not arrive from the future. That makes you two," he points to me and Ed, "Ed-B and Sam-B. Everyone get it?"

There is a pause.

"I don't understand," says Ed-B.

And this is Ed talking.

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