There's a trick to time travel. And sooner or later, I think, anyone who's unlinked figures it out. We're rare enough that two of us hardly ever cross paths, and when we do, there's seldom any time to share information. But we live long enough to where we'll try almost anything, and time travel eventually comes up.

You see, just because you can move between realities at will doesn't mean that you can move up and down the timeline in the same fashion. In my experience, most realities simply aren't built to allow for time travel in the first place. I believe–and I am no scientist, mind you, merely a very old traveler–I believe this is because time travel can be extraordinarily dangerous–not just for the temporal travelers themselves–but for reality itself. So the best way to think about reality without time travel is like a gun with the safety on.

There are realities, though, where the safety is off. The rules work differently from place to place and some realities allow for it. There are restrictions still: in some universes you cannot go back farther than your own birth, in others if you run into yourself your trip automatically ends with you at your starting point, and so on.

When you're unlinked, and you wish to move rapidly in time, you simply cross over into a world with active time travel–of a kind that will get you where you wish to go, of course–and then go when you need to go. This is primarily used for going back in time, mind you: the timestream runs at different paces in certain universes, so to move forward rapidly you can just duck into a fast-moving world and then back to the other world.

It's funny, now that I think about it–especially how dangerous the implications of time travel can be–I don't think I've ever run into a universe that appeared to have undone itself with time travel. Usually it's something far more mundane.

For example, did I ever tell you about the world in which time travel is so costly that one of the only industries that can actually justify doing it is the pornography industry?

They discovered time travel early in this world, but it was a lot like the dive to the bottom of the Marianas Trench that finally happened in 1956 after two fatally unsuccessful attempts: it was done and simply couldn't afford financially to be done again.

Then a pornographer latched onto an idea to make extremely unique sex movies. He sent back a couple, one man and one woman, to Dealey Plaza in 1963. They arrived in a room that historical research showed as abandoned during the time of the shooting. Then they set up a camera and started having sex with the window open. They paced themselves so that they both climaxed at the sound of the gunshots.

This video was a startling success. And it made the entrpreneur who had thought of the idea very, very rich. Enough that he kept trying to outdo himself with each video: having a threesome hidden in Ford's Theater but with the assassination of President Lincoln occurring in the background, all the way up to a small lesbian orgy on a rooftop in Pompeii in full view of Vesuvius. Each time, the orgasm–or orgasms–synched up perfectly with the history-altering event.

I'm digressing. My point with this is simply that reality has precautions in place–and perhaps it only lets down its guard when it knows it can. When it knows somehow that the result will be a best-selling two-men video that takes place while the Library at Alexandria burns down around them instead of a mistake that utterly undoes life as that universe knows it.

Of course, it stands to reason that any number of the broken universes I have ventured into could have been teeming with life we would recognize, but it was the wrong kind of life, and it found itself with a gun that had the safety off.