Fork (a short story)
We discovered time travel. Actually, we haven't yet, but it has happened. Or will happen. Possibly. The specifics are a little complicated.
We managed to resolve some misconceptions about time travel (the first being that it's actually possible). Things you change in the past don't affect the future; at least, not the future you came from. You can't stop a thing from having happened. History is immutable. It's because of this that you can't travel forward in time (at least, not faster than anyone else). The trip is backwards, and it's one-way.
A common thought in science fiction before time travel existed is that changes to the past will affect the future. It's a great literary device, but unfortunately caused massive paradoxes. The image of a person slowly fading out in the future was powerful, but we now know that it doesn't work like that.
Some philosophers postulated about the "Many Worlds" interpretation of the universe. The idea is that any point throughout history where a quantum decision could be made, the universe split, taking both options simultaneously. We live in a branch of the universe, and millions of times a second, for every possible undecided action, new universes are tearing off. The idea is nice, but that's not how it works. There is no reason, nor cause for the universe to take two paths instead of one, so it doesn't. Until someone invented time travel.
It was invented millions of years ago, or a few decades into the future, depending on how you look at it. The man who invented it didn't really know what he'd done. I suppose he never will.
The device transports it's contents back a fixed length of time. 63 years, approximately. It also transports the contents approximately 15 meters to the west. The theory is that we just lucked out, and actually landed on the earth 63 years earlier, instead of in the middle of space, or under the ground. If it was out by even a few minutes, none of the travellers would have survived.
When a person is transported back, the universe tears. History cannot be changed, but it can be forked. The appearance of something that didn't exist in the original time line causes the universe to fork at that point. The original time line has already happened, but a new one forms from that point. Changing anything from then will only affect the future in the new timeline, not in the old. So far as the old timeline is concerned, that matter simply stops existing as soon as the machine fires.
The original inventor of the device never knew what happened to the matter. He sold the device as a way to solve the pollution problem. Put in your junk, and it just disappears! We had six months of mysterious future rubbish appearing before the first child climbed in. She was the first known time traveller (well, living one. We had received some corpses).
She was fifteen. The poor kid spent a few months in an asylum before the internet picked up on the story, and launched a campaign to have her released. For once the conspiracy theorists were right (well, right-ish. It wasn't aliens, or the government, but she was genuinely from the future).
Working from boxes and other stuff found in the future rubbish, scientists have been working on recreating the device. We're sure that they will be successful because we got more travellers. Some of the travellers coming through now have knowledge about the device, which is leading to speculation that it may be created earlier here than in other realities. We hope it will be soon enough to stop the cycle.
The problem that was never considered is that when you remove matter from the universe, the universe itself gets lighter. The universe receiving the mass gets heavier. The amounts of matter were ultimately so tiny that it was nearly impossible to notice at first, but now we have. The gravity of earth is changing.
The mass of the earth was always changing, don't get me wrong - star-dust, meteorites, random space junk; it's landing in the atmosphere all the time. The fact that we were able to make this worse is a testament to both our ingenuity, and our stupidity. Millions of times going through the cycle to get to this reality, has led to billions of tonnes of junk being unceremoniously dumped, by us, in our future. The problem is that even if we resolved to never do it again, that wouldn't change all the rubbish that we've already dumped.
Navigation equipment was the first to tip us off. GPS slowly started wandering as the satellites started responding to the minuscule changes in earth's gravity. It started as funny stories about self-driving cars "acting aggressively" by cutting corners and parking badly. Soon everything had to be recalculated because roads apparently went through buildings, and people got hurt.
Our only hope is to create the device early enough that we can go back before this madness started, and warn them. Stop them from ever creating it. This won't help us at all, but maybe we can make one world a better place; a cosmic hard reset. We hope that this has already happened in other realities, too. We will never know. We just know that it hasn't happened for us. Our future keeps getting written, and every time something new appears, we know that once again, we failed.
One day we hope to get it right.