The first thing which happens is that someone invents something which is vaguely analogous to time travel. It's a weird mechanical resonation at first, a byproduct of the incredibly fine processes which go into the most advanced semiconductor manufacturing. The weird little resonance is easily worked around, and chip fabrication continues apace, but the source of the resonance becomes more and more difficult to explain, and people start scratching their heads. And the chip manufacturer has a healthy habit of investing in research and development, so some money is put aside, and the puzzle multiplies, and the practical possibilities start to multiply too, and then... then small objects begin appearing in the laboratory.
The first few aren't noticed, grains of sand, too small. But the marbles, they get a little puzzled attention. Especially the marbles appearing on one side of the chip fabrication area, which is the cleanest space in all of human spacetime and should be functionally impossible to smuggle rogue matter into. Security footage is checked, and the largest, most recent marble can be seen pinging into existence and clacking to the floor one afternoon, and rolling away underneath vital equipment.
And then a mouse. The mouse is a serious problem. The fabrication area is contaminated by the mouse's arrival and has to be half torn down to recover it and sterilise all the machinery.
It's impossible. It's impossible for a mouse to show up here.
And then... just as one of the researchers into the strange resonance phenomenon is starting to piece together what's happening, that same researcher arrives in the fabrication plant. She arrives in the foetal position. The shockfront from the displacement is visible to the people in the plant who see her show up. She coughs up a USB stick.
"I was the smallest," she says. It is the first thing she says. Her name is June, which everybody knows.
It's true, she does have the smallest physical stature of anybody working at the plant. "I know," the existing June says to her.
"I just survived a trip through a space machine," the new June claims. "It's a machine you're about to start building and testing. One year from now, this lab switches function, it becomes the space machine lab. You're about to figure out a way to travel up in space. Not back in time. Up through 4-space, up through the stack of timelines.
"There's a stack. That's what this resonation is. There's an infinity of them, actually, we think the stack is continuous, not discrete. But there's a resonation, an alignment, every sidereal year. This world, the world where it's the eighteenth of November, 2020, is precisely one sidereal year out of phase with the world where it's the eighteenth of November, 2021. Because that's how long it takes the world to return to its original point in space."
"But what about the motion of the Sun through galactic space?" June asks her newer self.
"We haven't worked that out yet," the new June says. "Something more pressing came up. The universe started tearing itself apart. Some kind of brilliant cosmic phenomenon. It started in infinitely deep space but then it closed up on our solar system, from every direction, like a fist. That's what it looked like from our perspective, anyway. It slowed down as it got closer. Over the course of September and October we saw it ruin three out of four gas giants by the time we got the machine up and running. November, we've been ramping up human trials. The world is set to end around midnight in the morning on the first of December, 2021."
"The good news is, we have about a year longer to prepare for it. And, if we run out of time, I brought schematics. We can skip a lot of the research. Get a head start."
They start work on the two problems, the cosmic destruction phenomenon and the new space machine, simultaneously. Things get strange, because the work proceeds so rapidly, and nobody manages to make any kind of formal announcement or create any kind of cover story. Word gets out, but the word is so strange that nobody does anything about it.
And it's just as well that the space machine works, because the cosmic wavefront shows up significantly earlier this time around, months earlier. So early that the end of the universe seems likely to arrive as soon as October 2021.
"We're out of time," the project declares. They test the machine out, just as before, and then send June-the-original and June-the-interloper through the machine first, and then spend the few remaining weeks attempting to widen the aperture, sending more, larger people through, back to autumn of the preceding year. But this time the plan is different. This time they get cracking on building the next space machine immediately, and even bring a few critical parts of the machine through. And instead of stopping in 2020, they go back another jump. And another jump, and another.
Until eventually, there are a dozen or so semiconductor physicists and chemistry researchers (and some marbles, and some experimental dust, and a mouse) arriving in a blank field in 2014, in the dead of night, before the chip fabrication plant was even built.
There is no way to step down again, to return to the previous timelines, the ones which are ahead. In fact, the original two timelines are probably gone now, completely consumed by the cosmic wavefront.
The gathered, heavily duplicated interlopers regroup, and attempt to establish some kind of workshop/project to save the world.
But the cosmic wavefront shows up again, less than a year after they arrive. It's going to smash the world to plasma before the end of 2015. It's as if the wavefront is following them all back in time, stalking them back through the timelines.
June is just the second-oldest of four Junes, now, and is a long way from her authentic, original, dead family and friends. It has been a miserable slog. All she still has is the mouse, the original mouse, which they sent back in time originally, only for it to be caught alive, so that when she eventually followed it back in time she was able to reunite with it. They have used the same mouse each time, and somehow the mouse always lingers in the area after being sent back, and zeroes in on June again. Maybe it's because she feeds it nice fruit.
"Mouse," she says, "this has been a complicated continuum. There is so much which happened which I can't even explain to you. What is this wavefront? I can see a diagram of the thing now, sweeping across spacetime. Well, spacespace, I suppose. All the layers of timelines stacked on top of one another, and this diagonal line, just a little steeper than the diagonal line which links each year with the same year in the previous timeline, cutting each timeline off earlier and earlier."
"The weird part," she says, still talking to the industriously exploring mouse, "is that nobody came back to our timeline to warn us. We developed the space machine, but in the future, in the deeper timelines, there would have been people who saw the wavefront coming much later, long after the space machine was developed. Surely they would have visited us in the past, simply of their own accord? Even before seeing any wavefront?
"But that didn't happen. Which means that either the wavefront consumed those deeper timelines much earlier than ours... or, there are no deeper timelines at all. The older June was the oldest one who exists on the entire skein of time, drawing the rest of time along behind her in her wake. What are the odds? I guess someone had to be first, though. I should name you, mouse."
She sits up, thinking.
"But what are the odds? The odds that we would develop the space machine just a few months before the universe would come to an end? Astronomical.
"Did we end the universe, mouse? Is that what does it? Is that why this wavefront follows us every time we jump back? Does the space machine cause the skein of time to start unravelling like some sweater? Then maybe... we can save the rest of the timelines by stopping here? By simply stopping running, and watching and waiting while the universe shatters?
"No. That can't be. Because then the timeline above ours proceeds as normal for six or seven more years, and finds the space travel technology again, just like the older June originally did. And the loop starts over again. Is that what's truly happening here? Is that why nobody from another timeline has ever come to see us, because they decided to abandon that technology forever to protect the rest of spacespace, even if only for a little longer?"
"No, June," the mouse says. "However, you are close."
June blinks. "Mouse?"
"My name is unimportant," the mouse says.
"Can I call you Nim for short?"
The mouse's facial expressions are difficult to read. "I am a fugitive, June, an incalculably old and complex inhabitant of the many-dimensional skein of... what in your language could be rendered as 'space-space-space-time-time-time', with many more 'space's and 'time's omitted. I have seen numerous cross-sections of human society as I have journeyed across your multidimensional existence, from past to future, from continent to continent, from planet to star. It was I who gifted the original timeline with the quantum-level resonation phenomena they needed, the loose thread to tug at. I was shipwrecked in that dimension, and fostered and accelerated their development of the space machine in order to be able to continue my journey. An escape craft, of a kind."
"A fugitive, mouse?" June asks. "A fugitive from what?"
"The diagram you imagine," the mouse says. "Draw it, on paper. The manifold of the worlds."
With some prompting, and some extra data given by the mouse, she draws it. There is the diagonal wavefront of ending universes, and then, when the loop breaks, a grace period of six or seven years, and then the diagonal begins again. Continuing the pattern, a rudimentary sawtooth formation appears.
The mouse enters the diagram, breaking a little lead off the tip of June's mechanical pencil and using it to draw further, adding the significant inter-universal detail, not visible due to the discrete movements of precisely one year which she and her cohort have been using up until now.
"And here, behind us. We are trapped, both coming and going."
"This," the mouse declares. "This is the shape of the universe."
The sawtoothed edge... looks exactly like teeth. The closing teeth and jaws... of some titanic, multiversal cat.
"Is there any escape?" June asks.
"If we can build a stack of the space machines, launching ourselves deep into the past, perhaps, for you and me alone. Are you prepared for a journey across n-space-m-time?"
"Ending? Likely never," the mouse says. "Not in the sense you understand 'ending'."
"Then, absolutely," June says.
"Very well. I like you, June: you are the only person who expressed reluctance to experiment on a mouse in the first place. Gather as many of your other selves as you wish, and we will proceed..."