Because this is so long for an OP, I'm splitting it into multiple posts, for ease of reading, and then linking them back to this one, or the Table of Contents post that follows. Also, there will be TL;DR summaries in bold red.
On December 21st, 2012, in flashes of light reminiscent of Q (from Star Trek) teleporting something, four new Earth-Moon systems appear, two on either side of us. Shortly thereafter, the Large Hadron Collider, and other cyclotrons and advanced particle sensors, get some very strange readings, and scientists discover a few new particles (said particles, being slightly slower than light, took a while to appear). Each system, including ours, is separated by precisely thirteen light-seconds from barycentre to barycenter (meaning that the two furthest Earths are 26 light-seconds away from our Earth-Moon system, and 52 light-seconds from each other), along the same orbit - the line of sight distance is of course a bit shorter (apparent sizes noted here; the flashes are a little under twice as big as each body, not the whole Earth-Moon system). This means that each Earth is a little less than a day and a half off from its nearest neighbors, and a little less than three days from the next furthest ones.
This should be incredibly dangerous, due to their gravitational fields perturbing the orbits of everything around them, but that isn't happening. Astronomers find, at first, no evidence of gravity coming off the other Earths, beyond things that appear with them orbiting them. After a while, though, a scientist watching one of the closer Earths notices a meteor appearing, passing through the system, being perturbed by its gravity, moving on, and then completely vanishing about three light-seconds away from that Earth. So, might these Earth-Moon systems be giant, incredibly detailed holograms?
No, that wouldn't be weird enough. What we see are Earths in alternate dimensions, visible through permanent 'portals' or 'quasi-wormholes' (for lack of a better term) created by a being or beings of immense power and boredom (who probably chose the date just to troll people). Any orbital path that leaves one Earth-Moon system, curved to intersect with another, will pass through the 'portal'. A vehicle on that course would note that at three light seconds from its starting Earth, three of the E-M systems vanish - specifically, the ones that are neither its starting point, nor its destination. Probes or other vessels not on a course for another Earth notice all four other Earths vanishing at three light-seconds out (unless that Earth's visible zone is behind, so to speak, their starting Earth), as they're out of the light cone emitted by the portals. When a vessel that is on course for another E-M system gets within three light-seconds of its destination, the stars and planets change, as the vehicle has passed into the new dimension.
This phenomenon is discovered when a large probe is launched from the system furthest ahead of us in the Earths' orbit (supposedly by a 'lone genius and his students', according to the radio, though they do have some wealthy sponsors) on a path for our system, and does not vanish when it gets three light seconds from that Earth. Instead, it continues on course, passing through a smallish asteroid without being damaged, nor damaging or perturbing it at all (it's still in the other dimension, and doesn't even see it), until it gets within three light-seconds of our Earth. At that point, another burst of those exotic particles occurs, much fainter than when the Earths appeared, and anything capable of detecting its gravity, does, as it's now on our dimension. The journey takes a little under a month for the unmanned probe, and that's only because they had a few solid rocket boosters in orbit, and mated them to the probe by remote control. It's very visible when slowing down. One observer described the probe's appearance as 'Like the Kerbal Space Program with a Dieselpunk mod.' It was originally planned as a Mars probe, but we're closer, thus both easier (could be done with just the boosters they already had in orbit) and more urgent (huge amounts of very sophisticated radio signals, not to mention what their telescopes are telling them). The probe was launched five days after the new Earths appeared, and broke orbit of its Earth two days later (it takes time to get the boosters mated, check everything that can be checked, and quintuple-check the math). After its last deceleration booster is expended (said booster falling into Earth's atmosphere, and burning up), it uses a few liquid-fuel rockets to settle into a stable orbit, 2,000 miles (3,218 km) above our Earth (the next-to-last booster settles into a lower, less stable orbit, near the ISS, and in range of a Soyuz; they might even use one of the ones that are already there by then, if it can be refueled in orbit, or has enough spare fuel).
Its arrival in orbit will have vast political consequences on all five Earths.
TL;DR: Four new Earths appear on December 21st, 2012. One of them sends a big, shiny, primitive-looking probe to our Earth.
You might already have guessed that the other Earths are not just copies of ours, a few days in the past or future. You'd be right. Even the ones ahead of us are in the past, but not the past as we know it.
Dieselpunk Earth
TL;DR: Dieselpunk Earth, in 1940, with elements of the Wold Newton Universe and some magic, supers, & potential supers.
Steampunk Earth
TL;DR: Steampunk Earth, 1878, with lunar and cislunar space travel, and strange magics.
Clockpunk Earth
TL;DR: Clockpunk fantasy Earth, 1555.
Fantasy Earth
TL;DR: Fantasy Dark Ages, 514 CE, with a little Middle Earth and 3.5-era Forgotten Realms.
Magic system notes
TL;DR: Magic, including weird science, consists of psionics used in simple or complicated ways, usually involving thoughtforms (often called spirits or gods) to do the work. Spellcasting mostly involves long rituals, and there are many styles of magic.
This is going to strongly affect public attitudes toward space travel, the existence of aliens, and the paranormal, among other things. The arrival of the probe from Dp-Earth, even more so, and in ways that will not be obvious, at first.
When the last booster burns up in OTL-Earth's atmosphere, either the thoughtforms within the booster's unspent fuel, or the entity that caused all this to happen in the first place, give OTL-Earth a 'gift': somewhat less than 1% of the population, all over the world (though not consitent with population density - North America, Europe, Japan, and Oceania (mostly in Oz and NZ) tend to have more psykers than either India or China, though India has more than China does) develop active psi powers - mostly low level, and mostly just thoughtform generation. The selection of exactly who gets powers is somewhat telling, as all of them tend to be considered strange within their own nations and cultures: Wiccans and other neopagans, New Age mystics, particularly 'spiritual' martial artists, tribal shamans (they're respected, but do tend to be seen as 'a bit off'), followers of Vodoun and SanterÃa, science fiction and fantasy fans, gamers, the more eccentric historical reinactors, would-be superheroes (well, the ones that are relatively serious about it), and mentally ill folks (OK, there's some overlap between the last two, particularly if chuunibyou is a real mental illness), in or out of asylums, with sufficiently 'interesting' derangements. Few of these people start with even enough power to hover, much less fly, nor are they going to be bouncing bullets off their chests any time soon. Those who get psi powers beyond thoughtform generation and manipulation tend to start with subtle abilities like probability alteration, ESP, and sometimes astral projection, meta-psi, psychic vampirism, and telepathy, rather than anything flashy, and even those with more obvious powers like PK or EK are rarely going to have something impressive - expect one to levitate a pencil, not oneself. Also, mind control is far from easy, more along the lines of telepathically-assisted RL hypnosis than the sort of things you see in the comics.
The abilities gained are generally consistent with the individual's beliefs, interests, and personality - someone who believes they have a particular power, or strongly wants it, will gain some version of it, albeit generally an unimpressive version, at first - Star Wars fans who 'feel the Force', martial artists with (weaker versions of) the legendary and/or cinematic powers of their style, D&D gamers with a Path style based on the Schools of Magic in that game, or a Book style based on the feats and powers of their favorite class (at barely first level), and so forth. Strong-willed individuals, as well as those who don't take orders well (which isn't quite the same thing), tend to start out stronger, and improve faster, as do those with greater intelligence (though a combination of all three is best). Those not given the gift can train for it, particularly if taught by someone who already has it (though again, the ones who don't take orders well tend to be better at it) - this is rather easier if the teacher is a telepath, or has learned or developed mind-affecting spells.
Some animals will also develop powers - mostly cats, and species native to Australia, though any animal of a species with significant local myths about it might merge with an appropriate spirit (e.g. foxes and tanuki in Japan, coyotes and ravens in North America, and so forth). This rarely happens to plants or locations, without at least semi-conscious human interference - at least until something arrives from Fa-Earth.
Being so affected by Dp-Earth, the spirits tend to behave 'in-genre' (though they don't all agree on what genre it is), and try to get machinery, animals, and people to behave in-genre, as well - this doesn't work well on people, especially as in-genre behavior is often the opposite of what a professional is trained to do (listen to a cop or soldier complain about cops or soldiers in most ficion, and you'll see what I mean). It rarely works well on animals, unless it's consistent with their instincts. This in-genre behavior may be a factor in who gains powers, and who is better at it.
It isn't going to take long before governments start asking their space agencies to draw up plans for both manned and unmanned visits to other worlds, and getting frustrating results. Then again, for an interplanetary spacecraft, the Nautilus-X looks surprisingly affordable, even if you add a second grav-wheel to neutralize the centrifugal force - how affordable it turns out to be is another matter, but given the gift of magic, 'less expensive' is actually fairly likely. Before that, though, the difficulty of space launches from OTL-Earth will be at RL levels - complex and very expensive. OTOH, the situation on Dp-Earth will likely lead to various nations' military budgets being diverted to space, even before the probe arrives.
On another note, the particles produced by dimensional transition have a sufficiently consistent speed, that observation of particle detectors can precisely locate any dimensional incursions heavier than 3 kilograms, even if astronomers somehow don't (which isn't so helpful with spirits, but while they lack mass, they do disturb whatever it is that's producing these sprays of particles as if they did - just not much).
I will be posting a timeline below, currently at one month per post, linked in the Table of Contents post. Fiction in the setting will also be linked there, including by other people, as long as you give me a title for it.
Threads on other forums:
Main thread on SB.com
Fiction thread on SB.com
Thread on AH.com
Thread on the SJGames forum
On December 21st, 2012, in flashes of light reminiscent of Q (from Star Trek) teleporting something, four new Earth-Moon systems appear, two on either side of us. Shortly thereafter, the Large Hadron Collider, and other cyclotrons and advanced particle sensors, get some very strange readings, and scientists discover a few new particles (said particles, being slightly slower than light, took a while to appear). Each system, including ours, is separated by precisely thirteen light-seconds from barycentre to barycenter (meaning that the two furthest Earths are 26 light-seconds away from our Earth-Moon system, and 52 light-seconds from each other), along the same orbit - the line of sight distance is of course a bit shorter (apparent sizes noted here; the flashes are a little under twice as big as each body, not the whole Earth-Moon system). This means that each Earth is a little less than a day and a half off from its nearest neighbors, and a little less than three days from the next furthest ones.
This should be incredibly dangerous, due to their gravitational fields perturbing the orbits of everything around them, but that isn't happening. Astronomers find, at first, no evidence of gravity coming off the other Earths, beyond things that appear with them orbiting them. After a while, though, a scientist watching one of the closer Earths notices a meteor appearing, passing through the system, being perturbed by its gravity, moving on, and then completely vanishing about three light-seconds away from that Earth. So, might these Earth-Moon systems be giant, incredibly detailed holograms?
No, that wouldn't be weird enough. What we see are Earths in alternate dimensions, visible through permanent 'portals' or 'quasi-wormholes' (for lack of a better term) created by a being or beings of immense power and boredom (who probably chose the date just to troll people). Any orbital path that leaves one Earth-Moon system, curved to intersect with another, will pass through the 'portal'. A vehicle on that course would note that at three light seconds from its starting Earth, three of the E-M systems vanish - specifically, the ones that are neither its starting point, nor its destination. Probes or other vessels not on a course for another Earth notice all four other Earths vanishing at three light-seconds out (unless that Earth's visible zone is behind, so to speak, their starting Earth), as they're out of the light cone emitted by the portals. When a vessel that is on course for another E-M system gets within three light-seconds of its destination, the stars and planets change, as the vehicle has passed into the new dimension.
This phenomenon is discovered when a large probe is launched from the system furthest ahead of us in the Earths' orbit (supposedly by a 'lone genius and his students', according to the radio, though they do have some wealthy sponsors) on a path for our system, and does not vanish when it gets three light seconds from that Earth. Instead, it continues on course, passing through a smallish asteroid without being damaged, nor damaging or perturbing it at all (it's still in the other dimension, and doesn't even see it), until it gets within three light-seconds of our Earth. At that point, another burst of those exotic particles occurs, much fainter than when the Earths appeared, and anything capable of detecting its gravity, does, as it's now on our dimension. The journey takes a little under a month for the unmanned probe, and that's only because they had a few solid rocket boosters in orbit, and mated them to the probe by remote control. It's very visible when slowing down. One observer described the probe's appearance as 'Like the Kerbal Space Program with a Dieselpunk mod.' It was originally planned as a Mars probe, but we're closer, thus both easier (could be done with just the boosters they already had in orbit) and more urgent (huge amounts of very sophisticated radio signals, not to mention what their telescopes are telling them). The probe was launched five days after the new Earths appeared, and broke orbit of its Earth two days later (it takes time to get the boosters mated, check everything that can be checked, and quintuple-check the math). After its last deceleration booster is expended (said booster falling into Earth's atmosphere, and burning up), it uses a few liquid-fuel rockets to settle into a stable orbit, 2,000 miles (3,218 km) above our Earth (the next-to-last booster settles into a lower, less stable orbit, near the ISS, and in range of a Soyuz; they might even use one of the ones that are already there by then, if it can be refueled in orbit, or has enough spare fuel).
Its arrival in orbit will have vast political consequences on all five Earths.
TL;DR: Four new Earths appear on December 21st, 2012. One of them sends a big, shiny, primitive-looking probe to our Earth.
You might already have guessed that the other Earths are not just copies of ours, a few days in the past or future. You'd be right. Even the ones ahead of us are in the past, but not the past as we know it.
Dieselpunk Earth
TL;DR: Dieselpunk Earth, in 1940, with elements of the Wold Newton Universe and some magic, supers, & potential supers.
Steampunk Earth
TL;DR: Steampunk Earth, 1878, with lunar and cislunar space travel, and strange magics.
Clockpunk Earth
TL;DR: Clockpunk fantasy Earth, 1555.
Fantasy Earth
TL;DR: Fantasy Dark Ages, 514 CE, with a little Middle Earth and 3.5-era Forgotten Realms.
Magic system notes
TL;DR: Magic, including weird science, consists of psionics used in simple or complicated ways, usually involving thoughtforms (often called spirits or gods) to do the work. Spellcasting mostly involves long rituals, and there are many styles of magic.
This is going to strongly affect public attitudes toward space travel, the existence of aliens, and the paranormal, among other things. The arrival of the probe from Dp-Earth, even more so, and in ways that will not be obvious, at first.
When the last booster burns up in OTL-Earth's atmosphere, either the thoughtforms within the booster's unspent fuel, or the entity that caused all this to happen in the first place, give OTL-Earth a 'gift': somewhat less than 1% of the population, all over the world (though not consitent with population density - North America, Europe, Japan, and Oceania (mostly in Oz and NZ) tend to have more psykers than either India or China, though India has more than China does) develop active psi powers - mostly low level, and mostly just thoughtform generation. The selection of exactly who gets powers is somewhat telling, as all of them tend to be considered strange within their own nations and cultures: Wiccans and other neopagans, New Age mystics, particularly 'spiritual' martial artists, tribal shamans (they're respected, but do tend to be seen as 'a bit off'), followers of Vodoun and SanterÃa, science fiction and fantasy fans, gamers, the more eccentric historical reinactors, would-be superheroes (well, the ones that are relatively serious about it), and mentally ill folks (OK, there's some overlap between the last two, particularly if chuunibyou is a real mental illness), in or out of asylums, with sufficiently 'interesting' derangements. Few of these people start with even enough power to hover, much less fly, nor are they going to be bouncing bullets off their chests any time soon. Those who get psi powers beyond thoughtform generation and manipulation tend to start with subtle abilities like probability alteration, ESP, and sometimes astral projection, meta-psi, psychic vampirism, and telepathy, rather than anything flashy, and even those with more obvious powers like PK or EK are rarely going to have something impressive - expect one to levitate a pencil, not oneself. Also, mind control is far from easy, more along the lines of telepathically-assisted RL hypnosis than the sort of things you see in the comics.
The abilities gained are generally consistent with the individual's beliefs, interests, and personality - someone who believes they have a particular power, or strongly wants it, will gain some version of it, albeit generally an unimpressive version, at first - Star Wars fans who 'feel the Force', martial artists with (weaker versions of) the legendary and/or cinematic powers of their style, D&D gamers with a Path style based on the Schools of Magic in that game, or a Book style based on the feats and powers of their favorite class (at barely first level), and so forth. Strong-willed individuals, as well as those who don't take orders well (which isn't quite the same thing), tend to start out stronger, and improve faster, as do those with greater intelligence (though a combination of all three is best). Those not given the gift can train for it, particularly if taught by someone who already has it (though again, the ones who don't take orders well tend to be better at it) - this is rather easier if the teacher is a telepath, or has learned or developed mind-affecting spells.
Some animals will also develop powers - mostly cats, and species native to Australia, though any animal of a species with significant local myths about it might merge with an appropriate spirit (e.g. foxes and tanuki in Japan, coyotes and ravens in North America, and so forth). This rarely happens to plants or locations, without at least semi-conscious human interference - at least until something arrives from Fa-Earth.
Being so affected by Dp-Earth, the spirits tend to behave 'in-genre' (though they don't all agree on what genre it is), and try to get machinery, animals, and people to behave in-genre, as well - this doesn't work well on people, especially as in-genre behavior is often the opposite of what a professional is trained to do (listen to a cop or soldier complain about cops or soldiers in most ficion, and you'll see what I mean). It rarely works well on animals, unless it's consistent with their instincts. This in-genre behavior may be a factor in who gains powers, and who is better at it.
It isn't going to take long before governments start asking their space agencies to draw up plans for both manned and unmanned visits to other worlds, and getting frustrating results. Then again, for an interplanetary spacecraft, the Nautilus-X looks surprisingly affordable, even if you add a second grav-wheel to neutralize the centrifugal force - how affordable it turns out to be is another matter, but given the gift of magic, 'less expensive' is actually fairly likely. Before that, though, the difficulty of space launches from OTL-Earth will be at RL levels - complex and very expensive. OTOH, the situation on Dp-Earth will likely lead to various nations' military budgets being diverted to space, even before the probe arrives.
On another note, the particles produced by dimensional transition have a sufficiently consistent speed, that observation of particle detectors can precisely locate any dimensional incursions heavier than 3 kilograms, even if astronomers somehow don't (which isn't so helpful with spirits, but while they lack mass, they do disturb whatever it is that's producing these sprays of particles as if they did - just not much).
I will be posting a timeline below, currently at one month per post, linked in the Table of Contents post. Fiction in the setting will also be linked there, including by other people, as long as you give me a title for it.
Threads on other forums:
Main thread on SB.com
Fiction thread on SB.com
Thread on AH.com
Thread on the SJGames forum