This second copy of Earth is a hundred years behind in technology and four thousand years diverged. A gloss of its name may be Tetratopia; the planet where there is intelligence able to metaphorically organise things into grids.

It takes Tetratopia longer to notice, because they don't have a lot of space infrastructure set up; their satellites came with them (otherwise they definitely would have noticed) but this solar system has Earth's rest of the Solar System including their deep space communciations network sufficient to talk to someone on the other side of the Sun.

The message comes as a radio transmission with a simple codec, starting with a classic 1-1-2-3-5 or thereabouts.

(The message came so fast because they want Callisto to be second to get there.)

Here's Callisto! Hi other Earth you look far less well-defended and I sure hope you lack the genocidal ideology of local Earth! 1-1-2-3-5 or thereabouts.

Haha, this is altogether too much, and Tetratopia finds itself in the kind-of-overwhelming break-of-routine of having contact with as many as several different alien civilisations, and this is probably related to how they've lost contact with the Mars base and the interstellar probes and in fact any sort of space infrastructure beyond their sphere of influence.

Top cryptanalysts are starting work on figuring out these alien transmissions, and prediction markets are trying to look ahead of them (though not too well, because the usual experts are trying to live at the object level) and also flying around without any clue what their prices should be on any other question. It happens to be daytime in the populated hemisphere, City 00 is just waking up a few hours after sunrise, which means Tetratopia is getting a fast early start on this matter.

They quickly find that these alien civilisations are made up of two: one from the third planet (this Solar System's version of Tetratopia?) on the other side of the Sun, and one from the fifth planet (this Solar System's version of Jupiter?). It's not long after that they become pretty confident and then almost certain that these civilisations are representing themselves as... biological humans of Earth, and digital people of Callisto.

Let us take this moment to talk about interplanetary travel, as one often does.

You might have heard that interplanetary travel is done using minimum delta-v Hohmann transfer orbits, which take between a quarter and a half of the orbital period of the outer body in the limits where the orbits are infinitely distant and the same respectively. Tetratopia sure does approximately that, though with nuclear rockets they allow themselves something a little bit faster.

Hohmann transfer orbits are slow. Hohmann transfer orbits are boring. It's the 22nd century, the way you're meant to do interplanetary travel is to point in a straight line directly at your target and then accelerate until you're halfway there, and then flip around and decelerate the other half of the way. For fixed reactor output it's actually more efficient to burn for longer, so if your rocket is a spherical-cow it does actually do this burn and flip exactly halfway.

Now suppose you were a diamondoid computing element that could resist pressures of however many Pascals; as with Pascal of the wager, name whatever number you like, it's going to be more than you need. But we're doing a gentle launch here, only 100 g, because Callisto doesn't have a Dyson sphere, it has to pathetically fuse hydrogen from the atmosphere of Jupiter. Then some simple SUVAT means that it takes about 13 hours to get from Callisto to Earth. This isn't a particularly expensive trip, your delta-v is only like 15% of the speed of light. That's, what, a spaceship that's half antimatter? Something like that. Regardless, not even hard.

Anyway so say your spaceship weighs a ton, you have an antimatter exhaust velocity of 100,000 km/s or whatever (a third the speed of light! If your exhaust velocity is bigger than your delta-v you don't even really have to worry about the rocket equation, vindicating our previous calculation. To order unity, because we're doing physics here), that's a power output of a hundred gigawatts. This is very detectable across the solar system!

Even Tetratopia can see that something's going on, and quickly infers that something really is going on (antimatter torchship. antimatter torchship).

Antimatter torchship! Antimatter torchship!

This about Tetratopia: its entire sizeable space program is staffed by insane people who started playing Rocket Spreadsheet at about the age they were old enough to physically press the buttons. They are so ready for antimatter torchships to be a real thing.

...wait, shits. Uh, antimatter torchships.

Anyway so your spaceship weighs a ton and it's going at 15% of the speed of light. 15%c is slowww so you can just use the Newtonian kinetic energy equation, and that gives you 200 megatons of TNT. That is... okay, that's actually fine? Tetratopia has designed bigger. They haven't built bigger, they haven't even built a thousandth as big, but they totally could if they wanted to. I mean, the airburst does level a city, that does suck for the city, but lots of things level cities you ain't special and you don't need a relativistic kill vehicle for that.

Unless it's half antimatter and half antimatter, which isn't much harder to do than something that's half antimatter. Now that gives you 20 gigatons of TNT upon annihilation with the atmosphere, which okay we've designed larger it's not like it's going to be knocking the moon out of orbit but it's starting to get a little worrying, there are a lot of places where 20 gigatons of TNT could flatten 00 and/or cause a firestorm.

Of course, if Callisto actually wants to cause problems, they keep open an apparently-benign communications channel with Tetratopia and in the meantime hack Modernity by using science Modernity doesn't understand or tactics Modernity doesn't understand and probably both. For all Callisto knows, seeing an antimatter flare like that might cause Tetratopia to panic and blow up the lightcone! Unless it definitely takes longer than 16 hours to do that from a cold start, which Tetratopia is starting to suspect, save for the fact that they only see one launch and not ten thousand.

Anyway, these considerations add up to lol, lmao, Callisto has all the optimisation power and Tetratopia has none. It seems like a good idea to decorrelate the positions of all the important people from all the important stuff, so that if Callisto just wants to kinetically wipe out leadership via huge explosions for insane reasons then they can do that and if they just want to destroy a city they can do that; it seems like a good idea but less good of a one to decorrelate everyone's positions, though shuffling around hundreds of millions of people on short notice is genuinely hard.

None of this looks like the actual opening moves in a game to eat Tetratopia, because alterCallisto keeps those statistically indistinguishable.