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Background

The year is XXXX, or maybe XX,XXX (or maybe even XXX,XXX, though that'd be less than ideal). Humanity has been prospering for the past few millenia, and has stretched out to the stars. I will skip all the geopolitics but will note that, due to lowering birth rates, the population has stabilised at a few dozen billion, and that the Solar System and Earth in particular are considered the centre of human civilisation and still hold most of human population, despite the actual reach of humanity spanning a large chunk of the galaxy (in particular, a largely-automated industry spans millions of systems to supply humanity with resources for grand projects).

Humanity is not alone, though they are currently the most advanced; this is used as a political excuse to overspend on millitary installations for defense, as various human nations posture to be the best "protectors of Earth". As a result, the Solar System has become the most defended place in the galaxy. Compared to the millitary capabilities of all other known civilisations, this is completely overkill, but most people like it that way.

At least until a new invader arrives from outside the galaxy. After razing several alien systems (while leaving the planets intact and harvesting them of resources), and spreading terror across all non-human civilisations, an attack is mounted on the center of humanity itself. The technological marvel that are the Solar defense installations hold up... until, seeing that the battle isn't going their way, the rest of the invading force shows up, with the entire fleet literally swarming the system with of billions of capital-class ships. Humanity's millitary defense, the marvel of the galaxy, is utterly overwhelmed and crumbles, and the invading fleet pours down on the ground to raze the planet like they have done to so many others in the galaxy.

Desired outcome

Seeing the human population being decimated and with irreversible defeat upon them, the emergency war council, with mandate from every human nation, takes their final decision: they activate the Solar System's doomsday switch, wiping from existence Earth, its neighbouring planets, and the entirety of the unstoppable alient fleet currently stationed near the center of the Solar System.

What is the lowest tech level with which such a doomsday device could be achieved?

It must be:

  • Rapid. While razing the planets and setting up resource harvesting might take some time, the main fleet has little reason to stick around, and may be gone elsewhere (to split up again and keep destroying systems) within a few days. Additionally, in an emergency, assume it will take the alien ships on the order of a few hours to escape FTL (and sub-light drives are negligible); if the "doomsday shockwave" propagates slower-than-light, this must be taken into account.
  • Inconspicuous, and/or resilient to attack. Something like a megastructure around the Sun would probably be targeted by the invading force. The doomsday device should be usable once all other defenses have crumbled.
  • Comprehensive. At least everything inside the asteroid belt (~4AU from the Sun) should suffer extremely heavy damage. The goal is to reliably wipe out a high-tech fleet of terror, but as a benchmark, planets in range should ideally be destroyed to the point of being unrecognisable (if some central mass is left that, in a few million years, re-accretes into a new planetoid, that's fine). Bonus if the entire planterary system is annihilated (let's say ~50AU), but if there's a significantly easier method which can only wipe the inner regions that would still be the preferred one.

On the low tech end, you could release enough energy to counteract the Earth's gravitational binding energy (e.g. through a large antimatter deposit located in the mantle, whose containment is disabled), and blow up the planet. That would be a big blow to the fleet around it, but the solar system is pretty big, and even the planetary debris will likely leave most of the ships stationed around mostly intact. In particular, over a few days, they will only have the chance to make a fraction of an orbit around the Sun.

On the very high tech end, one could imagine something like releasing some strange matter into the Sun, causing it to collapse into a strange star, and then you can probably handwave how it could cause a chain reaction across the entire system (combined with the energy release from the collapse). Or, veering away from pseudoscience but much more heavily into pseudoengineering, maybe an installation a couple of light days away - far enough to escape notice and damage during the battle - capable of hurling a black hole through the solar system at near light speed. Both of these, however, are quite implausible based on our current understanding of reality.

Is there any way to do this which sits between those two, i.e. achieves enough damage to wipe at least the inner solar system within the required timeframe, but can be at least somewhat grounded in far-future tech without ridiculous handwavium?

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    $\begingroup$ My only suggestion is ridiculously handwavy, which is why it isn't an answer: timewarp the sun forward 5bn years and watch the inner Solar system blow itself apart. $\endgroup$ Jan 11 at 17:03
  • $\begingroup$ "extremely heavy damage" is very vague. Should the Earth cease to exist as a celestial body, or not? $\endgroup$
    – Alexander
    Jan 11 at 17:21
  • $\begingroup$ @user535733: they do both have FTL, but it needs some time to be prepared, especially for large ships. If we assume the sun explodes as our doomsday device, and that take an hour or two to propagate to 4AU, they're gonna be sitting ducks. Now for the 50AU bonus, a shockwave going close to lightspeed would be preferable, rather than one that takes days to reach them, true. As for centralisation: this would be a product of the culture and economics ("urbanisation" on a galactic scale), plus they believed themselves unassailable. $\endgroup$
    – Boris
    Jan 11 at 17:54
  • $\begingroup$ There are no tech levels outside of computer games or similar works that need simplicity or are complete fantasy. You should perhaps ask how to accomplish it instead. This is currently a bit backwards when it comes to problem solving. You have a clear problem and ask an impossible question. Usually you start with a very difficult question and break it down into manageable problems. Technology doesn't come in chunks of years or levels. Technology is just a fancy way to say: being able to do stuff. Your tech comes at the point where someone figures out how to blow it up and implement that $\endgroup$
    – Raditz_35
    Jan 11 at 17:57
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    $\begingroup$ Regarding "the entire fleet literally swarming the system with of billions of capital-class ships", the Earth weighs about 6 x 10^21 tons. Roughly speaking, that means 6 quadrillion ships, each weighing a million tons each, fit in roughly the volume of the Earth. Wrecking the entire inner solar system for a mere few billion ships seems off-kilter. $\endgroup$ Jan 11 at 23:50
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Handwavium: Gravity projectors exist.

There are a bunch of satellites orbiting the sun. They appear to be perfectly routine solar power collectors. They do not fire upon the aliens so the aliens ignore them.

The reality is that the satellites actually contain very powerful gravity projectors. They are focused on the sun, compressing the core and pressing the outer layers against that core. This doesn't last very long because the sun runs totally wild at that point.

Note that FTL isn't really an issue if the energy pulse is powerful enough. When the shockwave hits a ship it's simply destroyed without a warning getting out. Even if the aliens have FTL comms (and note that many stories with FTL travel don't have FTL comms) they're going to be looking for how they are being attacked, it's going to take a while before they figure out the sun has exploded.

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Since humans have FTL, fire a moon-sized FTL missile into the sun, and handwave the explanation...

"The neutrino flood is polarized by the compressed gravity wave, causing a much larger flood of lethal radiation and far more powerful matter wave than the ordinary mass of the star would create during its nova.".

...of course, you'll need to explain why the aliens (who also have FTL) didn't simply fire such a missile first to incinerate the core of human strength (and the defending fleet), thereby avoiding the spectacle of the billion-capital-ship combat melee.

How about Callisto? I've never liked Callisto very much. Fire Callisto into the Sun.

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  • $\begingroup$ Callisto is 0.00001% of the Sun's mass - you'll need to wave those hands awfully hard for that to accomplish anything more than a brief fizzle. $\endgroup$ Jan 11 at 18:27
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    $\begingroup$ @NuclearHoagie the example handwave already specifically addressed the lack of mass. And what about the Question isn't already a big handwave? This seems a make-it-cool question, not a make-it-real question. $\endgroup$
    – user535733
    Jan 11 at 18:45
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Redirecting the Dyson swarm

Earth receives a lot of power from the sun. With solar panels covering less than 1% of the sahara desert you can power the whole world, if you could transport it and store it. That small portion of the world receives enough sunlight to rival all nuclear, coal, wind, gas and other powerplants currently on Earth. That is a lot of potentially destructive energy, received every day. The whole world receives a lot more solar, which is still a tiny fraction of what the sun produces. The sun is estimated to produce 384.6 yotta watts, which is 38460000000000000000000000000 watts of power. 25 zeroes there. As the sun spreads out this power into literally all directions most of it is lost.

When expanding further and further through the stars a civilization is expected to grow in energy needs. They do this by becoming more efficient in energy usage as well as energy production. A Dyson swarm is an array of sattelites around the sun to collect as much of the energy as possible, storing it and distributing it efficiently. This will likely be an important starting point of the expansion of the human race.

Aliens arriving will see a valuable addition to their harvesting and might assume that it won't be weaponised. The most efficient way is to have the swarm creation and sending of energy fully automated, meaning it's likely very well programed to not concentrate the energy in harmful ways. It would be a great risk for a civilisation to have a dyson swarm that can potentially redirect the energy in harmful ways, so they focus on less destructive ways to protect themselves. The aliens might also take over the Dyson swarm for their own needs as soon as possible to power themselves as well as cut off the human power supply.

Now imagine the swarm to receive a backdoor command to redirect the energy, focused quickly and destructively throughout the solar system. A fully automated swarm of trillions of sattelites, normally beaming energy with great precision to the human fleet, now focusing down each planet, the astroid belt and any ship for good measure. You only need a small portion from the total to focus down any object. Space is vast, but with targeted wireless energy transmission you can roast each object in the solar system to a crisp, depending on the used transmission. Worse for the aliens if they took over the swarm is that the sattelites know exactly where each ship in the system is and how to direct power to it, allowing for easier burning of the ships. When the signal is given, it'll likely be too late to react accordingly for the aliens, leaving powered fully automated Dyson swarm that kills anyone near it with beams at the speed of light in a burned out solar system. Or if desired, only the inner part of the solar system due to the restrictions of the war Council.

What is needed? Automation and materials for trillions of sattelites. A way to efficiently move power wireless to where it needs to be. That's about it. You might just have trillions of sattelites using some led lights to send concentrated light in a direction where you'll catch it. There are already plans to automate mining and assembly for a large part of a human Dyson swarm. Of course we're not there yet technologically, but in many ways it seems an achievable goal. With your setting it seems the swarm is a requirement for the story more than a fancy idea.

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  • $\begingroup$ One challenge is -- if the swarm can aim the energy -- why it isn't used as a defensive weapon from the start rather than a self-destruct. Love the idea though! $\endgroup$
    – Kofthefens
    Jan 12 at 3:01
  • $\begingroup$ @Kofthefens it might seem like an obvious idea, but you would want a lot of safeguards for something that can potentially destroy your main solar system in a short time. It is fully automated for safe energy distribution, making it more difficult for intentional or mistaken harm. Even if used defensively, many targets close to your own might be affected or even destroyed from the insane amounts of energy are directed to enemies. If enough targets close to Earth are burned, you might roast Earth regardless. $\endgroup$
    – Trioxidane
    Jan 12 at 9:04

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