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Something in orbit around an inhabited planet blasts it with a hefty dose of gamma rays/x-rays/something else, killing everyone within a relatively short time frame (hours or days) but leaving no obvious trace of what has occurred. When offworlders turn up a week later to find out what happened, they find a lot of corpses and not much else.

There is no requirement to kill everything single living thing, just enough members of enough species that the ecosystem and any extant civilisation collapses.

What form of radiant energy, if any, could achieve this effect?

some considerations

I was going to use gamma rays, but am concerned they wouldn't penetrate an Earth-like atmosphere. Or would the proximity and intensity make that not a problem? e.g. could an intense enough burst destroy the atmosphere so quickly that the hard radiation can then kill people directly?

When it comes to traces left behind by the blast, am I right in thinking that there wouldn't be residual radioactivity? Or would an intense enough burst of gamma or x-ray cause alpha/beta decay in materials on the planet's surface? (For the purposes of running scans to figure out what happened.)

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  • $\begingroup$ Doesn't have to be seconds (anything up to several days is fine) and it doesn't have to hit everyone on the planet at once. I was imagining the effect sweeping around the surface of the planet, in fact. So the simplification would be "what type and strength of EM radiation could be emitted from orbit and either kill a person immediately or give them a lethal dose that will kill them before help arrives a week later". $\endgroup$ – propaganamede Mar 29 at 19:06
  • $\begingroup$ However, quicker is better, as it leaves less time for someone to think of writing a message explaining what happened! $\endgroup$ – propaganamede Mar 29 at 19:08
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    $\begingroup$ @propaganamede I like your question, but I've proposed an edit - I think you need to make it clearer what your core question is, because every question is meant to be a single clear question. The current format with three questions might end up with the question getting close votes for being unclear what you are asking. $\endgroup$ – Ben Mar 29 at 19:18
  • $\begingroup$ If there is only 1 object releasing the energy, it could orbit the planet and reach practically all of it in less than an hour. If the object has one or more drones on board which can also release the energy, it could get them into place first so that it has complete coverage. This does not answer your question, but it addresses the point you and JBH are commenting on. $\endgroup$ – Aaron Mar 29 at 19:23
  • $\begingroup$ @ben thanks, you’re right, that’s a lot clearer! I’ve also changed the time requirement since I think seconds was misleading. $\endgroup$ – propaganamede Mar 29 at 19:27
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You cannot accomplish this without severely disrupting the planet's surface. Enough gamma or x-ray energy will complete your objective of "Kill enough members of the civilization to ensure a collapse" but not with the objective of "Not leaving a trace to figure out what happened".

Basically an Earth like atmosphere is really, really good at absorbing high energy wavelengths. See here: http://gsp.humboldt.edu/OLM/Courses/GSP_216_Online/lesson2-1/atmosphere.html

That means that shining a really bright gamma or x-ray beam will simply heat up the atmosphere. Heat up the atmosphere enough and you'll wipe out the civilization. You could save an insane amount of energy if you were okay with just wiping out enough plant life to cause crop failures and mass starvation, but that would hardly guarantee extinction.

This can occur naturally with a near-Earth Gamma Ray Burst, and it probably has caused a mass extinction event in the past. https://en.wikipedia.org/wiki/Gamma-ray_burst#Effects_on_Earth

The practicality of this is very questionable, as the energy requirements are insane. The most viable technique would be to inject anti-matter all across the sky to bombard everything with gamma, heating up the atmosphere.

See this question for more info: How much antimatter would we need in order to wipe out all humans, eradicate all animals and blow up the Earth?

TLDR: It isn't really feasible in the way you want. The rays won't reach the surface, so you'll have to bake the entire planet.

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    $\begingroup$ Thank you, that covers exactly the problems I had in mind. The first link you shared is particularly useful for figuring out how I want this to work. $\endgroup$ – propaganamede Mar 30 at 8:50
  • $\begingroup$ “and it probably has caused a mass extinction event in the past” - citation needed. Actually don’t bother, there is zero evidence for this. $\endgroup$ – Gimelist Apr 1 at 20:58
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You could use a particle beam.

https://en.wikipedia.org/wiki/Particle-beam_weapon

Particle accelerators are a well-developed technology used in scientific research for decades. They use electromagnetic fields to accelerate and direct charged particles along a predetermined path, and electrostatic "lenses" to focus these streams for collisions.... A particle-beam weapon is a weaponized version of this technology. It accelerates charged particles (in most cases electrons, positrons, protons, or ionized atoms, but very advanced versions can accelerate other particles such as mercury nuclei) to near-light speed and then shoots them at a target. These particles have tremendous kinetic energy which they impart to matter in the target, inducing near-instantaneous and catastrophic superheating at the surface, and when penetrating deeper, ionization effects which can be especially detrimental to electronics in the target.

A beam shooting massive particles would get a fair number through the atmosphere, delivering their punch to more massive things on the ground (like people). Charged particles would scatter from mutual repulsion on the way down, producing a shotgun like effect. You can cook things with the kinetic energy of these particles which they drop off as heat in their targets. I think this effect was best described in Repo Man by J Frank Parnell, discussing the neutron bomb. https://www.youtube.com/watch?v=3VKzqAefBVY

A problem, though, is that a beam energetic enough to thermally kill all the people would produce comparable thermal effects on everything else on the ground. Your alien visitors would figure out what had happened: stuff got cooked from space. A lesser dose of particles would not kill thermally but would kill by acute radiation poisoning. It is slower and messier and there might still be some folks lingering a week later when the aliens show up. If you can push the alien visitors out to 3 weeks that should be enough time. But if the aliens are familiar with Earth life they will still be able to figure out how everyone died.

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  • $\begingroup$ There is also the problem of people underground, in mines, bunkers or even underground cities, and people in submarines. $\endgroup$ – Eth Apr 1 at 9:28
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You need to use a frequency that the atmosphere is transparent to. I would suggest using 4 GHz microwaves, the same as is used in your kitchen appliance.

You would bath the world with enough microwaves to raise the temperature of the typical critter by about 10 degrees C. This would leave the corpse in roughly the same condition as dying from a fever would have. Critters that were under dense forest cover would have a higher survival rate. Critters in the water would be able to dissipate the heat.

This is still a not-trivial amount of energy. If you are trying to heat up an average of 30 cm thick layer of water by 10 C, you are spending 600 kCal/square foot or about 2.4 MJ or about 2/3 of a kWh/ft2.

Expanding that, about 7 kWh/m2 = 7 million kWh/km2 Earth is about 500 million square km so 3,500 trillion kWh. And that assumes efficient coupling.

This seems very wasteful. Food microwaves, after all, were designed to cook. It may be possible to come up with a frequency that hits a resonance in some protein. Imagine something that disrupted hemoglobin. Zzzzot! everything with a red blood cell falls over dead.

If you are willing to criss cross the planet within the atmosphere, you could do everyone in with sound. kWh per square meter will make a lot of noise. (Hifi speakers are terribly inefficient at converting energy to noise. The designers care about fidelity. even 1 watt of acoustic energy is LOUD. Suppose you generated infra-sound, say at 10 Hz, but at an amplitude that had peak over pressure measured in pounds per square inch. As a side effect it would treat most buildings like a bad earthquake.

However, overall, chemistry is more suitable. The ideal chemical is one that requires some time after exposure to be effective. This means that people won't run and hide when others start dying. Something that interferes with a precursor chemical for nerve transmitters may do. Circle the planet in a polar orbit, and kick the pellets out of the ship at a speed they are near motionless relative to the surface. This means they hit the atmosphere fairly slowly so they don't burn up.

A way to convert all or much of the carbon dioxide in the atmosphere to carbon monoxide would do the trick. CO has a residence time in the atmosphere of between 2 months and 5 years. (Estimates vary...) So discovery a week after the event would leave a big fingerprint. WHO exposure limits are 10 ppm for a shift, and 100 ppm for 15 minutes. This is sufficient to block 2% of the hemoglobin. Residence time in the blood is about 5 hours, so the 100ppm should eventually stabilize at around 40%. It may be possible to excite CO with the right frequency to give it the energy to reconvert it back to CO2.

Diseases could work too. Spanish flu was the most recent pandemic with high mortality. While somewhere between 50 and 100 million people died, overall mortality was a small percentage of the world's population. Perhaps another disease that kills with a Cytokine storm. (Flu was scary. A person could go to bed healthy and wake up dead.)

Diseases seldom are deadly to multiple species. Rabies is an exception, and may be a good candidate:

  • Most mammals are susceptible.
  • You can get it from inhalation. (Bat researchers now get vaccinated against it.)
  • It has a fairly long incubation time.

No disease is sudden enough to not leave records. Anything lethal enough that people can't write stuff down is too lethal to spread effectively.

Finally, if you truly want to leave a mystery, and have Startrek transporter technology, you circle the planet, stealthed, and beam people up, dropping them on the back side of the moon. Don't worry about maintaining the pattern buffers... At 100 people per second, 7 billion people would take 7 million seconds, or about 3 months. 10,000 people per second brings it down to 70,000 seconds or about a day. This is about the same time as a polar orbit will cover the earth.

Mind you, if you have transporter technology, tune it to a particular molecule, and just remove nerve transmitters. No one is going anywhere if their acetylcholine is missing.

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As for traces of radiation, you would not find any form of radioactivity deposited from a high energy gamma emission. Gamma or x-ray may excite atoms or cause ionization, but they will not activate nuclei. Neutrons will activate nuclei, not gamma or x-ray. If you used x-rays in a certain energy level you would boil the water out of living organisms, and that would be hard to ignore.

As for traces of something, the use of a massive gamma ray emitter from orbit would leave evidence in the atmosphere. The ozone layer protects the planet from high energy particles. You gamma ray weapon would destroy the local ozone before an appreciable amount of energy reaches the surface to kill people. This would leave a sort of smog in the area and a hole in the ozone that, depending on the area effected may take years to centuries to dissipate.

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