[animatethrow]

Aliens could plausibly sterilise earth from light years away using an electron beam, according to a great Kurzgesagt episode:

https://www.youtube.com/watch?v=tybKnGZRwcU

An electron beam seems to be the stealthiest of the three extermination options, the other two being a star laser and antimatter "rods from God."

[perihelions]

That's probably physically impossible for a charged-particle beam (like electrons). It would defocus itself from its own self-interaction, as well as from interaction with intervening magnetic fields.

On this theme: we know several types of natural, astrophysical accelerators of charged particles—but none of those are observed as a localizable source of charged particles, from the perspective of astronomy. We just see secondary photons.

[JumpCrisscross]

> probably physically impossible for a charged-particle beam (like electrons)

That’s the point of the relativistic part of a relativistic electron beam. Time dilation doesn’t give the beam internal time to self interact.

[FirmwareBurner]

>An electron beam seems to be the stealthiest of the three extermination options

Or they could just false flag a nuke strike and watch us sterilize ourselves.

[gridspy]

A lot of effort is currently put into tracking missile launches and predicting ballistic trajectories. The goal is to give some warning if a nuclear strike is launched.

It would be obvious if a strike came from an extraterrestrial source, there would be no terrestrial launch detection.

In addition, such a attack is unlikely to succeed. It would take a long time to arrive on earth and by the time the result of the attack was known there could be angry Earthlings counterattacking.

[krisoft]

> It would be obvious if a strike came from an extraterrestrial source, there would be no terrestrial launch detection.

There would be no launch detected, that is true. That doesn’t mean that it would be obvious that the attack was extraterestrial. The alternative hypothesis would be the that the known terrestrial enemies developed some technique to confuse your sensors, or cloak themselves, or bribe your watchdogs, or pre-position warheads in space, or any other similar deception. People would be sooner thinking that their enemies smuggled nukes in overland than to think that aliens attacked them.

[AtlasBarfed]

The state actor would probably assume a surreptitiously placed space-borne nuke, like on a "space telescope" or other larger satellite.

You don't need all that fuel to get it into space, it's already there. You just drop the warhead.

[sneak]

To deorbit things requires delta-v, not “just drop it”.

[Onavo]

If they entered the atmosphere stealthily and launched from sea level, then it would appear to be similar to a submarine launched missile.

[consumer451]

> A lot of effort is currently put into tracking missile launches and predicting ballistic trajectories. The goal is to give some warning if a nuclear strike is launched.

Less advanced nations such as North Korea and Pakistan have nukes. Do you think that their monitoring systems are really that good?

[gambiting]

No, but they won't destroy the earth with what they have - untold destruction of couple other nations, with millions dead and extreme humanitarian and economic catastrophe - sure, but not extinction of life on earth.

[consumer451]

To expand on my previous comment, one of the routes to armageddon that I have been imagining for years is something along the lines of:

1) A surprise meteor hits Pyongyang or Islamabad.

2) Their military hits their preprogrammed nuclear targets.

3) This draws larger powers into a nuclear exchange.

4) Fin.

But really, I think a bioweapon made by some kid in >2070 is probably more likely.

[gambiting]

I don't see why or how the step 3 would happen. Even if NK nuked South Korea, and even if theoretically US responded with a nuclear strike(and that is already really stretching it), why would other superpowers respond with their own nuclear strikes? China would be super unhappy, sure, but would they attack US with nukes over destruction of North Korea? I don't see it.

I can see Pakistan and India exchanging nukes, but why would anyone join in that?

[consumer451]

Yeah that's true, I just worry that things could escalate quickly once there is any exchange of nuclear weapons. It would certainly be destabilizing to a great extent.

My concern is that it would possibly snowball and draw in other actors.

[dylan604]

The aliens would have superior tech, so hacking into our systems would be trivial. They could just launch an actual nuke rather than lobbing one in from space.

[throwanem]

People assume this and I never understand why. Even if we postulate highly complex realtime computing as a necessity for controlling a superluminal engine, why assume an entire technological history unrelated to ours would make it trivial, or for that matter possible, that even FTL-capable extraterrestrials can compromise earthly systems?

'The thing about aliens is they're alien.' - why assume this only works in one direction?

[tzs]

> People assume this and I never understand why

There's at least one science fiction writer who had similar thoughts and wrote a story [1] where the FTL is pretty much the only technology in which the FTL aliens who visit Earth are ahead of us.

https://en.wikipedia.org/wiki/The_Road_Not_Taken_(short_stor...

[throwanem]

There's a good deal of Turtledove in my perspective on this, yeah. His academic study of history lends a perspective few if any other authors in my experience share; my own study of the same subject, though entirely amateur, tends to make his counterfactuals seem more plausible rather than less, by which I infer both they and their consequences in his stories are drawn with scholarly care.

[krisoft]

> People assume this and I never understand why.

Of course it is not a guarantee. It is possible that the alien invasion fleet arrives, they land, unload their main battle tanks and a passing puppy laps up their whole fleet accidentally.

What we know that by virtue of them being here they are either very good at faster than light travel, or they are good at traveling slow.

If they are good at FTL what else are they good at? We currently think that is impossible. What other things we think as impossible are practiced by them?

If they traveled slow, they must be also good at maintaining their equipment on crazy long timeframes. It also shows that they are the patient sorts who plan and execute things on the order of timelines our empires crumble. How long have they been with us then? How much preparation did they do beforehand? Did Pham Nuwen code the intel management engine?

But sure, it is possible that the Aliens arrive. They broadcast a TV signal threatening us, but unfortunately the sync is a bit off so basicaly no-one understands it. Then they enter our atmosphere. The high oxygen concentration rusts their equipment and they all die.

[throwanem]

Seeing something occur that we thought was impossible tells us our understanding of physics is incomplete, which we already know. Seeing how it is done would probably tell us more, but we haven't. Until we do, we're guessing, and to assume FTL mastery confers godlike powers is as much an assumption as any other. Turtledove addresses this in the story that a sibling commenter mentions; I can also recommend that story, which rewards the reader with considerable entertainment while making its point about what can reasonably be assumed in the total absence of information.

The same goes for the sublight option, only a lot more so. Unless they have FTL communications, which I believe we also consider effectively impossible, by the time they get here anything they think they know about us will be wildly outdated, in technological terms at the very least. Possibly also in terms of the dominant terrestrial species, but we can be generous here.

We can be generous about their information latency because that only matters if they want it to. Any species which wants us dead and doesn't care what state the planet's left in - a reasonable assumption, if we're talking about them popping our nukes at us - doesn't need to come close to landing, or even to orbit, to do it; a kinetic bombardment in passing will amply suffice to depopulate Earth to more or less any degree desired. For subluminal interstellar travel to be even remotely feasible, even for an individually long-lived species, implies access to the kind of delta-V budget where the only limiting factor in such a bombardment is the time it takes to accelerate impactors, which may be zero if those are released before or during deceleration to match velocity with our solar system.

In the former or FTL case, we don't know how FTL works or even could, and we therefore can assume anything we like - with all assumptions at equal risk of bankruptcy. In the latter or high-sublight case, they don't need to be more clever to kill us if that's all they're after, and it may be unreasonably charitable to assume we would even get a chance to see it coming.

[jorticka]

They would compromise the systems by dumping nanobots into them which rewire the whole thing, not by breaking the encryption.

[throwanem]

Okay, but if they can do that to our networks, why assume they can't about as easily do it to us?

[dylan604]

I always assumed they'd just use the gov't mandated encryption backdoor.

Then again, since the master code was 000000, it wouldn't really be to difficult to break without a backdoor

[dylan604]

You're assuming that the aliens are alien.

[throwanem]

If there's a reason to assume otherwise, I've yet to become aware of it. Or are you getting at something more earthly in scope?

[shkkmo]

Or just nudge a couple of big asteroids into collision courses

[randomname93857]

They probably can do that much cheaper: just come up with two opposing conspiracy theories, and people will naturally divide into 2 camps and will eagerly kill each other.