80,000 Hours Podcast
We can guess what intergalactic war would look like. And strangely, it matters.
18 Jun 2026
Transcript generated automatically by AI and may contain errors.
Chapter 1: What does intergalactic war look like according to physics?
Let's talk intergalactic war in space billions of years from now.
What would something like that actually look like according to our best understanding of physics?
I recently came across a really interesting analysis of exactly this question from the AI researcher Baron Millich and I want to walk you through it because it's incredibly fun, remarkable how much we can likely predict this far in advance and as I'll explain it might even be decision relevant to humanity in the relatively near future.
If you spend any time thinking about the far future, and by far future I mean billions of years from now, one question that really matters is whether our universe mostly ends up peaceful or mostly ends up violent. Does it settle into a patchwork of civilizations each quietly doing their own thing?
Or does it end up looking more like a science fiction movie with enormous space empires locked in some sort of endless conflict? The answer to this probably turns on a surprisingly answerable question. In an all-out war between two galaxy-spanning civilizations, does the defender or the attacker have the advantage?
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Chapter 2: What are the three viable weapons for intergalactic warfare?
If we want a peaceful future, we're going to want to hope that it's the defender, because in that case violence won't pay and so it probably won't exist, or at least not much of it anyway. Here's the setup. Imagine two civilizations, each occupying their entire home galaxy and harnessing the energy of every star inside it. Both have maxed out their technology tree.
There's nothing of meaningful use left for them to invent. These are what physicists call Kardashev 3 or K3 civilizations. To give the attacker their best shot and to keep their commute manageable, we're going to imagine that these two galaxies are positioned right next to one another.
And we're also going to assume no wormholes, no faster than light travel, no time machines, just known physics and engineering pushed to their absolute limits. What might a war between these two vast galactic powers actually look like? I'll show you now. There are really only three viable classes of weapon for civilizations at this scale, each with very distinct strengths and weaknesses.
First, beam weapons, specifically incredibly powerful lasers.
Chapter 3: How can civilizations defend against attacks from space?
It sounds crazy, but it's consistent with the laws of physics and material science to be able to target a laser at a planet-sized object in a neighboring galaxy. It simply seems possible to build mirrors large and precise enough, and there's almost no material in the intergalactic medium to scatter the laser or cause it to lose precision or strength.
The best case for an attacker is if a K3 civilization can successfully coordinate huge numbers of Dyson spheres across the entire galaxy to fire synchronized lasers at a single point in another galaxy. We can't be sure whether a K3 civilization can actually pull this off, but we're not aware of any engineering constraint that clearly prevents it.
If they can make it work, they would be able to deliver 10 to the 35 joules of energy at any given planet-sized target in a neighboring galaxy. The gravitational binding energy of Earth, the energy needed to completely blow the planet apart, is about 10 to the 32 joules, just one thousandth as much. So a laser like this would destroy Earth in a few microseconds.
And then, of course, the laser could be targeted somewhere else.
key benefit of such lasers for the attacker is that they come with no warning the photons travel at the speed of light the fastest speed possible the first photon that tells you you're being shot at is the laser shot so that sounds awfully offense dominant but this approach has a very important drawback lasers can't change trajectory once launched if you're firing from andromeda at the milky way
The round trip of information is about five million years.
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Chapter 4: What surprising advantages do defenders have in space warfare?
You see where something was two and a half million years ago, and then your laser takes another two and a half million years to get there. A lot can change in five million years. Star positions are sufficiently predictable over that kind of timeframe to hit them.
But anything that's deliberately moving, even a little, habitats, fleets, computational infrastructure set on eccentric orbits, they could be absolutely anywhere within an enormous volume of space by the time your beam arrived to try to hit them. That makes the intergalactic laser devastating against fixed targets.
Dyson spheres locked around immobile stars, infrastructure built around the supermassive black hole at the galactic center, planets on completely predictable orbits like Earth. But against a civilization that has deliberately chosen to spread out and move about entirely at random, you're basically going to end up trying to hit a needle in a haystack the size of an entire galaxy.
You can try to make your laser broader to hit more things, but its power just quickly becomes too low to do serious damage. The underlying issue is that space is just unfathomably empty, and that emptiness is a huge asset to any defender. Which brings us to our second weapon, usually referred to in the tiny literature that exists on all of this, as relativistic kill vehicles, or RKVs.
Chapter 5: What implications does intergalactic warfare have for humanity?
These are basically physical objects accelerated to some large fraction of the speed of light and hurled at the enemy galaxy, bullets more or less. The kinetic energy at these speeds is so enormous that they don't need to carry explosives, they just hit things and trivially obliterate them. Now, RKVs have one massive advantage over beams.
Because they're objects physically closing the distance, they can carry onboard sensors and steering equipment to update their targeting as they approach the thing that you want to destroy. They're not shooting blind from effectively two and a half million years ago. So in the final tens of thousands of years of flight, they can survey the defender's galaxy, pick a target, and adjust course.
And over such long distances, even really modest onboard thrusters would allow you to home in on some individual habitat and keep steering into it right up to the moment of impact. The downside? Well, naturally, they travel slower than light. And that means the Defender gets to respond.
Chapter 6: How can civilizations prepare for potential space attacks?
If you're shooting from 2.5 million light years away, an RKV launched at 90% the speed of light would give a Defender up to 250,000 years of warning, at least if they spotted the missile when it was launched. Even if it was only spotted when already inside the galaxy, that's still decades or centuries of reaction time.
And this reaction time is a big benefit to the Defender, because they don't actually need to entirely destroy an incoming RKV, which would be a little challenging. It's enough to simply nudge it slightly off course. Space, again, is almost entirely empty. So if you push it a little bit sideways enough to offset its own steering, It's going to sail past everything off into the empty void forever.
Defensive laser installations, the same kind that any responsible property owner would build on their Dyson spheres anyway, they can melt the surface of incoming RKVs, blind their sensors, and apply a small push to shove them off target. A push that adds up because they're traveling so fast and for so long.
Because the defender is sending pure energy a short distance, rather than mass an enormous distance, the energy required to deflect an RKV is just vastly less than the energy the attacker had to spend launching it and accelerating it.
Chapter 7: What lessons can we learn from hypothetical galactic conflicts?
That asymmetry, cheap to deflect, expensive to launch, is the heart of the defensive advantage against these intergalactic bullets. The third type of weapon is the classic science fiction invasion fleet. Ships that cross the void and then slow down, establish a beachhead in the opposing galaxy, and try to go on to conquer territory.
This is the least promising approach, at least in the early stages of a war. Invasion ships just have a fundamental problem that beam weapons and RKVs do not.
they've got to decelerate at the destination and decelerating is the opposite of stealthy you're going to end up ejecting an exhaust plume which is incredibly visible to the people you're trying to surprise you are essentially releasing a giant beacon screaming i am here come kill me for literally thousands of years as you slow down the deceleration issue also makes it energetically expensive to send a large functional payload because you have to accelerate to nearly the speed of light carrying with you not only your invasion fleet but also
all of the equipment you're using to speed up, and all of the equipment you need to slow down at the other end, which is basically the same amount again. On top of that, invasion ships, unlike RKVs, which are basically just bullets, they're going to be expensive, kind of internally complex, and therefore pretty fragile objects. So even a modest laser shot could probably break them apart.
And even if you get past all of those barriers, you arrive fighting an entrenched civilization with all of its industrial capacity right there, while your supply lines stretch back millions of light years, leaving you just completely isolated.
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Chapter 8: Why is the future of humanity significant in the context of cosmic warfare?
You'd effectively be expending a lot of resources to send over something that can be blown up by the defender for pennies on the dollar. So invasion fleets could at best become relevant late in a war, once the defender is already very weakened by the first two types of weapons we've talked about. So what should a sensible, paranoid, defending civilization do in response to all of this?
First and foremost, don't be a sitting target. Do not cluster your people or infrastructure around fixed, predictable points like stars and planets. Instead, you've got to starlift. Pull the hydrogen out of your stars and distribute it to billions of mobile habitats scattered across the galaxy on randomized shifting orbits.
Each one of these could easily carry enough fuel to support a fusion reactor that continues running for billions and billions of years. Apparently, this is somewhat less energy efficient than a nice orderly set of Dyson Spheres, but it would make your civilization virtually impossible to wipe out through distant bombardment.
Attacking a paranoid, diffuse K3 civilization set up in this sort of way would be, as Millage puts it in his essay, like trying to punch holes in a fog. Second, build massive redundant sensor networks. Giant interferometers spanning light years, deep space sentinel probes extending far out into the galactic halo and beyond. Anything unusual.
An object too big, moving too fast, or with the wrong spectrum for natural interstellar debris. That's got to get flagged and tracked and probably destroyed. It sounds expensive, but in the scheme of the resources that we're playing with here as a K-3 civilization, it's all very affordable, I assure you.
A paranoid civilization would also probably want to seed diffuse dust minefields along likely attacked vectors from nearby galaxies. This would slow down incoming RKVs and light them up for defensive systems to easily see. Third, maintain a powerful and visible second strike capability. Even if you're hit without warning, you should be able to launch a devastating counter bombardment.
And of course, this doesn't help you survive the first strike, but it raises the cost to the attacker, making war even less attractive to them than it otherwise would be. And fourth, maintain the ability to detect and neutralize any enemy self-replicating probes, you know, von Neumann machines that manage to slip through and start booting up inside your galaxy.
If they could get away with this, this would kind of be the attacker's ideal and their long game. You want to get replicators established internally where they can operate and attack with much shorter time lags. Stabbing these out, as we've talked about, is really doable with your galaxy-wide monitoring system. But the challenge is that you do need to have a bunch of redundancy.
So this system can work almost perfectly, even if you're facing a huge influx of lasers and RKVs targeting your defensive infrastructure. As best as we can tell, the fundamental dynamics here strongly favor the Defender, so long as they prepare ahead of time. The attacker has to cross millions of light years, expending enormous energy to accelerate mass or coordinate their beams.
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