Space Warfare Seems Mostly Defense Dominant
Epistemic status : I’ve done some thinking and research on this but I am not a physicist and I can easily be wrong about specific things.
Sometimes when thinking about the long term future, it is interesting to think about the offense-defense dynamics of a fully colonized ‘mature’ universe. E.g. suppose we succeed at aligning AIs and colonize a large section of the universe but there are some unfriendly aliens who have colonized the rest, or alternatively that we have colonized our galaxy with aligned AIs but there’s a bunch of rogue AIs running around in the other galaxies, how worried should we be? More generally, in the longer term evolution of the universe there is going to be a long period after colonization, but before all other galaxies become unreachable due to cosmic expansion, that there is wide room for ‘interactions’ across cosmic distances. Will the universe end up mostly or entirely peaceful with everybody just quietly maximizing utility as seems best to them or will there be e.g. more classic SciFi style interstellar or intergalactic conquests with e.g. ravenous hordes of replicators eating galaxy after galaxy?
To think about this, we’re going to try to assess what a hypothetical intergalactic war between two roughly equivalent galaxies would look like, where both host a mature AI Kardashev-3 style civilization – i.e. one that has maxxed out its ‘tech tree’ and can harness the mass/energy of its entire galaxy largely as it pleases. We are also going to assume no dramatic departures from known physical limits – i.e. no wormholes, time-travel, infinite-energy-glitches or exceeding the speed of light, since if any of these are possible then all bets are off1.
My general conclusion after thinking about this a fair bit is that in the end the dynamics are fairly strongly defense dominant, if both adversaries are equally capable and paranoid, in that an attacker would need to spend far more mass/energy to destroy the defender than a defender would need to expend to defend itself. An attacker is extremely unlikely to ever be able to fully wipe out a mature and prepared defending K3 civilization, although they could potentially seriously cripple them if the defender is taken by surprise or not careful, and that in general such intergalactic wars would last for millions and potentially billions of years, would be ruinously expensive and unpleasant for both sides, and would permanently hurt the long-term growth potential of both attacker and defender.
This makes large scale war between mature K3 civilizations ‘irrational’ in the sense that the attacker would likely spend more resources in the war than they could expect to gain by e.g. colonizing the remnant of the defender’s galaxy after they have been wiped out. However, there are exceptions to this which could still drive war. Firstly, if the attacker has an ‘irrational’ dislike of the defender civilization for e.g. ideological or other reasons this could motivate a prolonged campaign which does not pass purely economic cost/benefit considerations. Secondly, if the attacker assesses the defender to be a young or ‘inexperienced’ budding K3 civilization then they could be highly vulnerable to a first strike and obliterated much more cheaply than a mature and highly paranoid K3 would be. Intergalactic wars (unsurprisingly) seem likely to be highly visible to the rest of the universe which could prompt interventions, although over timescales again of millions of years.
To see how we get to this conclusion, let’s wargame out what this theoretical war would look like. For illustrative purposes let’s imagine that we are a Kardashev-3 civilization inside the Milky way and we are defending against a similar civilization in Andromeda, roughly 2.5 million light years away. Let’s assume both galaxies are roughly ‘evenly matched’ (i.e. to within an OOM) of energy density, number of stars, mass etc. Let’s break down what the offensive and defensive capabilities are for a sensible K3 civilization.
Offensive weaponry :
These come in three main classes:
First, pure electromagnetic beam weapons . Specifically, these will be laser or maser beams of incredibly high power and extremely well collimated which can cross intergalactic space to hit specific spots at the target galaxy.
At K3 scale, we can get extremely powerful with these. Around each star we assume there will be a Dyson sphere and stellar infrastructure capable of constructing massive phased arrays which can operate in perfect synchrony. With such arrays, it is well within the known laws of physics to create highly collimated lasers which can converge on relatively small targets (e.g. perhaps a few hundred kilometers) across vast intergalactic distances. This process is highly energetically efficient since the intergalactic medium is almost completely empty, and hence transparent across almost all frequencies. Surprisingly, because of this, once the laser...