Hidden universal expansion: stopping runaways

by Stuart_Armstrong1 min read11th May 201753 comments


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We have a new paper out, presenting the 'aestivation hypothesis'. It's another attempt to reconcile the fact that cosmic expansion seems very easy, yet we see no trace of any alien group doing it.

The idea is that civilizations expand rapidly, but then 'go to sleep', while they wait for the temperature to drop and it becomes possible to do computations with maximal efficiency.

There are some few problems with the theory, though - mainly, why would the civilizations conceal themselves? Even if they were sleeping, they should have some automated processes rounding up intergalactic gases, preventing stars from drifting out of galaxies, and so on.

But though it's hard to justify a civilization permanently hiding, there are reasons why a civilization might hide temporarily.

Consider the following diagram:

Here, a civilization is expanding from the red point, and will eventually reach Earth (drawn not entirely to scale). It's expanding at a decent fraction of light-speed. The red sphere is their physical expansion front, while the yellow sphere is the light expansion front. When that yellow reaches Earth, we will generally be able to notice their expansion, and have some time to react to it - unless they conceal themselves as they expand.

Why would they want to do that? It's not as if we could counter their expansion, or have any chance of resisting. But there is one thing that we might be able to do: flee. Imagine that we got a hundred years warning; we might be able to rush AI, Dyson the sun, build escape ships and launch them at a significant fraction of light speed, etc. They might never be able to catch us, and, as we or our AIs fled, we could develop technologies to reduce or damage our pursuers.

Therefore, it makes sense for the expanding civilization to conceal itself until it has any other civilizations completely surrounded. That means that Dyson swarms and other major feats of stellar engineering might be delayed by many years or decades by the red civilization. So that the 'noticeability front' - the distance at which other civilizations can see clear evidence of red's expansion - lags a bit behind their actual expansion front.


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Speculation about why alien civilizations are concealing themselves is approximately as reasonable as speculation about why angels and demons apparently are concealing themselves.

Easily asserted, easily denied.

That applies much more strongly to your comment than mine. I linked you to an essay's worth of supporting arguments, you just offered a flat assertion.

Well we have plausible reason to believe in aliens. The copernican principle, that the Earth isn't particularly special and the universe is enormous. There's literally no reason to believe angels and demons are plausible.

And god do I hate skeptics and how they pattern match everything "weird" to religion. Yes aliens are weird. That doesn't mean they have literally the same probability of existing as demons.

I imagine that a few centuries ago a RationalWiki page on microorganisms would describe them as: "A pseudoscientific belief that human diseases are caused by invisible beings. Coincidentally, exactly the same thing as described in the Bible. This dangerous myth is spread by opponents of scientific bloodletting and other uneducated people."

Microorganisms do not have to make any special effort to hide themselves (at least not from human beings, if they may do some hiding from certain other microelements of humans).

Likewise, if you propose a theory of aliens which does not involve hiding from humans, e.g. there are aliens, but too distant to be visible to us, that theory will be reasonable. The hiding theory is not.

In fact, I think it's exactly the same.

There are these powerful entities concealing themselves from us! Let's call them "angels" and "demons". No, let's call them "aliens".

Yes, they could be entirely the same -- theologies do not necessarily say that angels are not also aliens.

The main question is why they don't wage visible wars with each other? Because in the case of war for the territory the earlier you start, the more you gain (for the future computations).

One possible sign of such war would be sending messages addressed to naive civilizations as our, in order to enslave it by powerful AI - I called it SETI-attack. But we don't see such messages.

However, we still could find them, if medium distance between civilization is in order 100 mln -1 bln light years, and physical travel is below 0.5 c I explore these limitations on the civilization density here: https://www.academia.edu/30029491/The_Risks_Connected_with_Possibility_of_Finding_Alien_AI_Code_During_SETI

The main question is why they don't wage visible wars with each other?

Visible to whom and how do you know?

Letting a lot of energy escape while destroying something is crude and inefficient. E.g. a nanobot swarm or an information system takeover would leave nothing you'd recognize as war. Plus, are you quite sure some types of (super)novas are not starkillers in action?

Letting a lot of energy escape while destroying something is crude and inefficient.

Scorched earth is the most effective defence, and I would expect to see evidence of civilizations destroying their physical resources on a massive scale. But perhaps the threat of that is what keeps wars from happening.

Scorched earth is the most effective defence

That doesn't look true to me.

If you can pull it off, it works against any opponent no matter how powerful they are.

I don't know to which extent "I'll cut my throat and bleed on you" counts as an effective defense...

The rational reasons to go to war are to prevent a future competitor and to gain resources. Scorched Earth removes both of those reasons: if you can destroy your own resources AND inflict some damage on the enemy at the same time, then no-one has rational reasons to go to war. Because even a future competitor won't be able to profit from fighting you.

If advanced civilisations have automated disagreement resolving processes, I expect them to quickly reach equilibrium solutions with semi-capable opponents.

What happens when the committed scorched-earth-defender meets the committed extortionist? Surely a strong precommitment to extortion by a powerful attacker can defeat a weak commitment to scorched earth by a defender?

It seems to me this bears a resemblence to Chicken or something, and that on a large scale we might reasonably expect to see both sets of outcomes.

Scorched Earth removes both of those reasons

No, it doesn't. Invading you to force you to destroy your own resources is a good way "to prevent a future competitor". You are not going to do it on your own, so you need to be pushed into this by war.

Not to mention that, historically speaking, reasons to go to war are not often "rational". WW1 would be a classic example.

I think that in the case of star wars they may need allies in the remote parts of the universe and so will send SETI messages.

Messages sent at the speed of light..? To "remote parts of the universe"? Consider the time scale.

No problem. I explore a method to send images up to 1 billion light years (or more) in my article by drawing images from Dyson spheres on a galactic plane.

If your messages which take millions of years to arrive are relevant to a war, the war also goes on for millions of years. Why would you expect to see anything change over a mere century or so?

Intergalactic war may go for million of years. But humans now are naive civilization and the first who can affect it by ideas of a computer program will win it. Basically it would help to convert Earth in a remote fortress which will help to start colonization of our part of the universe. As a result, a territory of the message sender would grow .

Meh. For a war waged over millions of light years (and, necessarily, lasting millions of years) what can Earth offer? Some atoms? They are easily found elsewhere.

A new starting point to send probes with the half speed of light in all directions in the backland of the enemy.

Any star system will do for that purpose. All you need is star's energy.

No, you need a traitor behind enemy lines to receive a message and start an attack. Star system without a civ can't receive a message.

Ah, I see. You're thinking, basically, about an information plague which, moving at c, leapfrogs physical probes.

See astronomer Fred Hoyle's A For Andromeda for a fictional exploration of the idea (and a pretty good novel).

I was thinking more of Vernon Vinge's A Fire Upon the Deep, but yeah, this is not a new idea.

Very interesting! But I feel that this is very improbable:

physical travel is below 0.5 c

When I discussed the idea of Alien AI I met two points of view:

1) AI is impossible. 2) EY: AI will travel with almost the speed of light

I think it worth consider the intermediate point of view, that is 0.5 с as the total speed of colonization. Reasons:

  1. Colonization speed is lower than the speed of travel of the von Neumann Probes, as they need some time to replicate.

  2. It is lower than cruise speed of the probe, as it needs some time to accelerate and decelerate. Declaration of the high-speed probe is especially difficult.

  3. Closer to с speeds require much more energy because of the special relativity, so energy investments will produce diminishing returns in the speed of colonization.

  4. If a civilization is remote, the universe acceleration will also work against it perceived speed of colonization.

  5. High speed creates a lot of problems with space dust, which require heavier protection shields.

So, 0.5-0.7с speed are a realistic estimate for the maximum speed of space colonization.

I have a paper on that :-)


And though it mentions 0.5c for fission engines, most "realistic" probe designs will be much faster, because you can use more exotic things like bussard ramjets to decelerate (using them to accelerate, the way they were originally designed, is harder that it seems, but they are perfect to decelerate). Also Eric Drexlers's been working on some interesting dust shielding designs. So the paper is very much "conservative" in its assumptions.

Thanks for interesting link.

It is possible that total colonization wave speed is much higher, closer to с, but I suggest just to put some weight to the hypothesis that is around 0.5-0.7c because of some unknown tradeoffs. I would estimate such outcome as 30 per cent. Only in that case observing alien astroengineering and SETI is possible.

Second question:

Do you have a nice reference (speculative feasibility study) for non-rigid coil-guns for acceleration?

Obvious idea would be to have a swarm of satellites with a coil, spread out over the solar system. Outgoing probe would pass through a series of such coils, each adding some impulse to the probe (and doing minor course corrections). Obviously needs very finely tuned trajectory.

Advantage over rigid coil-gun: acceleration spread out (unevenly) over longer length (almost entire solar system). This is good for heat dissipation (no coupling is perfect), and maintaining mega-scale rigid objects appears difficult. Satellites can take their time to regain position (solar sail / solar powered ion thruster / gravity assist). Does not help with g-forces.

Disadvantage: Need a large number of satellites in order to get enough launch windows. But if we are talking dyson swarm anyway, this does not matter.

How much do we gain compared to laser acceleration? Main question is probably: How does the required amount of heat dissipation compare?

I have not seen any papers about it, but did look around a bit while writing the paper.

However, a colleague and me analysed laser acceleration and it looks even better. Especially since one can do non-rigid lens systems to enable longer boosting. We developed the idea a fair bit but have not written it up yet.

I would suspect laser is the way to go.

Interesting idea. No, I don't have any references, sorry!

Do you have a non-paywalled link, for posterity? I use sci-hub, but paywalls are a disgrace to science.

Also, do you have a nice reference for the bussard ramjet/ramscoop deceleration?

Obvious advantage: A priori you don't need nuclear fusion at all. You use a big em-field for cross-section and use, ultimately, drag against the interstellar medium for both deceleration and energy generation. No deceleration needed in (thinner) intergalactic medium. Entropy gain should be large enough to run mighty heat-pumps (for maintaining high field superconductors and radiating excess heat). No need to carry fuel or manage fusion; your kinetic energy at relativistic speeds has almost as much energy as antimatter. Antimatter sucks because production, containment, and difficulty of not frying yourself with the resulting radiation (light probe cannot shield against gamma), and probably a couple more reasons.

Disadvantage: not obvious whether this works. I would appreciate an actual engineer doing the computation. (I am just a mathematician, and have not seen a study of this deceleration design because I suck at searching the literature)

Probably at least three problems:

(1) How much impulse at what speeds? Determined by cross-section of collecting EM-field over required mass of collector.

(2) Might be good for decelerating from 0.9c to 0.05c over maybe 10k years (pulling numbers out of my ass). Would still need secondary system for the remaining deceleration, until slow enough for gravity assists. Could collect propellant over the long deceleration, but then would need to dissipate a shitload of heat; unclear whether net gain.

(3) Heat dissipation.

I agree that deceleration is the thing to care about; beat the rocket equation on deceleration by clever designs using the interstellar medium, and on acceleration by big machines.

Question: I suppose it's implicitly assumed that FTL travel is physically impossible even for extremely advanced civilizations, are there reasons to feel relatively secure in our belief that aliens will not have discovered some sort of exotic or unknown physics that allows them to achieve that?

Yes, I assume it. If not true, they must be here.

Exactly. FTL is allowed even by relativistic mechanics (who know what allows quantum gravity?), although time travel paradoxes ensue.

S. Jay Olson's work on expanding civilizations is very relevant here, e.g. https://arxiv.org/abs/1608.07522 and https://arxiv.org/abs/1512.01521 That work suggests that even non-hidden civilizations will be fairly close to their light front.

Now, the METI application: if this scenario is true, then sending messages so that the expanding civilization notices us might be risky if they can quieten down and silently englobe or surprise us. (Surprise is likely more effective than englobement, since spamming the sky with quiet relativistic probes is hard to stop)

When does this matter? If we happen to be far away from the civilization, then they will notice the message late and we could have done all sorts of things in the meantime - escape, becoming an equivalently powerful civilization, gone extinct etc. We would have done the same even if they had not been there. So there is no change.

If they are already here there is only an effect if they react to the attempt at messaging ("Only talk to civs that want to talk to you"/"Only wipe out civs that might pollute the ether with deliberate messages"), so there is no change.

[ In fact, a civilization that deliberately tries to conceal itself may be particularly concerned with signalling civilizations since they might act as beacons: if they get cut off, that might tell other listeners where something is going on. Keeping them signalling even when well inside their englobement might be good camouflage. Until they are discreetly replaced with decoy young civilizations...]

So the only case that leads to nontrivial effects is the nearby but not here case. Friendly civilizations react by doing something friendly, but that would have happened anyway when they came here. Unfriendly civilizations use the information to be more efficiently unfriendly (like quiet englobement). The main thing that changes is that the opportunity for running away (or other reactive responses) decreases.

So the overall utility change is U(METI)-U(no METI) = -Pr[nearby but not here] * U(running away).

Now, Pr[nearby but not here] seems to be small for rapidly expanding civilizations. If they spread at speed v, then it is (v/c)^3. For v=0.9c, it is only 27.5%, and for 0.99c it is 2.97%. So it is a small net negative, assuming U(running away) is positive.

All of the above is conditioned on that interstellar/intergalactic expansion is doable. If it isn't, we get informational game theory instead.

What are your scenarios for interstellar warfare? The question obviously depends on whatever turns out to be the technically mature way of violent conflict resolution.

Let me propose a naive default guess:

Small technically mature von-neumann probe meets primitive civilization or unsettled system: probe wins.

Small technically mature von-neumann probe meets system with technically almost-mature inhabitants: probe cannot even make problems.

System with dyson swarm + AI: Unassailable on short timescales. Impossible to profitably invade. Maybe sling another star at it if you control the stellar neighbourhood.

In this scenario, interstellar warfare is a matter of land-grabbing: Spam the entire sky with probes, moving as fast as possible, dyson a fraction of stars to keep up the expansion front, and fortify all other systems. "Fortify" might just mean "build and maintain a couple thousand tons of observatories & industrial base", i.e. almost nothing: One just needs enough headstart to win any inner-system race against later von-neumann probes. This is relevant if the colonizer has reasons to keep most systems mostly virgin, and is compatible with the silent sky.

In this scenario, if we saw an expansion front, we would rush to move from category (1) to (2); this is slightly bad for the big colonizer.

What does "flee" mean in this context? It would mean rushing to grab a bigger slice of the pie. Do I understand you correctly there?

On the other hand, the game-theory appears to suggest that colonization speed dominates stealth, all the time: The only reaction move we have is to do what we should do anyway, if we care about colonizing the universe (if we don't care then we don't need to react at all, just keep our system).

So, in summary, I do not understand how the red civilization intends to influence our decision processes by staying stealthy.

Why would they want to stop us from fleeing? It doesn't reduce their expansion rate, and we already established that we don't pose any serious threat to them. We would essentially be giving a perfectly good planet and star to them, undamaged by war (we would probably have enough time to launch at least some nuclear missiles, probably not harming them much but wrecking the ecosystem and making the planet ill-suited for colonization by biological life). Unless they're just sadistic and value the destruction of life as a final goal, I see no reason for them to care. Any planets and star systems that would be colonized by the escaping humans would be taken just as easily as Earth, with only a minor delay.

Why would they want to stop us from fleeing?

Because over the billions of years of our flight, we could develop technology that could be used to counter them, especially if interstellar warfare favours the defence or scorched earth is possible.

First of all, the Earth has been around for a very very long time. Even slowly expanding aliens should have hit us by now. The galaxy isn't that big relative to the vast amounts of time they have probably been around. I don't feel like this explains the fermi paradox.

If aliens wanted to prevent us from fleeing, this is a terribly convoluted way of doing it. Just shoot a self replicating nanobot at us near the speed of light, and we would be dealt with. We would never see it coming. They could have done this thousands of years ago, if not millions. And it would be vastly more effective at snuffing out competition than this weird strategy. No need to even figure out which planets might evolve intelligent life. Just shoot all of them, it's cheap.

You could time them so they all hit their targets at the same time and give no warning. Or have them just do the minimal amount of destruction necessary so they aren't visible from space.

First of all, the Earth has been around for a very very long time. Even slowly expanding aliens should have hit us by now.

Yes, this explains it only if we are in the very small window between the yellow and red fronts.

We would never see it coming. [...] Or have them just do the minimal amount of destruction necessary so they aren't visible from space.

Us seeing it coming is not the problem; it's the next civilization along not seeing our destruction that's important. And it's not clear at all that you can easily do "the minimal amount of destruction necessary", especially since we have nuclear weapons that are likely visible if fired en mass. More to the point "just shoot them all, it's cheap" is true if you don't care about being observed (you can Dyson suns for the energy, and have visible shielding mechanisms for probes that shoot through the very dusty interstellar - not intergalactic - space). I'm not yet convinced that it's easy or cheap to do it a c-comparable speeds and discreetly.

we have nuclear weapons that are likely visible if fired en mass.

Would we be able to detect nuclear weapons detonated light years away? We have trouble detecting detonations on our own planet! And even if we did observe them, how would we recognize it as an alien invasion vs local conflict, or god knows what else.

The time slice between us being able to observe the stars, and post singularity, is incredibly tiny. It's very unlikely two different worlds will overlap so that one world is able to see the other destroyed and rush a singularity. I'm not even sure if we would rush a singularity if we observed aliens, or if it would make any difference.

I think it's a lot more likely that an expanding civilization will want to conceal themselves after they have surrounded any given point, with that point being far away from their frontier of expansion. Because that's precisely when such advanced civilizations will gain the most simply from being as resilient as possible to the threat of other expanding entities. By contrast the frontier will see intense competition to simply expand into unpopulated space as fast as they can, so hiding is unlikely to be a priority. Even if their expansion might be checked by some hostile entity, this is not a big deal in expectation; it might just be what those farther back from the frontier were waiting for, a chance to try again and leap-frog the earlier colonists.

? I don't understand. Once they're surrounded us, they can take their time and eliminate humans in an unnoticeable fashion.