If life can organize into sentient beings around stars other than our own, there are few assumptions we can make about the civilizations that would emerge. We’ve long ago given up on the idea that such creatures would look like us, just as we abandoned the concept of life on every conceivable astronomical object. William Herschel, among others, thought life might exist on the Sun, a notion that in different form may be coming back around, as witness the growing interest in panpsychism and stellar consciousness. But let’s talk about physical life forms rather than energy fields.
Since we have to assume something somewhere, let’s posit that any civilization would select as its top priority its own survival. That seems obvious enough. Survival demands energy, and that demand increases as the civilization grows through the Kardashev scale, gradually using more and more of the energy of its star and ultimately going beyond that to look for energy sources elsewhere. Dyson sphere thinking comes out of this realization, although we have yet to identify such structures if they do exist.
We need to (temporarily) forget Dyson and acknowledge that energy collection could take forms that are on the edge of our own fancy. Even science fiction at its best can’t imagine everything, and the vast hordes of astronomical data swelling our databases may contain things that we can’t identify as artificial constructs or activities. But we have a universe to play around in, and let’s take this cosmos to its logical conclusion. We know that it is not only expanding, but that the expansion is in fact accelerating.
The Hubble Constant, which relates the rate at which galaxies are receding from us as space expands to their distance from us, is not so constant after all. Or put another way, this value (currently considered in the range of 67.4 km/s/Mpc to 73 km/s/Mpc) actually varies over time. Those figures show the discrepancy between observations with the Planck satellite of the Cosmic Microwave Background and measurements using Cepheid variable stars. Thus the ‘Hubble tension,’ which is all about trying to resolve this difference. Whatever that result, current thinking is that as the universe ages, the expansion rate increases thanks to the interplay between gravity and dark energy.
Now since we’re already getting into mind-blowing territory here, let’s do what Dan Hooper did in 2018 and ask ourselves what a civilization with the vast skills of a Kardashev Type III civilization (one that can control the energies of an entire galaxy) might do to ensure its own future. We’re assuming a civilization that intends to maximize its energy use and must thus overcome the problem of spatial expansion. Because as Hooper points out in a paper in Physics of the Dark Universe, within 100 billion years, all matter not gravitationally bound to our own Local Group of galaxies will become what he calls ‘causally disconnected’ from the Milky Way.
Image: Dan Hooper, in an image provided by the University of Chicago.
Beyond This Horizon is the wonderful title of a Heinlein novel first serialized in Astounding in 1942 and later released in book form. Heinlein was interested in reincarnation, but in this case, the horizon we’re talking about is the ‘edge’ (forgive the metaphor) of the rapidly expanding universe, assuming that dark energy as we currently describe it is correctly understood. That last is the other key assumption that Hooper makes in his paper, the first being the need for civilizations to keep finding new sources of energy. And that’s it as far as assumptions go in this intriguing paper – two and no more.
Hooper (Fermi National Accelerator Laboratory, University of Chicago) points out that a Kardashev III civilization will understand that dark energy will increasingly dominate the total energy density of the cosmos, so that expansion goes exponential. Distant galaxies begin to cross this horizon, rendering them forever out of reach. Energy collection could involve collecting stars – the stellar harvest I mentioned in an earlier post, here seen at its most stupendous scale. The more stars that can be harvested – i.e., propelled toward the civilization’s center – the more stars will become gravitationally bound and thus rendered safe from being lost to the effects of this dark energy.
But we’re not talking about all stars. We need a subset of them. After all, stars aren’t static; they continue to evolve as we go after them. Listen to this, as Hooper considers two different rates of travel for the civilization to reach and ‘rescue’ stars:
…very high-mass stars will often evolve beyond the main sequence before reaching their destination of the central civilization, while very low-mass stars will oftentimes generate too little energy (and thus provide too little acceleration) to avoid falling beyond the horizon. For these reasons, stars with masses in the approximate range of M ∼ (0.2−1)M☉ will be the most attractive targets of such an effort.
So we’re in Dyson sphere country after all, but a revised version of same. The author is talking about transporting usable stars by enclosing them to capture their energies and then applying that energy to the stars as thrust. We have to harvest stars, as the above quote makes clear, that are luminous enough to provide enough thrust, and avoid stars that are ‘fast burners’ and will not reach the central civilization in time to be useful. Hooper continues (again, notice that he’s running his calculations on two different velocities for moving stars; he deploys three velocities elsewhere in the paper):
A civilization that begins to expand in the current epoch, traveling at a maximum speed of 10% (1%) of the speed of light, could harvest stars in this mass range out to a co-moving radius of approximately 50 Mpc (20 Mpc). Unlike more conventional Dyson Spheres, these structures would not necessarily emit in the infrared or sub-millimeter bands, but would instead use the collected energy to propel the captured stars, providing new and potentially distinctive signatures of an advanced civilization in this stage of expansion and stellar collection.
Hooper plugs in a speed of transport of no more than 10% c. It’s an arbitrary choice that may raise an eyebrow or two given that we’re talking about civilizations that have had billions of years of experience in matters of science, technology and propulsion. But let’s take this conservative value for velocity, and the lesser ones he discusses as useful parameters. The idea is that the Dyson spheres being used can transfer all of their collected energy into the kinetic energy of the captured star.
What we wind up with is this: If we assume a maximum speed of 10% c, the advanced civilization could collect stars that are currently as far away from it as 65 Mpc. The numbers are spectacular. Remember, a megaparsec (Mpc) is one million parsecs, or 3.26 million light-years. There are a lot of stars to play with in that volume. Here’s Hooper’s Figure 2 portraying the results for the three velocities he considers in the paper.
Image: Figure 2. A summary of the prospects for an advanced civilization to transport usable stars to a central location, assuming that such efforts begin in the present epoch. Stars in the red (upper left) regions will ultimately fall beyond the cosmic horizon, while those in the blue (right) regions will evolve beyond the main sequence before reaching their destination, and thus not provide useful energy. The grey dashed lines denote the length of time that is required to reach and transport the star. We show results for transport that is limited to speeds below 10%, 1% or 0.1% of the speed of light, and assume that the Dyson Spheres transfer approximately 100% of the collected energy to the kinetic energy of the star (η = 1). The blue region in each frame has been calculated for the optimistic case of stars that are starting their main sequence evolution at the time that they are encountered… Credit: Dan Hooper.
The author then proceeds to adjust for stellar age, which takes into account the fact that we aren’t getting to each star at the beginning of its main sequence evolution:
Integrating our results over the initial mass function of Ref. [34] and the cosmic star formation rate of Ref. [35], we estimate that an advanced civilization (with vmax = 0.1c and η = 1) could increase the total stellar luminosity bound to the Local Group at a point in time 30 billion years in the future by a factor of several thousand relative to that which would have otherwise been available. Over a period of roughly a trillion years, the total luminosity of these stars will drop substantially, but will continue to produce substantial quantities of useable energy due to the longevity of the lightest main sequence stars.
So we can’t shut off dark energy but a sufficiently advanced civilization can extend its lifetime considerably. In the next post, I want to dig into the kind of technosignatures we might find if we happened across this kind of star harvesting in our observational data. In doing this, we’ll certainly not be limited to observations within the Milky Way, but have the entire visible universe to consider. Clément Vidal has examined pulsar imagery that could flag the kind of ‘spider’ pulsar engine described in a recent article here. The technosignatures involved in Hooper’s scenario may be even more tricky to find.
The paper is Hooper, “Life versus dark energy: How an advanced civilization could resist the accelerating expansion of the universe,” Physics of the Dark Universe Volume 22 (December 2018), pp. 74-79. Abstract / Preprint. In addition to several books and numerous scientific papers, Hooper has co-produced an excellent podcast called Why This Universe? whose archives are available here.
If we see any indication that this is occurring in our local group, that would be an interesting data point. Would they encourage us to join in their efforts, or destroy our civilisation as potential rivals?
Once we get to such [fanciful] ideas as harvesting stars, why stop at limiting the technology to just harnessing them with means to create a thrust direction? Remove some mass to make them smaller and longer lasting. Are the stars too small to provide enough thrust? Add mass from the large stars.
Optimize masses for harvesting, and then on “arrival” further manipulate mass to optimize for energy requirements.
Star-lifting may be unnecessary. Are there “fusion dampers” that can control the rate of fusion like control rods in nuclear reactors?
Dark Energy is still mysterious. Can the “energy” be harnessed rather than the matter it is accelerating away?
Or is it possible that DE is an effect of using some technology? Is it like “pollution” or “waste” of some process? If so, is the increasing rate of expansion due to the growth of a civilization[s] using that process with the effect of increasing the expansion rate of the universe? [making our acceleration of global temperatures from CO2 emissions look trivial by comparison.] This viewpoint would indicate that we are misinterpreting a technosignature for a natural process.
But please do not take this speculation as my view, it is just an out-there “what if?”]
Consider The Great Attractor as one such potential technosignature, albeit on a scale somewhat more grand than that cited.
I should also mention two philosophical bones of contention with the stated premises:
1) A civilisation capable of herding stars is likely to also have figured out how to counteract dark energy – at least locally.
2) I question whether survival auto-implies unlimited growrh.
Hi Paul
Happy Holidays!
Great article and the article in the link as well.
Do you recall Dyson had a thought or two on this topic?
https://suli.pppl.gov/2019/course/RevModPhys.51.447.pdf
Great reminder, Mark. Thanks! I haven’t looked at that paper in years, but now I will dig back into it. Happy Holidays to you as well.
Good job for us, being a K0.7 civilization, to ponder big ideas like these. Albeit in the same fashion as an amoeba in my left nostril would ponder Elon Musk’s Starships building a city on Mars, but we try nevertheless.
I’ll assume that as you level up on the Kardashev scale, these thoughts should become more informed and likely also more pressing.
Therefore I wouldn’t bet on a civilization quietly reaching KIII and then suddenly start sending ships out to go and rescue stars from the “edge”. They would probably start much earlier.
I would assume that when you become capable of building a spider stellar engine, you would do that with a goal in mind. One use could be to shepherd the stars of your empire closer together to improve speed of communication and transport of goods between stars. And then one might conceive of moving this collection of stars along the spiral arms against the rotation of the galaxy so as to add stars and gas clouds to the cluster more quickly.
Now there is a dead giveaway of a technosignature right there, for a civilization that is still working it’s way up to KIII.
As for travelling to other galaxies to go and rescue stars and send them in the direction of our cluster, even if you steal fuel from the heaviest stars to extend their life and add some horsepower to the smaller stars, you will only be able to steal about 15% of the mass of most galaxies, as the dark matter halo which is 6 times the mass of the visible stars, will not come along unless you accelerate very gently, and that takes too much time. So you will need to harvest 6 galaxies’ worth of stars in order to add one galaxy’s mass to the cluster.
So while you will be able to increase the luminosity of the cluster quite dramatically, once the effect of dark energy becomes more pronounced, your star cluster will be ripped apart because there is not enough mass to keep it together.
For me it would make more sense to identify the biggest clump of dark matter we could reach in time, let’s say the Virgo supercluster, and collect the stars of our galaxy in a compact cluster and head that way. You would travel through other galaxies as you go, adding more and more stars to this moving cluster, and eventually it will be massive enough to drag the dark matter halos of the galaxies you encounter along. You could also travel to galaxies adjacent to your direction of travel and build similar moving clusters of stars there and do the same.
That way you will rescue not only a huge amount of stars, but also add a lot of dark matter to the already massive supercluster, thereby not only increasing its luminosity, but also making it more difficult to pull apart.
That is of course, if you want to play it that way. Build a safe haven, normalize the mass of all the stars for maximum longevity, and then quietly wait for the universe to end. You’d certainly buy a lot of time but in the end Dark energy will prevail.
Maybe our KIII civilization will have even more radical ideas. Maybe they’ll gain some sort of understanding of how to counteract Dark energy, and maybe it will require a truly insane amount of energy to attempt this.
So, instead of converting all stars to a small weight to increase their lifespan, these guys may go for building stellar engines around black holes, and eventually even around the black holes in galactic centers. Maybe they will figure out how to accelerate whole galaxies by evaporating stellar material, gas and even Dark matter in these engines. And then merge these supermassive blackholes into something truly humongous, and construct their own quasar to provide the energy needed to, well, do pretty much anything they please.
So one could presumably study nearby galaxies which have rather active galactic nuclei, and check for odd goings-on and maybe catch an ambitious KIII civilization with their hand in the cookie jar.
@Tiens
If the Parenago Effect is real, maybe that is an explanation for these stars’ higher velocities.
See also a discussion with Greg Matloff in a 2019 CD post Probing Parenago: A Dialogue on Stellar Discontinuity
Thank you for this, Alex. It seems I have a bit of reading up and pondering to do after reading your discussion with dr Matloff.
Hi Paul
A lot to ponder and wounder about with this post.
Cheers Edwin
50 million parsecs is a big chunk — but even this hypothetical K-III civilization, starting in the present epoch, and moving stars at 10% of light-speed, could harvest and “rescue” from the disappearing horizon stars from no more than 0.000004% of the volume of the observable universe (14.2 BILLION parsecs in radius). How much is irretrievably lost, and all the treasures within it!
Intergalactic star harvesting would conflict with intergalactic population expansion. Since the economics of star harvesting favor lower speeds and population expansion higher, the latter should have the temporal advantage. Assuming DE leaves galaxies intact, I would predict expansion by duplication of “K3” peoples would be favored. Star harvesting would be considered aggressive and deep time investment would favor long range communication rather than aggression. With all the time to do anything, something new and unpredictable is unlikely to be sourced from the self.
Hello Andrew and on your 1 and 2
Countering dark energy may be at once possible and economically out of reach. The cost barrier may be a fundamental law similar to the light speed barrier imposed by GR. The cost may be low enough to be paid but impractical. It could also require star harvesting.
A consumption rate value never assumes unlimited growth. Perhaps more importantly and as long has entropy applies, zero population growth does not eliminate the Malthusian limit. The limit is defined by consumption rate and the practical economics of energy convergence loss. The Marvel character Galactus and a civilization of always one trillion could hit the wall simultaneously.
Dark energy may allow entropy to be overcome but I don’t understand the how and whether of dark energy avoiding energy conservation. Perhaps the practical economics of deep time and super-galactic free energy favor K4 people with centrally located infrastructure. Countering dark energy would look like dark matter but how would we identify areas of too much dark matter?
@Harold,
As you suggest, maybe moving the increased population to other galaxies is a better solution. While the K4 civ might be cohesive initially, DE would eventually irretrievably separate it into K3. But this approach delays the ultimate separation of each K3 civ from the next harvestable galaxy, as they chase the DE expansion.
Filling the universe with civilizations may be the better way of maintaining the growth of a species in an expanding universe.
It also allows for relatively low-cost intergalactic travel using a variety of already known, but still out of human reach, technologies.
A follow up to my reply on sending humans.
The mass of our sun ~2E20 kg
Human mas ~1E2 kg
So shipping humans in cryosleep would allow 1E28 humans to be shipped per star for the same energy. For a galaxy with 3E11 stars, that is 3.3E15 humans per star.
Using seed ships instead, with ~3E13 cells per human -> 3E41 potential humans.
If each egg cell becomes a human, this allows 1E30 humans per star system in a galaxy of 3E11 stars. Clearly more than enough to fill the target galaxy.
Therefore, sending humans, human eggs/DNA etc, or instructions for machine life, to another galaxy is far, far cheaper than transporting stars, allows the population to grow their economies in their new petri dish galaxy and to jump to the next nearest galaxy/galaxies for far less effort than star harvesting.
Will we even need stars if we get fusion right, perhaps we just need a very large fusion reactor in orbit that we use to beam light onto ‘earths’ floating through space or even O’Neil cylinders for that matter.
Good job this post is called Centauri Dreams. Sounds more like Centauri Nightmares; stars constantly being pushed and pulled around the universe like billiard balls!
I’ve never understood nor cared for the K-I, II and III principle. But given these supposed civilizations have been around for millions of years, surely it would be easier to make a sun from molecular hydrogen, than move existing ones around over billions of light years. If we can achieve it in our laboratories here on earth, albeit still in the rudimentary phase, it’s not beyond the realms of possibilty that they could do it on a much larger scale with technology as yet unavailable to us. Just a thought.
Hi Paul
Of course, the stars might have other ideas. Stephen Baxter’s “The Thousand Earths” explores the idea of Olaf Stapledon that the Stars are living beings who might resent such activities as star-lifting and Skhadov Thrusters. Greg Matloff has run with the idea with papers on the concept in recent years, and now Baxter has combined it with his ruminations on a Universe empty of all life but our own and the very distant Future prospects for humanity if we can learn to harvest Dark Energy. When the Sky empties of other Galaxies and we *mustn’t* practice stellar husbandry, what are our options?
Won’t that be a hoot. If our Sun is deemed to have caught an infection on its 3rd planet and it’s heading for a void between the stars to hinder the spread of the infection to other stars.
Or maybe it’s the other way around, since there are a few stars which will pass within a parsec between 28k and 40k years from now, and Sirius is also slowly inching towards us.
If all they want is the energy, why wouldn’t an advanced civilization merely collapse the stars into black holes where they are, recovering nearly half the mass as energy, and store it in antimatter batteries for shipment to wherever they need it?
Any BoE calculation of how many galaxies would have to be harvested compared to your BH strategy?
Other options include reducing the population size and/or per capita use of energy, and changing the way the civilization operates, e.g. living in a simulation rather than IRL.
As Asimov once wrote in a short story, unregulated population growth cannot be continuous. The same applies to energy use, no matter how much energy is harvested from stars. Continuous growth is physically unsustainable and must end and turn negative.
Would a mature, highly advanced civilization have the mentality equivalent to humanity’s rapacious extraction of ever greater quantities of fossil fuels?
If the best strategy was turning all (or most of the stars) the stars in a galaxy into BH, that would make a very obvious technosignature. An invisible galaxy that would only be optically detectable by its gravitational lensing. It might also have more than its fair share of merging BHs.
Hi Mike
Imploding a star requires a lot of mass. And a lot of fusion energy is liberated before it can be imploded into neutron stars or black holes. Supermassive stars might’ve imploded directly into black holes when cosmic metallicity levels were low, but that’s no longer true.
Incidentally accretion into a black hole liberates about 5.7% of mass-energy, which is better than fusion, but a long way from the 50% you quote. But how do you plan on turning it into antimatter???
We’re supposing this is an advanced civilization, capable of moving stars and colliding them. But I don’t think they would need to resort to crude measures. Rather, they could devise electromagnetic means to wheedle out and contain artificial stellar prominences, to process them by fusion to transmute any desired elements, and to drop waste iron in a controlled way to obtain all of its gravitational potential energy. (Though I imagine that they would actually live atop a thin layer of insulation on the surface of a star like Capella, a yellow giant with half the surface gravity of Earth, radiating away the star’s heat and generating usable energy by using pair production of thermalized neutrinos as a cold sink)
Perhaps I’m overlooking something, but I was thinking by lowering fusion waste to the Schwartzschild radius r = 2GM/c^2, they should release gravitational potential energy -GMm/r, yielding -0.5 mc^2. Once collected by any mundane means of power generation, this energy could be used to carry out any method of pair production known or unknown to man. I’m imagining they can figure out a way to deal with the technical problems involved in rounding up and storing antimatter.