Stars are a rounding error
Notes on some interesting factoids I learnt from Anders Sandberg’s draft book, Grand Futures.
“Starlight is heavier than worlds”—Anders Sandberg
Looking at the energy density of stuff in the universe, we find a few surprising, and not so surprising, facts. First, the obvious: baryonic matter itself is a rounding error, contributing 4.5% of the energy of the universe. Nine tenths of those sweet, sweet baryonic numéraire are stuck in the warm plasma floating between galaxies. About half the remainder forms the stuff of stars.
Planets don’t even match a thousand of the contribution of stars to the energy density of the universe. Somewhat surprisingly, supermassive black holes have a contribution. Regardless, the fact remains that planets are a rounding error to a rounding error to a rounding error of the energy of dark matter and energy. Even starlight contains more energy. So in a literal sense, starlight is heavier than worlds.
So, I hope that emphasizes just how important it is to figure out if we can make use of all that dark matter/energy. Or even that intergalactic plasma! From that perspective, stars are really only useful as a bootstrapping step to capture the real resources. Good thing too, given that even slow star-lifting can be done in Mega Years.
But there’s an obvious problem: you can’t really interact with it. Dark matter only interacts gravitationally, and gravity is weak. So very weak that in spite of huge great clouds of the stuff just lying around in clouds about galaxies, we can see the great devouring supermassive black holes at their heart haven’t even managed a nibble.
But yeah, black holes: that’s one way to make use of dark matter. Not via accelerating them in accretion discs till they break apart and bleed off great deluges of energy, like with ordinary matter. Instead, through black hole radiation. That’s right, we chuck the matter in and weight till strange eons past for black holes to die.
The efficiency of this is not bad. You can get half of the energy you dump in back out. A fifth of that is light, a half is electrons/positrons and the other fifth is heavier particles. Which, again, is more than that of all baryonic matter and radiation combined. The rest is neutrinos, which are also very hard to capture.
Of course, there’s the minor issue that you can’t make the black holes much heavier than kg or else they’ll mostly release light very light particles i.e. few baryons. Can’t make much with those. (Turing completeness, let alone error correction, has not been shown for leptonic matter on its own.)
On the plus side, that reduces the time it takes for the blackhole to evaporate. A black hole takes years to evaporate. A one solar mass black hole takes years. Here’s the black hole evaporation time formula, in case you’re wondering:
OK, so that’s converting dark matter taken care of. What about collecting it? The obvious idea is just to chuck some black holes at this. The only idea I’ve got, really.
Except there’s one little issue: it can’t suck enough. Remember, we want teeny-weeny black-holes. kg, remember? That means a radius of m. You’d need an extremely dense mesh of blackholes to get anywhere. But that causes a bunch of problems, one of which is the pain in the butt of making sure they don’t crash into one another and merge.
The other big issue is that black holes experience drag. Crazy, right? You’d think they’d just suck up everything in their way. But no, some stuff gets slingshotted away by gravity, sapping momentum from the blackhole. So somehow, you’ve got to co-ordinate countless bodies which destroy anything they touch, cannot be allowed to hit intersect, and you have to somehow use them to scoop up all the dark matter in the universe.
How very cursed.
So, as far as I’m aware, we don’t have a good solution to the problem of collecting dark matter, and we may have a small-scale, limited, slow solution to the problem of turning a lump of dark matter into useful stuff. So super-heated plasma floating through the inter-galactic void for dinner, anyone?
Acknowledgement: Everything I know about this subject comes from Ander’s excellent book. And many thanks to my Grand Futures reading group for stimulating discussions on the topic.
This is a minor comment but I think it would be useful to define terms like “baryonic matter”. I’m much more well-versed in particle physics than the average person (I have taken one (1) class on particle physics, which I think puts me in a pretty high percentile on particle physics knowledge) but I don’t remember what “baryonic matter” means. It’s also non-trivial to find out what it means: Wikipedia says a baryon is defined as having an odd number of valence quarks, but I have no intuition for what that means either.
From context, I think what you mean by “baryonic matter” is “matter that forms atoms”, and you mean to exclude dark matter, black holes, and force-transmitter particles (photons etc.).
Yeah, that’s fair. I assumed people would know for some reason, but TBH I mix up baryons and hadrons quite often, so it’s not like I wouldn’t have been confused, too.
The mass of a black hole is proportional to its radius, not to its volume like with rocks. So you can’t make a two-dimensional mesh of black holes, only a one-dimensional mesh.
To capture dark energy aka the expansion of the universe, take some masses and let them expand apart, there’s your potential energy. If you ever capture all of it, you can go collect the matter that has stopped expanding away from you.
That sure is a bound on how tight you can make a grid or lattice of black holes.
Yep, that is a way of capturing dark energy. But can you capture enough?
Sure, just put everyone in stasis until the batteries are refilled.
Couldn’t you use big black holes to suck up dark matter, and then harvest energy/momentum from those? Even if the large size is not ideal for the next step of conversion, making the dark matter capture even marginally easier seems like the more important consideration given the ratios involved.
Curious if you have a citation for this!
Anders make this claim in his book draft, with some arguments against stable leptonic structures, which you might think precludes them as a substrate for computations. But it’s hard to prove that a thing can’t be done. As for whether anyone else has proved it, I’m afraid I don’t know.
While reading that exact passage I had an idea which I don’t think will pan out but nevertheless has some cool confluence: use black holes as shielding for intergalactic probes to scoop up the plasma.
On the subject of “capturing dark energy” (I don’t think this technically captures any energy previously existing), my favorite proposal is to connect distant galaxies together with strings, and use the resulting tension to turn a turbine. In principle your limit would then only be the strength and length of your intergalactic rope.
See Mining Energy in an Expanding Universe for what I think is the earliest proposal for this idea.
Another way of acquiring useful energy from dark energy is to place two objects extremely far away from each other and give them a velocity towards each other that is somewhat less than their recessional “velocity”. The two objects will initially be transported away from each other because dark energy is creating new spacetime between them even though relative to spacetime they are moving towards each other. Then, mutual gravitational acceleration gradually increases the velocity of these two objects. The velocity of the two objects towards each other eventually overwhelms the creation of new space by dark energy. Thus, the objects return with a kinetic energy greater than what would be generated by the conversion of gravitational potential energy to kinetic energy alone.
Edit: Typos.
How do you connect a galaxy with a string? I don’t understand.
Anders writes about this! p. 783-785 (809-811 in the raw PDF). Unfortunately, even with a carbyne chain (“presumably close to the ultimate limits of molecular matter”) checks “the lost mass-energy from extending the cable will be a factor 1.39×10⁹ larger than the work done by the cable”
But a galaxy isn’t solid. How do you anchor the galaxy to the chain and vice versa?
¯\(ツ)/¯, but if you could...
Dark Energy seems pre-paradigmatic; it does appear we need some way to account for the blueshift of distant galaxies, but I’m hesitant to enshrine it as a ‘type of stuff’ and by implication the largest item on our ledger of future potential resources.
I’m a big fan of this sort of exploration about “what are the details of our cosmic endowment.” Thanks for posting.
It’s not the dark energy we want to harvest. It’s the dark free energy.