author: niplav, created: 2024-11-15, modified: 2025-07-08, language: english, status: in progress, importance: 6, confidence: unlikely
One might be able to solve two problems far-future civilizations will face (most baryonic matter is intergalactic hot plasma; intergalactic high-c probes are damaged by dust) with one idea—use black holes as shielding.
Baryons make up only 4.5% of
the current massenergy
of the universe. >80% of that baryonic massenergy is intergalactic
dust or plasma—mostly hydrogen and helium with temperatures from
$10^5$
to $10^7 K$
, and if compressed to water density would
have temperatures from $10^{31}$
to $10^{34}K$
(Sandberg
2023,
p. 746).
We or our descendants might care a lot about this massenergy, since if we can't use dark matter or dark energy, it would represent most of the resources available in the universe.
So it would be cool if we had a method of collecting this intergalacic plasma & dust, and ideally one that fulfills these properties:
For intergalactic
colonization,
an advanced civilization would want to move through intergalactic space at
very high speed—the closer to $c$
, the better. But at such velocities,
collisions with dust and small particles create explosions releasing as
much energy as a hand grenade.
For a black hole civilization, One option to fulfill some of the criteria for collecting the IGM and shielding a probe would be to create intergalactic "ships" that are composed of a coordinating, baryonic center and a "shell" of black holes orbiting that center, moving at high speed through the intergalactic medium, in a setup similar to a Dyson swarm. Alternatively, the setup could be two or more black holes orbiting each other, with the baryonic center stationary at a Lagrange point where it is shielded by one or more of the black holes in the direction of travel.
This, but with black holes instead of mirrors. Sandberg 2023, p. 460
The black holes would "mop up" the intergalactic medium as the craft would move through it, increasing in mass1. The heat of the intergalactic medium would be converted to the mass of the black hole, as black holes don't have any temperature and are excellent heat sinks; instead they emit Hawking radiation in the black body spectrum.
One option is to use such black hole mops in intergalactic colonization, where the black holes act as shielding for the center from collisions with intergalactic gas. I think this would slow this construction down, but usually not change its structure, though I haven't thought about it very much.
How does this fare on our list of criteria?
Black hole mops would have some other disadvantages:
Despite having mostly listed downsides of black hole mops, I think they're an attractive option to do further reseach on.
I don't know whether such constructions are possible or desirable, but I would like to hear some feedback by someone more knowledgeable about physics.
I don't know whether the intergalactic medium is charged, if so the black holes would also accumulate charge. I assume that on a medium scale the intergalactic medium is fairly evenly distributed, so I don't think they'd accumulate angular momentum. ↩