We’ve talked about this before but mostly in the context of Hawking Radiation from small, artificial black holes.
Today’s post is focused on large, long-lived black holes, where Hawking Radiation is incredibly tiny and other methods are needed.
So we’ll be discussing those other methods as well as
what the implications of living on minimal Hawking Radiation would be like.
2) The Fate of the Universe
In this section we’ll go over the timeline of ages of the Universe fairly quickly, and also quickly cover some of the other ideas for Civilizations far in the future, which we may expand on in future posts.
3) Black Hole Farming
In the last section we’ll get into the meat of things, trying to contemplate what civilizations would be like that essentially fed themselves off black holes.
It’s the concept of using black holes as the power source for your civilization, and actually creating or placing black holes to make that work best, which is the origin of the title.
I think it summons to mind the image of farmer in coveralls with a pitchfork literally farming black holes but we’re sticking with it anyway.
So without further ado, let’s dig in.
Our first topic, using Black Holes as power sources is, as I mentioned, something we looked at before in the twin posts discussing Hawking Radiation, Micro-Black Holes, and using them to power starships.
You may want to read those, or re-read those, before proceeding, but the quick summary is that Black Holes are thought to emit Hawking Radiation loosely in proportion to their size.
Except backwards from what you’d expect, the giant monster sized ones in the centers of galaxies emit so little of it you’d need a trillion, trillion years to collect enough energy to turn on a little LED light for a fraction of a second.
Alternatively the small ones gush out power so fast they burn out their tiny mass in very short times.
There’s two upshots of this.
First, that the lifespan of black holes is proportional to the cube of the mass, one twice as massive emits only a quarter of the power and lives eight times longer, one ten times as massive emits a hundredth of the power and lives a thousand times as long, etc.
Second, if we can make artificial black holes, and especially if we can feed matter into them to replace what they lose to Hawking Radiation, we have an excellent power source for things.
Black Holes are roughly on par with anti-matter, and vastly better than nuclear fission or fusion, in terms of energy per
unit-mass of fuel, and they don’t blow up unless you starve them to death, a process that would take years or centuries normally, making them a very attractive option for power
generation and storage.
This is assuming we can figure out how to make small ones and feed them, both of which are actually a lot harder than with their bigger, naturally occurring kindred.
Which again emit virtually no energy on timelines that can be measured without using scientific notation.
This doesn’t mean we can’t tap black holes for power in other ways though.
The preferred way to tap a black hole for power quickly, which also works on neutron stars, is to suck out their rotational energy.
Stars spin, same as planets, they have a lot of angular momentum and that is one of those conserved quantities in nature.
When they die and collapse they start spinning much faster for the same reason an ice skater twirling around with her arms out will spin much faster by just bringing her arms in toward her body.
Our sun rotates around once a month, neutrons stars often rotate many times a second, that is why pulsars make such handy clocks.
I was going to say pulsars are a type of neutron star but all neutron stars begin as pulsars, it’s just they have to be pointing in our direction for us to notice the pulsing and that effect diminishes with time.
This isn’t a post on pulsars so I’ll just simplify it for the moment by saying they emit two narrow beams from opposite directions and if you’re at the right angle each of those beams will pass over you every time it spins around, which again is many times a second.
They only do this for the first hundred or so million years of their life, and only about a tenth happen to line up with Earth so it is right to think of pulsars as a type of neutron star it’s just that the type is
A) Fairly young
and
B) coincidentally aimed our way.
Every neutron star was a pulsar for someone at some point.
Science fiction loves to say you can use pulsars to get navigational fixes off of, and that’s basically true, but you’d need a catalog of all the young neutron stars to do that properly.
And again it is only young neutrons stars you can use for this as they slowly lose energy and cool with time, something we’ll discuss a bit more in the second section of this post.
Anyway needless to say black holes spin too, and very quickly, and both them and neutron stars emit huge magnetic fields as a result, same as Earth does from having a giant molten ball of spinning metal in the core.
You can tap that power, sucking energy from spinning
magnets was how the first electric generator worked, the Faraday Disc, which was the precursor of dynamos.
The disc slowed down as it leaked power as electricity.
Stealing away that black holes rotational energy, which is a large chunk of it’s total mass energy, is thus a pretty attractive option.
And there’s various proposed ways of doing that.
The Penrose process is probably the best known of them, and relies on being able to remove that energy because a black holes rotational energy is thought to be stored just outside the event horizon in what’s called the ergosphere.
You obviously can’t dip under an event horizon and suck energy out, but we can from the ergosphere.
There’s also the Blandford–Znajek process which is one of the lead candidates for explaining how quasars are powered.
If you’re familiar with Quasars, and how they are brighter than most galaxies, this gives you an idea how much juice a black hole can provide.
It also taps the Ergopshere for power and does it by using an accretion disc, so you’d use this on a black hole that already had one or that you were feeding, we’ll come back to that in a moment.
You can also just dump matter into a black hole, it gains kinetic energy as it falls down, same as if we drop a rock off a tall building.
If you tied a spool of thread to that rock and ran an axle through the spool attached to an electric generator you’d get electricity.
And you could do the same with a black hole too.
Of course if you drop that rock off the building you’d get less power than you’d expect because the rock is falling through air, slamming into air particles, and transferring much of its momentum to them, actually heating the air up in the process.
This is how parachutes work, transferring all that kinetic energy into a wide swath of air as heat.
It’s not a lot, but if the object is moving fast enough, like a spacecraft on re-entry, it’s a lot more and can make the object and the air it’s hitting so hot it will glow.
You could gain some power with a solar panel that was nearby, drinking in that light.
And you can do the same with a black hole because as matter falls towards them and often ends up in orbit around the black hole rather than directly entering, it forms what we call an accretion disk.
And those glow quite brightly, giving off a lot of photons you can collect to use for power.
If you dump matter into a black hole you can collect that power.
It should be noted that when things approach large masses they usually don’t curve and slam down into them, and that’s as true for black holes as anything else.
Their path curves, depending on how close they get and how massive they are.
If they are very close to a very large mass they will hook right
in, but normally they either fly off at a different angle or enter an orbit.
And if there’s other stuff hanging around there for them to bump into their orbit will decay and they’ll eventually fall in.
All that bumping, again, generates heat and if there’s enough heat, lots of visible light too, same as a red hot chunk of metal.
That’s an accretion disc, for a black hole.
And everything that falls into a black hole will add to its rotational energy too, though if it goes in backwards it will subtract from it.
So if you’re dumping matter into black holes it pays to drop it in the right direction.
Now neither the rock on a string or the solar panels collecting light off matter dumped into a black hole is terribly efficient as these things go, but they are a lot conceptually easier for some then the other methods I mentioned.
Getting back to the Blandford–Znajek process, which I said was a prime candidate for how Quasars work and another black hole power method, and for our purposes it’s pretty similar to the penrose mechanism but happens to have an equation you can use to determine how much power you get out of the thing.
They aren’t the same thing, and if you want to explore the difference I’ll attach a link in the post description to Serguei Komissarov’s 2008 paper that detailed the differences for
those who are interested.
That equation shows us that the power output of a black hole via this process goes with the square of the magnetic field strength of the accretion disc and the square of the Schwarzchild radius of the black hole, both of which will rise if we increase the size of that accretion disc or if we increase the mass of the black hole, and in nature bigger black holes usually have much larger accretion discs.
Particularly the big ones near the center of galaxies, especially volatile young galaxies, as I mentioned this is usually considered a prime candidate for how quasars are powered and quasars frequently give off a hundred times the power of an entire regular galaxy.
We would presumably want to tap that power a lot slower, using much smaller black holes and matter flow rates.
Now any of the methods that involve extracting rotational energy will eventually cause that black hole to slow and finally stop rotating.
At that point while you can still dump matter in, you won’t get nearly as a good a return, and the black holes mass will increase, making it live longer and give off less power via
Hawking Radiation, which is the only option I’m familiar with that let’s you tap into the rest of that mass energy, as the black hole slowly evaporates.
And we do want that energy.
While lighter artificial black holes can emit useful sources of power via Hawking Radiation, the big massive ones essentially aren’t.
Not unless you can build ridiculously sturdy equipment that can operate without wear or tear needing power or replacement matter to fix over even more ridiculously long periods of time.
But we will have at least a hundred trillion years to get better at building sturdy material, and there aren’t many things around to cause external wear and tear by then, and it is the only game in town after you suck out the rotational energy and all the stars burn out, plus if you can do it there are some big potential advantages to waiting that long to pull out your energy, as we’ll discuss in part three.
But first, let’s hit Part Two and review the Fate and Chronology of the Universe.
Or I should say the primary current theory for a naturally aging and expanding universe.
I mention that for two reasons.
First that theory could be wrong, it probably is at least in part, or incomplete, and second because we don’t live in a universe that’s likely to continue along a natural path, because we live in it.
Intelligent critters can change their environment after all, and generally tend to, and we’ve spent a lot of time on this channel talking about ways to tinker with planets, stars, and whole galaxies so it would seem silly to ignore how that could affect the progression of the Universe.
So first we have the big bang, which doesn’t terribly interest us today, other than it being worth keeping in mind that the Universe began expanding then and continues to do so, and almost certainly has parts that are so far away from us that we will never detect any light from them since new space emerges between them and us faster than light can cover the distance.
This effect will only get worse with time and eventually only the galaxies in our local area close enough to be bound to us by gravity will remain.
As those galaxies get further away, and from all that emerging extra space seem to get further away faster and faster, the light from them red shifts and gets weaker and weaker.
That’s not the only red-shifting light out there though, and there’s one type that is of great interest to us today for our final section.
The Big Bang happened about 14 billion years ago, and just 400,000 years later an event called the last scattering took place.
Not a long time, an eyeblink compared to the age of the Universe, but still a hundred times longer than recorded history and about the duration of human existence.
The last scattering was an important event, and is aptly named.
Up until then the universe was a much smaller and denser place. And small and dense means hot.
Very hot, up until then the universe would have glowed like a star in every single direction you look, a big white haze.
But the light emitted didn’t go far because it was too hot for atoms to form yet and it that pre-atomic plasma soup light scattered much easier.
As the universe cooled down and suddenly atoms could form, and were further apart from expansion, photons could suddenly travel long distance without being likely to run into anything and that kept plummeting.
Most photons will never run into anything now.
As a result there are always photons left over from then still flying through space thus far uninterrupted in their journey.
Now when they started off the spectrum was pretty similar to what stars emit, visible light, but over time as they’ve traveled, with new bits of space emerging along their path red-shifting them, they’ve lost power.
They went through infrared and finally entered the microwave range just recently, this left over radiation that’s in the background of everything throughout the cosmos, is called cosmic microwave background radiation.
As more time passes it will grow weaker and weaker and the universe will keep expanding and cooling.
Eventually it will get so weak and cold that those bigger naturally occurring black holes will finally start giving off more Hawking Radiation then they absorb in background radiation and actually begin to slowly age.
Right now all naturally occurring black holes are actually growing in mass, even if there’s no matter nearby to feed
them.
That time, when things are that cold, is a long, long way off.
Before we get there we have our own sun slowly getting hotter until it eventually renders Earth uninhabitable and goes Red giant, swallowing Earth, then leaves behind a earth-sized dense corpse called a white dwarf, which generates no new energy from fusion but still gives off a lot of light compared to what our planet uses, and ought to still be warm enough to light many earths for even longer than its current remaining lifetime before going red giant.
That’s our first example of a civilization at the end of time, because normally we figure it’s the end of the road when our star goes red giant, at least here on Earth, and sooner than that too because the Sun is heating up and Earth will probably be uninhabitable inside a billion years.
Except it won’t be, because there are intelligent critters on it.
We may come back and explore this idea in more detail in the future but for now I want to use it as our first example of how you can’t look at the timeline for the natural Universe as particularly likely.
Not because the science is wrong but because it doesn’t contemplate the impact of us on that timeline.
We’ve talked a lot about moving planets or shielding them from light to cool them down.
We looked at that in the terraforming post and more recently in the Ecumenopolis post.
So a billion years from now without intelligence Earth might be rendered uninhabitable by a sun growing hotter, but that probably won’t be how it goes down.
We might sterilize our planet ourselves long before that, our track record when it comes to screwing up our planet on accident or blowing up chunks of it is not in my opinion quite as terrible as many naysayers think, but it certainly isn’t anything we’d want to brag about either.
Or we might disassemble it for building material.
In the megastructures series we’ve explored the basic idea that a planet, in terms of living area, is basically as efficient as mountain with a few caves on it is.
You get a lot more space by disassembling that planet to build megastructures, in the same way you would disassembling a mountain and its few cramped caves to use the rock and metal to build skyscrapers.
You could disassemble the average mountain, and it’s cramped few caves able to hold maybe a few hundred people, and build housing for the entire planet.
Similarly you can disassemble a planet and reassemble it as megastructures with thousands or millions of times the living area.
So we might do that and have no planet here in a billion years.
Or we could shade the planet, putting a large thin shade between us and the sun, decreasing the light we got, especially the infrared range that’s pretty useless for plants,
and keeping us from burning up.
Or we could just move the planet outwards.
Moving planets is pretty time consuming as we discussed in the Terraforming post but it is doable, requires no advanced technology, and we do have a billion years.
So in a billion years it would seem very unlikely the world will die, because it either will have long before from us screwing up or using it for building material, or because we valued it a lot and decided to preserve it.
And you can protect against red giant phase of a star and weather it and come back in to live around that white dwarf remnant for many billions of more years.
Of course even thirty billion years from now when that white dwarf is too cold to be of any further use to us, a black dwarf, the Universe will still be quite young and going full tilt.
Our galaxy will still be forming stars at the same rate as now, only a bit faster since we will have merged with the Andromeda galaxy by then and some of our other neighboring galaxies will have either merged in by then or be approaching.
It won’t be for 800 billion years, about 200 times the age of Earth and 60 times the age of the Universe, and 200 million times the duration of recorded human history, before that star formation starts dying off, and it will be an estimated 100 trillion years before it ceases entirely.
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