The logic of energy efficiency may, paradoxically, anchor the most advanced civilisations to their solar systems – helping solve the Fermi paradox

A hyper-efficient power-source that you can’t meaningfully accelerate (small black holes) may make space-faring surprisingly difficult for advanced civilisations

“In a galaxy assumed to be filled with clever beings, why don't we see any? This dissonance is known as the Fermi Paradox.”

― The SETI Institute
(SETI: the Search for Extraterrestrial Intelligence)

One of the arguments in the book I’m messily writing in public here, online, is that all technological civilisations will ultimately converge on the manufacture of lots and lots of small black holes for energy production, as they are, by far, the most efficient energy source in our universe. Black holes can, potentially, convert up to 42% of any mass that you chuck into them into energy. Fission can only convert roughly 0.1% of its fuel into energy, while fusion at its most efficient converts roughly 0.7%. And yes, in answer to the question from the smart-arse at the back: matter/antimatter annihilation would be 100% efficient, but there isn't any freely available antimatter in our universe, while making antimatter takes far more energy than you can get back, so black holes still win…

Yes I’m glossing over the technical difficulties of artificially manufacturing a small black hole (the task is far beyond our current technologies), and of safely running one as a power source, and then harvesting gravitational energy as well as electromagnetic energy, all of which are non-trivial problems. You’re not just sitting around the black hole, passively harvesting Hawking radiation: any decent sized black hole emits far too little Hawking radiation, far too slowly. Instead, you need to actively feed it matter as fuel, and then, as that infalling matter accelerates to close to lightspeed around it, forming the hot doughnut of an accretion disc, you harvest the radiated energy from all that disintegrating matter – so you are basically building and feeding a tiny quasar. Tricky! However, it’s all theoretically do-able within the known laws of physics, so given the staggering benefits in terms of energy efficiency, it will probably, eventually, be done.

“If something is permitted by the laws of physics, then the only thing that can prevent it from being technologically possible is not knowing how.”

― David Deutsch, British physicist, in The Beginning of Infinity: Explanations That Transform the World (a book I highly recommend)

This is a NASA artist’s impression of the first-ever micro-quasar to be discovered, SS433. It’s a deeply peculiar binary system, in which a stellar-mass black hole strips matter from its A-type companion star. (An A-type star is a bit bigger and a lot brighter and hotter than our sun – I’m not sure why the artist shows it so orange-y here, as it would be a brilliant blueish-white to our eyes.) This infalling gas heats up dramatically and causes the black hole to blast out relativistic jets in the manner of a full-sized quasar. No I’m not saying some civilization built this micro-quasar: just showing that relatively small and energetic quasars are possible. (Normal quasars, generated by supermassive black holes with the mass of millions or billions of suns, outshine entire galaxies.)

THE WEIRD IMPLICATION

But that may well anchor technological civilisations to their home solar system! A small artificial black hole, with roughly the mass of Mount Everest, can pump out a stunning, hard-to-conceptualise amount of power. Mount Everest has a mass somewhere between ten-to-the-fourteen to ten-to-the-fifteen kilograms (depending on your definition of where a mountain starts), i.e. between about one hundred trillion and one quadrillion kilograms. There’s a lot of asteroids floating around in that size range, so you could quite easily flood your local solar system with many, many sources of essentially unlimited power.

Question: Oooofff, that’s a big engineering project. Why not make them much less massive? Hey, boulder-mass black holes! Why not put one in every vehicle?

Answer: Because extremely low-mass black holes evaporate much too fast, through Hawking radiation. If a black hole’s mass is cut to a thousandth of its original size, its lifetime is a billion times shorter, because the lifetime shrinks with the cube of the mass. Small enough, and they go off like a bomb, in seconds. No, Mount Everest-sized black holes are at the sweet spot – they are manageably small, but large enough not to evaporate – and thus explode – inside the lifetime of any conceivable civilisation.

But then you can't use your most efficient energy source to power spacecraft, because you aren't going to be able to accelerate a black hole with the mass of Mount Everest to anything like the speed you would need to travel between the stars. One quadrillion kilograms (1,000,000,000,000,000 kgs) is an unreal amount of inertia to overcome. (And if you did manage to get it going, imagine the energy required to slow the bastard on arrival.)

So you would have to use far less efficient energy sources to drive any starships. Bussard ramjets, or Ram Augmented Interstellar Rockets, perhaps. How might that work? Well, once a ramjet spaceship can get up to speed, it can use magnetic fields to trap any ionised hydrogen gas in the space the ship is moving through, and funnel it to a point where it can fuse: so it’s a fusion drive that doesn’t have to carry all its fuel. But it’s still only getting 0.7% energy from that fuel, and you still have the problem of getting it up to a fast enough speed for the ramjet to work. And it is now generally thought that the drag generated by the magnetic “scoop” on a pure Bussard ramjet would outweigh the acceleration any fusion was able to cause. This has led to the proposal of hybrid designs, like Ram Augmented Interstellar Rockets, and Laser Powered Interstellar Ramjets… (Worth clicking on the link to that 1977 paper by D. P. Whitmire and Albert Jackson, for a nostalgic wallow in the crude, grainy typeset of the scanned PDF, and to remind yourself that people dreamed big back then. Let’s recover that energy, and dream bigger!)

Anyway, yes, you probably could do this, but civilisations do not like going technologically backwards. Using a draggy old ramjet, in an age of hyper-efficient and technologically advanced black hole energy production, would be as though, in a world of supercomputers and AI and unlimited fusion energy, you had to cross the Atlantic by steamboat.

Bussard ramjets already look weirdly old-fashioned, and we haven’t even built one yet. This is a NASA artist’s conception of one. The many-kilometers-wide electromagnetic scoop is invisible

We don’t have good intuitions for how big the true gap is between our current most energy-efficient production method here on earth (nuclear fission), and black holes as a power source: it’s not just that black holes can give you roughly 400 times more energy per gram of fuel, but that the fuel for fission reactors is extremely rare stuff like uranium-235 (just one atom in every fifty million on Earth is made of uranium-235). Fusion is a bit better: you squeeze out seven times more energy from your fuel than fission does, but again, the most easily used fuel for fusion is deuterium, and that is not very abundant either. (Maybe 0.015% of the hydrogen in Earth’s oceans is deuterium.)

EAT DIRT

Whereas the fuel for black holes could be literally dirt. You could throw anything at all into a small black hole, and have a huge percentage of those juicy, energy-packed protons and neutrons turned into energy. You turn your whole solar system into potential hyper-energetic fuel. (Landfill is no longer a problem! Yes, I joked about this already in my satirical short story, The iHole. No, the science in the iHole is not accurate, nor meant to be, it’s comedy.)

So, once you’ve moved your main energy source over to technologically-produced small black holes, many earlier, less efficient technologies (like fission and fusion and, oh yes, burning stuff you’ve dug out of the ground) will tend to wither away.

But now you have a main power source that cannot be used to drive spacecraft, because the intrinsically high mass of any black hole, relative to its power output, makes it grotesquely hard to accelerate. (You can accelerate it a little, because the power output is so astonishingly high – but not to anything close to lightspeed.)

That doesn’t rule out interstellar travel, but it does makes it much, much harder. I think we have a naive intuition that, as power sources improve, starship travel will automatically get faster and easier. That breaks down with black holes.

And it matters because stars are very, very, very, very, very far away. Light itself, travelling as fast as anything at all ever can in this universe, takes a second to get from Earth to the moon, eight minutes to get from there to the sun… and over four years to get to the next nearest star (which isn’t even a good one). Humans have only ever travelled one light second from Earth, and it took much of the resources of the richest country on earth to do that. And we couldn’t sustain it. For the past fifty years, no human being has made it out of Earth orbit. Four lightyears… is a stretch. Spaceflight is hard.

YOU CAN SPEED UP… BUT NOT SLOW DOWN

That’s not to say interstellar travel isn’t helped at all by small black holes.

One possibility is that you could use the ludicrous amounts of energy they produce to laboriously manufacture enough antimatter to power a matter/antimatter drive. That solves the accelerating-Mount-Everest problem (you only have to accelerate the fuel – antimatter – not the immense mass that is used to make the fuel – the black hole), but it’s an eye-wateringly expensive way to make small amounts of lightweight fuel. We actually know how to make antimatter – we make it in particle accelerators, a few atoms at a time. But currently it would take 25 million billion kilowatt-hours of energy to make one gram of antimatter. (Oh, and the current cost of doing that would be one hundred trillion dollars.) That’s for one gram of fuel.

Alternative possibility: you could have your efficient power source (=small black hole made out of an asteroid) in your native solar system, and use it to generate, say, an extremely powerful laser to drive a light sail, to accelerate your spacecraft towards a nearby star. You don’t have to carry fuel at all! Huge weight saving. But now you've got a horrendous deceleration problem at the other end, if you’re not going to just whizz past the star at close to the speed of light. How do you bring your lightsail-ship back DOWN from near-lightspeed? You would need another black-hole-powered laser already in place at the other star, to slow it. (Yes, Laser Powered Interstellar Ramjets might solve this by USING the otherwise unwelcome drag of their ramjets to decelerate. But it’s still a big problem.)

So you could, very slowly, using old-fashioned methods like ramjets (or a staggeringly expensive antimatter drive) reach other nearby stars, build out your civilisation around that new star, build small technologically-manufactured black holes, and set up a decelerator laser at the other side… But this still puts quite a brake on expansion rates. And it just makes the whole business far more of a slog, and less attractive.

Anyway, at first glance, this looks like an interesting local maximum that a civilisation could get trapped at. Not that it would be impossible to become a star-faring civilisation, but that there would be significant pressures against it. And significant pressure to explore and exploit (and beautify, and delight in!) your own solar system very fully before leaving it.

And of course that makes it a possible contribution to answering the Fermi paradox. (That is, if there are lots of aliens out there, then where are they, and why aren't they popping by for a cup of tea and some biscuits?)

Obviously, there are many, many more factors involved in the Fermi paradox. This is just one. But I haven’t seen this one stated before, so I thought I’d throw it out there. I’m not putting it forward as an answer, but as an intriguing new contribution to a complex ongoing conversation.

Anyone got any thoughts on this? What am I missing? Please do throw your ideas, and any feedback both positive and negative, into the comments.

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