Hi Robi,

For reference, Anders Sandberg discussed that on The 80,000 Hours Podcast (emphasis mine):

[Anders:] The best method [to extract energy after the era of starts], in my opinion, is to use black holes. I’m very fond of black hole power. And I am assuming that maybe in a few trillion years I’m going to be dealing with protesters saying, “No black holes in our neighbourhood,” and “Don’t build that power plant, Anders.” But they’re actually lovely. Black holes have accretion disks when they suck in matter. Or rather, it’s not that they suck in matter — that’s kind of a picture we get from science fiction — they’re just an object with gravity like anything else. But what happens when you put a lot of junk around a black hole? They form a disk, and the friction between parts of the disk heats up the matter. That means it radiates away energy and gets more tightly bound and slowly spirals in. There is also some angular momentum leaking out at the sides where some dust gets thrown off.

The effect of this is that the potential energy of that junk — and it can be anything: burnt-out stars, old cars, old space probes, planets you don’t care for, et cetera — gets ground down, and the potential energy gets released as radiation. So now you can build a Dyson sphere, a very big one, around this whole system, and get all of that energy.

How much total mass energy can you get? It turns out it’s almost up to 40% for a rapidly spinning black hole. The exact limit depends on where the inner edge of the accretion disk is, because eventually you get close enough that you essentially fall straight in without releasing any more energy, and that gets trapped inside the black hole. Now, converting 40% of the mass energy of old cars and space probes into energy is kind of astonishing: that is way more effective than fusion. So actually, the stars might not be the biggest energy source around. We might actually be able to make the galaxies shine much more if we dump things into black holes and gather that energy.

William, I am guessing you would like Anders' episodes! You can find them searching for "Anders Sandberg" here.

Hi Mo, thanks for the feedback.

1. Good thought, I've cross-posted it to my account there.
2. This post was spurred by a conversation I had about the upper limit of AI intelligence and the fact that it was likely very far above all humans combined. This is meant as, like you said, a pretty unobjectionable support for my then assumed conclusion. The conversion was heavily influenced by Cotra's Bioanchors report.
3. I was estimating the brains computation ability very roughly. I guessed that there would be more detailed estimations already done, but would take time to read through and understand their premises. I'll read through the document when I have some time.
4. These two look interesting to read.
5. Anders Sandberg is an interesting person. I speculated someone had done calculations similar to mine, I'm not surprised that he is one of such people.