Let's say, (in the far future), that humans have become really advanced beings and are at the verge of becoming a Type III civilisation. Now, they need a gargantuan energy source for various purposes, for e.g. making megastructures, weapons and for powering a energy-hungry civilisation. So most of the humans come up with a plan- Turn the Milky Way into a quasar.

The idea here is really simple. Black holes are really sloppy eaters. Only a fraction of the mass that comes near them is absorbed, while the rest is blasted away at nearly the speed of light, in the form of jets, radiation or light.

So, the civilisation here, comes up with this plan:

Various planets and stars are nudged into the proximity of Sagittarius A*, the Milky Way's supermassive black hole. These planets and stars are broken up by the powerful tidal forces, and form a dense accretion disk around the black hole. The black hole then blasts this matter out into space, creating powerful jets.

Now here comes the cheat-code (or trick). Paper-thin mirrors are installed near the black hole, at a sufficient distance from the event horizon. These mirrors reflect the jets and radiation of the black hole, and aim it towards a set of orbiting solar panels (black hole panels?). These panels absorb the radiation and the jets' energy, and the energy is used either directly, or stored in giant batteries, which I shall discuss in a future question. The mirrors themselves are kept in place by a delicate balance of radiation pressure from the jets, and the powerful gravity of the SMBH.

The mass being consumed in question is about 5-10 solar masses per month.

Diagram of the setup below:enter image description here The question here is:

How much energy would a civilisation get from a quasar Milky Way per day, in terms of wattage?

(Note: This question does not ask for the energy harvested from Hawking Radiation or anything similar)

  • $\begingroup$ Are you concerned about the intense radiation completely destroying the mirrors? In any event mirrors aren't effective for matter, only light. And are you assuming everything is 100% efficient? $\endgroup$
    – Rafael
    Oct 23, 2022 at 14:28
  • $\begingroup$ I don't see how to answer this. How much stuff comes out depends on how much stuff you throw in, and how quickly. Are you asking how energy efficient the process is? $\endgroup$
    – Daron
    Oct 23, 2022 at 14:33
  • $\begingroup$ No, actually how much energy this process releases $\endgroup$ Oct 23, 2022 at 14:33
  • $\begingroup$ Dude I will still answer the question when you clarify. But I am sure that I will say something like $4 \times 10^{26}$ Watts and it will mean nothing to you. How much energy? Enough. $\endgroup$
    – Daron
    Oct 23, 2022 at 14:34
  • $\begingroup$ @FuriousNukefrostArcturus Then it depends on how much stuff you chuck in. At the moment it is like saying I have a wind turbine, how much electricity does it generate? That depends on the wind. $\endgroup$
    – Daron
    Oct 23, 2022 at 14:35

1 Answer 1


100,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 Joules per Month

This paper claims the best conversion rate of matter to energy from the accretion disk is about 6%. At least for a nonrotating black-hole. I think Sag-A rotates only slowly.

enter image description here

I think the original source of that number is this paper

enter image description here

I have not read the second paper before, but it has a lot of recent citations. So might still be respected by modern physicists. The fact that it is not cited in the first suggests this figure is folklore to accretion disc people and it will be difficult conclude it is the original source.

In any case 5.7% falls out of the decrease in potential energy from a particle very far away moving into the Schwarzschild radius. For some black-holey reason the same energy must be sprayed back out in some form.

Energy is converted to matter by the law $E=mc^2$. One kilo of matter gives you $9 \times 10^{16}$ Joules of energy. A solar mass is about $2 \times 10^{30}$ kilos which is $18 \times 10^{46}$ Joules. Ten of those is $180 \times 10^{46}$ Joules. Since the efficiency rate is 6% we are left with about $11 \times 10^{46} \simeq 10^{47}$ Joules.

Is that a lot?

There is also something called the Eddington limit which controls how much mass you can possibly feed into a black hole before the stuff shooting out stops more stuff getting in. I will assume 10 Suns per month is below the EL for Sag-A since it is so big.

  • 1
    $\begingroup$ Maybe use scientific notation in the title? $\endgroup$
    – BillOnne
    Oct 23, 2022 at 16:01
  • 2
    $\begingroup$ @BillOnne I am trying to make a point that it is a stupidly big and useless number that and I am stupid and handsome for taking the time to calculate it. $\endgroup$
    – Daron
    Oct 23, 2022 at 16:03
  • $\begingroup$ It would be better to quote the text directly, for those users who cannot read images. $\endgroup$
    – L.Dutch
    Oct 23, 2022 at 16:32

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