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This question is scoped to natural philosophy.

The study of natural philosophy seeks to explore the cosmos by any means necessary to understand the universe

https://en.m.wikipedia.org/wiki/Natural_philosophy

Civilization types:

  • A Type I civilization is able to access all the energy available on its planet and store it for consumption. Hypothetically, it should also be able to control natural events such as earthquakes, volcanic eruptions, etc.
  • A Type II civilization can directly consume a star's energy, most likely through the use of a Dyson sphere.
  • A Type III civilization is able to capture all the energy emitted by its galaxy, and every object within it, such as every star, black hole, etc.
  • A Type IV civilization that can harness the energy of an entire universe, mastering the power of multiple galaxies
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    @Dmytro Brazhnyk We have not even registered a Typ I galaxy. Why do you exclude Typ I and Typ II from your question?
    – Jo Wehler
    Feb 27 at 15:53
  • @JoWehler If Type III, IV would exist in out galaxy - they would knocked our door already, and might not for good. Type I, II is harder to spot than Type III or Type IV. Feb 27 at 20:42
  • @JD This question related to Natural philosophy. Astronomy or biology will not be able to answer this question. The study of natural philosophy seeks to explore the cosmos by any means necessary to understand the universe. Feb 28 at 8:07

3 Answers 3

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Options:

  1. There aren't any close enough, or at all, so a rare-abiogenesis or great-filter scenario.
  2. Dark-forest scenario of highly advanced species hiding and eliminating emergent advanced species.

It's worth noting, we aren't as good at looking as people think, with SETI observations concentrated in the hydrogen-gap only, poorly funder, and that it's seriously plausible for there to be no other advanced life in our galaxy.

There is one other option that's interesting. There's an idea that the movements within neutron stars as they cool could be used for computation. That could mean the virtual-spaces within them exceed the complexity of the universe outside them. I haven't seen concrete figures estimating this though.

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  • Black hole hypercomputers might be last refuges for sentient life also. And it strikes me a complete Dyson swarm would be naturally “dark”; and in general barring large-scale astroengineering or (yes, presumably unwise!) omnidirectional signal propagation I’m not sure we have a particularly clear idea of what we would even be looking for as a technosignature. It’s nice to dream but yes — I would suppose the likelihood of species going for substrate transition over galactic expansion is rather high
    – Joseph Weissman
    Feb 27 at 15:30
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    @JosephWeissman: That's a misconception. In total, over time, a Dyson Sphere must emit as much radiation as it absorbs, to stay at a stable temperature. But, it would do it shifted towards the infrared, & from an expanded surface. Likely quite a distinct signiture. It's possible it might do it in a beam, reducing the chance of seeing it.
    – CriglCragl
    Feb 27 at 20:35
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    Fascinating, thanks for explaining!
    – Joseph Weissman
    Feb 27 at 20:59
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Perhaps for one or more of the following reasons:

  1. There are no such civilisations.
  2. They exist but are too far away.
  3. They deliberately cloak their presence to escape our notice.
  4. They only consume galaxies when we are not looking in their direction.

I favour reason 1), since I think the idea of galactic civilisations is one dreamed up by people who have watched too much Star Trek.

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  • Our estimates are that abiogenesis is reasonably like on an Earth-like planet in the habitable-zone, bolstered by how quickly life seems to have emerged after the Earth's crust cooled. It has been assumed that complexity then increases by default. A species that can make self-replicating machines, which we are on the brink of, can colonise galaxies at below lightspeed. Lack of any life is the more implausible outcome, & as Bostrom suggests might indicate a simulation
    – CriglCragl
    Feb 27 at 15:34
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Most likely we don't see them because there aren't any. The civilization scale makes some assumptions which probably don't hold in reality.

The only observable example of somewhat intelligent life that we know of is the human population on this planet earth, which as a whole doesn't match even the most basic premise of a civilization characterized by coordinated goals and actions.

Type I civilizations are somewhat conceivable under a very optimistic worldview. Type II and up are probably about as realistic as faster than light travel - according to our best scientific knowledge it's impossible. Everything else is science fiction material.

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  • A halo style belt-planet made from the asteroid belts by Von Neumann replicators is seriously plausible, & a simpler precursor of a Dyson Sphere.
    – CriglCragl
    Feb 27 at 15:27
  • I'm not talking about technology :-) Feb 27 at 16:03
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    @CriglCragl It really isn't. Go look up the various discussions that sprang up around the tensile strength of Ringworld floor material. Dyson structures are inherently unstable and inefficient to build. You'd have more luck with a Dyson swarm... although those have problems of their own.
    – Corey
    Feb 27 at 22:52

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