Sometimes people, even scientists, speculate about the eventual existence of "super-civilisations" - somewhere in outer space of course.
They should be able to control energies and forces, extremely far above what humans do today ("beyon imagination", it is sometimes paradoxically said, though it is exactly the opposite, at least at the moment: pure imagination).
So, beside the(very big) question about probability:should the consequences not be observable - indeed observed - already? Should not the absence of any such observations make, "would-be believers" think second?
Or did I miss something. Has any serious astronomer or other relevant scientist made any claim about any evidence at all?

Good question. I thought, but could be wrong, that someone has suggested that there is very little evidence for large scale engineering such as "Dyson spheres". I'm curious to know what, if anything, has been ruled out in the galaxy, and beyond.

Maybe it is staring us right in the face,
and we just don't recognize it for what it is.

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It's possible. I'll run by a few possibilities with the understanding that they are pure speculation and in some cases may not be physically possible.

If, as science seems to indicate, FTL is impossible and travelling between stars is expensive in energy, civilizations may tend to move slowly between stars, but develop massive populations in a limited number of star systems. Under these conditions, it wouldn't make sense to look around for the few planets you could terraform (or x-form). Space based civilization would be the norm.

By the way, keep in mind that cubic growth can never keep up with exponential population growth, so whether it starts on the originating planet or later, a civilization will have to control population growth one way or another. This takes away one of the arguments for continued galactic expansion.

Class M stars are very long lived and very common. They are also very hard to detect even nearby because they are relatively dim. Being common, they might make an excellent location to develop a space based civilization. Note that a dyson shell (an actual solid shell) is unlikely because it would be structurally unstable unless it used some very interesting design techniques.

So perhaps we can't see megacivizations because they tend to spread slowly between systems and are around hard to see (but common) stars.

But why build around a star? Purpose built reactors could make more efficient use of the material, so perhaps megacivilizations take stars apart and build very large (but very dim) megastructures.

And then, it may just make more sense to live slower, living in cometary bodies moving from Oort cloud to Oort cloud, spread throughout the galaxy.

And we can get a bit more speculative still: Most of the mass in the galaxy is dark matter. We don't know if dark matter life is impossible.

And here's another science fictional possibility: We could even find out that FTL is impossible but moving to other branes/universes/whatever is not. Perhaps we don't see ET because there are better places to go.

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Another purely hypothetical possibility:

Someone has to be the first to build something like that. Maybe it'll be us.

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Distances in space for even large objects, natural or otherwise are very very vast, even within one galaxy. If anything like a "Dyson sphere" or some large artificial structure existed just a hundred light years away, (which is a short distance within the Milky Way,) there is nothing in physics that absolutely demands that we would be able to detect it. If we did detect something, we may not recognize it as artificial.

Also, our concepts and expectations of what hypothetical "civilizations" might build or do could possibly be very limited within our own Sci-Fi stereotypes compared to what's really out there, if they exist.

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Does physics as such "demand" that humans observeansything?
The point here is not to "prove" there can be no such things. Least of all to "prove" there is nothing out there above our imagination (but if there is such phenomena in the litteral sense, it´s hard to see how we can discus them fom pure logical reasons).
It is those who make such hypotheses that should show something supporting their claims(if not "burden of proof" at least something from the real world, supporting their claims).

I suppose the standard "sufficiently advanced technology is indistinguishable from magic" applies, but then wouldn't we notice large scale magic?

Not necessarily. That's my point in an earlier post. If ET built huge structures or systems in deep space, something as big as our solar system, it isn't automatically a given that we can detect it, just because ET built it.

It also isn't automatically a given that we can detect it as artificial. (i.e. If it looks just like an oddly dense gas giant transiting a distant star.)

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By assuming that a lot of the dark matter consists of Dyson sphere's thus cloaking those stars, we have suggested that dark matter could contain a lot of technically competent critters as well as a way for a lot of civilizations to go unobserved. My guess is that those capble of building Dyson spheres would continue to do so and the commotion they would cause doing so would be difficult to ignore.

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For those inclined to oppose human meddling with the structure of the universe or the composition and configuration of objects and groups of objects within the universe, consider:
Whether there is a limit to the magnitude of a modulation of chaos below which order remains invariant? Or, is order but a fiction invented by perspectives applied over finite, however large, time intervals?

Actually, a Dyson Sphere would look more like some kind of Red Giant, than a piece of dark matter, you still have to radiate the entier energy production of the inner sun from the outer skin, with the large surface you only need a relatively low temperature compared the inner sun, but is still has to be radiated.

Are there any objects observed that produce about the same amount of energy as the Sun, but radiates it at a lower temperature from a larger surface?
Preferably with oddities in the spectra.
And at what distance could we expect to be able to observe them?

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Most of the stuff we can't see is nonbaryonic. That is, not conventional matter. If dark matter life were possible, it could be pretty much anywhere and we would be hard pressed to detect it. There still is a lot of conventional "dim" matter out there, much more than what we can see. We can't see most of the matter in the galaxy. There are, of course, good conventional explanations for dim matter.

As for what you would see with megastructures: As I mentioned, class M stars are hard to see even nearby. Something that would increase the radiating surface area and reduce the observed temperature would make it even harder to see. Larger stars would have to be a bit more distant, but it is a large galaxy.

Megastructures that made truly efficient use of energy would want to radiate at as low a temperature as possible and could be very difficult to see. Construction would have to be very rapid and energy inefficient to be obvious.

I don't assume there is any such thing, and I think the "Dyson sphere" is a technologically primitive concept. If we wanted to look for megastructures, we really don't have a good idea of what to look for - just guesses. This is another area where speculation is fun, but doesn't get you very far.

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Disclaimer: Avatar is not an official NASA image and does not imply any specific interplanetary or interstellar capability.

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What’s the matter with ET that he doesn’t contact us?

He must know we are here. We’ve been sending out methane signals for four billion years, oxygen signals for two billion years, agricultural and metal working signals for ten thousand years, coal burning for a thousand, and industrial chemicals for two hundred.

Even we have signaling plans, both radio and optical.

If ET can build a Dyson sphere, assuredly he could illuminate our Moon with appropriate images.

Bob

That speculative subject has been covered in painful detail in the "How many intelligent civilizations in the Milky Way?" topic.

Short version: We don't know if they exist. If they exist we don't know how far away they would be, and we don't know what they would do. You can come up with a thousand arguments for any position you want to take.

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I say there is an invisible elf in my backyard. How do you prove that I am wrong?

Disclaimer: Avatar is not an official NASA image and does not imply any specific interplanetary or interstellar capability.

The Leif Ericson Cruiser

Well, if there were these super civs... It's VERY possible that we just haven't discovered what they left behind, I mean... look at Central and South America... they find ruins all the time in the jungles that have been long-forgotten.

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Why would a super civilisation produce observable results?

If they are very advanced presumably their machines, though high energy, are very efficient and thus won't be pumping huge amounts of energy into space for us to observe.

Think of a Dyson Sphere - a very advanced piece of technology. That would very hard to observe, and would I think look like a dim star if you could see it.

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"I worry that, especially as the Millennium edges nearer, pseudo-science and superstition will seem year by year more tempting, the siren song of unreason more sonorous and attractive." - Carl Sagan, 1995

What's that "indistinguishable from magic" saying?

We're not yet even a Type I civilization. What's makes us think we'd be able to recognize say a Type III?

B. Haisch, an astronomer, appears to consider the ETH seriously:

UFO Skeptic.

The two most interesting articles, IMO, are the "Inflation Theory..." from last year's JBIS, and the "Speed of Light Limit Argument."


Edit to add: I just realized the "magic" remark was brought up prior to my post... oh well.

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"Where the telescope ends, the microscope begins. Which of the two has the greater view?" - Hugo

"Men occasionally stumble over the truth, but most pick themselves up and hurry off as if nothing had happened." - Churchill

Michio Kaku seems to be of a similar opinion.

Snip:

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I have nothing useful to say about your intriguing question, except to add that I think it's a version of Fermi's paradox. Here are a few earlier threads where this topic was discussed:

Where is everybody?
How is the Fermi paradox a paradox?
Fermi was Jedi. Sagan was Sith.
SETI institute on Fermi's paradox
Possible explanations for Fermi's paradox?
Are We Alone?
The Only Ones?

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