The strongest reason that aliens might be invisible to us is that they are deliberately hiding.
Couldn’t they just be really, really far away; or have a structure and value system such that their bustling activity looks like natural phenomena; or be trying like hell to get our attention but finding us kind of hard to talk to?
or have a structure and value system such that their bustling activity looks like natural phenomena
No; to see why you need to think about physics, engineering, thermodynamics and information processing. They’d want to capture low entropy sources of energy, and radiate away all that energy as high-entropy infra red radiation. This would be noticeable. Also, it would be an amazing coincidence if alien engineering just happened to look exactly the same as pristine nature.
Or if they entered a region of dust or gas that had a temperature differential to the sphere.
If fact, I just realized that if any such structures like Ringworlds existed, they would be trivially easy to spot if they were at the right aspect to us. It would look like a star with either a band across it, or like the star had a single ring. The ring on the far side of the star would also be tremendously bright (if angled to our solar system slightly) and much easier to spot with a telescope that blocked off the light of the central star.
Stars are too bright and too far away and the ringworld too thin (at least I think there would be stability problems if they were built too wide, but I don’t know enough). Worse, I thought the way we were trying to detect planets these days was by looking at the gravitational effect the planet has on the star. But wouldn’t a ringworld balance out its own gravitation effect on the star?
you need to think about physics, engineering, thermodynamics and information processing
Like this guy did. Short short version: to get the most out of your star, you drain the energy of the emitted photons until they are (almost) at the temperature of the CMB. Infra-red is still harvestable.
Getting rid of all the stars in one part of space and replacing them with near CMB doesn’t seem to produce what we see: it would produce a raised CMB signal in one direction (would it be strong enough to pick out? How close, exactly, to CMB would it be?), a reduced star density in one region, and presumably a bunch of half-eaten galaxies on the boundary of the region.
The point that “it would be an amazing coincidence if alien engineering just happened to look exactly the same as pristine nature” seems reasonable convincing to me.
I think the idea is not all the stars in a region—it’s one star per civilization. The basic idea is to maximally exploit energy transfer between the star and interstellar space, so optimal efficiency makes the artifact appear from the outside to be as close to interstellar space as possible.
Couldn’t they just be really, really far away; or have a structure and value system such that their bustling activity looks like natural phenomena; or be trying like hell to get our attention but finding us kind of hard to talk to?
Yes, they could lie beyond our cosmic horizon.
No; to see why you need to think about physics, engineering, thermodynamics and information processing. They’d want to capture low entropy sources of energy, and radiate away all that energy as high-entropy infra red radiation. This would be noticeable. Also, it would be an amazing coincidence if alien engineering just happened to look exactly the same as pristine nature.
Could we detect Dyson spheres if they were out there?
Might we be able to see them if they went in front of galaxies or other highly luminous body?
Come to think of it, wouldn’t they look a lot like black holes?
Not really. The black hole might have hawking radiation it should also gravitationally lens the surroundings more than a dyson sphere.
It might also have an accretion disc.
Or if they entered a region of dust or gas that had a temperature differential to the sphere.
If fact, I just realized that if any such structures like Ringworlds existed, they would be trivially easy to spot if they were at the right aspect to us. It would look like a star with either a band across it, or like the star had a single ring. The ring on the far side of the star would also be tremendously bright (if angled to our solar system slightly) and much easier to spot with a telescope that blocked off the light of the central star.
Just an OT thought.
Stars are too bright and too far away and the ringworld too thin (at least I think there would be stability problems if they were built too wide, but I don’t know enough). Worse, I thought the way we were trying to detect planets these days was by looking at the gravitational effect the planet has on the star. But wouldn’t a ringworld balance out its own gravitation effect on the star?
The Kepler Observatory also detects planets by masking the light from the star and then looking for any bright spots (Coronal Masking).
Even more advanced telescopes will use this technique in combination with the gravity wobble. There are also diffraction and coronal masking.
http://www.iop.org/EJ/article/0004-637X/662/1/738/65461.web.pdf
http://spie.org/x24241.xml?ArticleID=x24241
why did I get down voted on that post?
Like this guy did. Short short version: to get the most out of your star, you drain the energy of the emitted photons until they are (almost) at the temperature of the CMB. Infra-red is still harvestable.
Getting rid of all the stars in one part of space and replacing them with near CMB doesn’t seem to produce what we see: it would produce a raised CMB signal in one direction (would it be strong enough to pick out? How close, exactly, to CMB would it be?), a reduced star density in one region, and presumably a bunch of half-eaten galaxies on the boundary of the region.
The point that “it would be an amazing coincidence if alien engineering just happened to look exactly the same as pristine nature” seems reasonable convincing to me.
I think the idea is not all the stars in a region—it’s one star per civilization. The basic idea is to maximally exploit energy transfer between the star and interstellar space, so optimal efficiency makes the artifact appear from the outside to be as close to interstellar space as possible.