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A list of my thoughts/assumptions

  • we have a good understanding of how stars live and die, so a "healthy" star (I assume you would only build a Dyson sphere around a star that's going to last a long time) gradually fading to nothing (as the sphere was built) would be strange
  • it would be unlikely that a "dark" celestial body would be blocking it because space is so vast that for something to line up with and stay in line with the star would be vanishingly unlikely
  • as mentioned in the title, I assume we wouldn't see it unless we started paying close attention to all the stars we can see
  • I think you'd probably build the sphere around a relatively small star, so from Earth would we even have the resolution/clarity to see the star disappear and how it did so
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    $\begingroup$ To your first point -- wouldn't building such a sphere take generations, maybe centuries? I'm guessing the only way astronomers would notice a star getting dimmer is by comparing its current magnitude with photographic or other records. And I suspect pictures or data sufficiently accurate has only been available for a hundred years or less $\endgroup$ Commented May 12, 2023 at 22:16

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The general assumption seems to be that randomly looking and seeing a megastructure under construction is unlikely, instead looking at what a finished one might appear as. Some modelling suggest Dyson spheres would be easy to detect due the fact they will be radiating close to 100% of the parent star's energy output, but over a much larger area and lower temperature producing a unique spectra with some attempts to search for these.

A follow up paper suggests that a civilization capable of building a Dyson sphere may be pushing the limits of energy harvesting harder, making the outsides 'darker' in IR and therefore possibly missed in the data currently available.

Related:

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    $\begingroup$ @SamDean The idea is you capture 100% of the energy from your sun as light etc, then use it to do useful work. The result of that work will become heat one way or another, and the logical thing to do with it is radiate it away. In fact energy from sun MUST =energy radiated away, otherwise the whole thing melts. Exception here is if you are doing exotic things like large scale antimatter production to power rockets or similar. $\endgroup$ Commented May 12, 2023 at 13:36
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    $\begingroup$ @SamDean - It's the 'close to' part that's important. We currently use about one gazzilionth of the sun's energy. If we could increase that to 100 gazillionths, we'd live in a free-energy paradise. Now scale that up to a bazillion gazillionths. $\endgroup$
    – Richard
    Commented May 12, 2023 at 22:24
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    $\begingroup$ A Dyson sphere would show up as a pixel that's far too bright for it's spectrum on an IR telescope. Nothing natural could emit that much energy at that low a temperature. You don't need to know mass. $\endgroup$ Commented May 13, 2023 at 4:20
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    $\begingroup$ @SamDean All the radiative energy energy hitting Earth is also emitted, 100%. That's one of the key points of thermodynamics. This is a self-regulating fact: If less was emitted, a body would heat up and increasingly emit more, until emissions equal immissions, at which point -- by necessity -- the heating would stop. A dynamic equilibrium has been reached. Life on Earth is fueled by the steady flow of solar energy though Earth; specifically, energy of low entropy. $\endgroup$ Commented May 13, 2023 at 17:51
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    $\begingroup$ @SamDean To make an analogy: 100% of the water that goes into a waterwheel comes out again on the downstream side. No water is used up. And yet, a waterwheel can turn a mill. $\endgroup$
    – hobbs
    Commented May 14, 2023 at 2:05
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My answer is a partial yes. We can, theoretically, detect a lack of certain spectra from the star, indicating a consumption of energy on a large scale. However, this may not be detectable if the orbital plane of the star is tilted so that we see the "top" or "bottom" of it: this would cause the orbiting swarm or constructed sphere to be unnoticeable. (because building above the poles of a star is kind of bad, due to low orbital speed, or full zero orbital speed.

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    $\begingroup$ Assuming you have the tech to build an actual Dyson Sphere, building out of plane is no harder than building in plane (both require absurd structural strength). And if it is a Dyson swarm any inclinations are possible as long as you plan the orbits correctly. $\endgroup$ Commented May 15, 2023 at 7:59

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