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My understanding is that a completed Dyson Sphere would be detectable because we would see a star that seemed to produce only infra-red light, and much less than would be expected from an object of that size.

But this doesn't really work for a Dyson Swarm. We would still see the full spectrum in the gaps between the billions of satellites. Further, it would be challenging to parse out any signal in the occlusions each satellite would make because there would be so many; they would blend in with the star's natural variability. Or perhaps the satellites aren't even satellites, but rather statites and the occlusions never change.

Am I wrong? Or is there something else we would look for to determine if someone built a Dyson Swarm around a particular star?

Ryan_L
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If there were a gargantuan amount of large satellites surrounding a star that were packed densely enough, we would probably notice subtle yet abundant fluctuations in brightness. However, with such an observation, consensus will likely pass it off as a large asteroid or dust belt rather than any exotic or artificial phenomena.

In fact, we have found asteroid belts around other stars, Zeta Leporis being one example. The first sign was that the star emits significantly more infrared light that what is considered typical - similar to the convention that you suggested regarding Dyson spheres, though not entirely blocked out. However, if we theoretically observe a strict pattern in fluctuations, or directly measure its shape and notice peculiar uniformity, for example, scientists may raise an eyebrow, and the idea of a Dyson swarm could eventually be brought up.

So, using conventional methods, you are correct that the detection of a Dyson swarm would be difficult. Following such an observation, the most likely conclusion, at least initially, would be attributed to a natural phenomena more than anything else.

4NT4R3S
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    If the swarm were laid out in a pattern, we'd probably be able to notice that the flickering is particularly regular, though a regular pattern is not particularly required for a dyson swarm. – Darth Pseudonym Mar 11 '24 at 21:09
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    Thank you for the note; I only suggested regular fluctuations as a potential sign pointing towards an artificial object, not the only convention to imply such, as it depends how the swarm is composed. I've edited my response to better clarify that it is just one possibility. – 4NT4R3S Mar 11 '24 at 21:44
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A Dyson swarm spectrum would have three components: the starlight escaping through gaps, the heat emissions from the swarm, and reflected light from the interior.

The star spectrum would be normal. The heat emissions are plausibly a blackbody spectrum with a temperature set by the sphere design (you might run it hot or cold depending on what is being optimized for, and there might even be subsystems at different temperatures like habitats and computers generating their own waste heat spectra).

The reflected light is most interesting: it would be dependent on the starlight and what kind of internal surface the swarm has. If it is photovoltaics there will be absorption lines for them, if it is reflectors heating thermoelectric systems a different set of lines. It would also have polarization. The ratio of this light to the starlight is dependent on the radius of the sphere relative to the stellar radius (and how reflective the interior is).

So I would expect there to be a kind of three component spectrum, dominated by the waste heat but with a part normal starlight and a part reflected light that might be separated by polarization. There are a bunch of degrees of freedom in the design one can parametrize by (radius, working temperature, material, gap size etc.)

Anders Sandberg
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