Okay, so the title is a bit off. They’re hunting for partial Dyson spheres using infrared and optical.

I was confused on how they would detect something completely blocking a sun from millions of light-years away.

Even a Dyson sphere, which is technically unlikely anyway, would be possible to spot. You would look for something very bright in the infrared spectrum with almost no light in the visible spectrum. It would also be larger than a normal star of the same energy, but that would be hard to tell given all the other issues.

A partial swarm is easier because it will have variability towards more infrared and then back to a more normal spectrum.

And, of course, all this is speculation until we find a candidate and determine it doesn’t have a natural source for that behavior.

Why would there necessarily be strong infrared emissions? Since a Dyson Sphere is meant to harvest all energy produced by a star, any leakage would be unnecessary inefficiency, wouldn’t it?

Thermodynamics says that energy can’t be destroyed (mass-energy, but generally that won’t matter). So after the work of running your stellar civilization is done, you will radiate out waste heat. There is no real way around this without breaking thermodynamics or having a handy black hole to dump all your waste heat into. Therefore, the energy of the star will still be released, but it will be released as infrared.

If you’re using the Dyson sphere purely as a power plant and e.g. charge batteries, the thermal radiation will be distributed over the whole area covered by the civilization.

A solar panel, or any other power generator we use, doesn’t radiate away all the generated energy either. It’s radiated from the point of use.

So you heat habitats, which radiate heat. And run computers, which radiate heat. And move objects around, which radiates heat (among other things). And if you merely absorb energy from your star…it radiates as heat. This is the whole idea of entropy. Unless your lasers are particularly efficient and you use them to beam the energy elsewhere, your Dyson swarm is going to radiate heat equivalent to the energy your star puts out.

You’re ignoring my example - what if you charge up batteries at the Dyson sphere, and use the energy anywhere else? There’s no physical reason the energy must be used around the Dyson sphere.

So all you need is a perfect charging system. We don’t have those, and physics doesn’t allow for them. This would be no different than the laser example I gave, and this only makes sense after you have a second Dyson swarm.

Why perfect? As long as the efficiency is high enough, you wouldn’t see the sphere itself as very bright, it would be quite dim. Do we know any hard, physical limitations for this, like we do for speed?

I don’t think you have any appreciation for just how much energy even a dim star provides. A Kardashev 2 civilization has access to a billion times the energy we (Earth) have, and we only use about 70% of the energy we have access to. Even if you use all that energy, there will still be waste heat. Now you’re proposing that this hypothetical civilization has a second star (at least) that it’s importing energy from, which means it will be a larger area emitting infrared in their home system, because thermodynamics still has to be obeyed.

And yes, the laws of thermodynamics have to be obeyed. They are as rigid as the speed of light, meaning there might be shortcuts but they are very advanced. To put it in perspective, we are almost capable of starting a Dyson swarm, and we have no options for bypassing the laws of thermodynamics and only have the barest ideas of how to bypass the speed of light.

I don’t think you have any appreciation for just how much energy even a dim star provides. A Kardashev 2 civilization has access to a billion times the energy we (Earth) have, and we only use about 70% of the energy we have access to.

We also have no idea what such large amounts of energy could be practically used for. Just as one possible example, the recent approach for warp drives would consume large amounts of energy - and it would cause the energy to be used over a large area, going against your assumptions. Of course there are many other options, e.g. creating matter from energy.

Even if you use all that energy, there will still be waste heat.

Yes, and as I keep repeating, the waste heat would not necessarily be produced at the location of the Dyson sphere.

Now you’re proposing that this hypothetical civilization has a second star (at least) that it’s importing energy from, which means it will be a larger area emitting infrared in their home system, because thermodynamics still has to be obeyed.

First: why must there necessarily be a second star? They could live inside ships in-between solar systems, which would only need one star to import energy from, and no more. And my whole point is that this would make the Dyson sphere itself much dimmer than you’re assuming it to necessarily be.

And yes, the laws of thermodynamics have to be obeyed. They are as rigid as the speed of light, meaning there might be shortcuts but they are very advanced. To put it in perspective, we are almost capable of starting a Dyson swarm, and we have no options for bypassing the laws of thermodynamics and only have the barest ideas of how to bypass the speed of light

You haven’t shown that the laws of thermodynamics actually pose limits here. Nothing I’m proposing goes against the laws of thermodynamics.

Sorry, all I’m seeing are reasons how you could take all the energy from a given star and move it elsewhere without a reason to do so, even to the point where virtually none of that energy is being used locally. This is the classic solution looking for a problem idea.

There are plenty of resources on the internet that have already responded to all your questions. Feel free to look it up.

Yes, I was only focusing on the “physically possible” part. I don’t think it makes sense for us to inherently limit our search for such things to the most obvious solution - focus on that first, sure, but don’t rule out that non-physically based assumptions are wrong. We can’t assume that a civilization capable of producing a Dyson sphere would exactly follow what we assume to make the most sense.

But I can gladly provide some possible reasons:

• A dark forest scenario would mean that you’d want to hide your energy harvesting as much as possible, while also not living close to megastructures that could be discovered from afar
• A civilization focused on exploring surrounding star systems would automatically spread their energy usage around a wide area
• They could be building things they don’t want lower civilizations to see (e.g. prime directive), which would necessitate building them in voids (as long as they don’t have technology to cloak it)

You’re writing as if the assumption of local energy usage is physically given and can’t be wrong, but we simply can’t know for now. It could be right, or it could be wrong. Again, I agree that it makes sense to assume it to be correct, as it would be a much more easily recognizable marker, but that doesn’t mean it’s the only option.