Even a Dyson sphere can't just swallow the star's radiation. It makes use of a potential gradient, which means it would have to radiate away some energy on the outside. This would probably show up as, say, a brown dwarf. Still, if it's not in our direct neighborhood the chances of us discovering it are even lower.

Unless the engineers are concerned about detection and choose somehow to radiate in only one narrow direction, pointed away from any nearby stars.

That's true, and stealth is an interesting proposition. However, there are probably easier methods of preventing detection, such as not building a giant structure in the first place. Also, even if they manage to radiate away most of their waste heat, they'd have to assume eventual detection by an advanced civilization. Sure, with exponential effort you can always get infinitesimally closer to being virtually undetectable, but is that really feasible?

In the end, it's certainly not impossible that a civilization would go to extreme lengths in order to hide its enormous artificial structures. But the real issue is: why would all civilizations hide in this way? And if it turns out they do all have elaborate stealth tech in operation, that means there must be an incomprehensibly huge threat out there - which in itself might be the answer to why nobody is calling us up.

Have you read Revelation Space by Alistair Reynolds? If you haven't, you really need to.

No. why do I need to?

Revelation space is a speculative fiction novel set in the far future during the golden age of space colonization. Spoilers ahead.

Throughout the novel there's alien civilizations that use different kinds of way to cloak constructs and settlements. Examples include devices like artificial moons and holographic, multi-spectral holograms that emulate gas giants to hide stuff inside.

The one I think andrewflnr may be referring to is a device that one race used to cloak entire regions of space to hide their civilization from technologically superior aggressors. Essentially they set up variable-gravity devices at strategic points around their region of space, and had the devices resonate to build 'waves' of gravity that would constructively interfere at a massive scale at the borders of their space, which would essentially tear any intruders apart using the amplified+alternating gravity waves. Several million years in the future, observers/scientists of other races on the outside would dismiss these chaotic space regions as an unexplained anomaly.

Mostly, I was thinking of that whole theme in relation to the "incomprehensibly huge threat" thing Udo mentioned. If you're talking about Shrouders, I don't remember that method of using gravitational waves being mentioned. Did you hear that somewhere else?

So, first you build a Dyson sphere, then you build a system to cool the outer surface of your Dyson sphere to near 3 Kelvin (except some narrow directions in space, where you radiate your excess heat).

Now the interesting question is, is the energy you capture from the star sufficient for running this cooling system of yours. I have no clue.

Let's see if someone in Physics StackExchange could answer this:

http://physics.stackexchange.com/questions/69649/thermodynam...

The Wikipedia article defines a Dyson sphere as a system of orbiting solar-power satellites meant to completely encompass a star and capture most or all of its energy output.

I think there is a misunderstanding here about what that means in practice. Thermodynamics still applies. Energy has to be harnessed across gradients. Even if they wanted to only collect visible light, they'd have to radiate the heat somewhere.

The black space is at about 3 Kelvin temperature. A Dyson sphere would necessarily (the star inside would heat it) have a much higher temperature than mere 3 K above the absolute zero. Thus it would radiate heat according to whatever its temperature, and be quite distinct compared to the cold back space background.

This is assuming that the creators of the Dyson sphere haven't also figured out how to absorb the heat energy with extremely high efficiency.

You are assuming they'd have figured out how to break the laws of thermodynamics as we know it.

> break the laws of thermodynamics as we know it

Not a giant leap to imagine for a species that's been able to construct a Dyson Sphere in the first place ;)

I don't understand why any civilization would ever build a Dyson sphere.

The sphere would be dynamically unstable in the gravity field of the star and station-keeping would be expensive and hard to coordinate over the lightspeed delay.

And stars are terrible energy sources--they're huge, diffuse, inefficient, and not portable. It would be much more useful to build fusion generators to order.

If you've got the technology and infrastructure to build a Dyson sphere, is there that much difference between a star and a fusion generator? Maybe the reason we've had so little success with fusion generators is because they have to be built very big to keep them lit.