First, a back-of-the-envelope calculation:

power output of sun / surface area of sphere of radius 1 AU = ~1368 W/m^2

You MUST radiate this much (on average) from every square meter of your sphere. MUST, as in, mathematical imperative.

Under the (typical) assumption of a perfect Lambertian emitter, you can invert the Stefan"“Boltzmann Law to get an average temperature of 394.1°K (or 121°C). That's not visible light, but it is pretty hefty across infrared and near-radio.

The Problem

This temperature is inconveniently high (it's death for most terrestrial life, for example), and it is unavoidable under these design constraints. If we decide it's unacceptable, we have several options. You've bitten off a lot to chew, so the best that can realistically be done is propose several classes of solutions.

Approach 1: Reduce Power Output of Star

(Insert magitech.) Stars have stable fusion rates. Maybe you can chuck a whole solar-system's worth of iron into it to slow it down. Or choose a dimmer star. Or siphon off material with a wormhole or something equally scientifically dubious.

Approach 2: Make Your Dyson Sphere Bigger

Because irradiance follows an inverse square, at 2 AU, the energy dissipated per area is 1/4 that at 1 AU. This is much more manageable.

Gravitational stresses also decrease on the poles of a larger Dyson sphere, and you get even more surface area. So it's good in other ways too. It can't reduce your heat signature, but that isn't really possible anyway (see Stealth, below). So, I recommend this approach.

Approach 3: Let Some Light Through

Poke holes in your Dyson sphere. The sideways areas give you more area to radiate. This doesn't work very well for a full sphere (the surfaces tend to radiate into each other), but lesser (and more practical) megastructures (e.g. Niven rings) can radiate nicely. Consequently, I recommend this approach.

Approach 4: Concentrate Energy

This cannot be done without reversing entropy. We can do that by expending more energy (say some fraction collected from the star), so it's not impossible in this case.

There's probably a thousand ways to do this, but they'll all be difficult because fundamental laws of optics make it essentially impossible to focus light from a large object like a star. The energy will need to go through an intermediate stage (e.g. solar panel -> electricity -> laser), with tremendous losses along the way. Those losses translate into waste heat, which is a tremendous problem in an enclosed space. Consequently, this is a bad class of approach to take.

Another thing to consider is that directional emission must be done carefully to avoid unintended consequences. For example, If you find a way to emit all the star's energy in a laser, then the whole assembly will rapidly start accelerating the opposite direction (because you've just made a stellar-scale photon rocket). You need another laser in the other direction if you don't want to move.

Stealth

The only even remotely possible way to do this is with something from approach 1 or 4 (because otherwise either light from the star is getting out, or your waste heat in aggregate is literally exactly equal to the star's output in the first place. If you can direct a beam, you have a fighting chance at aiming it. But, no matter what, energy input equals energy output; it is mathematically impossible to hide your Dyson sphere from all directions all at once if they could detect the original star.

posted almost 8 years ago

imallett‭

11 reputation 0 1 1 0

I think the best solution would be a Matrioshka Brain. This is effectively a layered set of Dyson spheres. Once it reaches equilibrium, each shell has a particular temperature differential across it, which can be used to generate work. These shells feed eachother, so the total temperature drop across the entire brain is equal to the temperature of the star minus the temperature of interstellar space.

The neat thing about Matrioshka Brains is that the temperature of the radiating surface of each layer is lower than the previous. With some careful effort, you could eventually radiate all the power of the star in a spectrum that is closer and closer to that of background radiation. You can't get it perfect (there's no such thing as stealth in space), but you can get close.

This makes it substantially harder to detect the brain. It is very hard to distinguish a signal whose spectrum is close to that of noise. You have to listen for a long time. It gets even harder from a distance. Most observations of thermal characteristics like this are from a distance where the Matrioshka brain would subtend only the tiniest fraction of a pixel. This means the temperature of the brain would be averaged with the temperature of the background radiation, and that average would be heavily weighted towards the background radiation.

EDIT: A key that I failed to mention is that there is an assumption here that there is value in harnessing the energy. Once energy is no longer in thermal form, there's many more options. For example, one might export high-potential-energy materials to be emitted elsewhere. It might be amusing to go collect the results of solar fusion from other stars and use the energy from the Matrioshka brain to un-fuse them back into hydrogen to be used in fusion reactors! A lot of options open up once you've sucked as much of the energy as possible out of the solar radiation instead of emitting it as heat!

posted almost 8 years ago

Cort Ammon - Reinstate Monica‭

91 reputation 2 5 9 0

You don't understand the concept of the Dyson Sphere if you are trying to "get rid" of all that heat. Heat is energy, and the DS is meant to capture the energy. The "sphere" requires many new technologies, engineering marvels, etc. chief among them is some sort of super material that absorbs and stores the energy for use when you need it, or targets the energy where you need it. If you can't handle all the excess heat, then you are not in Dustin Sphere territory. Such a structure is reserved for civilizations that both require all that energy, and can actually use it. IOW: if "too much heat" is a problem for the materials you are using, then you are building a massive structure just to vent out the energy you are capturing, which makes no sense at all.

posted almost 6 years ago

Robert C. Mustain‭

11 reputation 0 1 1 0

A heat pipe system should be able to handle what your load.

The advantage of heat pipes over many other heat-dissipation mechanisms is their great efficiency in transferring heat. A pipe one inch in diameter and two feet long can transfer 12,500 BTU (3.7 kWh) per hour at 1,800 °F (980 °C) with only 18 °F (10 °C) drop from end to end. Some heat pipes have demonstrated a heat flux of more than 23 kW/cm², about four times the heat flux through the surface of the sun.

So on the inner surface of the sphere you put a lot of heat collection pads hooked up to heat pipes that travel through the sphere structure and to the outside where there would be huge thermal radiators.

The suns head would be absorbed, transferred through the heat pipe arrays, and then dissipated out through the radiators.
The sphere would glow in the infrared spectrum, but it would be diffuse.

posted almost 8 years ago

AndyD273‭

1 reputation 8 213 1 0

Being careful with your aim

You can't not radiate, so if you want to not be observed then you need to make sure you aren't radiating in the direction of your nearest neighbours. To do this you'll need to somehow channel all the energy into a series of directional beams that are aimed in-between the stars closest to you. The amount of power you'll need to pump through those emitters will vary depending on the number of emitters you have, so let's have a look at some numbers:

How much energy have we got to lose?

Lets assume we're talking about our sun for now.

3.8×10²⁶ W

OK. Just to be clear that's an awful lot of zeroes.

How can we absorb it?

There are a couple of answers to this, some on this very site. Let's assume that we use a couple of shells to create some vast thermal power plant and we can then shuffle the electricity around. It's not going to be perfect, there will still be some ambient radiation, but with decent insulation and design it should be well below the amount that's noticeable at interstellar distances. If you want to decrease the energy leakage then increase the number of shells (like adding very big, complicated blankets to a very big incandescent baby).

How can we get rid of it?

Let's turn to the ever excellent Randall Munroe for this one.

The Boeing YAL-1 was a megawatt-class chemical oxygen iodine laser mounted in a 747. It was an infrared laser...

There are bigger lasers, but they all fire for fractions of seconds.

So we've got a laser that can fire on the order of megawatts of power. Lets assume they each fire at 3.6MW and we can run them continuously and see how many of them we need...

10^21 lasers. Assuming they each take up a meter squared that's 1/281'th of the space we have to play with on the outside of our sphere. Not great, but it's usable. If we can get more laser apertures in then we can improve upon that, but preferably we want to be able to shift their aim.

Now, if we aim them at the darkest bit of sky we can find from their individual location (parallax is going to be important at these scales), then we'll be minimising our chances of being noticed. Again: This isn't perfect. The lasers won't have perfect beam coherence, so the beam will be more of a cone spreading out into the stars. This in turn means that stars further away might be clipped by an intense and regular source of radiation, spawning all sorts of wild theories about rapidly rotating stars and suchlike.

What about the bits that aren't laser arrays?

Clad them in long wavelength EM absorbent materials, refrigerate them and pump the heat back into the inner shell of your stellar power plant. Sure, you're going to hugely increase the entropy of the system as a whole, but when you're engineering a Dyson sphere I think minimum-entropy concerns are pretty much out the window...

Naturally this has some problems (heat buildup on the laser arrays being a large one) and some potential improvements (the IR lasers probably aren't the best way to direct the energy, some form of particle beam apparatus may well be better) but it should suffice to mask you from a casual observer.

One last note: Make sure the lasers are evenly spread or you'll end up pushing your shell ever so slowly out of position, and crashing a Dyson sphere into a sun is pretty embarrassing...

A quick note: This would also require some 'hotspot' exhausts from which to extract the electricity to power the lasers. Clever positioning of these and suitable focusing systems could again minimise the chance of detection. In fact, if you really wanted to you could probably get away with just using the exhausts, though the focusing requirements would be a lot more tricky to deal with that aiming a laser.

posted almost 8 years ago

Joe Bloggs‭

26 reputation 41 2 3 0