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View Full Version : Dyson spheres and Black hole emit no light; How do we tell the difference?



neilzero
2012-Dec-25, 01:40 AM
All the black hole candidates are thousands of light years or more away. Much closer we could likely tell the difference easily. The lensing effect of a Dyson sphere (or Dyson swarm) would not be detectable except very close such as ten light years or less. A search for Dyson spheres did not produce any likely candidates, but perhaps the search was flawed. Dyson earlier decided a Dyson swarm was more probable , which might capture 1% instead of 99% of the stars energy. Some of our search for planets of other stars might produce evidence of a Dyson swarm, but we have not checked a millionth of 1% of the stars for planets.
Unless a super strong material is available, the Dyson sphere would have about the mass of a super Jupiter, plus the mass of the star at the center. Area = 4 times 3.414 times r squared: If the radius is a trillion meters = 10*12; then Area = 12.56 times 10*24 cubic meters of material if one cubic meter of material is required for the sphere per square meter of surface. The sphere can possibly be smaller for a class M star, which is just barely fusing hydrogen. The inside of the Dyson sphere will reflect some of the energy back at the star, increasing the temperature of the photosphere of the star, but not change the rate of energy production much.
I have not seen any speculation on how the energy would be extracted, but concentrating solar with steam turbines would work. This would require heat exchangers covering most of the outer area of the sphere, so I suppose it would look like a red giant star in infrared, except much cooler. Please correct, refute and or embellish with unrelated comments. Neil

Selfsim
2012-Dec-25, 01:54 AM
Please correct, refute and or embellish with unrelated comments.Sure .. Dyson spheres are science fiction, and are not practically feasible.

Frank Merton
2012-Dec-25, 02:08 AM
Sure .. Dyson spheres are science fiction, and are not practically feasible.Yes they are SF and yes they are not practically feasible -- as far as we can see, but how are you so sure?

My problem with them has always been that I can think of easier ways to do basically the same thing -- why build a structure around a star (which is going to evolve and therefore cause trouble) when one can just build a structure in space and use distributed energy sources.

Nowhere Man
2012-Dec-25, 03:03 AM
Dyson spheres* will be emitting IR radiation. Black holes will be emitting gamma radiation and have an accretion disc and polar jets.

*(Dyson's original concept (IIRC) was not a solid sphere, but a whole bunch of satellites of the star, in orbits of various inclinations, intercepting as much of the light as possible.)

Fred

Van Rijn
2012-Dec-25, 07:46 AM
Sure .. Dyson spheres are science fiction, and are not practically feasible.

E-book readers used to be science fiction. So what? And what makes Dyson spheres "not practically feasible"? What do you even mean by "not practically feasible"? It sounds like you're just making another argument from personal incredulity, but I'm curious what your argument would be. Are you just pointing out that it isn't practical quite yet (which again, would make me ask "so what"?) or are you thinking of a particular version of a Dyson sphere? If the second, I'd suggest reviewing the subject, especially focusing on "Dyson swarms."

Van Rijn
2012-Dec-25, 08:23 AM
The lensing effect of a Dyson sphere (or Dyson swarm) would not be detectable except very close such as ten light years or less.


What lensing effect are you referring to?



A search for Dyson spheres did not produce any likely candidates, but perhaps the search was flawed. Dyson later decided a Dyson swarm was more probable , which might capture 1% instead of 99% of the stars energy.


As Nowhere Man points out, the original concept was for what is now typically referred to as a "Dyson swarm." A Dyson swarm could completely surround a star, you just need enough objects.



Unless a super strong material is available, the Dyson sphere would have about the mass of a super Jupiter, plus the mass of the star at the center.


How did you determine this?

Anyway, if you really want a solid Dyson sphere, it could potentially be dynamically supported by having objects moving about at higher than orbital velocity on (for instance) electromagnetic tracks within the structure.


The inside of the Dyson sphere will reflect some of the energy back at the star, increasing the temperature of the photosphere of the star, but not change the rate of energy production much.


Are you thinking of a "Dyson bubble" here? (This would use radiation pressure, and could be a single, but very thin, structure.) This is still another type of Dyson sphere.



I have not seen any speculation on how the energy would be extracted, but concentrating solar with steam turbines would work.


It depends on what it is for. If you want a lot of electricity, something like thin film solar panels would be an option. But there just may be a lot of habitats and their industrial hardware.

Anyway, the exact details generally aren't important for detection. As Nowhere Man mentioned, you look for IR. They could be hard to spot if large enough relative their stars, so that their temperature is similar to the CMB. Or for class M stars, they'd be fairly hard to find just because the stars are dim.

By the way, there's a new Dyson sphere search (read about it a couple months ago).

Paul Wally
2012-Dec-25, 09:06 AM
What about partially constructed Dyson spheres - work in progress so to speak? How long would it take to construct a Dyson sphere, 1000, Million or Billion years?

Glom
2012-Dec-25, 12:48 PM
Depends on how dense your swarm is I guess.

If it is to be say a 10^12 metres in radius, that's an area of 10^25 square metres. If a satellite covers an area of 10^5 and coverage will be expected to be 1%, then it will require 10^18 units.

If each unit takes 1 year to build and deploy and they have a million facilities working on them, it will a trillion years to complete.

You know, I don't think this idea is practical like it sounds.

neilzero
2012-Dec-25, 03:30 PM
The gravity of any object in space produces some gravity bending of radiation, plus I believe there is a knife edge effect at long wave lengths and possibly short also. Except for compact stars neither effect is easily detected. A 5% Dyson swarm would have a very low mass to radius ratio, but might produce significant knife edging.
With an area of 10^25 square kilometers, we might use 10^25 almost one kilometer orbiting asteroids as the swarm. Average mass one billion tons = total mass 10^34 tons. A super Jupiter might be radius 100,000,000 meters = 1.33 times 10^24 average density 8 = 10^25 tons, so we need to costruct "asteroids" with a billion times less average density for my super Jupiter mass guestimate to be in the ball park, unless I made a likely arithmetic error. Avoiding collisions between 10^25 "asteroids" will be challenging, and that is considerably short of capturing all the energy.
A Dyson bubble or solid structure would likely reflect several percent of the energy back to the photosphere, the swarm somewhat less.
I think Glom is correct = we likely won't find any Dyson swarms that capture even a millionth part of the star's energy. For the dynamically supported solid Dyson sphere we likely need to dismantal brown dwarfs to get enough mass within 4 light years of the construction site, unless we have construction material about a million times a million times stronger than steel.
From a class M star, the radiation may be longer wave length than infrared, but part of the star radiation will be unaltered if this is a partial swarm.
Likely many Sirus mass blackholes have negligible acretion disk and negligible polar jets and are too small to signifcantlly ocult or lens a distant star which may be why we have found few of them. Neil

cjameshuff
2012-Dec-25, 05:31 PM
Depends on how dense your swarm is I guess.

If it is to be say a 10^12 metres in radius,

6.7 AU? Why?



If each unit takes 1 year to build and deploy and they have a million facilities working on them, it will a trillion years to complete.

A year to build each 300 m square mass-produced module?



You know, I don't think this idea is practical like it sounds.

With the assumptions you base your conclusion on, certainly not. They are hardly reasonable assumptions however.

Say the sphere is 0.3 AU in radius (could be even smaller). You have a million factories each building 1000 1 km square modules a day. Coverage reaches 100% in a bit under 70000 years.

Of course, that's still an unrealistic way to approach it. A civilization harnessing just 1% of the star's output can likely construct new modules a great deal faster than that. Assume production scales with area already produced, the total area being expanded by 10% a year. Start with 1 km^2. After 10 years, you have 2.6 km^2. After 100, about 13000 km^2. That 0.3 AU radius sphere is covered in just under 400 years. At a tenth that growth rate, just 1% a year, a bit under 3800 years. This is for total coverage, not 1% coverage.

Van Rijn
2012-Dec-26, 04:13 AM
If each unit takes 1 year to build and deploy and they have a million facilities working on them, it will a trillion years to complete.


One of the main ideas behind Dyson spheres is "what happens if you have an exponentially growing interplanetary civilization?" On an exponential growth curve, they'd start out slow, but go very fast at later stages.


You know, I don't think this idea is practical like it sounds.

A lot of the stuff we do today wouldn't have looked practical even just decades ago if you used a linear production model.

IsaacKuo
2012-Dec-26, 06:51 PM
All the black hole candidates are thousands of light years or more away. Much closer we could likely tell the difference easily.
Dyson spheres would emit light. The spectrum would be lower, meaning they look more like brown dwarfs or star systems with heavy protoplanetary discs. Dyson sphere candidates wouldn't look anything like black hole candidates.

The lensing effect of a Dyson sphere (or Dyson swarm) would not be detectable except very close such as ten light years or less.
No idea what you're thinking here. The lensing effect of a Dyson sphere would only be detectable when it is very far away. It bends light by the same amount as a normal star, but the sphere itself essentially increases the radius of blocked light. Since the deflection angle decreases with distance, the minimum distance for lensing increases with increasing radius of blocked light. Depending on the radius of a Dyson sphere, it may be impossible to detect it via gravitational lensing unless it's outside our own galaxy.

A search for Dyson spheres did not produce any likely candidates, but perhaps the search was flawed. Dyson later decided a Dyson swarm was more probable , which might capture 1% instead of 99% of the stars energy. Some of our search for planets of other stars might produce evidence of a Dyson swarm, but we have not checked a millionth of 1% of the stars for planets.
Actually, Dyson's original concept was for a "swarm", and searches so far have assumed this sort of Dyson sphere. Broadly, the appearance of a satellite swarm and a statite/bubble shell would look similar, so this is not a big deal unless we find Dyson sphere candidates worth looking more closely at.

In fact, some of the Dyson sphere searches so far have assumed a partial sphere, despite the implausibility, because they would be easier to detect. A partial sphere would be easier to see because the host star would still shine in visible light, and the spectrum of a partial Dyson sphere would be easier to distinguish from a natural source (like a brown dwarf). It doesn't seem very plausible to be lucky enough to find a Dyson sphere during an intermediate partial phase, but the relative ease of detection/identification made such searches worth trying.

Unless a super strong material is available, the Dyson sphere would have about the mass of a super Jupiter, plus the mass of the star at the center.
Usually, a Dyson sphere is considered to be a large collection of satellites. However, a more efficient possibility is a statite shell, which would actually be very lightweight since it is limited in weight to what can be supported by photon pressure.

I have not seen any speculation on how the energy would be extracted, but concentrating solar with steam turbines would work. This would require heat exchangers covering most of the outer area of the sphere, so I suppose it would look like a red giant star in infrared, except much cooler. Please correct, refute and or embellish with unrelated comments. Neil
Photovoltaics would work.

The size of a Dyson sphere also depends upon what the host star is like. For example, a white dwarf only requires a Dyson sphere the size of a planet.

neilzero
2012-Dec-26, 11:32 PM
Even with 50% coverage of the energy of a G star; aren't collisions difficult to avoid and about half of the units shading another unit at least partially, and frequently? A trillion circular orbits each one kilometer larger radius puts the larger ones in the Oort cloud. Is there an arrangement for a trillion units, all 0.3 AU orbits or smaller that avoids 1000 collision avoidance manuvers per week? Is missed by one meter dependable? Isn't a 100,000 unit pile up likely to result from the first collision pair? Can we adapt this arrangement to permit a million flying cars over a city simultanously? Neil

Glom
2012-Dec-26, 11:36 PM
I presume there'd be a little depth to the orbits that are ultimately insignificant in the grand scheme of things.

neilzero
2012-Dec-26, 11:48 PM
Even with 50% capture of the energy of a G star; are not collisions difficult to avoid and about half of the units shading another unit at least partially, and frequently? A trillion circular orbits each one kilometer larger radius puts the larger ones in the Oort cloud. Is there an arrangement for a trillion units, all 0.3 AU orbits or smaller that avoids 1000 collision avoidance manuvers per week? Is missed by one meter dependable? Isn't a 100,000 unit pile up likely to result from the first collision pair? Is this arrangement adaptable to permit one million flying cars over a 1000 square kilometer city simultanously?
In any case they may want a large radius to accomodate a human population of 10^40 Disposing of waste heat will be a serious problem even if photovoltaic cells and most everything else are 90% efficient. Perhaps, the outer surface won't be a lot cooler than a red giant star.
Glom, please rephrase #14 or give more details. Neil

Glom
2012-Dec-27, 12:29 AM
It's tough to rephrase. I've had too much.

cjameshuff
2012-Dec-27, 01:13 AM
You don't need a separate orbital sphere for every single element, you don't even need trillions of separate elements. For example, you could use continuous hoops passing within meters of each other at crossing points, using short range electromagnetic interactions to regulate their distances, apply slight side forces to hold them out of their natural orbital plane, transfer power, etc. Not all elements need be in circular or even Keplerian orbits. You might have an arrangement with habitats banding the equator, and polar statites supported by photon pressure and reflecting light to the equatorial bands from the poles. You might have one giant solar sail bubble with habitats supported by radiation pressure or by ribbons moving at faster than orbital velocity. Etc...

Van Rijn
2012-Dec-27, 01:27 AM
Even with 50% coverage of the energy of a G star; aren't collisions difficult to avoid


Why would they be? Orbit radius can be quite varied, and there's no depth to speak of for these structures. And naturally this would be an actively controlled system. Solar sails could be used for maintaining orbital relationships.


and about half of the units shading another unit at least partially, and frequently?


If you live on the Earth, you get shaded by the planet about half the time on average.


A trillion circular orbits each one kilometer larger radius puts the larger ones in the Oort cloud.


Why would you do that?



Is there an arrangement for a trillion units, all 0.3 AU orbits or smaller that avoids 1000 collision avoidance manuvers per week?


Sure. Space out individual units along an orbit to form a ring, with other rings at other inclinations and differing orbital radii. With a minimum orbital radius difference of 100,000 km, you could have 200 levels within 20,000,000 km total.

John Mendenhall
2012-Dec-27, 01:41 AM
Ringworld, Ringworld. Better, maybe. Certainly more fun. I want to join the character that was journeying to the base of the arch.

Hey, you can't be serious all the time.

No signature on this post.

Van Rijn
2012-Dec-27, 02:15 AM
Ringworld, Ringworld. Better, maybe. Certainly more fun.


At one time I really liked the idea of a ringworld, but I actually think it would be pretty boring (and never mind the likely non-physical superstrong material requirements). The entire thing would have the same gravity, same light (and day/night cycle), mostly the same climate, etc. A Dyson swarm with many, many possible variations in habitats from zero G to heavy gravity, with millions of different habitat architects, could make a ringworld look dull by comparison.


Hey, you can't be serious all the time.


Oh, I know, but it's also fun to think about what you could do with the "many habitats" concept.

eburacum45
2012-Dec-27, 08:29 AM
To make a Dyson Swarm that doesn't collide with itself you can place all the satellites in a series of steeply inclined orbits, slightly ofset from each other; the pattern it makes is a hollow torus, looking like this.
http://www.orionsarm.com/im_store/JenkinsDysonSwarm.jpg
As you can see, the torus doesn't intercept all the light from the star. I think Dyson's own estimate of a swarm collecting 1% of a star's light is quite reasonable. This would provide ten million times more power than the amount of solar energy incident upon the Earth. From a distance this would look like a rather strangely shaped dust cloud; perhaps that's what we should be looking for.

Frank Merton
2012-Dec-27, 11:22 AM
At one time I really liked the idea of a ringworld, but I actually think it would be pretty boring (and never mind the likely non-physical superstrong material requirements). The entire thing would have the same gravity, same light (and day/night cycle), mostly the same climate, etc. I dunno; Niven managed to make it a pretty exciting place.

eburacum45
2012-Dec-27, 05:11 PM
A ringworld around a reasonably cool white dwarf might be do-able with real world materials, especially if you don't mind a bit of low gravity. However it would be a lot easier to make millions or billions of smaller rotating habitats each with a distinct environment, as Van Rijn suggests. The main drawback is that you couldn't walk from one environment to another.

IsaacKuo
2012-Dec-27, 05:34 PM
To make a Dyson Swarm that doesn't collide with itself you can place all the satellites in a series of steeply inclined orbits, slightly ofset from each other; the pattern it makes is a hollow torus, looking like this.
http://www.orionsarm.com/im_store/JenkinsDysonSwarm.jpg
As you can see, the torus doesn't intercept all the light from the star.
I don't feel there's a compelling reason to stop there. A second torus or a partial "seashell" configuration can cover the caps. This augmentation doesn't even need to have any solar collection hardware, it could just passively reflect light toward the main torus.

I used to be a fan of the torus configuration, but really it's less efficient than the more obvious "seashell" configuration.

cjameshuff
2012-Dec-30, 01:57 AM
A ringworld around a reasonably cool white dwarf might be do-able with real world materials, especially if you don't mind a bit of low gravity. However it would be a lot easier to make millions or billions of smaller rotating habitats each with a distinct environment, as Van Rijn suggests. The main drawback is that you couldn't walk from one environment to another.

A bunch of smaller habitats would also allow more habitable volume for the materials. To clarify a point of possible confusion, the continuous hoops I mentioned were not ringworlds, but orbital structures under only minor tension, largely as a means of control and damping of oscillations.

eburacum45
2013-Jan-02, 10:12 AM
I used to be a fan of the torus configuration, but really it's less efficient than the more obvious "seashell" configuration.
If anyone wonders what the 'seashell' configuration looks like, here is an image of it by Anders Sandberg;
http://www.aleph.se/Trans/Tech/Megascale/d_late.gif
note that this has nested rings of collectors with different radii. There should be no problem with stability in such a configuration, so long as the masses of the satellites are negligible.

Glom
2013-Jan-02, 12:02 PM
Is there an animated version?