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pie33
2008-Apr-09, 10:43 AM
A common description of a Dyson sphere would be: "A Type II Dyson sphere would be totally opaque (unless it had openings). The spheres would hence be invisible from a distance, just a black disk on the sky."

Is it possible to know if a mini black hole is really a Dyson sphere?

TobiasTheViking
2008-Apr-09, 12:20 PM
Yes, by the gravitational pull it gives on objects around it.

A black hole would be a lot more massive.

Also, a black hole will(if it is sucking in matter) have an accretion disk, which would be very obvious, and give off a lot of energy we can see.

Swift
2008-Apr-09, 01:15 PM
A Dyson sphere would also not be completely invisible in all wavelengths, but would probably give off some infrared radiation.

eburacum45
2008-Apr-09, 05:39 PM
That's right. Unless the dyson sphere somehow contains a mechanism to convert the emitted energy of the star into stored energy, all the luminosity of the star inside it will eventually be emitted by the surface of the sphere, albeit at a lower wavelength because of the cooler temperature.

The outside temperature of the sphere will be dependent on the surface area of the sphere and the luminosity of the star (minus any energy subtracted for storage).

galacsi
2008-Apr-09, 06:03 PM
A common description of a Dyson sphere would be: "A Type II Dyson sphere would be totally opaque (unless it had openings). The spheres would hence be invisible from a distance, just a black disk on the sky."

Is it possible to know if a mini black hole is really a Dyson sphere?

Difficult if both are a fiction !

Noclevername
2008-Apr-11, 03:46 AM
Difficult if both are a fiction !

...Or not.

JustAFriend
2008-Apr-13, 08:10 PM
Basically, if you can putt up to it and knock on a port.... it's a Dyson Sphere.

If you putt up to it and it sucks you in and crushes you, it's a black hole.

(OK, OK, so you're a scaredy cat... shoot a laser beam at it or a stream of gas or particles....)

Acolyte
2008-Apr-14, 01:21 AM
Um... if you can see it, it's a Dyson Sphere - if it's pulling you off course & invisible it's a Black Hole. (or of course if you see star systems being ripped to plasma in a circle around nothing *grins*)

Black Holes wouldn't have any size.

Veeger
2008-Apr-14, 01:59 AM
A common description of a Dyson sphere would be: "A Type II Dyson sphere would be totally opaque (unless it had openings). The spheres would hence be invisible from a distance, just a black disk on the sky."

Is it possible to know if a mini black hole is really a Dyson sphere?

I find the question a bit perplexing. If you mean, is it possible to know from earth based systems and measurements, I would say neither has definitively been found, only a relatively small number of AGN/black hole candidates.

On the otherhand if you were close enough to actually see or otherwise definitively measure the object, the distinction would be obvious. Wouldn't it?

-Veeger

RalofTyr
2008-Apr-15, 06:48 AM
Dyson sphere are nice, but why isn't the universe full of them?

Acolyte
2008-Apr-15, 06:52 AM
Dyson sphere are nice, but why isn't the universe full of them?It might be... there does, after all, seem to be a LOT of missing mass around. :lol:

Tog
2008-Apr-15, 07:07 AM
Okay, I know what a Dyson Sphere is, but I have some questions about them.

Where would a culture get the raw materials to make one?

I assume they would have to spin to generate gravity, even if they do it pretty slowly. Wouldn't that mean that the top and bottom (bits on the axis) would have virtually no gravity? If so, would it be possible to "fall" from the surface into the star?

Doesn't the radiation from the star create a pressure on the surface? Would that pressure ever be enough to "pop" the sphere, or would there be release valves to bleed it off?

If there is pressure, would it be enough to counteract the lack of gravity in the question above? Assuming that lack of gravity is an issue at all.

Neverfly
2008-Apr-15, 09:03 AM
Okay, I know what a Dyson Sphere is, but I have some questions about them.

Where would a culture get the raw materials to make one?
I can't help but speculate about Bio-tek or Nano-tek on this buut... The raw materials could be avaible in a solar system abundantly. What's mind boggling to me is the extraction of the materials, moving them such distances at such masses, construction of the sphere and maintenance of it.


I assume they would have to spin to generate gravity, even if they do it pretty slowly. Wouldn't that mean that the top and bottom (bits on the axis) would have virtually no gravity? If so, would it be possible to "fall" from the surface into the star?

That might be a very long descent:p
But I think a dyson shpere would still have mass, and therefore some gravity of its own. Its own gravity- at that distance may hold more influence than the star would.

Doesn't the radiation from the star create a pressure on the surface? Would that pressure ever be enough to "pop" the sphere, or would there be release valves to bleed it off?

I don't think it would create that much pressure. Even so, that energy would be collected and converted for other uses.


If there is pressure, would it be enough to counteract the lack of gravity in the question above? Assuming that lack of gravity is an issue at all.
Get a really big umbrella up near the poles...:whistle:

ETA: It was a while that no one answered Tog, so I threw something up. Even if I'm wrong on somethin'- Someone's BOUND to come along to correct my errors:p

eburacum45
2008-Apr-15, 02:19 PM
Okay, I know what a Dyson Sphere is, but I have some questions about them.
Where would a culture get the raw materials to make one?
Dyson didn't describe a 'shell', as far as I can tell; the idea is just to 'intercept' the light so that it can be collected for use. This could be done with a swarm of satellites with solar panels, or with a thin shell held up by light pressure. The thin shell would have to be very thin and very light- so light that the mass of a single large asteroid would suffice.

As for the idea of people living on the inside; this is pretty much impossible, as they would fall off into the star, unless gravity generators are possible (which seems unlikely). Spinning the sphere would only make a small fraction of the sphere habitable.

See this FAQ
http://www.nada.kth.se/~asa/dysonFAQ.html

eburacum45
2008-Apr-15, 02:22 PM
But I think a dyson sphere would still have mass, and therefore some gravity of its own. Its own gravity- at that distance may hold more influence than the star would. If the sphere has a uniform thickness it will exert no pull due to gravity on anything inside. Wierd, but true. The only gravity will come from the star.

Swift
2008-Apr-15, 03:13 PM
Okay, I know what a Dyson Sphere is, but I have some questions about them.
All my answers are to the best of my knowledge and a lot of that knowledge is from science fiction. I'll skip the ones I don't have a clue about.

First, as eburacum45 said, Dyson's original idea wasn't to build it so much to live on it, but to use it to collect all the possible solar energy as you could.

Where would a culture get the raw materials to make one?
You use the rest of the material in the solar system. Depending on what exactly you built, and the tensile strength of your sphere (and thus the thickness of material you need), you might have to go "farm" material from other near-by systems. You'd probably also want to clean out all the asteroids and comets, even if you didn't need the materail, so they don't hit your nice sphere.

I assume they would have to spin to generate gravity, even if they do it pretty slowly. Wouldn't that mean that the top and bottom (bits on the axis) would have virtually no gravity? If so, would it be possible to "fall" from the surface into the star?
Yes. There exist calculations for what the spin rate would need to be for a 1 AU radius sphere to have 1 g at the inside surface of its equator. The force would decrease as you head to the poles. And you'd still need to do something to contain the air, like very high walls around the pressurized areas. All of that is part of the idea behind the "design" of Larry Niven's Ringworld (http://en.wikipedia.org/wiki/Ringworld) (spinning it, walls on the edge to contain the atmosphere).


Spinning the sphere would only make a small fraction of the sphere habitable.
Yes, but as Ringworld shows, that "small fraction" is still a lot of area; 3 million times the area of the Earth!

eburacum45
2008-Apr-15, 03:24 PM
As Niven realised, that makes the rest of the structure pretty much redundant.

Swift
2008-Apr-15, 03:31 PM
As Niven realised, that makes the rest of the structure pretty much redundant.
I don't disagree. It all goes back to why are you building the object, power or living space? For living space, the Ringworld seems a better solution. For power, the sphere, but probably as a "Swarm" not as a single, solid sphere.

I wonder if you could do both - a Ringworld at the equator, with two hemispheres of swarming satellites to collect solar energy.

Lastly, there is another idea of why a civilization would build objects such as these (its not originally my idea, but I like it) - because THEY CAN! Maybe a massively advanced civilization would do things like this, just for the fun, or to put their mark on the Universe. Lego toys, but your blocks are the size of minor planets! :dance:

Noclevername
2008-Apr-15, 04:20 PM
I don't disagree. It all goes back to why are you building the object, power or living space? For living space, the Ringworld seems a better solution. For power, the sphere, but probably as a "Swarm" not as a single, solid sphere.

I wonder if you could do both - a Ringworld at the equator, with two hemispheres of swarming satellites to collect solar energy.


A swarm for living space also makes more sense, as you could produce a lot more habitats from the same mass of material as you can making a ringworld from it (and from conventional materials too, no need to invent supertough "scrith" or other unobtanium as you would need to make a working ringworld.)

Swift
2008-Apr-15, 04:57 PM
A swarm for living space also makes more sense, as you could produce a lot more habitats from the same mass of material as you can making a ringworld from it (and from conventional materials too, no need to invent supertough "scrith" or other unobtanium as you would need to make a working ringworld.)
I guess the only downside, from a story telling standpoint, is that the civilzation has to retain some rudimentary spaceflight, if you want to travel from one satellite (swarmlet?) to another. In Ringworld, which postulated a post-collapse civilization (at least in the first book), you could always just walk.

Still, a Swarm meta-civilization would make for a nice SF story collection. Each writer could create his own civilization on a satellite, and then you could introduce interactions between them, after a long period of no-communications. For example, the nano-technologists go visit a civilization on a Swarmlet that has reverted back to early Renaissance technology. If enough time had progressed, you could even start to get divergent evolutions of humanoid species (as in Ringworld).

Noclevername
2008-Apr-15, 05:11 PM
I guess the only downside, from a story telling standpoint, is that the civilzation has to retain some rudimentary spaceflight, if you want to travel from one satellite (swarmlet?) to another. In Ringworld, which postulated a post-collapse civilization (at least in the first book), you could always just walk.

Still, a Swarm meta-civilization would make for a nice SF story collection. Each writer could create his own civilization on a satellite, and then you could introduce interactions between them, after a long period of no-communications. For example, the nano-technologists go visit a civilization on a Swarmlet that has reverted back to early Renaissance technology. If enough time had progressed, you could even start to get divergent evolutions of humanoid species (as in Ringworld).

I was thinking of it from an engineering viewpoint, but if you want a story vehicle, it's best to combine the swarm/rigid shell idea; a sun-girdling mesh with spinning habitats linked to each intersection. They can therefore remain separate yet still traversible without advanced technology. The linking mesh and spinning mechanisms are maintained by the remains of automated self-replicating robots, while the inner habitats are left alone. And some life forms from the habitats have escaped and adapted to the low/zero G linking corridors...

Neverfly
2008-Apr-15, 07:13 PM
If the sphere has a uniform thickness it will exert no pull due to gravity on anything inside. Wierd, but true. The only gravity will come from the star.

That is weird, considering that mass has gravity. Thanks for clarifying this- albeit I will go crazy until I read up on it and understand it:p

eburacum45
2008-Apr-16, 07:19 PM
I was thinking of it from an engineering viewpoint, but if you want a story vehicle, it's best to combine the swarm/rigid shell idea; a sun-girdling mesh with spinning habitats linked to each intersection. They can therefore remain separate yet still traversible without advanced technology. The linking mesh and spinning mechanisms are maintained by the remains of automated self-replicating robots, while the inner habitats are left alone. And some life forms from the habitats have escaped and adapted to the low/zero G linking corridors...
That is an idea I have considered. One good 'mesh' to place your spinning cylinders in is a ladder, making a rungworld; two examples of this type from OA are shown in these pictures,
http://www.flickr.com/photos/25577499@N07/2414984991/in/set-72157604502272263/
and
http://www.orionsarm.com/worlds/Evermore.jpg

The 'rungworld' concept was originally invented by Niven fans, as a kind of satire on his ringworld concept; personally I thnk it is a workable idea, assuming self-replicating technology and a good supply of carbon to make the cylinders. No Scrith required.

Swift
2008-Apr-16, 08:08 PM
Rungworld, I love it, as both a joke and a serious concept! Brilliant!

Glom
2008-Apr-17, 01:12 PM
What about a static shell around a star using the heat from the star as a kind of hypocaust? People could live on the outside with free patio heat. The gravity of both the star and the shell would be enough to hold down an atmosphere maybe?

eburacum45
2008-Apr-17, 06:00 PM
Eric Max Francis calls that sub-type an 'Outside Dyson'.
He has some calculations to go with the concept here.
http://www.alcyone.com/max/writing/essays/outside-dyson-shells.html

Basically a small red dwarf between 0.054 and 0.079 masses solar could have an 'outside dyson' and maintain an Earthlike environment over its surface.

RBG
2008-May-18, 08:34 AM
If these ETs can build a device from planet raw materials and place it around a sun to wring out it's energy, wouldn't it just be easier and more effective to turn the planets themselves into energy via E=MC^2?

RBG

eburacum45
2008-May-18, 02:04 PM
If these ETs can build a device from planet raw materials and place it around a sun to wring out it's energy, wouldn't it just be easier and more effective to turn the planets themselves into energy via E=MC^2? Only if they can actually find a way to convert mass directly into energy.

To do that you could use antimatter annihilation- but the process of making antimatter is very inefficient and requires more energy than you'd get back. Even if you could dump the planets in a big fusion reactor you'd only get a fraction of the matter converted into energy- and any element higher than iron in the periodic table would absorb more energy than they give out.

There is a way of converting mass directly into energy using a tiny black-hole, but making the black hole in the first place uses a heck of a lot of energy. Perhaps eventually this method would become available (or even more speculative methods using monopoles perhaps, or maybe cosmic string). This would allow you to convert planets into energy directly, giving vast amounts of energy from planetary materials- amounts which would make dyson spheres irrelevant.

The sun produces the same amount of energy every second as a million tons of antimatter or so, IIRC; annihilating a planet could produce much more than that (depending on how quickly you did it).

RBG
2008-May-18, 04:34 PM
You know that the alternative being discussed here is building a structure around a sun, right? ;-)

RBG

neilzero
2008-May-18, 06:48 PM
Perhaps dyson spheres have less utility than we think. It seems to me they are best adapted to least massive class m stars. Even here a sphere with a radius of ten million kilometers will get extreemly hot on the inner surface as the solar energy is reflected back to the stars photosphere. A class M photosphere is about 2000 degrees k. In a few years the photosphere color temperature would be 7000 degrees k, unless the reflected energy was reduced?
As someone typed where will they get enough material for a sphere with a radius of 10,000,000 kilometers? area = 4 times 3.14 times radius squared = 12.56 times 100 trillion = 1.256 quadrillion square kilometers. At one kilogram per square kilometer that came to 10 nano meters thick on my other thread, which we decided was too flimsty to support itself in free fall with minor tide forces. That means the dyson sphere makers need many quadrillion (10E15) kilograms of material, even if it is thousands of times stronger than any material we can presently make. A hotter star needs a larger radius and the material required increases as the square (perhaps the cube) of the radius. A dyson sphere with lots of openings might look like a g or F star because of the much hotter photosphere, even though it is only class M mass. Neil

Kaptain K
2008-May-18, 08:53 PM
1) The size of a late class M star is closer to that of Jupiter than that of the Sun.

2) The whole point of a Dyson sphere is to capture and use the energy of a star. Reflecting it back to the star defeats the whole purpose.

neilzero
2008-May-19, 03:37 AM
I think Kaptain K is correct on both points, but I find it hard to imagine how a closed sphere could avoid reflecting 1% of the energy back into the photosphere. How long would it take for the photosphere to heat from 2000 k to 7000k with 1% of the energy being returned to the photosphere? Forever perhaps?
The 99% needs to go somewhere. Laser beams powering space craft? million gigawatt radio transmitters? 1000 times the number of far infrared photons our Sun radiates, radiated from the outer surface of the dyson sphere?
Is a "late" class M star cooler than an average class M star? Neil

eburacum45
2008-May-19, 08:05 AM
You know that the alternative being discussed here is building a structure around a sun, right? ;-)

RBG

Yes, but the Dyson swarm, as originally envisaged by Dyson, is relatively simple technology which has no requirement for speculative technology. In theory any civilisation could build one eventually.

On the other hand no method of converting mass to energy efficiently is known to exist. In order to create enough antimatter to annihilate the Earth you would need a Dyson swarm just to collect enough energy to manufacture the antimatter. Even the use of microscopic black holes to generate power has a whole range of associated engineering difficulties which may be impossible to overcome. So it may be the case that no civilization, however advanced, can ever develop mass -> energy technology which creates a surplus of energy, and the Dyson swarm is the only practical option for large scale energy generation.

eburacum45
2008-May-19, 08:12 AM
A lightweight Dyson bubble would be supported by light pressure; it could have the mass of a large asteroid and still cover the star. A Dyson bubble supported by light is one of the more plausible options for a shell design, but to me the swarm is much more likely. A partial swarm would look very like a shell of dust, or maybe a disk of dust similar to those already observed around many stars.

eburacum45
2008-May-19, 08:16 AM
The whole point of a Dyson sphere is to capture and use the energy of a star. Reflecting it back to the star defeats the whole purpose.Unless you want to increase the temperature of the star, and thereby increase its output. Use half of the energy emitted by the star, and return the rest to heat the star up; eventually you will increase the luminosity of the star severalfold.

neilzero
2008-May-19, 01:24 PM
I'm having problems with the theory. It seems reasonable that feeding energy back to the photosphere would increase the luminocity several fold from class M to class G?
But the core temperature will not increase significantly for thousands, perhaps millions of years due to the feed back, so the core output will not increase. Over centuries, the photosphere output can only increase 50% if you are returning 1/2 of the energy. How much does the luminosity increase with a 50% energy increase? 50%? Neil

eburacum45
2008-May-19, 02:38 PM
The photosphere could get quite a bit hotter than you might think if you reflect 50% of the energy; after all, almost exactly 100% of the energy emitted by the star is currently lost. Heating the outer layers increases the temperature, there fore the luminosity- you then reflect 50% of the increased luminosity back to the star.

By reflecting that energy back to the star you are heating the star up from the surface inwards. The photosphere will get hotter, therefore more luminous, well before the increase in temperature reaches the core.

Since the star expands when it is heated in this way, that would affect the density- I wonder how that would affect the rate of reaction at the core?