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?

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.

A Dyson sphere would also not be completely invisible in all wavelengths, but would probably give off some infrared radiation.

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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).

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Difficult if both are a fiction !

...Or not.

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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....)

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.

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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

Dyson sphere are nice, but why isn't the universe full of them?

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It might be... there does, after all, seem to be a LOT of missing mass around.

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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.

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She might. You never know.

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.

That might be a very long descent
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.
I don't think it would create that much pressure. Even so, that energy would be collected and converted for other uses.

Get a really big umbrella up near the poles...

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

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

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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.

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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 (spinning it, walls on the edge to contain the atmosphere).

Yes, but as Ringworld shows, that "small fraction" is still a lot of area; 3 million times the area of the Earth!

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As Niven realised, that makes the rest of the structure pretty much redundant.

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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!

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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.)

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
Illuminati's Razor-The most complicatedly evil answer is usually the most correct answer. - Fazor
"Every book is a children's book if the kid can read." - Mitch Hedberg
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort