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Dyson Sphere

Hm, the stellar wind's going to be a problem for a Sphere, as that radiation will impinge on the inner surface. For a ring world, presumably a sufficiently large magnetic field could deflect the wind.
 
nathan said:
I'm not sure what you're trying to say in your parenthesized sentences.
The first one is wrong and the second makes no sense.
Click to expand...


The first one should've read that the gravitational attraction of the sphere
from the inside does not hold you to it. And the second one also assumes
no rotation on the sphere, so if a person stands on the outside of the sphere
the gravitational attraction of the star will, very slowly, pull the person
towards the star.

P.S. If there were a hole in the sphere it appears the star would escape
through it. Another one of my middle of the night epiphanies. :)
 
Solitaire said:
The first one should've read that the gravitational attraction of the sphere
from the inside does not hold you to it. And the second one also assumes
no rotation on the sphere, so if a person stands on the outside of the sphere
the gravitational attraction of the star will, very slowly, pull the person
towards the star.
Click to expand...

Thanks,

On the outside of the sphere the gravitational attraction will be the sum of the stellar attraction and the sphere attraction. Both can be modelled as point sources at their respective geometric centers. The stellar mass is going to dominate, and the centers should be coincident. At 1AU from a solar-mass star, the gravitational attraction is minuscule wrt normal Earth surface gravity. If it wasn't, we'd've all been taught about the importance of weighing objects using spring scales at local dawn and dusk, where the solar attraction would be at right angles. (and as evidence you can go measure some object's weight at local noon and local midnight on such a force scale and see if there's a difference).

So yes, you're right but I doubt the attraction would be sufficient for practical purposes.

P.S. If there were a hole in the sphere it appears the star would escape
through it. Another one of my middle of the night epiphanies. :)
Click to expand...

But the net gravitational force between the star and sphere would be towards the inside of the sphere opposite the hole.
 
Sorry to wake this up, but I realized my logic about the gravitational attraction of the Sun at 1AU is faulty, and this is the first chance I've had to touch the internet for a while. I was considering a static system, and completely ignoring that Earth is in *orbit*. So it's in free-fall wrt the Sun. The relative attraction of the Sun is:
[latex]M_s / M_e . r^2_e / 1AU^2[/latex]

I don't have those numbers handy right now.
 
Wonder how many civilizations that reach the point where they can build a Dyson Sphere would instead chose to go non-corporial ?
Have you seen the new estimates for planets in the “goldilocks zone” numbers in the 10 of billions, due to the finding that a much larger amount of red dwarf stars (90% of the stars in a given galaxy) have planets.
There has got to be somebody out there, and some of them are bound to be doing “Macro engineering”.
 
kedo1981 said:
Wonder how many civilizations that reach the point where they can build a Dyson Sphere would instead chose to go non-corporial ?
Click to expand...

I think going non-corporeal would be SLIGHTLY less plausible than building a Dyson Sphere. By which I mean both would be freaking impossible.

I estimate, by stating some numbers that I just made up, that you would have to raid about a billion star systems to get enough building materials for such a project.
 
aggle-rithm said:
I estimate, by stating some numbers that I just made up, that you would have to raid about a billion star systems to get enough building materials for such a project.
Click to expand...

First, one of the major objections to a dyson sphere seems to be the strength required for the material. That, it seems to me, is unnecessary as you don't need a rigid sphere, rather that you can have a swarm of smaller objects orbiting the star that manage to absorb most of the light from the star. The last 10% is probably the most expensive and it's not a big deal to lose that.

I suspect that you'd start by putting up your solar panels as close to the star as feasible, and only when orbital considerations came in would you put them further and further out. Of course, if you're also building habitat you'd probably want to put that in an ideal zone, but it seems to me that you could instead just use your solar panels to get as much energy as possible from the star and then use that energy to heat, power, etc. your habitats and put them wherever convenient. Logistically it may work out better than trying to fit habitat into a specific part of your swarm.

As to how much material is required, that's a reasonable question, but I doubt it's more than the mass of, say, all our solar system's rocky planets and asteroids.

There's an issue though of whether or not it would end up being cheaper to get your energy from fusion reactors rather than directly from the sun. If you master fusion you may be able to build a more efficient one than that fusion reactor in the sky, and I suspect that the mass of, say, jupiter is enough to give off a great deal more power than the sun for quite a long time.
 
Let's clear up some of the thinking about Dyson spheres. First, the cannot rotate around a star as this generates huge torsional stresses as one moves in latitude toward the pole. So consider them stationary. Second, a stationary object of anywhere near these sizes (literally) gravitates toward a solid sphere. Hence, a Dyson sphere will fall into it's sun, in the absence of huge amounts of energy being expended to hold it "up". It can be figured whether that amount of energy is in excess of that output to the sphere by it's sun, I suspect that it is but that's a moot point except for one tangent. If it is in excess and the Dyson sphere existed, then the builders had a more powerful source of energy than their sun, and had no reason to capture their sun's energy. If it is lower than the output of the sun, it is still a huge fraction, and thus the gain from building the Dyson sphere is not so great (in other words).

Here's the simplified case: For a satellite in vicinity of the Sun, capturing Solar energy, can it hold it's altitude from the Sun with only that energy?


Now for the structural issues.

Remember "solids" are not "Solid" at these scales - that's why the planets Mercury, Venus, Earth, and Mars are round. The gas giants and the liquid planets are round for understandable reasons. But at these scales, first semester statics will show that "Solids" exhibit plasticity to the extent they can be considered a liquid. I'm ignoring the somewhat oppositional forces of the solar wind, the light pressure, solar flares and radiation as they although oppositional are largely chaotic and thus do not assist, but make the problem yet harder, and assist in collapsing the Dyson Sphere.

So there is no possiblity of "Dyson Spheres" existing. However, there could be stars with a large number of artificial objects of huge sizes in the solar equatorial orbital plane, for sure.

Assume they are either in the same orbit as the planet of the intelligent species, or in another orbit.

If these were sufficient in number, either there would not be a dimming during transit, or there would be a high frequency dimming and spectral change, or there would be a pronounced dimming and/or spectral change.

Just as we know there can only be a few stable orbital positions around the Earth eg the Lagrange points L4, L5, in relation to the Moon's orbit, so it would be the case with another star, with planets, to a first approximation. But any additional objects placed in points alongside the host planet's orbital plane would have to be truly, truly hugh to see them from our position.

An example of what to expect B=dimming from planet passing C=dimming from artificial object at Lagrange point ...

B,c,c,B,c,c

That's just off the top of my head. Someone that's studied it may improve on this.
 
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Roboramma said:
First, one of the major objections to a dyson sphere seems to be the strength required for the material. That, it seems to me, is unnecessary as you don't need a rigid sphere, rather that you can have a swarm of smaller objects orbiting the star that manage to absorb most of the light from the star. The last 10% is probably the most expensive and it's not a big deal to lose that.

I suspect that you'd start by putting up your solar panels as close to the star as feasible, and only when orbital considerations came in would you put them further and further out. Of course, if you're also building habitat you'd probably want to put that in an ideal zone, but it seems to me that you could instead just use your solar panels to get as much energy as possible from the star and then use that energy to heat, power, etc. your habitats and put them wherever convenient. Logistically it may work out better than trying to fit habitat into a specific part of your swarm.

As to how much material is required, that's a reasonable question, but I doubt it's more than the mass of, say, all our solar system's rocky planets and asteroids.

There's an issue though of whether or not it would end up being cheaper to get your energy from fusion reactors rather than directly from the sun. If you master fusion you may be able to build a more efficient one than that fusion reactor in the sky, and I suspect that the mass of, say, jupiter is enough to give off a great deal more power than the sun for quite a long time.
Click to expand...
Would this be a good time to bring up the differences between the Dyson Sphere (a network of solar energy collectors as you've described) and the rigid Dyson Shell.
Anyway I have to agree with your suggestion that fusion, fueled by gas giant sourced hydrogen, might be a more practical energy source. Though a network of solar collectors would probably be cheaper to operate, especially if you can't fuse light hydrogen and are limited to burning 2H and 3He.
 
nathan said:
The relative attraction of the Sun is:
[latex]M_s / M_e . r^2_e / 1AU^2[/latex]

I don't have those numbers handy right now.
Click to expand...

Now I do, as the Griffiths Observatory happened to have a display board! Ms is about 1/3 million Earth masses, so we have:

[latex]1e6/3 . 6.3e6^2 / 150e9^2)[/latex]

[latex](6.3/150)^2 / 3[/latex]

[latex]0.042^2 / 3[/latex]

[latex].0018 / 3[/latex]

6/10000 Earth's gravity.

So not much then :)
 
Roboramma said:
As to how much material is required, that's a reasonable question, but I doubt it's more than the mass of, say, all our solar system's rocky planets and asteroids.
Click to expand...

OK, I did the math, and according to my calculations (which could easily be way off), it would take 300 planets the size of Earth to build a hollow sphere one meter thick with a radius of one AU. That's less than I expected, but unless there are HUGE amounts of hidden materials in the asteroids, I don't think we have enough.

A bigger problem might be, how do we harvest the materials we need? My mind keeps going back to Phil Plait's explanation for why it isn't realistic to blow up a planet, the way it is depicted in science fiction movies. You have to imagine how much energy it would take to accelerate a single rock to escape velocity. You use this much energy to fling a rock skyward. Then you repeat until the planet is gone.
 
mhaze said:
Let's clear up some of the thinking about Dyson spheres. First, the cannot rotate around a star as this generates huge torsional stresses as one moves in latitude toward the pole. So consider them stationary. Second, a stationary object of anywhere near these sizes (literally) gravitates toward a solid sphere. Hence, a Dyson sphere will fall into it's sun, in the absence of huge amounts of energy being expended to hold it "up". It can be figured whether that amount of energy is in excess of that output to the sphere by it's sun, I suspect that it is but that's a moot point except for one tangent. If it is in excess and the Dyson sphere existed, then the builders had a more powerful source of energy than their sun, and had no reason to capture their sun's energy. If it is lower than the output of the sun, it is still a huge fraction, and thus the gain from building the Dyson sphere is not so great (in other words).
Click to expand...

could a dyson sphere be constructed thinly enough that solar ejecta would keep it inflated?
 
EdipisReks said:
could a dyson sphere be constructed thinly enough that solar ejecta would keep it inflated?
Click to expand...
That's certainly an interesting question. The easiest way to answer it would be to compare two factors say at earth's distance to sun:

Acceleration on a solar sail
G force of sun on a suspended object, not in orbit

I'm going for "no worky" on that one.

Next you could look at the force of the solar ejecta, protons and such.

Finally, one could vary the distance from the sun to see if it might work at some optimal distance.

But basically, photons are so tiny and ineffective as drivers, that this isn't really practical.

Now if someone could harness anti-matter, clearly you could do some very interesting things.

But "Dyson spheres" are not something to look for as evidence of intelligent life elsewheres. Having said that, as I mentioned earlier, a modulation pattern in light and/or spectra could well provide clues.

It certainly would not be impossible say to modulate a star's output in a particular direction to do morse code. Just one (silly) example.

Now where's that intersteller router and what's its IP address?

:)
 
aggle-rithm said:
....Phil Plait's explanation for why it isn't realistic to blow up a planet, the way it is depicted in science fiction movies. You have to imagine how much energy it would take to accelerate a single rock to escape velocity. You use this much energy to fling a rock skyward. Then you repeat until the planet is gone.
Click to expand...
That's why it's so much fun!
 
aggle-rithm said:
OK, I did the math, and according to my calculations (which could easily be way off), it would take 300 planets the size of Earth to build a hollow sphere one meter thick with a radius of one AU. That's less than I expected, but unless there are HUGE amounts of hidden materials in the asteroids, I don't think we have enough.
Click to expand...

Fair enough, but I'm not clear why we need to assume a radius of 1 AU. Couldn't it simply be closer in?
 
aggle-rithm said:
OK, I did the math, and according to my calculations (which could easily be way off), it would take 300 planets the size of Earth to build a hollow sphere one meter thick with a radius of one AU. That's less than I expected, but unless there are HUGE amounts of hidden materials in the asteroids, I don't think we have enough.
Click to expand...
That's about the mass of Jupiter.

aggle-rithm said:
A bigger problem might be, how do we harvest the materials we need? My mind keeps going back to Phil Plait's explanation for why it isn't realistic to blow up a planet, the way it is depicted in science fiction movies. You have to imagine how much energy it would take to accelerate a single rock to escape velocity. You use this much energy to fling a rock skyward. Then you repeat until the planet is gone.
Click to expand...
That's one of the reasons I'm skeptical about Dyson Shells; their creation seems to require such a level of technology that other avenues should be possible and easier.
 
catsmate1 said:
That's about the mass of Jupiter.
Click to expand...
Jupiter is mostly hydrogen though. I doubt it's a good building material for what we're talking about.

You could use fusion to turn the hydrogen into heavier elements, but in that case you're already getting your energy from the fusion. Mind you, in that case to turn that much hydrogen into heavier elements we have to take into consideration what the fusion devices themselves are made of. Either it will take a very long time or you're again dealing with energy levels similar to what the sun is putting out, in which case if you can harness that, why can't you harness the sun, again?
 
The thread started off being about detecting a Dyson sphere as way of doing SETI.
If there are civ's that can do it they would have to leave some kind of footprint if they can re-org entire solar systems.
Seems like there would be massive amounts of waste heat, gravitational disturbances, energy trails from ships.
 
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