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Not to be confused with Dyson, the vacuum cleaner company.
October 25, 2012 8:54 AM   Subscribe

Dark matter, or DYSON SPHERES?

What is a Dyson Sphere, and with all the dramatic possibilities surrounding (ha) them, Who's written about them?
posted by fnerg (70 comments total) 17 users marked this as a favorite

 
Named after the great scientist and inventor Miles Dyson, I assume.
posted by nathancaswell at 8:57 AM on October 25, 2012 [3 favorites]


Known to our friends across the pond as Kilometers Dyson.
posted by gauche at 9:00 AM on October 25, 2012 [16 favorites]


Known to our friends across the pond as Kilometers Dyson.

That's what he usually went by, but his full name was 1.609344 Kilometers Dyson.
posted by jcreigh at 9:04 AM on October 25, 2012 [12 favorites]


Ah, but if a civilization could produce the sphere, why not a Dyson tree? Equally advanced, although in a different discipline.
posted by boo_radley at 9:05 AM on October 25, 2012 [1 favorite]


Miles Dyson.
The actual creator, Freeman Dyson.
No relation.
No relation.
posted by IAmBroom at 9:10 AM on October 25, 2012 [1 favorite]


It's worth reading Niven's "Bigger Than Worlds" on this. It's, um, available online.
posted by Chrysostom at 9:10 AM on October 25, 2012


Quoted from the second article, for irony: "This wild speculation about futuristic alien tech probably seems unscientific, but the search for extraterrestrial civilizations has always depended upon such speculation."
posted by Nelson at 9:12 AM on October 25, 2012


Awesome theory Fnerg!

*There's no dark matter. We just live in a universe/galaxy in which most of the stars are actually invisible because the millions of alien civilizations naturally wrap their stars in Dyson Spheres the minute they get the technology together to do so.

Any physics minded Mefites want to educate me a little bit on a question I've had, though? Would you have to spin a Dyson Sphere up in order for it to stay in place relative to its star, perhaps in the plane of the elliptic? In that case you'd get nice artificial gravity on the plane of the elliptic but 99 percent of the sphere's surface area would be useless because the artificial gravity would only work there. Or if you didn't spin it up, then there'd be no artificial gravity at all...

Or am I just being anthropocentric thinking gravity matters all that much?
posted by jackbrown at 9:12 AM on October 25, 2012


You know, as whacky theories go, this one is pretty cool. The odds of that much of the universe being covered up by Dyson spheres is ... unlikely, to be generous. But we can't (currently) prove it's not true. Completely unverifiable and unrefutable hypothesis.
posted by IAmBroom at 9:13 AM on October 25, 2012


The Sphereworld is unstable!
The Sphereworld is unstable!
The Sphereworld is unstable!
 
posted by Herodios at 9:16 AM on October 25, 2012 [7 favorites]


The connection between Dark Matter and Dyson spheres is only made by the OP. Not in the articles posted.
posted by vacapinta at 9:16 AM on October 25, 2012 [3 favorites]


Herodios: "The Sphereworld is unstable!"

They fixed that with the attitude jets in The Sphereworld Engineers.
posted by Chrysostom at 9:19 AM on October 25, 2012 [5 favorites]


I've always thought Dyson Spheres would be rare. If you could control incredible energies, would you stay and work on a giant construction boondoggle with your uncle or go cruise the galaxy with your antimatter drive?
posted by RobotVoodooPower at 9:21 AM on October 25, 2012 [1 favorite]


Here's a blog post discussing a scientist who plans to use data from the Kepler telescope to look for Dyson sphere type objects.
posted by audi alteram partem at 9:23 AM on October 25, 2012


The connection between Dark Matter and Dyson spheres is only made by the OP.

Yes, and accordingly, it should be deleted as original research.


Am I on the wrong website?
posted by goethean at 9:25 AM on October 25, 2012 [4 favorites]


What if you could control huge energies, but light speed is still the limit?
posted by Chrysostom at 9:25 AM on October 25, 2012


I hate Dyson Spheres so much. That smug English guy in the TV commercials pretends that having that stupid thing on your vacuum instead of wheels will make it work better or something. Look: A vacuum cleaner is not a racing car. It doesn't need to go around chicanes and hairpins and whatnot. I want to vacuum the whole floor, and that means I need to go in straight lines, back and forth. Stupid, stupid, stupid. It's no wonder that advanced alien civilizations would get so fed up with them that they would eject them all into space.
posted by The World Famous at 9:25 AM on October 25, 2012 [23 favorites]


If you could control incredible energies, would you stay and work on a giant construction boondoggle with your uncle or go cruise the galaxy with your antimatter drive?

Yes, but I would do it in invisible mode so as not to disrupt the primitives.
posted by goethean at 9:26 AM on October 25, 2012


RobotVoodooPower: I've always thought Dyson Spheres would be rare. If you could control incredible energies, would you stay and work on a giant construction boondoggle with your uncle or go cruise the galaxy with your antimatter drive?
Because:
Chrysostom: light speed is still the limit
1. Build an eden here around Sol, or
2. Build a ship, and spend the next thousand years in transit.
posted by IAmBroom at 9:27 AM on October 25, 2012


Or am I just being anthropocentric thinking gravity matters all that much?

Not a biologist or a physicist, but I'd imagine gravity plays a pretty considerable role in the physiological development of any, say, multicellular organism that evolves on, in, or just above the crust of a planet.
posted by gauche at 9:27 AM on October 25, 2012 [1 favorite]


I hate Dyson Spheres so much. That smug English guy in the TV commercials pretends that having that stupid thing on your vacuum instead of wheels will make it work better or something. Look: A vacuum cleaner is not a racing car. It doesn't need to go around chicanes and hairpins and whatnot. I want to vacuum the whole floor, and that means I need to go in straight lines, back and forth. Stupid, stupid, stupid. It's no wonder that advanced alien civilizations would get so fed up with them that they would eject them all into space.

And, FWIW, they do lose their suction. Surely aliens would come up with something better like a 'Vax' or a 'Henry'?!
posted by run"monty at 9:31 AM on October 25, 2012 [3 favorites]


But we can't (currently) prove it's not true. Completely unverifiable and unrefutable hypothesis.

That sounds like some sort of belief system to me.
posted by Old'n'Busted at 9:32 AM on October 25, 2012


Not a biologist or a physicist, but I'd imagine gravity plays a pretty considerable role in the physiological development of any, say, multicellular organism that evolves on, in, or just above the crust of a planet.

I am a physicist married to a biologist and as far as I can tell gravity seems to be irrelevant (or super duper close to it) for basic life processes. Large scale things would need to change somewhat, but when you're building a Dyson sphere, that seems like it would be a small correction.
posted by Shutter at 9:36 AM on October 25, 2012


Previously on the blue: How to build a Dyson sphere in 5 easy steps.
posted by James Scott-Brown at 9:36 AM on October 25, 2012


I hate Dyson Spheres so much. That smug English guy in the TV commercials pretends that having that stupid thing on your vacuum instead of wheels will make it work better or something.

And what's up with his silly ring-fan? It's major point is that it eliminates "buffetting", but sometimes I'm really fucking hungry, you know?
posted by LionIndex at 9:37 AM on October 25, 2012 [11 favorites]


gravity seems to be irrelevant (or super duper close to it) for life processes

No gravity, no liquid water. No liquid water, no life. Yeah?
posted by Egg Shen at 9:40 AM on October 25, 2012


The dark matter connection is a total red herring. Here's why:

The missing matter that dark matter explains cannot be Dyson spheres. This is for several reasons. First, we have multiple measures of the energy density of the Universe. The Universe is composed of "matter," which is energy that is non-relativistic - this means that as the Universe expands, the energy density of matter drops as the volume (length^3). There is also "radiation:" relativistic energy (photons or very light particles). This means that it's energy density drops as length^4 (3 powers for volume, one extra one because radiation red-shifts as the Universe expands). Then there is dark energy, which does not change with expansion (length^0, if you want to be specific).

Today, "matter" makes up about 30% of the total energy density of the Universe. However, we know from looking at the cosmic microwave background (CMB) that not all of the matter interacts with light. Roughly, the CMB is a snapshot of the Universe at one particular moment in time (~300,000 years post-Big Bang, when things cooled enough that the Universe just became transparent to light), and we already see in the CMB the formation of large structures. If all matter coupled to photons, then that structure would not have been able to form in the way it does (the photons would have been blowing proto-proto-galaxies apart for too long). Thus, there must be at least two forms of stuff that is non-relativistic: one that interacts with photons and one that doesn't.

Another thing that tells us the same thing is the primordial abundance of elements beyond hydrogen. In the very early Universe (~ < 11 minutes), the free neutrons and protons combined to form elements beyond simple hydrogen. This is a very delicate process, as free neutrons decay with a half-life of 11 minutes, and so the number of neutrons as well as the expansion of the Universe come into play in the prediction. If all the matter were in protons and neutrons AND the expansion of the Universe was as predicted by the total matter density, we'd get a different amount of heavy elements like lithium then we observe (though those observations are difficult). Again, we need two different types of matter: one that makes elements and then more that controls the expansion.

Both these separate measurements (along with measurements of galaxy rotation and clusters of galaxies) give the same results for normal (baryonic) matter: the stuff like us that makes up stars and interacts with photons etc, and dark matter, the rest that doesn't. Today, the Universe is ~70% dark energy, ~25% dark matter, ~5% baryonic matter, and a negligible percentage of radiation and neutrinos. These numbers change over the age of the Universe: at the start it was radiation dominated, moved to matter domination "today," and in the far future will be dark energy dominated. It is strange and perhaps intriguing that we live in a relatively short (only a few 10's of billions of years) when all the types of energy are important in the Universe.

What does this have to do with Dyson Spheres? To point is, just cause we can't see something doesn't mean it can be hiding behind a wall, so there simply cannot be a huge number of stars that we just aren't observing. If that wall is made out of normal matter, than it would have contributed in the early Universe, and we would know about it (unless you're postulating either that your aliens grabbed the matter and hid it in the first 11 minutes of the Universe, in which case: dude, heavy elements didn't even exist yet). The evidence for dark matter is much more extensive than "the galaxies are missing stars."

As for Dyson spheres around a star, they are in an unstable configuration. Like a ball at the top of a very steep spire, the star will stay in the center only as long as it is EXACTLY in the center: the smallest perturbation will not correct itself, and will eventually cause the sphere to collide with the star. As with the Niven Ringworld, you'd need active correction systems (basically attitude jets) to keep the star centered. Spinning wouldn't help; spin stabilization would keep the Dyson sphere pointed in the same direction (relative to the "fixed stars" of the rest of the Universe), but wouldn't prevent drift.

Also, the spin would require materials of incredible tensile strength. Just building a non-rotating Dyson sphere is ridiculous from a material science perspective; I vaguely recall a spinning one would require a tensile strength on the order of the strong nuclear force, but I could be overestimating that. This sort of problem is why the original Dyson sphere idea was more of a cloud or swarm: a bunch of energy-collecting satellites that orbit separately rather than a solid wall. Amusingly, the TVtropes page covers this very nicely.
posted by physicsmatt at 9:41 AM on October 25, 2012 [53 favorites]


"If you could control incredible energies, would you stay and work on a giant construction boondoggle with your uncle or go cruise the galaxy with your antimatter drive?"

Maybe if you're descended from curious apes, but if you come from a long line of bowerbirds or mollusks or something the first instinct might be to build a really nice nest. Make the home system into a paradise and keep that star burning long after it would have normally cooled by stirring things up with magnetic fields from time to time. Huddle closer as the light dims. Maybe they just aren't as interested in exploration as us.
posted by Kevin Street at 9:43 AM on October 25, 2012 [2 favorites]


Shutter I am a physicist married to a biologist and as far as I can tell gravity seems to be irrelevant (or super duper close to it) for life processes.

For basic biochemistry/molecular biology, it's unimportant.

For the physiology of multicellular organisms it's very important. To give a few examples: bone density is reduced in astronauts in low-gravity; the necessity of supporting an animal's weight determines the size and shape of its skeleton; standing up requires physiological changes to maintain constant blood pressure (similarly, an animal like the giraffe needs various modifications to its circulatory system to pump blood up to it's head against gravity, and cope with the head being lowered form above to below the body during drinking).
posted by James Scott-Brown at 9:44 AM on October 25, 2012 [2 favorites]


physicsmatt, I can always count on you to tell me the truth. Even though it hurts.
posted by digitalprimate at 9:47 AM on October 25, 2012 [1 favorite]



No gravity, no liquid water. No liquid water, no life. Yeah?


Just put a lid on your proverbial bucket and you're good to go. Water works just fine without gravity.
posted by Shutter at 9:48 AM on October 25, 2012 [1 favorite]


The simplest explanation for why dark matter can't be Dyson spheres is just:

Dyson spheres don't actually reduce the amount of energy radiating from a star, they just change its spectrum to have lower frequency and higher entropy. We can see (and have apparently specifically searched for) such "Dyson sphere waste heat" spectra. The Atlantic link actually covers this, briefly.
posted by roystgnr at 9:49 AM on October 25, 2012 [5 favorites]


light speed is still the limit

Time dialation! Everyone and everything you know will be long dead, but you'll be just a few months older. This is how teenagers in advanced alien civilizations will rebel against their parents.
posted by RobotVoodooPower at 9:51 AM on October 25, 2012 [2 favorites]


IAmBroom: "You know, as whacky theories go, this one is pretty cool. The odds of that much of the universe being covered up by Dyson spheres is ... unlikely, to be generous. But we can't (currently) prove it's not true. Completely unverifiable and unrefutable hypothesis."

Can you explain why you believe this to be unlikely? Also, how is it unverifiable if there are astronomers actively looking for the evidence, as noted by the second article? Isn't that the exact opposite of "unverifiable?"
posted by danny the boy at 9:58 AM on October 25, 2012


In the previous fpp that James Scott-Brown linked to, Blackanvil makes a really good point that may be relevant to anyone looking for Dyson Spheres. That is, heat dissipation might be a problem, especially over giant time scales. So maybe the best solution to that is to build your Dyson Cloud really big, maybe even so big that the waste heat looks a lot like the cosmic background radiation. So even if aliens existed and aren't trying to hide, physical restrictions might force them to build their structures in ways that are hard to detect from a distance.
posted by Kevin Street at 9:59 AM on October 25, 2012 [2 favorites]


Intergalactic dark matter filaments or FUNERARY BARGES MADE OF DYSON SPHERES SEEKING COMMUNION WITH THE HEAT DEATH OF THE UNIVERSE.
posted by ennui.bz at 10:04 AM on October 25, 2012


physicsmatt, I agree with your comment that Dyson spheres cannot be the dark matter that appears in CMB analyses. However, isn't 'dark matter' of some type also invoked as a possible solution to the fact that observed galaxy rotation profiles are inconsistent with observed matter distributions? I would think that Dyson spheres *could* be a possible (though, unlikely) solution to that inconsistency.
posted by BlueDuke at 10:04 AM on October 25, 2012


I hadn't seen Blackanvil's point; thanks. Dyson sphere equilibrium temperature (in Kelvin) is inversely proportional to the square root of the radius of the sphere, though, right? For a star like ours that's something like 400K at 1AU, so getting partially camoflagued in the CMB would require something like a 10,000 AU wide sphere. Not impossibly ludicrous by cosmological engineering standards, but at that point I'd think that other exotic solutions (dumping waste heat into a black hole, maybe? even the CMB is radiating heat back at you...) might become more attractive.

Landauer's Principle suggests that, as much as we worry about heat generation when computing now, we haven't seen anything yet. We're nowhere near the theoretical efficiency of what our computers (or our brains) can do, but if we ever get there, it will probably be important that that efficiency is inversely proportional to temperature.
posted by roystgnr at 10:20 AM on October 25, 2012


The missing matter that dark matter explains cannot be Dyson spheres.

I think the Dark Matter being discussed is the Missing Baryonic Matter.
posted by Slap*Happy at 10:35 AM on October 25, 2012 [3 favorites]


Okay, so what if we find* one?

I think I'd be terrified of them in exactly the same way I was terrified of black holes when I five.

I think I might even refrain from going outside at night on the off chance that these incomprehensibly enormous objects constructed by inconceivably advanced civilizations separated from us by unimaginable distances away might eat me.

* Locate an object that we're pretty sure could only be a Dyson Sphere.
posted by RonButNotStupid at 10:41 AM on October 25, 2012


Yes, but can it lift a bowling ball with the power of its suction?
posted by radwolf76 at 10:45 AM on October 25, 2012


The odds of that much of the universe being covered up by Dyson spheres is ... unlikely, to be generous. But we can't (currently) prove it's not true. Completely unverifiable and unrefutable hypothesis.

How many stars are known to have simply disappeared* in the past 10,000 years or so?

*Glenn Miller doesn't count, smart-ass.
posted by Herodios at 10:47 AM on October 25, 2012


danny the boy: IAmBroom: "You know, as whacky theories go, this one is pretty cool. The odds of that much of the universe being covered up by Dyson spheres is ... unlikely, to be generous. But we can't (currently) prove it's not true. Completely unverifiable and unrefutable hypothesis."

Can you explain why you believe this to be unlikely? Also, how is it unverifiable if there are astronomers actively looking for the evidence, as noted by the second article? Isn't that the exact opposite of "unverifiable?"
Unlikely, because it would mean 30% of the Universe is already covered in Dyson spheres. That's a buttload of civilization!

And, I'm wrong about "unverifiable". physicsmatt made that painfully obvious! Dammit... Back to plotting the overthrow of governments, and the nuclear strong force.
posted by IAmBroom at 11:06 AM on October 25, 2012


roystgnr, yeah just go with the Matrioshka brain idea and make progressively bigger Dyson spheres to extract the maximal entropy from the star's light for computation. Make the outer shell big enough and you can get your waste heat arbitrarily close to the CMB 2.7 K limit.

Blue Duke, yes dark matter was originally invoked in order to explain the rotation curve problem for spiral galaxies and the velocity dispersion of the Coma cluster. Basically, visible matter in the Universe is moving too fast to be gravitationally bound by the stuff we see. We can extract a number for the amount of dark matter necessary for the rotation curves to match, and this dark matter could just be regular baryons that we can't see. MAssive Compact Halo Objects (MACHOs) were the first idea for this kind of dark matter, and absolute shitloads of Dyson spheres (outmassing the visible galaxy) is a completely kosher explanation. However, the other measurements I mentioned give separate measurements for NON-baryonic matter in the Universe, and these match within the error bars the amount of matter needed for rotation curves. So these combinations are pretty conclusive in saying that whatever the dark matter is, it cannot be made up of normal atoms. Since Dyson spheres are supposed to be normal stuff, they cannot be the entire component of dark matter (nor can any other MACHO candidate).
posted by physicsmatt at 11:06 AM on October 25, 2012 [1 favorite]


Once again, physics ruins everyone's fun.

(though seriously, the physics that the Universe realizes is far more intricate and interesting than most of the alternatives we've come up with over the years. Newtonian physics is way less interesting than relativity, for example. Though this intricacy means that things you might want - like super-light travel - are impossible)
posted by physicsmatt at 11:17 AM on October 25, 2012


I still like my idea: a Dyson hemisphere, made of light enough material that the solar pressure on it is enough to keep it in place. Since it's only half a sphere, it will generate thrust and you can move the whole solar system around.

Of course, if the dark matter is because of alien civilizations doing that, it means THEY'RE ALL COMING FOR US
posted by ckape at 11:31 AM on October 25, 2012


You're not really ruining anyone's fun, physicsmatt. If I understand right, you're saying that the op's assertion dark matter = Dyson Spheres (which he probably wasn't too serious about in the first place) is incorrect, and that's cool. If there were more Dyson Spheres than visible stars the universe would be a pretty weird place anyway. But isn't it still possible for Dyson Spheres or Clouds to be out there, making up some percentage of baryonic matter?
posted by Kevin Street at 11:32 AM on October 25, 2012


Kevin Street: isn't it still possible for Dyson Spheres or Clouds to be out there, making up some percentage of baryonic matter?

*Could* there be one or more Dyson spheres in our Galaxy? Almost certainly yes.

Might there be enough of them to make up a significant fraction (10% or even 5%, say) of the normal baryonic matter? Probably not...

We've semi-seriously batted around the idea of using surveys like 2MASS and other optical / infrared all-sky surveys to look for "weird" spectral signatures. One of those long shot projects with high payoff if it works, but the details are really very messy. Stars move, detector sensitivity varies, cosmic rays ping detectors...
posted by RedOrGreen at 11:52 AM on October 25, 2012


Kevin, absolutely, some Dyson spheres could exist. In fact, I was cheating a bit in my previous explanations (though any non-technical description is inherently rife with oversimplifications), so let me correct that a bit.

The visible stars make up only a fraction of a percent of the energy density of the Universe, most of the baryons are "dark" in the sense that they are not hot or near a galaxy. So there's plenty of baryons you could scoop up (over the scale of megaparsecs) to build as many Dyson spheres as you damn well please. Some of that gas is what the other links in this post are talking about - they might glow dimly if they have an orphan star in them. This reminds me of Ian Banks' Against a Dark Background, actually.

However, this doesn't really avoid the baryon problems I was talking about as to why we no longer believe in MACHOs as the galactic dark matter, for four reasons. First, we have good measurements of the total dark matter in galaxies, and it agrees within the errors of the independent measures of non-baryonic dark matter. Second, we now have pretty accurate computer simulations of how galaxies form in the presence of dark matter, and they are in very good agreement with observations (though there are some interesting deviations, but those occur where we might expect resolution problems to creep in). This agreement requires there to be lots of dark matter in the galaxy, with a very different spatial distribution (no disk and much much larger than the place where the stars are), leaving little room for hidden baryons. Third, we can see dark matter in other galaxies through gravitational lensing; and that distribution matches the predictions of the previous two points. Finally, we are beginning to measure directly the local dark matter in our Galaxy through measuring the motion of nearby stars. The results have huge error bars still, but again, it appears to match the non-baryonic predictions.

But yeah, take 1 out of a million stars and drop a sphere around it, and we wouldn't know from any of the dark matter arguments I just described. We'd also have 200,000 Dyson spheres in the Milky Way. So things can still be interesting out there.
posted by physicsmatt at 12:01 PM on October 25, 2012 [1 favorite]


physicsmatt: "MAssive Compact Halo Objects (MACHOs) were the first idea for this kind of dark matter"

A mention of MACHOs and no mention of Weakly Interacting Massive Particles (WIMPs)?
posted by Chrysostom at 12:07 PM on October 25, 2012


ckape, didn't something like that show up in an E. E. "Doc" Smith story? I could swear I've seen it in SF somewhere.

Taken to its logical conclusion, a thin enough Dyson shell could 'float' on photon and wind pressure. Dunno what the dynamics of that would be.
posted by hattifattener at 12:36 PM on October 25, 2012


Morgan Freeman Dyson. Now there's a movie!
posted by blue_beetle at 12:37 PM on October 25, 2012


Today, the Universe is ~70% dark energy, ~25% dark matter, ~5% baryonic matter, and a negligible percentage of radiation and neutrinos. These numbers change over the age of the Universe: at the start it was radiation dominated, moved to matter domination "today," and in the far future will be dark energy dominated.

Why do you say that matter dominates today if it's only 5% (or 30% counting dark matter)?
posted by straight at 12:47 PM on October 25, 2012


In that case you'd get nice artificial gravity on the plane of the elliptic but 99 percent of the sphere's surface area would be useless because the artificial gravity would only work there.

I think the point of the Dyson sphere is more efficiently capturing the energy from your star than increased living space. Although that strip of gravity you'd get at the equator of a spinning Dyson sphere would by itself give you an enormous amount of room.
posted by straight at 12:50 PM on October 25, 2012


straight: baryonic chauvinism. Yes, dark energy is more dominate today, but it's the same order of magnitude, and there are still large structures built of matter around (clusters of galaxies and so on), so we'll say we're still in the matter era. When you work on things of this scale, as long as you can see both components on a linear plot, you might as well call them equivalent.

As a side note, getting accustomed to calling things "the same" when they are within a factor of 2 or so of each other makes balancing your checkbook a bit tricky.
posted by physicsmatt at 12:59 PM on October 25, 2012 [1 favorite]


Dork Matter, more like.
posted by bicyclefish at 1:04 PM on October 25, 2012 [1 favorite]


the cover of this book provided my most memorable encounter with Dyson spheres.

Later editions were much less graphically striking.
posted by mwhybark at 1:09 PM on October 25, 2012


The amount of material needed for such a sphere would be incredible. If you spread the whole earth out to the surface area of a sphere of a 93million mile radius it would be barely thick enough to hold together, if it would reach at all. In addition, we'd still be able to detect the stars by their gravity. Not that it's a serious hypothesis, but still..

Also from Wikipedia - "Such a shell would have no net gravitational interaction with its englobed sun (see Shell theorem), and could drift in relation to the central star. If such movements went uncorrected, they could eventually result in a collision between the sphere and the star—most likely with disastrous results. Such structures would need either some form of propulsion to counteract any drift, or some way to repel the surface of the sphere away from the star."
posted by estuardo at 4:06 PM on October 25, 2012


Wouldn't the solar wind exert outward pressure on the dome that would increase on any part of the dome that moved closer to the sun?
posted by straight at 5:01 PM on October 25, 2012


Wouldn't the solar wind exert outward pressure on the dome that would increase on any part of the dome that moved closer to the sun?

The thing is that any kind of pressure that works according to an inverse square law with respect to a point source - be that inward pressure from gravity, or outward pressure from radiated particles - gets neatly cancelled out when applied to the inside surface of a sphere.
posted by flabdablet at 5:47 PM on October 25, 2012


In addition, we'd still be able to detect the stars by their gravity.

Trying to explain all the unseen mass needed to explain the gravitational mechanics of galaxies is exactly the point of the dark matter hypothesis, isn't it? Dark matter is pretty much defined as stuff we can only detect gravitationally.
posted by flabdablet at 5:50 PM on October 25, 2012 [1 favorite]


Also, Dyson vacs do indeed totally suck, but they're noisy as hell.
posted by flabdablet at 5:51 PM on October 25, 2012


What makes you think Dyson Spheres would be dark? If they're dark they're taking on the entire radiant output of their star, and completely absorbing it. Think we have a greenhouse effect on earth? Ha!

Even if they're "using" all the energy of their star, they still have to cope with waste heat. Or the laws of thermodynamics are really just guidelines.
posted by Kid Charlemagne at 5:56 PM on October 25, 2012


Good point flabdablet, in both cases.
posted by estuardo at 6:01 PM on October 25, 2012


Even if they're "using" all the energy of their star, they still have to cope with waste heat.

No problem there. All you need is a simple wormhole heat pump and you can dump all that stuff back out anywhere you like. Stuffing it back into the core of a nearby non-enclosed star would be tidy.
posted by flabdablet at 7:21 PM on October 25, 2012


Or if you don't like my wormholes (but who doesn't like a wormhole?) perhaps the civilizations occupying these Dyson spheres are just so good at the whole energy ladder thing that they only radiate at a temperature indistinguishable from the cosmic microwave background.
posted by flabdablet at 7:26 PM on October 25, 2012


Yeah, that's the part that I didn't quite get about the waste heat comments. Unless I'm missing something, why are we assuming that a civilization technologically advanced enough to build a sphere wouldn't also have energy collection technology that has near perfect efficiency?

Also it suddenly occurs to me that the dyson sphere episode of ST:TNG doesn't make sense because you wouldn't be able to see the sphere on the viewscreen because there's no nearby light source to illuminate it. Artistic license, of course.
posted by danny the boy at 8:33 PM on October 25, 2012


Flabdablet: Stuffing your waste heat into the core of a star would take more work than you got out of it in the first place (unless it came from somewhere hotter than the core of a star, that is). Generally, your heat sink has to lead to somewhere colder than you are or it won't work. This is a thermodynamics thing. If it helps, think of "waste heat" as the way your civilization gets rid of excess entropy (generated by irreversible processes like computation, life, etc.). The most effective setup is to absorb all your star's output and then re-radiate it at as low a temperature as possible: the temperature difference determines how much work you can do with that energy, and the size of the radiating surface determines how cold it can be and still radiate enough.

Hence the discussion above about 1-AU spheres radiating at roughly Earthlike temperatures (visible in the infrared), or CMB-camouflaged spheres having to be as big as the Oort cloud.
posted by hattifattener at 10:39 PM on October 25, 2012


You have some problem with spheres as big as the Oort cloud? Think big, man.
posted by flabdablet at 11:19 PM on October 25, 2012 [1 favorite]


Who said they had a problem with large spheres? I'm just talking about the properties of "waste heat".
posted by hattifattener at 1:20 AM on October 26, 2012


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