Dammit! Somebody’s on to me!
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the moving masses in a hollowed planet are better comprehensible and imaginable for laymen like me, I prefer them over your zero mass matter : P
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So long as you only get one [edit: non-pregnant] watchmaker…
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Sure, but that’s not a very interesting construct (unless one is a mathematician, I guess). What’s interesting for those with a more engineering bent is when we go just a little beyond what is practical for our present state of technical and industrial capacity. What could we build if we were a bit smarter and had space-based industrial capacity?
The limit for solid objects of rock before which they collapse into spheres is reckoned to be about 400 km diameter. If steel were used — say to make an orbiting statue of the immortal Trump, last and greatest president of the US — since steel is about 4 times stronger in compression, but also about 2.5 times as dense, the thing would collapse into a sphere at about 600km average diameter.
One could get increase this limit by making the thing hollow, like the Statue of Liberty, but my smattering of engineering knowledge fails me in deciding just how large that could be.
On the other hand, if we take a spacecraft to be mostly empty space (even with the planet destroying machinery, the iconic Death Star would need access space to adjust and maintain it, and space for the personnel to live in), the average density would lower, raising the theoretical (in the sense of I’m-just-spitballing-it) limit. From my extensive experience building spaceships for Traveller and GURPS roleplaying games 
and by comparison with wet navy ships, I think we could put the average density of a Star Wars or Star Trek spaceship at around 0.1 tonnes/m3, to within an order of magnitude. This is 80 times less dense than steel, so I think you could make a non-spherical spaceship as big as 48,000 km, though I’d want a generous safety margin. So say, 24,000 km. (Which at 0.1 tonnes per cubic metre would be around 700 billion tonnes edit: _giga_tonnes.)
The interesting thing is that as one goes deeper into an object, the gravitational field lessens (less mass below, more mass above), so while the supporting structure has to bear the increasing weight of the structure above it, that weight will not increase steadily. The very centre of the ship could be fairly open, while the surface would be fairly heavily constructed, I think.
Of course, Star Trek/Wars technology includes manipulation of gravity fields, which, without any idea of the limitations of such, makes speculation on size limits difficult. As long as the power doesn’t give out. Since loss of power could result in an embarrassing collapse, though, Imperial/Federation engineers might want to stick to the mechanical limits when tempted to build megaships, and at that stay with fairly conservative structures.
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More great related sci-fi:
IIRC, Star Trek also had structural integrity fields to somehow strength the molecular bonds between atoms. It sounded neat as a kid, but when I took quantum physics in college, I realized this couldn’t work because of various issues such as degeneracy pressure. Then I took quantum field theory in grad school and realized it really couldn’t work because altering the strength of those bonds, even if you had some technomagic means to acomplish it, would mess with their physical and chemical properties.
In short, chemistry and materials science works the way it does for good logical reasons based on the most fundamental laws of the universe, and altering them willy-nilly isn’t in the cards. Altering the spacetime metric, on the other hand, a la gravity manipulation and artificial warping of spacetime, is a little less fantastical in that we don’t really know what spacetime is, but we know the relativistic differences between reference frames can lead to relative differences in mass and size (Lorentz transformation for example).
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We mustn’t forget the most powerful force in the universe, the music of John Williams 
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Huh. That’s not how I thought of structural integrity fields at all.
I thought of them more like force fields that prevented the material from warping or breaking apart under stress, like a compression bandage.
Which, if you’re doing that, why don’t you just hold the whole ship together with force fields? But then, maybe they only activate when the support structure is under unusual stress. And there’s less risk of the whole ship destroying itself during a power outage, too.
But, yeah, I never considered it as actually holding things together at an atomic or molecular level.
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[quote=“nimelennar, post:29, topic:82982”]
But, yeah, I never considered it as actually holding things together at an atomic or molecular level.[/quote]
Yeah, I had always sort of envisioned it as an artificially-applied force buttressing the existing physical structure of ship, a la magnetic ball toy sculptures, rather than changing the material properties of the existing structure.
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I was a serious closet nerd in my misspent youth, so I did highly productive things like memorize the Star Trek encyclopedia and technical manuals. The way the the TNG tech manual described the structural integrity fields on the Enterprise D was more like a molecular reinforcement than the force-fields they used on things like the cargo bay pressure curtains. But if it is a force field then you have to ask a few other problematic questions.
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If the field is adjacent to the molecular structure, then you have the same problem faced by fighter pilots. The flight suits can compress the blood flowing on the outside of the skin, but blood still flows freely through the internal organs. Or another analogy is dropping a wrench in a submarine. The wrench can still dent the deck even though the hull is under buoyancy.
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What is the force field made of? If it’s a electromagnetic field that’s somehow been constricted without subjecting the crew and equipment to fields that make an NMR/MRI machine seem mild by comparison, the whole hull would still have a powerful electromagnetic charge. If it’s an as-yet unknown “subspace” force beyond the five fundamental forces (electromagnetism, weak force, strong nuclear force, gravity and whatever causes the “cosmological constant” accelerated expansion of the universe), then we’re either back in spacetime metric engineering or we’re dealing with a completely made-up force.
And of course I think it’s pretty open to interpretation. Treknology was made up by some very clever and dedicated art department geeks, but the shows’ writers and most of the audience didn’t much concern themselves with how the light switches functioned.
ETA: Also, and this is one of the lessons of quantum field theory, energy fields are actually the exchange of bosons between particles with certain properties, including those that give them inertial mass (Higgs field interaction and mass-energy density). Consequently the idea of an electromagnetic force-field compressed in a given dimension without atoms to act as field conductors (electrons and the protons holding them from flying apart) itself reflects a bit of a misunderstanding the laws of physics. So either the force field is of a sort that reinforces the electromagnetic bonds between atoms (AKA chemical bonds) by flowing through them, or it’s separate and acts like a wall, in which case what holds the wall of the force-field together? The answer is that you need opposing charges, which can’t be entirely mass-less because of the laws of QFT.
There is a speculative type of matter called positronium composed of a positron and electrons (two very low-mass particles) in a mutual orbit. Unfortunately, this kind of “atom” is about as unstable as possible, and the particles attract and annihilate immediately. If it exists at all, it’s in the moment before those two antiparticles annihilate in particle colliders.
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OK, all of this has compelled me to insert the following clip
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Noone is proposing to alter them will nilly, but to alter them with style.
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The good news is that if the universe is a simulation, they might be reprogrammable. Does that count?
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if the universe is a simulation, i’m pretty sure the operators are currently getting soused on a three martini lunch.
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As long as they use gin. If they’re drinking vodka, I’m crawling up the cable to knock some sense into them.
ETA: If we do hack the simulation, I should probably not be out first ambassador to the universe upstairs 
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Segfault
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Hmmm…now I want to figure out if a protective wrapper is viable on a quantum operating system.
Just remember, when they turn us off, it was your fault. [JK…how you gonna remember?]
