What is the largest structure that could be built in this universe?
The first step would be space elevators: ultra-strong cables that rise over 36.000 kilometers into the sky to geostationary orbit and counterweights beyond.
A Dyson Sphere is a swarm of space stations orbiting or hovering around a star, capturing and converting all the radiation it emits.
One concept out of millions of unimagined ones is Larry Niven's 'Ringworld'.
But would it be possible to build a structure the size of a galaxy, or even the whole universe?
That would depend on the universe.
Space is expanding at an accelerating rate, but the speed of light stubbornly remains the same. At the moment, the boundaries of the observable universe are still receding. Every day new objects and patterns become visible. The cosmic background radiation is changing by the moment, though it would take many lifetimes to notice any difference.
Relatively 'soon', the expansion will be so rapid that galaxy clusters at the edge of visibility will begin to recede and vanish forever. Eventually, our local cluster of a few dozen galaxies will be cut off from the rest of the universe, surrounded by eternal darkness in all directions.
It seems such a waste to lose the stars, planets, and energy beyond the horizon, all the things we don't know we need yet. If mankind wants to exploit them, it will have to start expanding into intergalactic space within the next billion years.
The ultimate destiny of civilization is too great for humans to imagine. Unless civilizations are somehow compelled to destroy themselves, there is no limit to what can be accomplished.
One day, all the matter in the observable universe may be converted into the ultimate space structure. Like the rings of Saturn, it would be composed of many small components held together by gravity and by electromagnetic and perhaps nuclear forces. From the inside, it would look like a cloud of almost colliding particles.
This would be the home of humanity's remote descendants. The universe itself would have become self-aware.
How large such a structure can be depends on how soon construction will begin. Only a few billion years remain to mine the universe. Not including dark matter, it could weigh trillions of solar masses.
Computation improves as components are miniaturized. Ideally, this space structure would be composed of atomic-scale computing elements designed for maximum efficiency. A single element the size of a grain of sand would have more processing power than all computers on Earth today.
Building it would take much longer than gathering the required matter in the first place. Even disassembling stars into their atoms might be child's play compared to programming a transgalactic operating system.
What would be the design constraints of such a structure?
The active elements should be as close together as possible, but not too close. If all the matter in the observable universe were concentrated in a sphere ten light-years in diameter, it would instantly form a black hole. The components need to stay just outside the Schwarzschild radius, in ultra-high speed orbits or energy beams. Some would move near light-speed. Computations would be carried out by the particles and the distorted space-time surrounding them.
Such a universe-spanning structure might not even be visible: a black sphere against a black background. Beyond the cosmic horizon, there would be infinitely many other spheres, but they would have no way to communicate.
Even that can't be ruled out completely. Their particles might have become quantum entangled when the universe was younger, or they might use black holes or a network of wormholes.
A major problem would be cooling the network of tiny computing elements. Any process generates waste heat that carries away energy that can't be replaced. The waste heat must somehow be recycled.
No process is 100% efficient. As the cloud slowly loses energy, its computational processes will slow down. That wouldn't necessarily matter. The universe will go on forever, so the civilization could still be immortal.
Matter itself, however, does not last forever. Because of proton decay (if there is such a thing) or other quantum improbabilities, the structure will slowly lose mass. Imperceptibly, its components will start to fail. It could adapt in several ways: by increased efficiency and compressing data, but that only goes so far. Valuable information will be lost forever.
Eventually, the space civilization must die.
At the very end, shrinking down to the intelligence level of a single human being, there would be one 'final rational thought', the summation of everything that came before, a sense of great achievement and its ultimate purpose. What would it be? No one could imagine.
The best hard SF novel ever written: Infinite Thunder by Jack Arcalon.
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