- A physical system that exchanges neither matter nor energy with the outside.
- In later galactic physics, the term came to mean a consensus about what should be called “outside.”
- After the extinction, it came to refer to a world closed so completely that those inside could not discover what lay beyond it.
Energy does not disappear. Galactic civilization never abandoned that sentence, even at the end. Stars lost mass by emitting light, and planets absorbed that light, warmed, and returned it to space as radiation of longer wavelengths. Life captured part of that flow to build bodies, and when those bodies died, they returned the energy they had stored as heat and matter. The form changed, but the total amount of energy did not. What decreased in one place increased somewhere else. Even when the first photon deficit was discovered, physicists did not doubt the conservation law. What they doubted were the instruments, the reference time, and the definition of the observer. They considered it more likely that every device installed across the galaxy was wrong in the same way than that the law itself was wrong. Yet the deficits appeared regardless of the type of instrument. Unexplained shortfalls were found in light emitted by stars, light passing through wormholes, heat from gas orbiting black holes, and radioactive decay inside planets. The magnitude and pattern of the shortfalls differed. What had first appeared to be a phenomenon affecting only light was later found, as measurements became more precise, to exist in mass and momentum as well. A single atom did not become lighter than it had been the day before. No planet suddenly left its orbit. In large-scale measurements, the deficit became most visible when comparing energy flowing between objects. Slightly less was absorbed than had been emitted. Yet photons also disappeared inside closed cavities where energy was not being transferred to another object. The outflow was not limited to energy passing through wormholes or power networks. The universe seemed to collect a toll whenever energy flowed. The missing share was not heat. If it had been heat, the surrounding temperature should have risen, but no such effect was observed. If it had possessed mass, gravity should have remained. The explanation that the energy had been stored in the vacuum was also abandoned. As the deficit accumulated, the expansion rate of the universe did not change within measurable limits. One possibility remained: the observed galaxy was not a closed system. If so, civilization would have to deny its own understanding of itself. The problem appeared in language before it appeared in physics. The word universe meant everything that existed. By definition, there could be no outside to everything. To say that energy had gone outside, humanity first had to acknowledge a place that was not the universe.
Humanity had long tried to create small closed systems. Thermos flasks, vacuum chambers, and interstellar storage facilities reduced exchange with the outside, but they never eliminated it completely. Gravity passed through walls, quantum fields filled both inside and outside, and over long periods every shield emitted radiation. Perfect isolation was an ideal used for calculation. Wormholes overturned that ideal. Instead of dividing inside from outside with a wall, they bound two distant locations into a single boundary. There was no space between the entrances, yet energy crossed from one to the other. Civilization had learned to eliminate distance, and the last scientist used that technology to create a place where the energy already leaking away could gather. The storage space he created was not a room inside the universe. It was not a hidden planet somewhere in the galaxy, nor an empty region inside a black hole. It was a new causal domain connected to existing spacetime by a single throat. Events inside it could not affect the outside without passing through that throat. No external light illuminated its interior, and no gravity generated inside could be measured directly from outside. The size of the space did not matter. Measured from outside the storage space, its entrance could be smaller than an atomic nucleus. The volume inside was not determined in the same way as the surface area of the entrance. A structure that looked like a cat’s collar from one side could contain a region larger than a galaxy on the other. What mattered was not size but time. Time inside the storage space was designed to pass more slowly than outside. While billions of years passed outside, only a few seconds elapsed within. Light that entered was preserved almost unchanged, and matter had no time to decay. The structure was completed near the end of galactic civilization. The stars had aged, rotating black holes had been mined, and civilization’s maintenance costs had long since exceeded its productive output. The last scientist traced the flow of energy whose destination had remained unknown until then. The first deficit had been recorded long before he was born. He did not reach into the past or send matter backward through time. Using the accumulated time differences of wormholes throughout galactic history, he completed the storage space so that energy already in motion could gather in the future. What he created was not the beginning of the outflow, but the destination of the energy that had already escaped. Photons from the past always moved into their own future. Light did not travel backward through time. Time folded before the light.
The conservation law had never been broken. Energy that disappeared in the past accumulated in the storage space in the future. If both sides were calculated as a single physical system, the amount entering and the amount leaving matched. But an observer in the past could not include a storage space that did not yet exist within the boundaries of the system. An observer in the future could not measure the accumulated energy without entering the storage space. Each age called “the universe” only what it could see. That was why energy appeared to be missing in every age. Researchers studying the deficit searched for a boundary. If energy was leaving, there had to be an exit somewhere. They examined the outskirts of the galaxy, the event horizons of black holes, and the throats of wormholes. The exit was not located in any one place. Every process in which energy was emitted and absorbed became part of the boundary. The storage space was not somewhere beyond the galaxy. In the age when the deficit was being observed, it had not yet even been created. Researchers could see the moment when energy disappeared, but they could not see the future in which that energy would arrive. Yet because they could not see the future, their faith in the conservation law became stronger, and that faith reassured civilization. If the energy had not truly disappeared, they believed, then one day it could be recovered. The same logic was applied when deficits were confirmed around black holes. The state of a black hole could be known from its externally measured mass, spin, and charge. Civilization extracted rotational energy and supplied it to the power grid, and small deficits also occurred while that energy was being transferred elsewhere. At first, these were treated as errors in the calculations of extraction and output. Later, the spin and mass of black holes declined faster than expected, but the surrounding power facilities compensated for the lower output by drawing more deeply on the black holes’ energy reserves. As extraction and loss continued, the event horizons also shrank. Civilization at the time recorded this as the collapse of black holes. The conservation law took no side. It required only that the total amount of energy remain the same. It did not record who lost it and who received it.