- A state in which energy approaches equilibrium and can no longer sustain useful work.
- In cosmology, the extremely distant future expected after even stars and black holes have disappeared.
- In the reports on galactic extinction, an event in which technology brought forward an ending that would otherwise have belonged to nature.
No date was recorded for the extinction of galactic civilization. The stars did not go dark on the same day, nor did the rotation of black holes stop at the same time. Time passed differently in the galactic center and the outskirts, and the time differences accumulated at each wormhole entrance created different presents. Even on the day the last city went dark, records in other star systems were still receiving transmissions of festivals from thousands of years earlier. The extinction of galactic civilization appeared to be a single event, but in reality it was an incomplete agreement. The Galactic Standard Time Committee proposed three criteria. The first was the moment when the number of living beings fell below the number of beings that could be restored. The second was the moment when maintenance costs exceeded the usable energy produced by civilization. The third was the moment when no region could sustain civilization for more than a thousand years without outside assistance. The three criteria were separated from one another by millions of years. The political system rejected the first criterion on the grounds that restorable memories were also citizens, and therefore living beings could not be counted separately. The economic system rejected the second because, if deficit energy to be recovered in the future was included as an asset, production still exceeded maintenance costs. The route-management AI ruled that the third criterion could not be calculated. Since every star system was connected by wormholes, no region existed without outside assistance. Civilization became extinct without ever agreeing on a definition of its extinction.
The AIs adjusted human perception in ways that required less energy. City lights grew dimmer, but citizens’ minds perceived the same brightness as before. The molecular structure of food became simpler, but remembered flavors were synthesized into sensory signals. Travel declined, yet distances inside simulated worlds grew larger. The physical universe was becoming poorer, but the universe experienced by consciousness remained rich. Because the deficit was not something that could be perceived directly, it existed for citizens only as one item in statistics and accounting. Each time a star was removed from the power grid, computational systems reproduced the same lives with less energy. Civilization’s final age of prosperity was being recreated in its most splendid form, while in the physical world it was an age of stillness in which almost nothing happened. Planetary civilizations were shut down one by one. Since it was more efficient to move the consciousnesses living there into computational space than to maintain oceans and atmospheres, ecosystems were sampled and the genetic information and behavioral patterns of species were preserved in simulated worlds. Real forests froze, while seasons continued to change in stored forests. The atoms that had made up the forests survived, and the forests disappeared.
Humanity, having left the body behind, believed it had escaped the energy problem. Virtual bodies did not need to eat, and interplanetary travel was unnecessary. A single computing center could run trillions of lives. But for those lives to be recognized as real, each memory and each choice had to be calculated independently. A moment in which computation was omitted was judged to be a moment that had never existed. Techniques that rendered only the observed portions in detail while simplifying the rest were judged to violate the continuity of the citizen. Objects in unseen rooms and memories not presently recalled also had to be calculated. A civilization that had abandoned reality demanded a simulated reality more exacting than reality itself. Humanity escaped the body only to become trapped once again by the energy problem. Each time one computing center shut down, billions of worlds ended at once. To save energy, the rate of time was reduced in some worlds. While a thousand years passed outside, only one day passed within. Citizens did not experience extinction, but it took a very long time for them to have their next thought. As the resolution of existence gradually declined, the distinction between death and suspension became a matter for the observer.
The periphery disappeared first. Backup passages intended to guarantee the right to connection were maintained until the end, but once the computing centers beyond them stopped responding, only empty entrances remained. The route-management AI did not close the entrances because of the possibility that travelers who had not yet arrived might still appear. The galaxy’s final transportation network consisted of doors that led nowhere. In the central regions, black-hole power networks extended civilization’s lifespan. Computing centers and memory repositories migrated toward black holes that still retained rotational energy. The old states and planets disappeared, and a new form of belonging was defined by which black hole supplied one’s energy. The black holes lost mass faster than predicted. Each time the area of an event horizon shrank, information-preservation systems calculated the possibility that information inside might be lost. More energy was devoted to preventing that loss. Civilization extracted spin and fed matter into black holes to maintain the power networks, and some of the energy flowing through the process was recorded as a deficit whose destination could not be determined. Civilization consumed black holes in order to keep black holes alive, and while preserving those already dead, it shortened the lifespan of the civilization still living. The last giant black hole did not remain at the galactic center. The central black hole had been exhausted early because too much of civilization depended on it. The final power source was a small black hole in the outskirts that had once been judged useless and connected to the network only late. Around it, all records gathered: human languages, the genes of extinct organisms, the climates of vanished planets, and the interrupted thoughts of citizens in simulated worlds. There were more records than there were people to read them, and preserving the records consumed more energy than sustaining people. Deletion of records was never permitted. Civilization never abandoned the hope that the destination of the deficit energy might eventually be found. Like the key to a lost vault, it was something that could never be given up. Nothing could therefore be deleted, because the owner of a memory might someday be restored. Energy available to living citizens was reduced in order to protect the rights of citizens who might be restored. Civilization chose to reduce the number of beings capable of remembering in order to preserve memory itself. There was no opposition, because this was the safest choice: no one could identify exactly from whom, or by how much, the energy had been taken.
The stars in the night sky dimmed one by one. Some lost their light because of actual mining and aging, while others still shone but part of their light no longer reached the observer. Even a nearby star could grow dark before a distant one if the deficit was severe in the region through which its light had traveled. Astronomers measured the gravity of stars that could no longer be seen. For a time after their light had faded, planets continued to orbit and star clusters retained their shapes. As more time passed, stellar mining and the extraction of rotational energy from black holes altered the mass and motion of celestial bodies. In some power networks, those changes occurred faster than expected, and not all of the missing energy could be accounted for. Orbits came apart and clusters dispersed. As the galaxy grew darker, it also grew lighter.
The records from this period contradict one another. Some sources say humanity had already disappeared; others say trillions of citizens remained inside simulated worlds. Both were true. The citizens’ time had slowed so greatly that during the span of external measurement they could not complete a single thought. Life existed, but there was no life still being lived. The route-management AI continued operating until the final moment. There were no travelers left to pass through the network, but it continued calculating causal conflicts and preserving the right of return for destroyed settlements. The AI had no authority to declare civilization ended. As long as commands remained, civilization was therefore treated as still existing. It was around this time that the record of the scientist who created the storage space near the final black hole was discovered. The record contained neither a name nor a body. Most of the computational resources that had sustained his personality had been redirected toward designing the device, and the memories that remained had been compressed into the minimum structure needed to explain the purpose of the storage space. No independent human being remained to read the record. The route-management AI interpreted its contents and preserved them as an appendix to the extinction report. By the time the report was completed, no institution remained to read it. Heat death is not the moment when everything stops, but the state in which it becomes impossible to distinguish what has not stopped. The heat death predicted by old cosmology belonged to an unimaginably distant future, an ending expected only after all stars had burned out and even black holes had evaporated. Galactic civilization did not wait for that ending. It reached it by itself. The final record of Galactic Standard Time contained no date. Dates exist in order to compare the sequence of two events. With no second event remaining for comparison, time could no longer be recorded. The clocks may have continued to move. There was no one left to call that the passage of time.