This question comes from Alan, who asks: If every human somehow simply disappeared from the face of the earth, how long would it be before the last artificial light source would go out?
We'll start with the obvious: most lights wouldn't last long, because the major power grids would go down relatively quickly.
Without people, there would be less demand for power, but our fridges and air conditioners and lava lamps would still be running.
Fossil fuel plants, which supply the vast majority of the world's electricity, require a steady supply of fuel, and their supply chains do involve people doing things.
As coal and oil plants started shutting down in the first few hours, other power sources would get hit with the extra load.
This kind of situation is difficult to handle even with human guidance. And the result would be a rapid series of cascading failures, leading to a blackout of all the major power grids.
Nuclear reactors, of course, don't require a steady supply of fuel: one reactor operator I talked to said that if their core settled into low-power mode, it could continue running almost indefinitely; a cube of uranium contains about six million times as much stored energy as a similar-sized cube of coal.
Unfortunately, although there's enough fuel, most nuclear reactors wouldn't keep running for long. As soon as something went wrong, the core would go into automatic shutdown.
Every part of a reactor control system is designed so that a failure causes it to rapidly shut down.
This would happen quickly; many things can trigger shutdown, but the most likely culprit would be the failure of the power grid.
