What would happen if a gamma-ray burst hit the Earth?

Gamma-ray bursts are the most energetic explosions known, releasing in seconds what the Sun emits over its entire lifetime, but they are tightly beamed, so the danger requires one to be both close and pointed at us. The gamma rays themselves would not reach the ground — the atmosphere absorbs them — but that absorption is the problem, because it drives nitrogen and oxygen into nitrogen oxides that catalytically destroy ozone. Modelling suggests a burst within a few thousand light years could remove a substantial fraction of the ozone layer for years, admitting ultraviolet at levels damaging to surface and near-surface marine life, which is the base of the food chain. One hypothesis links the Late Ordovician mass extinction 440 million years ago to such an event, though it remains speculative. Bursts of this kind come from rare stellar events, and no candidate progenitor is close enough and oriented toward us.

How confident this is

This is a reasoned extrapolation rather than an observation. The mechanism is understood, but the scale of the consequences depends on assumptions that could reasonably be argued differently.

Physics

• What would happen if an asteroid hit the Earth? It depends enormously on size — most burn up, a 20-metre one broke windows across a city, and a 10-kilometre one ended the dinosaurs.
• What would happen if gravity disappeared? The Earth itself would come apart — gravity is the only thing holding a planet together.
• What would happen if the Sun became a black hole? Earth's orbit would not change at all — the mass would be the same. It would simply go dark and cold.
• What would happen if you fell into a black hole? Into a small one, tidal forces would stretch you apart before you arrived; into a supermassive one, you would cross the horizon without noticing.
• What would happen if you fell into a hole through the Earth? You would oscillate from one side to the other and back, with a one-way trip of about 38 minutes.
• What would happen if you travelled at the speed of light? You cannot — accelerating any mass to light speed requires infinite energy. Approaching it, time dilates dramatically.
• What would happen if gravity doubled? You would weigh twice as much, most tall structures would need rebuilding, and no bird larger than a sparrow could fly.
• What would happen if you were shrunk to the size of an ant? The square-cube law would break you: you would freeze, suffocate, and be unable to drink from a surface of water.
• What would happen if the speed of light were slower? Chemistry, nuclear fusion and the mass-energy relationship would all change — stars would not work as they do.