What would happen if gravity doubled?

Doubling surface gravity doubles every weight without changing any mass. The immediate human effect is cardiovascular and skeletal: the heart would work considerably harder to push blood upward, standing for long periods would be exhausting, and falls would carry four times the impact energy for the same height because both weight and impact speed increase. Structures are designed with safety factors that are typically well under two, so a large fraction of bridges, towers and roofs would fail, and trees would be limited to a fraction of their current height because the compressive strength of wood is unchanged. Flight becomes the sharpest constraint, since lift scales with wing area while weight scales with volume, so the maximum size of a flying animal drops steeply. The atmosphere would compress into a thinner, denser layer near the surface, raising sea-level pressure and lowering the height at which air becomes unbreathable.

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 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.
• What would happen if a gamma-ray burst hit the Earth? A nearby one aimed at us would strip a large fraction of the ozone layer within seconds, with the damage arriving as ultraviolet over the following years.