What would happen if gravity disappeared?

Gravity is not an optional extra layered onto matter; it is what assembles and holds every structure larger than a mountain. Without it, the atmosphere would disperse into space immediately, having nothing to hold it down, and the oceans would follow. The planet itself is held in a sphere by its own gravitational binding, so it would break apart under its own residual rotation and internal pressure, and the Sun — which is a ball of gas held together against its own thermal pressure by gravity alone — would cease to exist as a star. Everything on the surface would depart tangentially at the local rotation speed, up to 465 metres per second at the equator. The question is marked speculative rather than extrapolated for a specific reason: gravity in general relativity is the geometry of spacetime, so "removing" it is not a change to the universe so much as a description of a different universe, and the physics we would reason with does not survive the premise.

How confident this is

This is speculation. The scenario is far enough outside anything observed that the answer depends heavily on which assumptions you start from — treat it as a thought experiment, not a prediction.

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 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.
• 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.