What would happen if you fell into a black hole?

The lethal quantity near a black hole is not gravity but the difference in gravity between your head and your feet, and that tidal gradient is stronger for smaller black holes. Falling into a stellar-mass black hole a few kilometres across, you would be stretched into a filament — the effect physicists genuinely call spaghettification — well outside the event horizon. Around a supermassive black hole such as the 4.3-million-solar-mass object at the centre of our galaxy, the horizon is so large that the tidal gradient there is mild, and you would cross it without any local sign that anything had happened. From outside, an observer would never see you cross: your image would slow, redden and dim asymptotically as light climbing out lost energy, fading rather than vanishing. Once inside, all futures lead to the singularity, and general relativity stops giving usable answers before you arrive.

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 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.