What would happen if the speed of light were slower?

The speed of light is not just a traffic limit; it appears throughout physics as a conversion factor between space and time and between mass and energy. Lower it and the energy released by nuclear fusion falls with the square of the change, since E = mc², so the Sun would produce far less energy from the same fuel and stellar structure would be entirely different. The fine-structure constant, which governs the strength of electromagnetic interaction and therefore the size and energy levels of atoms, is inversely proportional to c, so chemistry itself would change: bond energies, molecular structure and the colours of everything. Relativistic effects would become part of ordinary life, with noticeable time dilation and length contraction at everyday speeds if c were low enough. This is marked speculative because c is entangled with the constants that the reasoning uses, so changing it changes the rules mid-argument — physicists generally regard only dimensionless constants such as the fine-structure constant as meaningfully variable.

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 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 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.