Opening the book…
The speed of light in vacuum, c, is the same for every observer, which forces space and time to trade off so that no material object or signal can exceed it. If information could travel faster, some observer would see an effect precede its cause; the light-speed limit is really the guardian of causality. It is a property of spacetime itself, not merely of light.
Treat c as an absolute ceiling on any transfer of energy or information. When speeds approach it, stop adding velocities the everyday way and use the relativistic addition rule, which always keeps the total below c. Discount any scheme claiming instant action at a distance; check whether it actually moves a usable signal, or only an appearance.
The limit governs signals, not appearances. Phase velocities, the expansion of space, and quantum-entanglement correlations can all outpace c or span distance instantly, yet none of them carries usable information, so causality survives untouched.