Part 5/10:

This leads us to a pivotal realization: if light does not travel through a medium, then what precisely defines its speed? Approximately 300,000 km/s is a figure associated with light, yet it turns out that this speed is a universal constant not exclusive to light alone. Gravitational waves and other particles also move at comparable speeds.

The principle that the speed of light is consistently measured as 300,000 km/s regardless of the motion of the source or observer sets it apart from other waves. For instance, if you're in a spaceship traveling at nearly light speed, and you switch on a flashlight, the beam's speed does not add to your ship's velocity in the anticipated way. It still measures exactly 300,000 km/s—a groundbreaking concept that Einstein solidified.