If you are in a closed cabin on a smoothly moving ship, no experiment inside can tell you whether you are moving or standing still.

Uniform motion cannot be detected by experiments inside the system.

Galileo asked readers to imagine tossing a ball or pouring water below deck on a perfectly smooth ship. Everything behaves exactly as it would on land. From this he drew the bold conclusion that Earth could be moving through space without us feeling it directly.

Why it matters That same idea about relative motion led, centuries later, to Einstein's theory.